CONCRETE TECHNOLOGY

concrete:

Concrete is a mixture of binding material, aggregates and water in a definate proportion.

Types of concrete:

1. Cement concrete: It is a mixture of cement, fine aggregates, coarse aggregates and water in a definite proportion.

2. Lime concrete: Here binding material is lime (CaO)

3: RCC: Steel reinforcing is done in the Cement Concrete.

4: Prestressed cement concrete:This concrete is a form of concrete used in construction which is "pre-stressed" by being placed under compression prior to supporting any loads beyond its own dead weight. This compression is produced by the tensioning of high-strength "tendons" located within or adjacent to the concrete volume, and is done to improve the performance of the concrete in service

Uses of concrete:

1. Building Foundations: Provides strong support for residential, commercial, and industrial structures.
2. Road Construction: Used in highways, streets, and airport runways due to its durability.
3. Bridges and Dams: Essential for structural integrity and water resistance in large infrastructure projects.
4. Sidewalks and Driveways: Offers a long-lasting, low-maintenance surface for pedestrian and vehicle access.
5. Marine Structures: Used in docks, piers, and offshore platforms because of its ability to withstand harsh environments.
6. Architectural Elements: Allows for creative designs in modern architecture, including sculptures and decorative features.
7. Tunnels and Underground Constructions: Ensures safety and stability under varying geological conditions.

.Benefits of concrete:

There are numerous positive aspects of concrete:
1. It is a relatively cheap material and has a relatively long life with few maintenance requirements.
2. It is strong in compression
3. Before it hardens it is a very pliable substance that can easily be shaped.
4. It is non-combustible

Limitations of concrete:

The limitations of concrete include:
1.Relatively low tensile strength when compared to other building materials.
2. Low ductility.
3. Low strength-to-weight ratio.
4. It is susceptible to cracking

Cement:

cement is a binding material used in the masonry.

Types of Cement with Uses, Strength, Functions, and Major Ingredients:

Type of Cement	Uses	Strength (MPa)	Function	Major Ingredient
Ordinary Portland Cement (OPC)	General construction, pavements, bridges	30-50	Provides high early strength	Lime, Silica
Portland Pozzolana Cement (PPC)	Marine structures, sewage works, dams	25-35	Improves durability and reduces permeability	Fly ash, Silica
Rapid Hardening Cement	Road repairs, precast concrete	40-60	Achieves high strength in a short time	High Lime content
Low Heat Cement	Mass concrete structures like dams	20-30	Reduces heat of hydration	Low C3S, high C2S
Sulfate Resisting Cement	Structures exposed to sulfate attacks	30-45	Resists sulfate attack	Low C3A content
White Cement	Decorative works, tiles, flooring	35-45	Provides aesthetic finish	Low Iron oxide

Physical Properties of Cement

Different blends of cement used in construction are characterized by their physical properties. Some key parameters control the quality of cement. The physical properties of good cement are based on:

1. Fineness of cement
2. Soundness
3. Consistency
4. Strength
5. Setting time
6. Heat of hydration
7. Loss of ignition
8. Bulk density
9. Specific gravity (Relative density)

Test names associated with these physical properties.

1. Fineness of Cement

The size of the particles of the cement is its fineness. The required fineness of good cement is achieved through grinding the clinker in the last step of cement production process. As hydration rate of cement is directly related to the cement particle size, fineness of cement is very important.

2. Soundness of Cement

It refers to the ability of cement to not shrink upon hardening. Good quality cement retains its volume after setting without delayed expansion, which is caused by excessive free lime and magnesia.
Tests: Unsoundness of cement may appear after several years, so tests for ensuring soundness must be able to determine that potential.

Le Chatelier Test

This method, done by using Le Chatelier Apparatus, tests the expansion of cement due to lime. Cement paste (normal consistency) is taken between glass slides and submerged in water for 24 hours at 20+1*C. It is taken out to measure the distance between the indicators and then returned under water, brought to boil in 25-30 mins and boiled for an hour. After cooling the device, the distance between indicator points is measured again. In a good quality cement, the distance should not exceed 10 mm.

3.Consistency of Cement

The ability of cement paste to flow is consistency. It is measured by Vicat Test. In Vicat Test Cement paste of normal consistency is taken in the Vicat Apparatus. The plunger of the apparatus is brought down to touch the top surface of the cement. The plunger will penetrate the cement up to a certain depth depending on the consistency. A cement is said to have a normal consistency when the plunger penetrates 10±1 mm.

4. Strength of cement

Three types of strength of cement are measured - compressive, tensile and flexural. Various factors affect the strength, such as water-cement ratio, cement-fine aggregate ratio, curing conditions, size and shape of a specimen, the manner of molding and mixing, loading conditions and age. While testing the strength, the following should be considered:
Cement mortar strength and cement concrete strength are not directly related. Cement strength is merely a quality control measure.
The tests of strength are performed on cement mortar mix, not on cement paste. Cement gains strength over time, so the specific time of performing the test should be mentioned.

5. Compressive Strength

It is the most common strength test. A test specimen (50mm) is taken and subjected to a compressive load until failure. The loading sequence must be within 20 seconds and 80 seconds.

Tensile strength

Though this test used to be common during the early years of cement production, now it does not offer any useful information about the properties of cement.

Flexural strength

This is actually a measure of tensile strength in bending. The test is performed in a 40 x40 x 160 mm cement mortar beam, which is loaded at its center point until failure.

6. Setting Time of Cement

Cement sets and hardens when water is added. This setting time can vary depending on multiple factors, such as fineness of cement, cement-water ratio, chemical content, and admixtures. Cement used in construction should have an initial setting time that is not too low and a final setting time not too high. Hence, two setting times are measured:

Initial set:

It is the time taken by cement paste to start hardening after mixing with water.
During this period, the cement remains in a plastic state and can be molded into any shape.

Ordinary Portland Cement (OPC): Minimum 30 minutes .
Rapid Hardening Cement: Around 5-10 minutes.

Final set:

It is the time taken by cement paste to completely harden and gain sufficient strength. After this time, the structure can bear loads without deformation.
Ordinary Portland Cement (OPC): Maximum 600 minutes (10 hours)

7. Heat of Hydration

When water is added to cement, the reaction that takes place is called hydration. Hydration generates heat, which can affect the quality of the cement and also be beneficial in maintaining curing temperature during cold weather. On the other hand, when heat generation is high, especially in large structures, it may cause undesired stress. The heat of hydration is affected most by C₃S and C₃A present in cement, and also by water-cement ratio, fineness and curing temperature. The heat of hydration of Portland cement is calculated by determining the difference between the dry and the partially hydrated cement (obtained by comparing these at 7^th and 28^th days).

8. Loss of Ignition

Heating a cement sample at 900 - 1000°C (that is, until a constant weight is obtained) causes weight loss. This loss of weight upon heating is calculated as loss of ignition. Improper and prolonged storage or adulteration during transport or transfer may lead to pre-hydration and carbonation, both of which might be indicated by increased loss of ignition.

9. Bulk density

When cement is mixed with water, the water replaces areas where there would normally be air. Because of that, the bulk density of cement is not very important. Cement has a varying range of density depending on the cement composition percentage. The density of cement may be anywhere from 62 to 78 pounds per cubic foot.

10. Specific Gravity (Relative Density)

Specific gravity is generally used in mixture proportioning calculations. Portland cement has a specific gravity of 3.15, but other types of cement (for example, portland-blast-furnace-slag and portland- pozzolan cement) may have specific gravities of about 2.90.

Chemical Properties of Cement

The raw materials for cement production are limestone (calcium), sand or clay (silicon), bauxite (aluminum) and iron ore, and may include shells, chalk, marl, shale, clay, blast furnace slag, slate. Chemical analysis of cement raw materials provides insight into the chemical properties of cement.

1. Tricalcium aluminate (C₃A)

Low content of C₃A makes the cement sulfate-resistant. Gypsum reduces the hydration of C3A, which liberates a lot of heat in the early stages of hydration. C₃A does not provide any more than a little amount of strength.
Type I cement: contains up to 3.5% S0₃ (in cement having more than 8% C₃A)
Type II cement: contains up to 3% SO₃ (in cement having less than 8% C₃A)

2. Tricalcium silicate (C₃S)

C₃S causes rapid hydration as well as hardening and is responsible for the cement's early strength gain an initial setting.

3. Dicalcium silicate (C₂S)

As opposed to tricalcium silicate, which helps early strength gain, dicalcium silicate in cement helps the strength gain after one week.

4. Ferrite (C₄AF)

Ferrite is a fluxing agent. It reduces the melting temperature of the raw materials in the kiln from 3,000°F to 2,600°F. Though it hydrates rapidly, it does not contribute much to the strength of the cement.

5. Magnesia (MgO)

The manufacturing process of Portland cement uses magnesia as a raw material in dry process plants. An excess amount of magnesia may make the cement unsound and expansive, but a little amount of it can add strength to the cement. Production of MgO-based cement also causes less CO₂ emission. All cement is limited to a content of 6% MgO.

7. Iron oxide/ Ferric oxide

Aside from adding strength and hardness, iron oxide or ferric oxide is mainly responsible for the color of the cement.

