8

CONCRETE

8.1 INTRODUCTION

Concrete can be easily prepared by mixing binding material (lime or cement), fine aggregate (sand) and coarse aggregate (crushed stone or brick) and water in the required proportion.

The fine aggregates fill the voids of coarse aggregate, and voids of fine aggregate are filled with lime/cement and also the lime/cement reacts with water and binds the aggregates together to form a hard material called concrete. The quality of a concrete depends on the grade of lime/cement, type of aggregates, quality of water, the mix proportion, method of mixing, placing and compacting, temperature and curing method and its duration.

Concrete produced by mixing cement, coarse and fine aggregates and water is called cement concrete. If lime is used in the place of cement it is called lime concrete. Surkhi concrete is one when surkhi (brick powder) is added along with lime.

Cement concrete has been a construction material used in large quantities for several decades. The reason for its popularity can be attributed to the excellent technical properties and the economy of the material. Thus, cement concrete and lime concrete are discussed further in this chapter.

8.2 PROPERTIES OF CEMENT CONCRETE

The properties of cement concrete are as follows:

1. It has high compressive strength and varies with the grade of concrete.
2. It is weak in tension.
3. It hardens with age, and the rate of hardening decreases after it has attained adequate strength.
4. During the process of hardening, it shrinks due to loss of water.
5. It is not impervious perfectly unless formed with special additives.
6. It is free from corrosion.
7. It can sustain all the normal atmospheric effects.
8. It forms a hard surface, which is capable of resisting abrasion.
9. It has fire resistance capacity.
10. It is more economical and highly durable.

8.3 TYPES OF CEMENT CONCRETE AND USES

Different types of cement concrete are discussed along with their uses.

8.3.1 Plain Cement Concrete

It is the simplest form of concrete made with cement, sand, pebbles or crushed rock and water. It is used for a variety of works where the structural members are subjected to compression. Simple foundations to mighty dams may be constructed with plain cement concrete.

8.3.2 Reinforced Cement Concrete

The cement concrete provided with steel reinforcement for taking on tension, bending, torsion, fatigue, etc., is referred to as Reinforced Cement Concrete (RCC). The steel used in RCC works are: mild steel bars, medium tensile steel bars, hot-rolled deformed bars, cold twisted bars, hard-drawn steel wire fabric or rolled steel. It is used in various structural members, viz., columns, beams, footings, slabs, staircases, shelves, basements, water tanks, retaining walls, folded plates, shells, domes, etc.

8.3.3 Pre-stressed Cement Concrete

Here high tensile steel wires are used as reinforcement instead of mild steel bars. High tensile steel wires are stretched initially to the desired level and concrete is placed. After setting and completion of curing, the concrete is put into use. As the steel wires are initially stretched, the concrete so casted is known as pre-stressed concrete. This concrete is used in the manufacturing of railway sleepers and electric poles and in the construction of large span beams, large span bridge girders, etc.

8.3.4 Light-weight Cement Concrete

This concrete has less density compared to the conventional concrete. Light-weight concrete may be prepared (i) by omitting the fine aggregate, (ii) by using light-weight concrete or (iii) by aerating concrete.

The concrete prepared by omitting the fine aggregate is called no-fine concrete. It has low drying shrinkage. This is used for cast-in-situ external load-bearing walls, small retaining walls, where repetitive loads are expected to occur, etc. This has better insulating properties.

The light-weight aggregates used are pumice, expanded slates, furnace clinker, etc., and the fine aggregates used are crushed light-weight aggregate or natural sand. This type of light-weight concrete has better insulating and fire-resistant properties, is highly water absorptive and economical. This can be used for all the works stated above.

The light-weight concrete obtained by aeration is known as gas, porous or foamed concrete. This is prepared by (i) mixing air-entraining agents with cement or cement mortar in high-speed machines, (ii) mixing a specified quantity of pre-formed foam with cement or cement mortar in an ordinary mixer or (iii) adding aluminium or zinc powder to cement mortar or hydrogen peroxide to concrete. It has a very high drying shrinkage and is used for the manufacture of building blocks.

8.3.5 Precast Cement Concrete

The concrete used for the casting of various structural components with or without reinforcement is called precast cement concrete. Fresh concrete is placed and compacted in moulds made for different structural units. After setting and curing they are removed from moulds and used. This is discussed in detail in the next chapter.