8. Alkalis

The amounts of potassium oxide (K₂O) and sodium oxide (Na₂0) determine the alkali content of the cement. Cement containing large amounts of alkali can cause some difficulty in regulating the setting time of cement. Low alkali cement, when used with calcium chloride in concrete, can cause discoloration. In slag-lime cement, ground granulated blast furnace slag is not hydraulic on its own but is "activated" by addition of alkalis. There is an optional limit in total alkali content of 0.60%, calculated by the equation Na₂0 + 0.658 K₂O.

9. Free lime

Free lime, which is sometimes present in cement, may cause expansion.

10. Silica fumes

Silica fume is added to cement concrete in order to improve a variety of properties, especially compressive strength, abrasion resistance and bond strength. Though setting time is prolonged by the addition of silica fume, it can grant exceptionally high strength. Hence, Portland cement containing 5- 20% silica fume is usually produced for Portland cement projects that require high strength. Cement containing high alumina has the ability to withstand frigid temperatures since alumina is chemical-resistant. It also quickens the setting but weakens the cement.

Ingredient	Function	Percentage (%)	Excessive Use Causes
Lime (CaO)	Provides strength and soundness	60-65%	Excess causes unsoundness and expansion
Silica (SiO₂)	Increases strength and durability	17-25%	Excess makes cement slow-setting
Alumina (Al₂O₃)	Improves setting time and provides resistance to chemical attacks	3-8%	Excess reduces strength
Iron Oxide (Fe₂O₃)	Provides color and helps in fusion of raw materials	0.5-6%	Excess reduces setting time and strength
Magnesia (MgO)	Increases strength and hardness	0.1-4%	Excess causes cracks and unsoundness
Sulphur Trioxide (SO₃)	Controls setting time	1-3%	Excess causes expansion and disruption
Alkalies (Na₂O, K₂O)	Increases reactivity with silica	0.2-1%	Excess causes efflorescence

Aggregates:

Aggregates are the important constituents of the concrete which give body to the concrete and also reduce shrinkage. Aggregates occupy 70 to 80% of total volume of concrete. So, we can say that one should know definitely about the aggregates in depth to study more about concrete.

Classification of Aggregates as per Size and Shape

1.Classification of Aggregates Based on Shape

We know that aggregate is derived from naturally occurring rocks by blasting or crushing etc., so, it is difficult to attain required shape of aggregate. But, the shape of aggregate will affect the workability of concrete. So, we should take care about the shape of aggregate. This care is not only applicable to parent rock but also to the crushing machine used.

2.Classified Of Aggregate according to shape into the following types

1. Rounded aggregates
2. Irregular or partly rounded aggregates
3. Angular aggregates
4. Flaky aggregates
5. Elongated aggregates
6. Flaky and elongated aggregates

Rounded Aggregate

The rounded aggregates are completely shaped by attrition and available in the form of seashore gravel. Rounded aggregates result the minimum percentage of voids (32-33%) hence gives more workability. They require lesser amount of water-cement ratio. They are not considered for high strength concrete because of poor interlocking behavior and weak bond strength.

Irregular Aggregates

The irregular or partly rounded aggregates are partly shaped by attrition and these are available in the form of pit sands and gravel. Irregular aggregates may result 35- 37% of voids. These will give lesser workability when compared to rounded aggregates. The bond strength is slightly higher than rounded aggregates but not as required for high strength concrete

Angular Aggregates

The angular aggregates consist well defined edges formed at the intersection of roughly planar surfaces and these are obtained by crushing the rocks. Angular aggregates result maximum percentage of voids (38-45%) hence gives less workability. They give 10-20% more compressive strength due to development of stronger aggregate-mortar bond. So, these are useful in high strength concrete manufacturing.

Flaky Aggregates

When the aggregate thickness is small when compared with width and length of that aggregate it is said to be flaky aggregate. Or in the other, when the least dimension of aggregate is less than the 60% of its mean dimension then it is said to be flaky aggregate.

Elongated Aggregates

When the length of aggregate is larger than the other two dimensions then it is called elongated aggregate or the length of aggregate is greater than 180% of its mean dimension.

Flaky and Elongated Aggregates

When the aggregate length is larger than its width and width is larger than its thickness then it is said to be flaky and elongated aggregates. The above 3 types of aggregates are not suitable for concrete mixing. These are generally obtained from the poorly crushed rocks.

3.Classification of Aggregates Based on Size

Aggregates are available in nature in different sizes. The size of aggregate used may be related to the mix proportions, type of work etc. the size distribution of aggregates is called grading of aggregates. Following are the classification of aggregates based on size:
o Fine aggregate
o Coarse aggregate

Fine Aggregate

When the aggregate is sieved through 4.75mm sieve, the aggregate passed through it called as fine aggregate. Natural sand is generally used as fine aggregate, silt and clay are also come under this category. The soft deposit consisting of sand, silt and clay is termed as loam. The purpose of the fine aggregate is to fill the voids in the coarse aggregate and to act as a workability agent.

Coarse Aggregate

When the aggregate is sieved through 4.75mm sieve, the aggregate retained is called coarse aggregate. Gravel, cobble and boulders come under this category. The maximum size aggregate used may be dependent upon some conditions. In general, 40mm size aggregate used for normal strengths and 20mm size is used for high strength concrete.

Bulking of Sand

The increase in moisture of sand increases the volume of sand. The reason is that moisture causes film of water around sand particles which results in the increase of volume of sand. For a moisture content percentage of 5 to 8 there will be an increase in volume up to 20 to 40 percent depending upon sand. If the sandisfinertherewillbemoreincreaseinvolume. This is known as bulking of sand.

When the moisture content of sand is increased by adding more water, the sand particles pack near each other and the amount of bulking of sand is decreased. Thus, it helps in determining the actual volume of sand, the dry sand and the sand completely filled with water will have the exact volume.

The volumetric proportioning of sand is greatly affected by bulking of sand to a greater extent. The affected volume will be great for fine sand and will be less for coarse sand. If proper allowance is not made for the bulking of sand, the cost of concrete and mortar increases and it results into under-sanded mixes which are harsh and difficult for working and placing.

SPECIFIC GRAVITY AND WATER ABSORPTION TEST

i) To measure the strength or quality of the material
ii) To determine the water absorption of aggregates

APPARATUS:

The apparatus consists of the following
(a) A balance of capacity about 3kg, to weigh accurate 0.5g, and of such a type and shape as to permit weighing of the sample container when suspended in water.
(b) A thermostatically controlled oven to maintain temperature at 100-110° C.
(c) A wire basket of not more than 6.3 mm mesh or a perforated container of convenient size with thin wire hangers for suspending it from the balance.
(d) A container for filling water and suspending the basket
(e) An air tight container of capacity similar to that of the basket
(f) A shallow tray and two absorbent clothes, each not less than 75x45cm.

THEORY:

The specific gravity of an aggregate is considered to be a measure of strength or quality of the material. Stones having low specific gravity are generally weaker than those with higher specific gravity values.

PROCEDURE:

(i) About 2 kg of aggregate sample is washed thoroughly to remove fines, drained and placed in wire basket and immersed in distilled water at a temperature between 22- 32º C and a cover of at least 5cm of water above the top of basket.
(ii) Immediately after immersion the entrapped air is removed from the sample by lifting the basket containing it 25 mm above the base of the tank and allowing it to drop at the rate of about one drop per second. The basket and aggregate should remain completely immersed in water for a period of 24 hour afterwards.
(iii) The basket and the sample are weighed while suspended in water at a temperature of 22° – 32°C. The weight while suspended in water is noted =W₁g.
(iv) The basket and aggregates are removed from water and allowed to drain for a few minutes, after which the aggregates are transferred to the dry absorbent clothes. The empty basket is then returned to the tank of water jolted 25 times and weighed in water=W₂g. .
(v) The aggregates placed on the absorbent clothes are surface dried till no further moisture could be removed by this cloth. Then the aggregates are transferred to the second dry cloth spread in single layer and allowed to dry for at least 10 minutes until the aggregates are completely surface dry. The surface dried aggregate is then weighed =W₃ g
(vi) The aggregate is placed in a shallow tray and kept in an oven maintained at a temperature of 110° C for 24 hrs. It is then removed from the oven, cooled in an air tight container and weighted=W₄ g.

Specific gravity = (dry weight of the aggregate /Weight of equal volume of water)

Apparent specific gravity = (dry weight of the aggregate/Weight of equal volume of water excluding air voids in aggregate

Hydration of Cement

Portland cement is a hydraulic cement; hence it derives its strength from chemical reactions between the cement and water. The process is known as hydration.
Cement consists of the following major compounds :
1.Tricalcium silicate, C₃S
2.Dicalcium silicate, C₂S
3.Tricalcium aluminate, C₃A
4.Tetra calcium aluminoferrite, C.AF
5.Gypsum, CSH:

Water-cement ratio

The water-cement ratio is the ratio of the weight of water to the weight of cement used in a concrete mix. A lower ratio leads to higher strength and durability, but may make the mix difficult to work with and form. Workability can be resolved with the use of plasticizers or super-plasticizers. Often, the ratio refers to the ratio of water to cement plus pozzolan ratio, w/(c+p). The pozzolan is typically a fly ash, or blast furnace slag. It can include a number of other materials, such as silica fume, rice husk ash or natural pozzolans. Pozzolans can be added to strengthen concrete.