8.4 PLAIN CEMENT CONCRETE

Concrete is an important construction material consisting of ingredients that are inert materials, viz., coarse and fine aggregates and binding materials, viz., cement or lime. Cement concrete is widely used as plain concrete, reinforced concrete, pre-stressed concrete and precast concrete members.

Properties of coarse and fine aggregates and the quality of water to make quality cement concrete is discussed in the following sections.

8.4.1 Fresh and Hardened Concrete

Cement concrete is prepared to a plastic consistency by adding water and this hardens on curing. Plastic concrete is called fresh concrete, which is a freshly mixed material that can be moulded into any shape. The concrete formed to the required shape and cured is said to be hardened concrete. The properties of the concrete, in both the stages, depend on the relative quality and quantity of cement, aggregates and water.

8.4.2 Aggregates

Aggregates are the major and important constituent of concrete. They form the body of the concrete as it occupies 70–80% of the volume of concrete. Although aggregates were originally considered to be inert material, they have recently been found to be chemically active to some extent. Cement is the only factory-made component, whereas aggregates (both coarse and fine) and water are naturally available materials.

Concrete can be considered as a two-phase material, viz., paste phase and aggregate phase. Paste phase is the role played by cement and water in concrete whereas aggregate phase is the role played by aggregates (both fine and coarse) in concrete. The requirement of paste phase has been dealt in previous chapter. Now the aggregates phase is discussed.

1. Classification

Aggregates may be classified as (i) normal weight aggregates, (ii) light-weight aggregates and (iii) heavy-weight aggregates.

Normal weight aggregates can be further classified into natural and artificial aggregates. They are also classified as coarse or fine aggregates depending on the size. Natural and artificial aggregates are as follows:

2. Coarse Aggregate

As stated above, coarse aggregates are produced by the disintegration and crushing of rock. Coarse aggregates are usually those particles which are retained on an IS 4.75 mm sieve.

Stones that are hard and durable such as granite, basalt, quartzite provide good coarse aggregate. Naturally available river gravels and shingle obtained from sea beaches after removing shale may be used.

Blast furnace slag is used for making light-weight concrete. Brick-bats are used for lime concrete or for temporary or cheap concrete work. For reinforced concrete only crushed rock aggregates of size 20 mm are generally considered as satisfactory.

The aggregates should be absolutely clean, free from organic matter and other impurities. The aggregate must be capable of resisting weather.

The grading of coarse aggregate is very important for getting good quality concrete. Good grading of coarse aggregate implies that the quantity of aggregate used should contain all standard fractions of aggregate in required proportions such that the sample contains minimum voids. Thus, a sample of well-graded aggregate (both coarse and fine) containing minimum voids requires a minimum paste phase. Minimum paste require less quantities of cement and water. This is further to mean increased economy, higher strength, lower shrinkage and greater durability.

Fineness modulus is obtained by adding the cumulative percentages of aggregates retained on each sieve (ranging from 80 mm to 150 micron) and dividing their sum by 100. Fineness modulus is an empirical factor, larger the figure, coarser is the material. For a good concrete, the value of fineness modulus of coarse aggregate should be between 6 and 8.5.

One of the methods of arriving at the practical grading is by the trial and error method. Different size fractions of aggregates can be mixed, and the one that gives the maximum weight or minimum void may be used.

3. Fine Aggregate

Fine aggregate is sand, which is usually obtained from rivers or lakes. Sometimes beach sand is also used. In places where sand is not available or a large quantity of sand is needed, crushed stone dust is used. The fineness modulus (F.M.) of sand should be around 2 to 3.2. The following limits may be used as guidance.

8.4.3 Water

Water is an important ingredient of concrete. As a general guidance, if the water is fit for drinking, it is fit for making concrete. However, some water containing small traces of sugar is not suitable for concrete. Another yardstick adopted is that if the pH is between 6 and 8 the water is considered to be suitable.

The best method is to use the water from the particular source and sample concrete is prepared for testing. The concrete is tested for 7 days and 28 days compressive strengths; if 90% result is obtained it can be considered to be suitable.

The following guidelines may also be adopted:

1. Neutralizing 100 ml sample of water using phenoplhaline as an indicator should not require more than 5 ml of 0.02 N NaOH.
2. Neutralizing 100 ml of sample of water using a mixed indicator should not require more than 25 ml of 0.02 N H₂SO₄.

BIS (IS: 456–2000) recommends the permissible limits for solids enlisted in Table 8.1.

Table 8.1 Materials and permissible solids

Source: IS: 456–2000.