Duff Abrams' law

The notion of water-cement ratio was first developed by Duff A. Abrams and published in 1918. Concrete hardens as a result of the chemical reaction between cement and water (known as hydration, this produces heat and is called the heat of hydration). For every pound (or kilogram or any unit of weight) of cement, about 0.35 pounds (or 0.35 kg or corresponding unit) of water is needed to fully complete hydration reactions.
However, a mix with a ratio of 0.35 may not mix thoroughly, and may not flow well enough to be placed. More water is therefore used than is technically necessary to react with cement. Water- cement ratios of 0.45 to 0.60 are more typically used. For higher-strength concrete, lower ratios are used, along with a plasticizer to increase flowability. Too much water will result in segregation of the sand and aggregate components from the cement paste

Effect of Water Cement Ratio on Strength of Concrete:

The water-cement ratio is one of the most important aspect when it comes to maintaining the strength of Concrete. The ratio depends on the grade of concrete and the structure size. We generally prefer a W/C ratio of 0.4 to 0.6, but it can be decreased in case of high-grade concrete, we reduce the amount of water and use plasticizers instead.
W/C ratio affects the workability of concrete and thus should be taken into careful consideration. Also, if the ratio exceeds the normal value, segregation of concrete occurs and the coarse aggregate settles at the bottom, thus affecting the strength of concrete greatly.

Limitation of Water Cement Law:

1. The internal moisture condition of hydration of cement continues till the concrete gain full strength.
2. The concrete specimen is cured under standard temperatures.
3. The concrete specimens should of same size.
4. The concrete specimens should of same age.

Workability

Workability is defined as the amount of energy required to overcome internal friction and cause complete compaction. Workability is completely depending upon the properties of various ingredients of concrete.

Factors Affecting Workability

1.Cement content of concrete
2.Water content of concrete Mix
3.proportions of concrete
4.Size of aggregates
5.Shape of aggregates
6.Grading of aggregates
7.Surface texture of aggregates
8. Use of admixtures in concrete
9.Use of supplementary cementitious materials

Following are the general factors affecting concrete workability:

1. Cement Content of Concrete

Cement content affects the workability of concrete in good measure. More the quantity of cement, the more will be the paste available to coat the surface of aggregates and fill the voids between them. This will help to reduce the friction between aggregates and smooth movement of aggregates during mixing, transporting, placing and compacting of concrete.
Also, for a given water-cement ratio, the increase in the cement content will also increase the water content per unit volume of concrete increasing the workability of concrete. Thus, increase in cement content of concrete also increases the workability of concrete.

2. Type and Composition of Cement

There are also affect of type of cement or characteristics of cement on the workability of concrete. The cement with increase in fineness will require more water for same workability than the comparatively less fine cement. The water demand increased for cement with high Al203 or C2S contents.

3. Water/Cement Ratio or Water Content of Concrete

Water/cement ratio is one of the most important factors which influence the concrete workability. Generally, a water cement ratio of 0.45 to 0.6 is used for good workable concrete without the use of any admixture. Higher the water/cement ratio, higher will be the water content per volume of concrete and concrete will be more workable.
Higher water/cement ratio is generally used for manual concrete mixing to make the mixing process easier. For machine mixing, the water/cement ratio can be reduced. These generalized method of using water content per volume of concrete is used only for nominal mixes.
For designed mix concrete, the strength and durability of concrete is of utmost importance and hence water cement ratio is mentioned with the design. Generally designed concrete uses low water/cement ratio so that desired strength and durability of concrete can be achieved.

4. Mix Proportions of Concrete

Mix proportion of concrete tells us the ratio of fine aggregates and coarse aggregates w.r.t. cement quantity. This can also be called as the aggregate cement ratio of concrete. The more cement is used, concrete becomes richer and aggregates will have proper lubrications for easy mobility or flow of aggregates.
The low quantity of cement w.r.t. aggregates will make the less paste available for aggregates and mobility of aggregates is restrained.

5. Size of Aggregates

Surface area of aggregates depends on the size of aggregates. For a unit volume of aggregates with large size, the surface area is less compared to same volume of aggregates with small sizes.
When the surface area increases, the requirement of cement quantity also increases to cover up the entire surface of aggregates with paste. This will make more use of water to lubricate each aggregate. Hence, lower sizes of aggregates with same water content are less workable than the large size aggregates.

6. Shape of Aggregates

The shape of aggregates affects the workability of concrete. It is easy to understand that rounded aggregates will be easy to mix than elongated, angular and flaky aggregates due to less frictional resistance.
Other than that, the round aggregates also have less surface area compared to elongated or irregular shaped aggregates. This will make less requirement of water for same workability of concrete. This is why river sands are commonly preferred for concrete as they are rounded in shape.

7. Grading of Aggregates

Grading of aggregates have the maximum effect on the workability of concrete. A well graded aggregates have all sizes in required percentages. This helps in reducing the voids in a given volume of aggregates.
The less volume of voids makes the cement paste available for aggregate surfaces to provide better lubrication to the aggregates.
With less volume of voids, the aggregate particles slide past each other and less compacting effort is required for proper consolidation of aggregates. Thus, low water cement ratio is sufficient for properly graded aggregates.

8. Surface Texture of Aggregates

Surface texture such as rough surface and smooth surface of aggregates affects the workability of concrete in the same way as the shape of aggregates.
With rough texture of aggregates, the surface area is more than the aggregates of same volume with smooth texture. Thus, concrete with smooth surfaces are more workable than with rough textured aggregates.

9. Use of Admixtures in Concrete

There are many types of admixtures used in concrete for enhancing its properties. There are some workability enhancer admixtures such as plasticizers and superplasticizers which increases the workability of concrete even with low water/cement ratio.
They are also called as water reducing concrete admixtures. They reduce the quantity of water required for same value of slump.
Air entraining concrete admixtures are used in concrete to increase its workability. This admixture reduces the friction between aggregates by the use of small air bubbles which acts as the ball bearings between the aggregate particles.

Measurement of Workability:

Slump Test.

Concrete slump test is to determine the workability or consistency of concrete mix prepared at the laboratory or the construction site during the progress of the work. Concrete slump test is carried out from batch to batch to check the uniform quality of concrete during construction.
The slump test is the simplest workability test for concrete, involves low cost and provides immediate results. Due to this fact, it has been widely used for workability tests since 1922.
Generally concrete slump value is used to find the workability, which indicates water-cement ratio, but there are various factors including properties of materials, mixing methods, dosage, admixtures etc. also affect the concrete slump value.

Factors which influence the concrete slump test:

1.Material properties like chemistry, fineness, particle size distribution, moisture content and temperature of cementitious materials.
2.Size, texture, combined grading, cleanliness and moisture content of the aggregates,
3.Chemical admixtures dosage, type, combination, interaction, sequence of addition and its effectiveness,
4.Air content of concrete
5.Concrete batching, mixing and transporting methods and equipment,
6.Temperature of the concrete
7.Sampling of concrete, slump-testing technique and the condition of test equipment
8.The amount of free water in the concrete
9.Time since mixing of concrete at the time of testing.

Slump Test Procedure & Result Interpretation

Procedure:
Apparatus Required:

1.Slump cone (300mm height, 200mm base diameter, 100mm top diameter)
2.Tamping rod (600mm length, 16mm diameter)
3.Base plate
4.Measuring scale

Steps:

Place the slump cone on a leveled surface.
Fill the cone in three layers, each layer being tamped 25 times using a tamping rod.
Remove excess concrete from the top and smooth the surface.
Lift the cone vertically and allow the concrete to slump.
Measure the difference in height (slump value) using a scale.

Slump Test Result Interpretation

Slump Value (mm)	Workability	Application
0-25 mm	Very low	Dry mixes, road pavements
25-50 mm	Low	Mass concrete, foundations
50-100 mm	Medium	General reinforced concrete
100-175 mm	High	High workability applications
>175 mm	Very high (collapse)	Overly wet, may cause segregation

1. Compaction Factor Test

Objective:

To determine the workability of fresh concrete by calculating the compaction factor.

Apparatus Required:

Compaction Factor Apparatus (consisting of two hoppers and a cylindrical mold)
Trowel
Weighing balance
Tamping rod
Steel scoop

Test Procedure:

Prepare the Sample: Mix the fresh concrete properly.
Filling the Upper Hopper: Fill the top hopper with fresh concrete without compacting it.
Allow Free Fall: Open the trap door at the bottom of the top hopper so that the concrete falls into the lower hopper.
Second Free Fall: Open the trap door of the lower hopper, allowing the concrete to fall into the cylindrical mold below. Leveling and Weighing:
Cut off the excess concrete on top of the cylinder using a trowel.
Weigh the concrete filled in the cylindrical mold. This gives the partially compacted weight (W1).
Fully Compacted Weight:
Fill the same cylindrical mold with the same sample of concrete in layers, compacting each layer using a tamping rod. Weigh this fully compacted concrete. This gives the fully compacted weight (W2).

Calculation:

Interpretation of Results:

Compaction Factor Range	Workability	Use Case
0.85 – 0.95	Low workability	Used for RCC and heavily reinforced structures
0.95 – 1.0	Medium workability	Suitable for normal concrete works
Above 1.0	High workability	Used for flowing concrete or self-compacting concrete

VEE-BEE CONSISTOMETER TEST

The Vee-Bee Consistometer Test is a method used to determine the workability of concrete, particularly for mixes with low workability, such as those used in mass concrete and heavy reinforcement. It measures the time required for the concrete to flow from a conical shape to a fully compacted shape under specific vibration.

Objective

To determine the workability of concrete by measuring the time required for the concrete to flow in the Vee-Bee Consistometer.

Apparatus Required

1. Vee-Bee Consistometer: A cylindrical glass tube mounted vertically on a vibrating platform.

2. Conical Mould: A mold for shaping the concrete before testing.

3. Tamping Rod: To compact the concrete in the mold.

4. Measuring Tape: To measure the height of the conical sample.

5. Vibration Source: Usually a mechanical or electrical vibrator.

Test Procedure

1. Prepare the Concrete Sample: Prepare the fresh concrete sample as per standard guidelines.

2. Fill the Conical Mold: Place the conical mold in the Vee-Bee Consistometer apparatus.
   • Fill the mold in three layers, each compacted with a tamping rod.

3. Start the Vibration: Once the mold is filled, initiate the vibration.
   • Vibrate the concrete for a specific duration.

4. Observe the Concrete Flow: As the concrete starts to flow and settle in the cylinder, measure the time it takes to reach a certain compacted height.

5. Record the Time: The time taken for the concrete to flow is recorded as the Vee-Bee Time (in seconds).

Calculation and Results

The Vee-Bee Time is measured in seconds and indicates the workability of the concrete. The shorter the time, the higher the workability.

Interpretation of Vee-Bee Time:

Vee-Bee Time (seconds)	Workability	Use Case
0-5	High workability	Self-compacting concrete
5-15	Medium workability	Normal concrete
15-30	Low workability	Heavily reinforced concrete

COMPARISON OF WORKABILITY MEASUREMENTS BY VARIOUS METHODS

Workability Description	Slump in mm	Vee-Bee Time in Seconds	Compacting Factor
Extremely dry	32 – 18	–	0.70
Very stiff	18 – 10	0.70	0.75
Stiff	0 – 25	10 – 5	0.75
Stiff plastic	25 – 50	5 – 3	0.85
Plastic	75 – 100	3 – 0	0.90
Flowing	150 – 175	–	0.95

CONCRETE MIX DESIGN

Concrete mix design may be defined as the art of selecting suitable ingredients of concrete and determining their relative proportions with the object of producing concrete of certain minimum strength & durability as economically as possible.

Objectives of Mix Design

•To achieve the designed/ desired workability in the plastic stage
• To achieve the desired minimum strength in the hardened stage
• To achieve the desired durability in the given environment conditions
• To produce concrete as economically as possible.

Factors to be considered for mix design

• The grade designation giving the characteristic strength requirement of concrete.
• The type of cement influences the rate of development of compressive strength of concrete.
• Maximum nominal size of aggregates to be used in concrete may be as large as possible within the limits prescribed by IS 456:2000.
• The cement content is to be limited from shrinkage, cracking and creep.
• The workability of concrete for satisfactory placing and compaction is related to the size and shape of section, quantity and spacing of reinforcement and technique used for transportation, placing and compaction.

Considerations
• Cost
• Specification
• Workability
• Strength and Durabilit

TYPES OF CONCRETE MIXES

1.NOMINAL MIX

Nominal Mix refers to a fixed proportion of the ingredients, typically used for concrete preparation. This mix is suitable for small construction works where the exact proportions are not critical. It is used for general-purpose concrete where high precision is not necessary.

Common Nominal Mix Ratios

Grade	Mix Ratio
M5	1:5:10
M7.5	1:4:8
M10	1:3:6
M15	1:2:4
M20	1:1.5:3

Applications

• Small construction works such as footpaths, pavements, etc.

• General-purpose concrete where the exact mix ratio is not critical.

• Concrete for non-structural elements.

Advantages

• Simple and easy to prepare.

• Cost-effective for small-scale works.

• Used where the exact strength requirements are not specified.

Disadvantages

• Less control over the mix ratio.

• May lead to variability in the strength of concrete.

• Not suitable for large or critical structural works.

2. Design Mix

Design Mix is a mix prepared based on the requirements of strength, durability, workability, and other specific parameters. Unlike the Nominal Mix, the proportions in a Design Mix are determined through trial and error based on the specific needs of the construction. It is used for large-scale construction where precise control over the concrete mix is necessary.

Characteristics of Design Mix

• Strength-Based: The mix proportions are based on the desired strength of the concrete (e.g., M25, M30, etc.).

• Precise Control: The mix is designed to meet specific requirements such as workability, durability, and shrinkage control.

• Trial Mix: The mix is determined through a series of trials to achieve the required properties.

Factors Influencing Choice of Mix Design

Grade Proportions (Design Mix)

• Grade of Concrete
• Type of Cement
• Maximum nominal Size of Aggregate
• Grading of Combined aggregate
• Maximum Water/ Cement Ratio
• Workability
• Durability
• Quality Control.

Grade	Proportions (C: FA: CA)
M25	1:1:2
M30	1:0.75:1.5
M35	1:0.5:1.5
M40	1:0.5:1.2
M45	1:0.45:1.1

Applications of Design Mix

• Large-scale construction works such as multi-story buildings, bridges, and dams.

• High-performance concrete where durability and strength are critical.

• Infrastructure projects that require precise control over the properties of concrete.

Advantages of Design Mix

• Customized Strength: The mix is designed for specific strength requirements, ensuring higher performance.

• Optimized Material Usage: Proper proportioning reduces wastage of materials and ensures cost-effectiveness.

• Consistency: Consistent quality of concrete is maintained with precise mix design.

Disadvantages of Design Mix

• Complexity: The design mix involves calculations, trials, and testing, making it more complex than a nominal mix.

• Higher Cost: Due to the need for testing and trials, design mixes can be more expensive than nominal mixes.

• Time-Consuming: It requires more time for trial mixes and quality control processes.

Aspect	Nominal Mix	Design Mix (Controlled Concrete)
Definition	Produced by taking standard arbitrary proportions of ingredients.	Process of selecting suitable ingredients and determining their relative proportions to produce concrete of a certain minimum strength and durability.
Proportions	Ratio in volume (strength and cost vary).	Ratio in weight (strength constant, cost reduced).
Type of Concrete	Prescriptive type concrete.	Performance-based concrete.
Usage	Used for ordinary concrete, typically grades not higher than M20.	Used for higher-grade concrete (greater than M20).
Quality Control	No quality control.	Quality control applied.
Water-Cement Ratio	Based on durability criteria, experience, and practical trials.	Based on concrete grade, 28-day compressive strength, and durability of concrete.
Water Content Adjustment	Can be modified by slump value (field-based test).	Can be modified based on compaction factor value (laboratory-based test).
Entrapped Air Content	No entrapped air content considered.	Entrapped air content considered according to nominal maximum size aggregates.
Trial Mixes	Trial mixes concept is mentioned.	Not much consideration for trial mixes.

Admixtures are chemicals, added to concrete, mortar or grout at the time of mixing, to modify the properties, either in the wet state immediately after mixing or after the mix has hardened. They can be a single chemical or a blend of several chemicals and may be supplied as powders but most are aqueous solutions because in this form, they are easier to accurately dispense into, and then disperse through the concrete.

• Admixtures are ingredients other than basic ingredients cement, water and aggregates that are added to concrete batch immediately before or during mixing to modify one or more of the specific properties of concrete in fresh and hardened state.
• Added in small quantity either in powder or liquid form
• Combination is used when more than one property to be altered.

Categories of Admixtures:

• Active materials are those which react chemically with a component within the Cementations materials.
• Surface active admixtures (surfactants): These are generally split into two components (one positively charged and the other negatively charged) and react with the air - water - solid material interface within the mortar thereby resulting in orientation and adsorption.
• Passive or inert admixtures: These do not change their form but have a physical effect such as light absorption and reflection as in the case of pigments.

Function of Admixtures

• Increase slump and workability
• Retard or accelerate initial setting
• Reduce or prevent shrinkage
• Modify the rate or capacity for bleeding
• Reduce segregation
• Decrease weight of concrete
• Improve durability
• Decrease the rate of heat of hydration
• Reduce permeability
• To make porous concrete
• To make coloring concrete
• To protect chemical attack
• Increase bond of concrete to steel reinforcement
• Increase strength (compressive, tensile, or flexural)
• Increase bond between existing and new concrete

Types of Admixture

1. Accelerating admixtures
2. Retarding admixtures
3. Water-reducing admixtures
4. Air-entrainment admixtures
5. Super plasticizers admixtures
6. Pozzolana admixtures
7. Grouting admixtures
8. Waterproofing admixtures
9. Air-detraining admixtures
10. Bonding admixtures
11. Corrosion inhibiting admixtures
12. Gas forming admixtures
13. Colouring admixtures
14. Alkali-aggregate expansion inhibiting admixtures
15. Fungicidal, germicidal, insecticidal admixtures

1.Accelerators:

Main objective of using accelerators in concrete is to increase speed of setting and hardening.