8.5 CONCRETE CHEMICALS AND APPLICATIONS

Admixture and construction chemicals are chemicals added to the ingredients of concrete to obtain the desired strength and durability.

8.5.1 Admixtures

Of late concrete is used for varied purposes and is made suitable for different occasions and environments. Ordinary concrete is not as versatile and does not suit all purposes. Thus, admixtures and chemicals are added to the ingredients of concrete. On the other hand, additives are materials that are added at the time of grinding cement clinker at cement factories.

The effect of admixture depends on the brand of cement, grading of aggregate, mix proportion and richness of mix. Thus, it is with caution that admixtures should be selected in correctly predicting the behaviour of concrete. There are several admixtures available, some important ones are discussed below.

1. Plasticizers and Superplasticizers

High degree of workability is needed in different situations. The addition of excess water will only help the fluidity and not the workability of the concrete. But the addition of plasticizers will improve the desirable qualities requires for plastic concrete.

Plasticizers are based on the following constituents:

1. Anionic surfactants such as lignosulphonates and their modifications.
2. Non-ionic surfactants, such as polyglycol acid of hydroxylated carboxylic acids and their modifications.
3. Others such as carbohydrates.

Among the plasticizers, calcium, sodium and ammonium lignosulphonates are mostly used. The quantities used are 0.1–0.4 % by weight of cement.

Superplasticizers constitute a relatively new and improved form of plasticizers. These are chemically different from conventional plasticizers. The special quality of superplasticizers is their powerful action as dispensing agents, and they are high-range water reducers. They are chemically different from plasticizers. Superplasticizers permit reduction of water up to 30% without reduction in workability. Superplasticizers are used for the production of flowing, self-levelling, self-compacting, high-strength and high-performance concrete.

2. Retarders

A retarder is an admixture which slows the process of hydration because of which the concrete remains plastic and workable. If concreting is prepared in hot weather, retarders overcome the accelerating effect of high temperature. Retarders are used in consolidating large number of pours without the formation of cold joints and in grouting oil wells.

Calcium sulphate is the commonly used retarder. Other admixtures used as retarders are lignosulphonic acids and their salts, hydroxylated carboxylic acid and their salts. These admixtures increase the compressive strength by 10 to 20%.

Retarding plasticizers are available in the market. These important admixtures are often used in the ready-mixed concrete industry for purposes of retaining the slump loss, during high temperatures long transportation distances and to avoid cold joints.

3. Accelerators

Accelerators are another very useful type of admixture which is added to get early strength. The various purposes are:

1. Early removal of formwork
2. Reduction of period of curing
3. Putting the structure early to use
4. Accelerating the setting time during cold weather
5. Energy repair work

The commonly used accelerator in the past was calcium chloride. However, it is not in use now, instead soluble carbonates, silicates, fluosilicates and some organic compounds are used. Fluosilicates and organic compounds like triethenolamine are comparatively expensive. Some of the accelerators available of late can set cement into a hard state in a matter of five minutes. The availability of such accelerators has made underwater concreting very easy. Further, waterfront structures which need repairs in short time may be done using accelerators. These materials could be used in a cold environment up to 10°C. Accelerating plasticizers are also available.

4. Air-entraining Admixture

Air-entrained concrete is made by using air-entraining cement or by adding an air- entraining agent. Air-entraining agents produce a large quantity of air bubbles which act as flexible ball bearings and modify the properties of concrete regarding workability, segregation, bleeding and the finishing quality of concrete. Further, the hardened concrete gains resistance to frost action and permeability.

Natural wood resins, animal and vegetable fats, various wetting agents (such as alkali salts), water-soluble soaps of resin acids, etc., are various air-entrained agents used.

Different air-entraining agents behave differently depending on the elasticity of the film of the bubble formed and the extent to which the surface tension is reduced.

5. Pozzolanic Admixtures

Pozzolanic or mineral admixtures have been in use since the advent of concrete. The application of pozzolanic admixtures modify certain properties of fresh and hardened concrete. The proper addition of pozzolanic admixtures to cement enhances many qualities of concrete, such as lowering the heat of hydration, increasing the water tightness, reducing the alkali–aggregation reaction, resisting sulphate attack, improving workability, etc.

Siliceous materials and aluminous materials do not possess any cementitious materials. But on reacting with cement and moisture, they chemically react with calcium hydroxide liberated on hydration and form compounds possessing cementitious properties. This reaction is called as pozzolanic reaction.