Advantages of using Accelerators:

• To remove formwork quickly
• To reduce the curing time
• To use structure earlier
• To finish the surface fast
• To increase the speed of construction
• For quick repairing work

Main Accelerators:

• Calcium Chloride (CaCl2)
• Soluble Carbonates
• Silicates

CaCl2 is the most commonly used accelerator in construction. If 2% CaCl2 by weight of cement is added in concrete, then it decreases the setting time from 3 to 1 hour and final setting time from 6 to 2 hours. We can get a two-day strength in 1 day at 21°C temperature. If the proportion of CaCl2 is more than 3% then flash set of concrete takes place, and drying shrinkage and creep will increase.

2. Retarders:

To decrease the speed of Hydration and setting, Retarders are used in concrete. Retarders make concrete plastic and workable for a long time.

Objectives of using Retarders:

• To decrease the setting time
• To increase strength by decreasing W/C Ratio
• To do concreting in hot areas
• For grouting of oil wells

Main Retarders:

• Calcium Sulphate (Gypsum)
• Starches
• Sugars
• Cellulose Products
• Acids or Salts

Gypsum is the most commonly used retarder. Generally, 2 to 3% gypsum is added. We can also use gypsum as a plaster of Paris. By using more gypsum, expansion of concrete will occur, and setting of concrete will be very slow.

3. Plasticizers:

Workability is the most important property of concrete. Workability of concrete will change according to the situation of construction. For deep beams, light partition walls, beam-column junctions, concrete pumping, tremie concreting, high workable concrete is used. Many times, plasticizers are also known as “Water Reducing Admixture.” Plasticizer increases the workability without adding much water. It decreases the W/C Ratio which increases the strength.

Main Plasticizers used in Construction:

• Calcium lignosulphonates
• Sodium lignosulphonates
• Ammonium lignosulphonates

Uses of Plasticizers:

• To achieve a higher strength by decreasing the water-cement ratio at the same workability as an admixture-free mix
• To achieve the same workability by decreasing the cement content to reduce the heat of hydration in mass concrete
• To increase the workability to ease placing in accessible locations
• Water reduction more than 5% but less than 12%

The commonly used admixtures are Ligno-sulphonates and hydrocarbolic acid salts. Plasticizers are usually based on lignosulphonate, which is a natural polymer, derived from wood processing in the paper industry.

Effects of Plasticizers on the Properties of Concrete

The effect of water-reducing admixtures is dependent on: dosage of the admixture, cement type, aggregate type and grading, mix proportions, and temperature.

Workability:

Typically, an initial slump in the range 25—75 mm can be increased by 50-60 mm.

Compressive Strength:

The compressive strength of concrete is increased by using water-reducing admixtures to reduce water content while maintaining workability. The increase in strength is a direct result of the lower water/cement ratio.

4. Super Plasticizers:

In 1960, Japan made the first Super plasticizer and then in 1970, Germany made it. By using Super Plasticizers, we can reduce 30% of water. It is also called “High Range Water Reducers.” It is a strong dispersing agent.

Advantages and Disadvantages of Super-plasticizers:

• Advantages:
  • Significant water reduction
  • Reduced cement contents
  • Reduce water requirement by 12-30%
  • Increased workability of concrete
  • Reduced effort required for placement
  • More effective use of cement
  • More rapid rate of early strength development
  • Increased long-term strength
  • Reduced permeability of concrete
• Disadvantages:
  • Additional admixture cost (the concrete in-place cost may be reduced)
  • Slump loss greater than conventional concrete
  • Modification of air-entraining admixture dosage
  • Less responsive with some cement
  • Mild discoloration of light-colored concrete

Polymers used as Super Plasticizers:

• Sulphonates melamine formaldehyde condensates (SMF)
• Sulphonates naphthalene formaldehyde condensates (SNF)
• Modified Ligno-sulphonates (MLS)
• Acrylic Polymer (AP)
• Poly Carboxylate Ester (PC)

5. Air Entraining Admixtures:

Air entraining agent is used to get an air-entrained concrete. This air entraining agent produces small air bubbles in concrete and works as ball bearings. So the properties of the concrete like workability, segregation, bleeding, etc. will change.

Some Air Entraining agents:

• Aluminium Powder
• Hydrogen Peroxide
• Alkali Salts
• Vegetable Oils and Fats
• Zinc Powder
• Natural Wood Resin

Effect of Air Entraining Agents:

• Decrease in strength of concrete.
• Decrease in volume of concrete.
• Increase in permeability of concrete.
• Workability increase.
• Decrease in Alkali-aggregate reaction.
• Resistance against Sulphate attack.
• Decrease in heat of hydration.

6 Silica Fume Concrete

• Silica fume is an artificial pozzolana having high pozzolanic activity.
• It is a by-product from an electric arc furnace used in the manufacture of silicon metal or silicon alloy.
• It has a high silica content of more than 80%.
• It is excellent for use as a Portland cement supplement.

Chemical Composition

The chemical composition depends on the product being made by the furnace.

• Composition is also influenced by the furnace design.
• It is mostly made of silica having a percentage of more than 80%.
• Other chemical compositions include Fe2O3, Al2O3, CaO, Na2O, K2O in small percentages.
• Unlike other by-products like Fly Ash, Silica fume from a single source has little or no variation in chemical composition from one day to another.

Physical Characteristics

• It should be in premium white and standard grey colors.
• The specific gravity of the silica fume concrete is 2.2.
• Its specific surface area is 20,000 sq. meters / kg.
• Particle size is less than 1 micron with an average diameter of 0.1 micron.
• The shape of the particle is spherical.
• It should be amorphous in nature.

Advantages of Silica Fume

• High early compressive strength.
• High tensile flexural strength and modulus of elasticity.
• Very low permeability to chloride and water intrusion.
• High bond strength.
• High electrical resistivity and low permeability.

7 Rice Husk

1. They are the hard-protecting coverings of grains of rice.
2. It is used as fuel in boilers for processing of paddy, producing energy through direct combustion and/or by gasification.
3. For every 1000 kgs of paddy milled, about 220 kgs (22%) of husk is produced.

Applications

• High-performance concrete
• Insulator
• Green concrete
• Bathroom floors
• Industrial factory flooring
• Concreting the foundation
• Swimming pools
• Water proofing and rehabilitation

Advantages

• Improves compressive strength, flexural strength, and split tensile strength of concrete when mixed with RHA.
• RHA mixed with concrete shows better bond strength as compared to OPC concrete.
• RHA helps to increase resistance to chemical reactions.
• RHA would result in a reduction of the cost of concrete construction.
• Reduction of the environmental greenhouse effects.

Disadvantages

• Suitable incinerator/Furnace as well as grinding method is required for burning and grading rice husk in order to obtain good quality ash.
• Transportation problem.
• Unburnt RHA is not suitable for concrete production for this too.

Introduction

Special concretes are prepared for specific purposes like lightweight, high density, fire protection, and radiation shielding. Concrete is versatile with good compressive strength but suffers from drawbacks like low tensile strength, permeability, and corrosion.

To overcome these deficiencies, advancements in material and construction technology have improved concrete's performance, making it more economical and durable. Research is ongoing to enhance its mechanical properties, durability, impermeability, and resistance to environmental factors.

Different Types of Special Concrete:

• Lightweight Concrete
• High Strength Concrete
• Fibre Reinforced Concrete
• Ferrocement
• Ready Mix Concrete
• Shotcrete
• Polymer Concrete
• High Performance Concrete

Lightweight Concrete

Conventional concrete has a density of 2200-2600 kg/m³, making it heavy. Lightweight concrete, with a density of 300-1900 kg/m³, offers advantages like reduced dead load, lower foundation costs, and better thermal insulation.

Types of Lightweight Aggregates: Natural Lightweight Aggregates:

• Pumice:These are rocks of volcanic origin. They are light coloured or nearly white and has a fairly even texture of interconnected voids. Its bulk density is (500-800 kg/m³).
• Scoria: Dark-colored volcanic aggregate, slightly weaker than pumice.
• Rice Husk: Use of rice husk or groundnut husk has been reported as light weight aggregate.
• Sawdust: Used in flooring and precast elements but affects cement setting.
• Diatomite: Derived from aquatic plants, used as a pozzolanic material.

Artificial Lightweight Aggregates:

• Sintered Fly Ash: Made by sintering fly ash at high temperatures (sold as 'Lytag').
• Foamed Slag: By-product of pig iron production, porous like pumice.
• Bloated Clay: Formed by heating special clay, expanding due to gas formation.
• Exfoliated Vermiculite: Expands on heating, used for lightweight insulation.
• Cinders & Clinkers: Partly fused particles from coal combustion.

No-Fines Concrete

No-fines concrete is made by omitting fine aggregates, using only cement, coarse aggregates, and water. It has a density of 1600-2000 kg/m³ (or as low as 350 kg/m³ with lightweight aggregates). It provides structural efficiency but has low bond strength, requiring pre-coated reinforcement.

Advantages of Lightweight Concrete:

• Smaller structural sections.
• Lower haulage and handling costs.
• Faster construction progress.
• Lower foundation costs, especially on weak soil.
• Better thermal insulation, reducing air-conditioning costs.
• Utilizes industrial waste like fly ash and slag.
• High fire resistance.
• Economic benefits and enhanced seismic performance.

Methods to Achieve Lightweight Concrete:

• Using lightweight aggregates.
• Introducing air bubbles (Aerated Concrete).
• Omitting fine aggregates (No-Fines Concrete).

High strength concrete

High strength concrete can be defined by compressive strength of concrete at 28 days of water curing. When the grade of concrete exceeds M35, then the concrete may be called as high strength concrete.

In general, producing HSC is difficult with the use of conventional materials like cement, aggregate, and water alone. It can be achieved by using chemical and mineral admixtures or any one of the following methods:

1. Seeding
2. Re-vibration
3. High speed slurry mixing
4. Use of admixtures
5. Inhibition of cracks
6. Sulphur Impregnation
7. Use of cementitious aggregates

A. Seeding:

In this method, a small percentage of finely ground, fully hydrated Portland cement is added to the fresh concrete mix.

B. Re-vibration:

Mixing water creates continuous capillary channels, leading to bleeding and water accumulation. Controlled re-vibration after a suitable time increases the strength of concrete.

C. High Speed Slurry Mixing:

This process involves advanced preparation of a cement-water mixture, which is then blended with aggregate to produce HSC.

D. Use of Admixtures:

High strength can be achieved by adding chemical admixtures such as superplasticizers and mineral admixtures like fly ash and silica fume.

E. Inhibition of Cracks:

Replacing 2-3% of fine aggregate with polythene (0.025 mm thick and 3-4 mm in diameter) acts as a crack arrester, improving strength up to 105 MPa.

F. Sulphur Impregnation:

Low-strength porous concrete is impregnated with sulphur, increasing strength up to 58 MPa.

G. Use of Cementitious Aggregates:

Certain clinkers (e.g., ALAG) are used as aggregates, providing high strength up to 125 MPa with a low water-cement ratio of 0.32.

Fibre Reinforced Concrete (FRC)

FRC is a composite material consisting of concrete and uniformly dispersed fine fibres. It improves ductility, flexural strength, toughness, impact resistance, and fatigue strength.

Types of Fibres:

• Steel Fibre
• Polypropylene Fibre
• Glass Fibre (GFRC)
• Asbestos Fibres
• Carbon Fibres
• Organic Fibres
• Natural Fibres (Coir, Cotton, Sisal, Jute, Wool)

Steel Fibres:

Steel fibres are round and range from 0.25 mm to 0.75 mm in diameter. They significantly improve flexural impact and fatigue strength and are used in overlays, pavements, bridge decks, and flooring.

Glass Fibres:

Produced as roving, strands, or woven mats, glass fibres are used for decorative rather than structural applications. Alkali-resistant glass fibres have been developed to combat degradation.

Plastic Fibres:

Fibres like polypropylene, nylon, acrylic, and aramid have high tensile strength, improving impact resistance and reducing crack sizes.

Carbon Fibres:

Carbon fibres have high tensile strength and modulus of elasticity. While promising, they are costly and not widely available in India.

Asbestos fibers:

Asbestos is a mineral fibre and has proved to be most successful fibre, which can be mixed with OPC. The maximum length of asbestos fibre is 10 mm but generally fibers are shorter than this. The composite has high flexural strength.

Factors Affecting Properties of Fiber Reinforced Concrete

The important factors affecting properties of FRC are as follows:

• Volume of fibers
• Aspect ratio of fibers
• Orientation of fibers
• Size of coarse aggregate
• Workability and compaction of Concrete
• Mixing

Necessity of Fiber Reinforced Concrete:

• It increases the tensile strength of the concrete.
• It reduces the air voids and water voids the inherent porosity of gel.
• It increases the durability of the concrete.
• Fibres such as graphite and glass have excellent resistance to creep.
• Deferential deformation is minimized.
• It acts as a crack arrester.
• It substantially improves its static and dynamic properties.

Ferrocement

Ferrocement is a type of thin wall reinforced concrete, commonly constructed of hydraulic cement mortar, reinforced with closely spaced layers of continuous and relatively small size wire mesh.

Materials for ferrocement

• Cement mortar mix
• Skeleton steel
• Steel mesh reinforcement

Advantages of ferrocement

• Highly versatile and can be formed into almost any shape.
• 20% savings on materials and cost.
• Suitability for pre-casting.
• Flexibility in cutting, drilling, and jointing.
• Good fire resistance and impermeability.
• Reduction in self-weight & minimum skilled labour required.
• Reduction in expensive formwork, achieving economy and speed.

Ready Mix Concrete

Ready mix concrete is concrete mixed away from the construction site and then delivered to the construction site by truck in a ready-to-use condition.

Advantages of Ready Mixed Concrete

• Produced under controlled conditions using consistent quality raw materials.
• Speeds up construction.
• Reduces cement consumption by 10 – 12%.
• Better durability of structure.
• Minimizes human error and reduces labour dependency.

Shotcrete or Gunite

Shotcrete is the process of conveying dry (or damp) sand and cement by means of compressed air through a hose to a nozzle where water is added before the material is sprayed on the construction surface.

Methods

• Dry mix - Cement and sand are mixed thoroughly in dry state.
• Wet mix - Concrete is mixed with water before conveying through delivery pipe.

Applications of Shotcrete

• Used to repair the damaged surface of concrete.
• Bridge deck rehabilitation.
• Fire and earthquake damage repair.
• Strengthening marine structures.
• Used in underground excavations in rock.
• Used for constructing concrete swimming pools.

Polymer Concrete

Polymer concrete is created by impregnating monomer into hardened concrete and polymerizing it via thermal processes, significantly improving its strength.

Types of Polymer Concrete

• Polymer Impregnated Concrete
• Polymer Cement Concrete
• Polymer Concrete
• Partially Impregnated and Surface Coated Polymer Concrete

Advantages of Polymer Concrete

• High impact resistance and compressive strength.
• Highly resistant to freezing, thawing, chemical attacks, and abrasion.
• Lower permeability than conventional concrete.

Applications of Polymer Concrete

• Nuclear power plants.
• Kerbstones and prefabricated structural elements.
• Precast slabs for bridge decks and roads.
• Marine works and irrigation structures.
• Sewage works and waterproofing of buildings.
• Food processing buildings.

CONCRETING OPERATIONS

The operations which are followed in actual practice in the making of concrete and in improving and maintaining the quality of concrete are known as concreting operations.

The following operations are involved in concrete making:

• Storing of materials
• Batching of materials
• Mixing of various ingredients
• Transportation of concrete mix
• Placing of concrete
• Compaction of concrete
• Finishing of concrete surface
• Curing of concrete
• Joints in concrete

Storing of concrete materials

The process of keeping the ingredients of concrete in their proper place to protect them from the effect of weathering is called storing.

OBJECT TO STORING

Maintaining the quality and grading of materials is the main objective of storage of materials. Every effort should be made that the quality of cement does not deteriorate during storage in warehouses or at the site of work.

Storage of cement

Cement is a finely ground material. It is highly hygroscopic and absorbs moisture, which may be in the form of free water. An absorption of 1 to 2% of water has no effect, but further absorption reduces the strength of cement. If the absorption exceeds 5%, the cement is ruined for ordinary purposes. During storage and transportation, care is taken to keep it away from moisture.

METHOD OF STORING CEMENT IN WAREHOUSE

• The cement bags should be placed directly over the floor if it is dry; otherwise, they should be placed on a raised platform made of wooden planks.
• The space between the exterior walls and piles should be 0.30 m.
• Bags should be placed close together to avoid air circulation.
• The height of the pile should not exceed 2.7m (15 bags stacked one above the other).
• The width of the pile should not exceed 3m.
• If the pile exceeds 1.44m (8 bags), the bags should be placed in header and stretcher courses alternately to prevent toppling.
• The oldest cement should be used first (FIFO system).
• Storage buildings should be weather-proof with water-proof masonry walls, RCC or leak-proof roofs, and minimal, closed windows to prevent moisture ingress.
• The floor should be concrete, at least 15 cm thick, and designed to drain rainwater away.
• Avoid water accumulation around the storage shed.
• Cement should be stored at least 15-20 cm above the floor using wooden planks to prevent moisture penetration.

Temporary Storage at Site

For short-term storage at the site, cement bags should be placed on a dry wooden platform raised about 15 cm above the ground. The stack must be covered with a tarpaulin or polythene sheet to protect against atmospheric moisture. Open storage should not be adopted in wet weather.

Removal of Cement Bags

• Use the 'First In, First Out' (FIFO) system to use the oldest cement first.
• Each consignment should be stacked separately and labeled with the arrival date.
• Remove bags from two or three tiers at the back instead of removing from only the front tier.

Effect of Storage on Strength of Cement

When stored for long periods, cement loses its strength. After one year, its strength may reduce by 30-40% compared to freshly produced cement.

CONCRETING OPERATIONS

The operations which are followed in actual practice in the making of concrete and in improving and maintaining the quality of concrete are known as concreting operations.

The following operations are involved in concrete making:

• Storing of materials
• Batching of materials
• Mixing of various ingredients
• Transportation of concrete mix
• Placing of concrete
• Compaction of concrete
• Finishing of concrete surface
• Curing of concrete
• Joints in concrete

Storing of concrete materials

The process of keeping the ingredients of concrete in their proper place to protect them from the effect of weathering is called storing.