Naturally available pozzolanic materials are clay and shale, diatomaceous earth, volcanic tuffs and pumicites. Artificial pozzolanic materials are fly ash, blast furnace slag, silica fume, rice husk ash, metakaoline and surkhi. Other mineral admixtures are finely ground marble, quartz and granite powder.

6. Damp-proofing Admixtures

Two important properties that concrete should possess with reference to water are:

1. To resist seepage when subjected to the pressure of water and
2. To protect the absorption of surface water by capillary action. In general, properly designed and constructed concrete should be impermeable. But it has been accepted that the addition of some damp-proofing admixture may prove to be of some advantage in reducing the permeability.

Damp-proofing admixtures are available in powder or liquid form. They have the property of filling pores or being water repellents. The prime materials in pore-filling admixtures are silicate of soda, aluminium and zinc sulphates, and aluminium and calcium chloride. These are also more active and render the concrete more impervious and accelerates the setting time. Hence, mineral oils free from fatty or vegetable oils are used.

The production of low-permeability concrete depends on the uniform spreading of the admixture.

8.5.2 Construction Chemicals

The discussion so far has been centered on the modification of the properties of concrete using admixtures. Other chemicals that are used to enhance the performance are discussed in this section. Such chemicals are referred to as construction chemicals or building chemicals.

1. Concrete-curing Compounds

The measure adopted to prevent the loss of water from the surface due to evaporation or any other means and to ensure retention is called curing. Surface loss of water from concrete depends upon air temperature, relative humidity, fresh concrete, temperature and wind velocity.

Liquid membrane-forming curing compounds are used. Curing compounds are made with bases such as synthetic resin, wax, acrylic and chlorinated rubber.

Resin and wax-based compounds effectively seal the concrete and prevent surface evaporation. After 28 days of curing, these compounds peel off.

Acrylic-based membrane compounds have the additional advantage of better adhesion of plaster. The membrane does not need to be removed, but the plastering can be done over it. Because of the acrylic emulsion the bonding of the plaster is better.

Chlorinated rubber-curing compounds form a thin film on the surface of the concrete, which prevents drying and at the same time fills the pores on the surface of the concrete. The surface film will wear out.

2. Polymer-bonding Agents

Many a times new concrete is required to be placed over an old concrete surface. In such cases a perfect bond is required. By providing a bond coat between the new and old surface of concrete a bond can be achieved. A mixing of a bonding agent with the new concrete helps to provide a better bond. Such mixtures also improve the workability and reduces shrinkage. Many types of commercial products such as Roof-Bond ERB, Nitobond PVA, etc., are available.

Polymer-modified repair materials are available for the repair of concrete work. Such repair works include ceiling of concrete roof, hydraulic structures, prefabricated members, pipes, poles, etc.

3. Waterproofing Chemicals

Many of the admixtures discussed in the previous section directly or indirectly reduce the permeability of concrete thereby making the material waterproof. However, waterproofing of roofs, walls, bathrooms, toilets, kitchens, basements, swimming pools, and water tanks, etc., still poses some problems.

Different materials are available to make the concrete perfectly waterproof. They are integral waterproofing compounds, acrylic-based polymers, mineral-based polymers, chemical DPC, waterproofing adhesives for tiles, silicon-based water-repellent materials, injection grouts, joint sealants and protective and decorative coatings.

8.6 GRADES OF CEMENT CONCRETE

During 1976 there was only one type of cement available. Later OPC had been graded and the gain in strength after 28 days had been based on the grade and type of cement. The design was based on 28 days’ characteristic strength of concrete unless there was evidence to justify a higher strength for a particular structure due to age (IS: 456–2000). Different grades of cement are given in Table 8.2.

Table 8.2 Grades of cement concrete

Source: IS: 456–2000.

8.7 MIX DESIGN CONCEPT

Mix design of concrete is the process of selecting the required ingredients of concrete and finding their relative proportions with the aim of producing an economical concrete of certain strength and durability.

It has been discussed earlier that concrete is based on two phases, viz., aggregate phase and paste phase. Workability of the concrete depends on the lubricating effect of the paste phase. The strength of concrete is predominantly governed by the aggregate paste and the contribution by the paste phase is limited. But the permeability of concrete is based on the quality and continuity of the paste phase. Further, the paste phase fully contributes to the control of drying shrinkage of the concrete.