OBJECT TO STORING

Maintaining the quality and grading of materials is the main objective of storage of materials. Every effort should be made that the quality of cement does not deteriorate during storage in warehouses or at the site of work.

Storage of cement

Cement is a finely ground material. It is highly hygroscopic and absorbs moisture, which may be in the form of free water. An absorption of 1 to 2% of water has no effect, but further absorption reduces the strength of cement. If the absorption exceeds 5%, the cement is ruined for ordinary purposes. During storage and transportation, care is taken to keep it away from moisture.

METHOD OF STORING CEMENT IN WAREHOUSE

• The cement bags should be placed directly over the floor if it is dry; otherwise, they should be placed on a raised platform made of wooden planks.
• The space between the exterior walls and piles should be 0.30 m.
• Bags should be placed close together to avoid air circulation.
• The height of the pile should not exceed 2.7m (15 bags stacked one above the other).
• The width of the pile should not exceed 3m.
• If the pile exceeds 1.44m (8 bags), the bags should be placed in header and stretcher courses alternately to prevent toppling.
• The oldest cement should be used first (FIFO system).
• Storage buildings should be weather-proof with water-proof masonry walls, RCC or leak-proof roofs, and minimal, closed windows to prevent moisture ingress.
• The floor should be concrete, at least 15 cm thick, and designed to drain rainwater away.
• Avoid water accumulation around the storage shed.
• Cement should be stored at least 15-20 cm above the floor using wooden planks to prevent moisture penetration.

Temporary Storage at Site

For short-term storage at the site, cement bags should be placed on a dry wooden platform raised about 15 cm above the ground. The stack must be covered with a tarpaulin or polythene sheet to protect against atmospheric moisture. Open storage should not be adopted in wet weather.

Removal of Cement Bags

• Use the 'First In, First Out' (FIFO) system to use the oldest cement first.
• Each consignment should be stacked separately and labeled with the arrival date.
• Remove bags from two or three tiers at the back instead of removing from only the front tier.

Effect of Storage on Strength of Cement

When stored for long periods, cement loses its strength. After one year, its strength may reduce by 30-40% compared to freshly produced cement.

SL No. Age in months Loss of Strength (%) 1 3 5 to 10 2 6 20 to 30 3 12 30 to 40 Storing of Aggregates

Storing aggregates should ensure uniform grading, prevent segregation, maintain consistent surface water conditions, and avoid mixing with harmful materials.

Precautions for Maintaining Uniformity of Grading

• Select a hard and dry ground for storing aggregates. If unavailable, provide a platform using planks, G.I. sheets, bricks, or a weak concrete layer.
• Store aggregates of different sizes separately.

Precautions for Prevention of Segregation

• Do not drop successive consignments in the same place to avoid segregation.
• Place aggregates in layers not thicker than one truckload.
• Avoid dropping aggregates from heights during transportation.
• Store aggregates close to the mixer to minimize transport distance.

Precautions for Preserving Uniformity of Moisture Content

• Piles should be large and 1.25m to 1.75m in height.
• Allow piles to stand for at least 24 hours before use to let moisture settle.
• Avoid using the bottom 300mm of sand piles as they become saturated.

Precautions for Cleanliness of Aggregates

Keep aggregate piles free of leaves, debris, and animal refuse.

STORING OF WATER

Water is stored at site in a masonry tank built for the purpose or in other clean containers. The walls of the tank should be somewhat higher than the surrounding ground. Sufficient quantity of water should be stored in advance to ensure the continuity of concreting operations. If the water obtained from a source contains dust etc., it should be collected a day in advance to allow such suspended impurities to settle down before use.

BATCHING

The process of measurement of ingredients (cement, fine aggregate, coarse aggregate and water) for making concrete is called batching. Batching is done in two ways:

• Volume batching
• Weigh batching

Volume batching

(a) Batching of cement: Cement is always batched by weight. Cement should never be batched by volume, because its weight per unit volume varies according to the way container (forma) is filled.

(b) Batching of aggregates: Wooden batch boxes known as formas are used for batching of fine and coarse aggregates by volume. The formas should be made of 30 mm thick timber.

(c) Batching of water: It is practice in the field to add water by tin cans or buckets. It is not an accurate method. It results in variable strength of concrete. Some of mixers are equipped with calibrated water tank attached permanently to the mixers. For the mixers not provided with water tank, a calibrated syphon system can be easily installed, such as the one used in cisterns of water closets. If, however, there are no such automatic devices, water should be measured in calibrated cans very accurately and then only should be added in the mixer.

Weigh Batching

Strictly speaking, weigh batching is the correct method of measuring the materials. For important concrete, invariably, weigh batching system should be adopted. Use of weight system in batching facilitates accuracy, flexibility, and simplicity. Different types of weigh batchers are available, the particular type to be used depends upon the nature of the job. Large weigh batching plants have automatic weighing equipment. The use of this automatic equipment for batching is one of sophistication and requires qualified and experienced engineers. In smaller works, the weighing arrangement consists of two weighing buckets, each connected through a system of levers to spring-loaded dials which indicate the load.

MIXING OF CONCRETE

The process of mixing of various ingredients of concrete in specified proportions is termed as mixing of concrete.

Methods of mixing:

• Hand mixing
• Machine mixing

HAND MIXING

The process of mixing the ingredients of concrete by manual labour is called hand mixing. Hand mixing is adopted for small and unimportant works and where quantity of concrete used is small. Hand mixing method requires more cement (10% more) than machine mixing for obtaining the same strength of concrete.

Following is step-wise procedure for mixing by hand:

• A platform of bricks, lean concrete, or iron sheets is constructed. The size of the platform depends upon the quantity of concrete to be mixed at a time.
• Spread out a measured quantity of sand evenly on the mixing platform.
• Spread the cement uniformly on this sand and mix it till the color of the mixture is uniform.
• Spread this mixture evenly again on the platform.
• Spread the measured coarse aggregate evenly on the mixing platform.
• Mix the material dry.
• Make a hollow in the center of the mixed material. After this, 75% of the required quantity of water based on the water-cement ratio is added and then start remixing taking care that no water escapes the mixture.
• The remaining water is added with the continuation of the mixing process.
• Normally mixing time should not exceed 3 minutes.
• The platform should be cleaned at the end of the day's work so that it is ready for use the next day.

MACHINE MIXING

The process of mixing the ingredients of concrete by a machine is called machine mixing. In this case, where a large quantity of concrete is to be produced, hand mixing becomes costly even if the labor is cheap. The machine mixing becomes essential. The concrete can thus be produced at a faster rate and at a lesser cost. The quality of concrete by machine mixing is also better.

TRANSPORTATION OF CONCRETE

The process of carrying concrete mix from the place of its mixing to final position or deposition is called transportation of concrete. Transportation of concrete mix is very important because time is a factor involved. The mix should be transported as quickly as possible.

Precaution in Transportation of Concrete

The following precautions should be taken during transporting concrete from the mixing place:

• Concrete should be transported as quickly as possible to the formwork within the initial setting time of cement.
• Efforts should be made to prevent segregation.
• Transportation cost should be as low as possible.
• The concrete mix should be protected from drying in hot weather and from rain during transportation.
• The concrete should be kept agitated in truck mixers so that it does not become stiff when transportation is likely to take more time.
• No water should be lost from the mix during transportation.
• The permissible duration of transport of concrete should be determined in the laboratory.

Methods Adopted for Transportation of Concrete

• Mortar Pan
• Wheel Barrow, Hand Cart
• Crane, Bucket and Rope way
• Truck Mixer and Dumpers
• Belt Conveyors
• Chute
• Skip and Hoist
• Transit Mixer
• Pump and Pipe Line
• Helicopter

Concrete Pumps

The modern concrete pump is a sophisticated, reliable, and robust machine. In the past, a simple two-stroke mechanical pump consisted of a receiving hopper, an inlet and an outlet valve, a piston, and a cylinder. The pump was powered by a diesel engine.

Choosing the Correct Pump

For choosing the correct pump, one must know the following factors:

• Length of horizontal pipe
• Length of vertical pipe
• Number of bends
• Diameter of pipeline
• Length of flexible hose
• Changes in line diameter
• Slump of Concrete

Placing Concrete

It is not enough that a concrete mix is correctly designed, batched, mixed, and transported. It is of utmost importance that the concrete must be placed in a systematic manner to yield optimum results. The precautions to be taken and methods adopted while placing concrete in the undermentioned situations will be discussed.

• Placing concrete within earth mould (example: Foundation concrete for a wall or column).
• Placing concrete within large earth mould or timber plank formwork (example: Road slab and airfield slab).
• Placing concrete in layers within timber or steel shutters (example: Mass concrete in dam construction or construction of concrete abutment or pier).
• Placing concrete within usual formwork (example: Columns, beams, and floors).
• Placing concrete under water

Placing Concrete within Earth Mould:

• Concrete is invariable as foundation bed below the walls and columns before placing concrete.

• All loose earth must be removed.

• Roots of trees must be cut.

• If surface is dry, it should be made damp.

• If it is too wet or rain soaked the water, then slush must be removed.