For a given set of materials, the four factors to be considered in the design of concrete mix are:

1. Water–cement ratio or cement content
2. Cement–aggregate ratio
3. Gradation of aggregate
4. Consistency

In general, all the four factors are inter-related and can not be dealt with individually to get the best concrete. However, two or three factors are fixed and the others are adjusted to get the required workability and economy.

The water–cement ratio represents the dilution of the paste, and cement concrete depends on the amount of paste. The gradation of the aggregate is done by adjusting the quantity of given fine and coarse aggregates. The required consistency or workability is obtained on the site of placement of concrete.

The effort in proportioning is to use a minimum quantity of paste which will lubricate the mixture while fresh, harden afterwards, will bind the aggregate particles together and fill the space between them. Excess cement should be avoided as it leads to greater cost, high shrinkage, high permeability, and more weathering. All these defects can be nullified by proper gradation.

There are over twelve mix designs of which two are explained, viz., the American Concrete Institute Method and the Bureau of Indian Standards Method. These two methods are popular in India.

8.7.1 American Concrete Institute (ACI) Method of Mix Design

This method has been used since 1944 and has undergone several revisions continuously. It has developed an identical procedure for angular or rounded aggregates, regular or light-weight aggregates and air-entrained or non-air-entrained concretes. The ACI mix design is based on certain factors which are established by field experience on large works. The factors are as follows:

1. The fresh concrete of a given slump with a well-graded aggregate (of a given maximum size) will have constant total water content irrespective of variations in water–cement ratio and cement content.
2. It considers the relationship that the optimum dry-rodded volume of coarse aggregate per unit volume of concrete is based on its maximum size and the fineness modulus of the fine aggregate (Table 8.3) irrespective of the shape of the particles.
3. Irrespective of the method of compaction, there is a definite percentage of air that exists which is inversely proportional to the maximum size of the aggregate.

Table 8.3 Dry bulk volume of concrete aggregate per unit volume of concrete

Source: ACI 211.1–91.

Reproduced with permission from the American Concrete Institute, Farmington Hills, MI (www.concrete.org)

The following step-by-step procedure has to be adopted:

1. All the required data, viz., (i) given modulus of the selected fine aggregate, (ii) unit weight of dry-rodded coarse aggregate, (iii) specific gravity of coarse and fine aggregates in SSD condition, (iv) absorption characteristics of both coarse and fine aggregates and (v) specific gravity of cement.
2. The maximum size of the aggregate has to be decided. Generally 20 mm and 10 mm are recommended for RCC and pre-stressed concrete work, respectively.
3. Workability has to be decided based on the slump depending on the work. General guidance may be taken from Table 8.4.

   Table 8.4 Recommended value of slump for different works

   

   Source: ACI 211.1–91.

   Reproduced with permission from the American Concrete Institute, Farmington Hills, MI (www.concrete.org)

   Note: Upper limit of the slump may be increased by 20 mm for compaction by hand.

4. Based on the selected slump and maximum of aggregate, the total water in kg/m³ of concrete can be read from Table 8.5. Also the amount of accidentally entrapped air in non-air-entrained concrete may be obtained from Table 8.5.

   Table 8.5 Approximate water content requirement for mixing and air content for different workabilities and nominal maximum size of aggregates.

   

   Source: ACI 211.1–1994.

   Reproduced with permission from the American Concrete Institute, Farmington Hills, MI (www.concrete.org)

5. The cement content is calculated by dividing the total water content by the water–cement ratio.
6. The bulk volume of dry-rodded coarse aggregate per unit volume of concrete is selected from Table 8.5 for the particular maximum size of coarse aggregate and the fineness modulus of fine aggregate.
7. On multiplying the bulk volume by bulk density, the weight of the coarse aggregate in one cubic meter of concrete can be calculated.
8. With the knowledge of the specific gravity of coarse aggregate, the solid volume of the coarse aggregate in a cubic meter can be calculated.
9. Similarly, the solid volume of cement, water, and volume of air is calculated in one cubic meter of concrete.
10. The solid volume of sand is calculated by subtracting the volumes of cement, coarse aggregate, water and entrapped air from the total volume.
11. The weight of the fine aggregate is calculated by multiplying the solid volume of the fine aggregate by the specific gravity of the fine aggregate.

8.7.2 Bureau of Indian Standards (BIS) Method of Mix Design

Bureau of Indian Standards (BIS) has recommended a Mix Design Concept mainly based on the research works performed in national laboratories. This method can be applied for both medium-strength and high-strength concretes. The step-by-step approach of the BIS mix design is given below.