Placing Concrete in Layers within Timber or Steel Shutter:

This can be used in the following cases:

1. Dam construction

2.Construction of concrete abutments

3.Raft for a high-rise building

• The thickness of layers depends on:

1. Method of compaction

2.Size of vibrator

3. Frequency of vibrator used

It’s better to leave the top of the layer rough so that succeeding layer can have a good bond.

Placing Concrete within Usual Formwork:

• Adopted for columns, beams, and floors. Rules that should be followed while placing the concrete:

• Check the reinforcements are correctly tied and placed.

• Mould releasing agent should be applied.

• The concrete must be placed carefully with a small quantity at a time so that they will not block the entry of subsequent concrete.

Placing Concrete Underwater:

Concrete having cement content at least 450kg/m³ and a slump of 10 to 17.5cm can be placed underwater.

Methods:

1. Bagged method

2. Bottom dump method

3. Tremie

4. Grouted aggregate

5. Concrete pump

Formwork:

Formwork shall be designed and constructed so as to remain sufficiently rigid during placing and compaction of concrete.

The joints are plugged to prevent the loss of slurry from concrete.

Stripping Time: Formwork should not be removed until the concrete has developed a strength of at least twice the stress to which concrete may be subjected at the time of removal of formwork.

Compaction of Concrete:

Compaction of concrete is the process adopted for expelling the entrapped air from the concrete. The lower the workability, the higher the amount of air entrapped.

The following methods are adopted for compacting the concrete:

Hand Compaction:

1.Rodding

It is a method of poking with a 2m long, 16 mm dia. rod at sharp corners and edges.

2. Ramming:
- It is generally used for compaction on ground in plain

Tamping: - It is a method in which the top surface is beaten by wooden cross beam of cross section 10 cm x 10 cm.
• Both compaction and levelling are achieved simultaneously.
• It is mainly used for roof slabs and road pavements.

Compaction by Vibration:

• Vibration is imparted to the concrete by mechanical means.

• Mechanical vibration can be of various types as given under:

1. Internal vibrator

2. External vibrator

Curing:

Curing is the process in which the concrete is protected from loss of moisture and kept within a reasonable temperature range. The result of this process is increased strength and decreased permeability Curing can be done by various methods, including immersion, ponding, spraying, and covering with wet sand.

Importance of Testing:

Testing plays a crucial role in ensuring the safety, durability, and performance of materials and structures. Both non-destructive testing (NDT) and destructive testing methods have their own importance. While NDT preserves the integrity of the tested material, destructive testing provides detailed data on the material’s behavior under stress, often used for research and development purposes.

Non-Destructive Testing (NDT) Methods:

Non-destructive testing methods allow the inspection of materials without causing any harm, and they are commonly used in industries to evaluate the condition of critical structures.

1. Visual Inspection (VT):

A simple method that involves visually inspecting the surface of a material for any signs of flaws or defects, such as cracks or corrosion.

2. Ultrasonic Testing (UT):

High-frequency sound waves are used to detect internal defects in materials. The echoes of these sound waves help to locate defects or irregularities within the material.

3. Radiographic Testing (RT):

X-rays or gamma rays are used to penetrate materials, and the resulting images help identify internal flaws such as voids, cracks, or discontinuities.

4. Magnetic Particle Testing (MT):

Magnetic fields are applied to ferromagnetic materials. Defects on the surface or near the surface can be detected by the pattern formed by magnetic particles.

5. Dye Penetrant Testing (PT):

A liquid dye is applied to the surface of materials. The dye penetrates cracks, and after cleaning and applying a developer, defects become visible under UV or visible light.

6. Eddy Current Testing (ET):

Eddy currents are induced in conductive materials to detect surface and sub-surface defects. Changes in the flow of currents can indicate the presence of flaws.

7. Acoustic Emission Testing (AET):

This method detects the sound emitted by materials under stress. It helps to identify and locate sources of internal cracking or other failures in real-time.

8. Leak Testing:

Used to detect leaks in sealed systems, such as pipes and tanks, through various techniques like pressure testing or helium leak detection.

Destructive Testing Methods:

Destructive testing involves testing materials or components to failure in order to understand their properties, behavior, and performance under stress. This is useful for research and quality assurance.

1. Tensile Testing:

A sample material is pulled apart until it breaks to measure its tensile strength, elongation, and overall behavior under tension.

2. Impact Testing:

This test measures a material's ability to absorb energy during a collision or impact. It provides insight into a material's toughness.

3. Hardness Testing:

In this method, the material’s surface is indented using a specified force, and the depth or size of the indentation is used to determine the hardness of the material.

4. Fatigue Testing:

This test evaluates how materials respond to repeated stress and strain over time, which helps to determine the material's endurance limit.

5. Creep Testing:

Creep testing measures the long-term deformation of a material when exposed to constant stress over a prolonged period, often at high temperatures.

6. Fracture Toughness Testing:

This method determines a material's resistance to crack propagation and fracture under stress.

Comparison Table: Non-Destructive vs Destructive Testing

Aspect	Non-Destructive Testing (NDT)	Destructive Testing
Purpose	To detect internal and external defects without causing any damage to the material.	To determine the material's strength and properties by testing it until failure.
Damage to Material	No damage to the material or component being tested.	Material is destroyed or permanently altered during the test.
Cost	Generally more cost-effective since it doesn’t require material replacement.	Can be expensive due to the destruction of materials and the need for new specimens.
Use Case	Quality control, safety inspection, and regular maintenance.	Research and development, material certification, and failure analysis.
Speed	Fast and efficient, providing immediate results in many cases.	Generally slower due to the complexity and nature of the tests.

Rebound Hammer Test:

The Rebound Hammer Test is a non-destructive testing method used to assess the surface hardness of concrete. It helps to estimate the compressive strength of the concrete based on its hardness. The rebound number is measured by the hammer's spring-driven mechanism that strikes the concrete surface and returns, providing a numerical value that correlates with the strength of the material.

Procedure for Rebound Hammer Test:

1. The test surface should be smooth and free of any coatings or impurities.
2. Hold the rebound hammer perpendicular to the concrete surface.
3. Push the hammer against the concrete surface, allowing the spring mechanism to strike the material.
4. Record the rebound number displayed on the scale.
5. Repeat the procedure at different points of the surface to get accurate readings.
6. Compare the rebound number with the established correlation charts to estimate the compressive strength of the concrete.

Advantages of Rebound Hammer Test:

• Quick and easy to perform.
• No need for sample preparation or external power sources.
• Can be used for testing large areas of concrete surfaces.
• Helps to identify uniformity and detect variations in concrete strength.

Limitations of Rebound Hammer Test:

• Surface roughness and moisture can affect accuracy.
• Only measures surface hardness, not internal defects or strength at depth.
• Requires correlation with other tests for more accurate results.

Ultrasonic Pulse Velocity (UPV) Test :

The Ultrasonic Pulse Velocity (UPV) Test is a non-destructive method used to assess the quality of concrete by measuring the velocity of an ultrasonic pulse passing through the material. The pulse velocity depends on the density and elastic properties of the material, which are influenced by the presence of cracks, voids, or other defects.

Procedure for Ultrasonic Pulse Velocity (UPV) Test:

1. Place the ultrasonic transducers (transmitter and receiver) on opposite sides of the concrete specimen.
2. The transducer emits an ultrasonic pulse that travels through the concrete to the receiver.
3. The time taken for the pulse to travel through the concrete is recorded.
4. The pulse velocity is calculated using the known distance between the transducers and the time taken for the pulse to travel.
5. The pulse velocity value helps to assess the quality of the concrete, with higher velocities indicating better quality and lower velocities indicating defects or deterioration.

Advantages of Ultrasonic Pulse Velocity (UPV) Test:

• Can detect internal defects such as cracks, voids, and honeycombing.
• Can be used for testing both fresh and hardened concrete.
• Non-destructive and requires minimal preparation.
• Can be applied on structures that are already built without causing damage.

Limitations of Ultrasonic Pulse Velocity (UPV) Test:

• Accuracy depends on the contact between transducers and the surface.
• Requires a smooth surface for optimal measurement.
• Cannot give a direct measurement of concrete strength, only an indirect assessment of material quality.

Comparison Table: Rebound Hammer Test vs Ultrasonic Pulse Velocity (UPV) Test

Aspect	Rebound Hammer Test	Ultrasonic Pulse Velocity (UPV) Test
Purpose	Assess surface hardness and estimate compressive strength.	Assess internal quality and detect defects in concrete.
Testing Method	Spring-loaded hammer strikes the concrete surface and rebounds.	Ultrasonic pulse travels through concrete and measures the time taken.
Test Duration	Quick and simple, typically within a few minutes per test location.	Relatively slower, as it involves setting up transducers and measuring pulse velocity.
Material Tested	Only surface hardness is tested.	Tests the internal quality and detects internal flaws.
Surface Preparation	Requires a smooth, clean surface for accurate readings.	Requires smooth surface contact for optimal results, but can be used on rough surfaces.
Data Accuracy	Provides surface strength estimation with moderate accuracy.	Provides internal material quality assessment with higher accuracy.
Limitations	Can be affected by surface conditions like moisture or roughness.	Accuracy depends on the contact between transducers and concrete surface.
Advantages	Portable, cost-effective, quick results.	Can detect internal defects and assess material quality in depth.