1. The target mean compressive strength at 28 days is given by

   = f_ck + tS

   where f_ck = characteristic compressive strength

   t = a statistical value

   S = the standard deviation

2. It is desirable to establish a relationship between concrete strength and water–cement ratio at the site. If such a relationship is not available the water–cement ratio corresponding to the target strength may be determined from the relationship given in Table 8.6. It is possible to design in an effective way if the strength of the cement is incorporated in the concrete mix. This is done in the BIS design. Thus, if the 28-days strength is known, the water–cement ratio can be obtained from Fig. 8.1.

   Table 8.6 Strength and water–cement ratio

   

   Source: ACI 211.1–91.

   Reproduced with permission from the American Concrete Institute, Farmington Hills, MI (www.concrete.org)

3. The air content can be obtained from Table 8.7 for the normal size of the aggregate used.

   Table 8.7 Approximate air content

   
   

   Figure 8.1 Relationship between free water–cement ratio and concrete strength for different cement strengths

   Table 8.8 Approximate sand and water content per m³

   

   Source: IS: 10262–82.

4. The water content and percentage of sand are determined from Table 8.8 for medium-strength (less than M 35 grade) and high-strength (greater than M 35 grade) concrete.
5. The cement content per unit volume of concrete is obtained as

   Mass of cement =

   The cement calculated from the above relationship has to be checked against the minimum cement content requirement for durability from Table 8.9, and the greater of the two has to be taken.

   Table 8.9 Details for different exposures of concrete

   

   Source: IS: 456–2000.

6. The quantity of aggregate can be calculated from the following relationship:

   (8.1)

   (8.2)

   where V = absolute volume of fresh concrete

   = (gross volume) – (volume of entrapped air) m³

   W = mass of water per m³ of concrete, kg

   C = mass of cement per m³ of concrete, kg

   S_c = specific gravity of cement

   P = ratio of fine aggregate to total aggregate by absolute volume

   f_a, c_a = total masses of fine and coarse aggregates per m³ of concrete, respectively, kg

   S_fa, S_ca = specific quantities of saturated, surface-dry fine aggregate and coarse aggregate, respectively.

7. The mix proportions have been calculated based on the assumption that the aggregates are saturated and surface dry. When there is any deviation, necessary correction has to be incorporated.
8. The above calculated mix properties have to be checked by means of trial batches.
9. First the workability is checked, this forms Mix No. 1. If the measured workability is different from the assumed one, then the water content is changed (Table 8.10), and the whole mix design is modified keeping the new water–cement ratio constant.

   (b) A minor adjustment in the aggregate quantity may be made to improve the finishing quality or freedom from bleeding and segregation. This forms Mix. No. 2.

   (c) Now the water–cement ratio is changed by ±10% and the mix proportion is recalculated. This will form trial Mix Nos. 3 and 4.

   (d) Testing for trial Mix Nos. 2, 3 and 4 are done simultaneously.

   (f) These test results will provide adequate information including the relationship between compressive strength and water–cement ratio from which the correct mix proportion can be arrived at.

8.8 MANUFACTURING OF CONCRETE

While manufacturing concrete, it should be ensured that every batch of concrete has the same proportions. This is a mandatory requirement so as to satisfy two aspects, viz., same workability and uniform strength. In the manufacturing of concrete the following steps are followed:

1. Proportioning of concrete
2. Batching of materials
3. Mixing of concrete
4. Conveyance of concrete
5. Placing of concrete
6. Compaction of concrete
7. Curing of concrete

8.8.1 Proportioning of Concrete

Selection of the proper quantity of cement, coarse aggregate, sand and water to obtain the desired quality is known as proportioning of concrete. Concrete is formed by successive filling of voids in aggregate by sand, the voids in sand by cement and voids in cement by water and undergoing a chemical reaction.

The concrete formed by proper proportioning of ingredients should satisfy the following properties:

1. The fresh concrete should have adequate workability for uniform placement.
2. The hardened concrete after setting should have the desired strength and durability.
3. The concrete should be cheap considering the materials and labour.

There are two approaches in proportioning concrete. In the first method no preliminary tests are conducted. But based on experience, arbitrary ratios such as 1:2:4; 1:1½:3; 1:1:2, etc., are used. This method of proportioning by adopting an arbitrary ratio is called the mix method, and the concrete formed by this method is called ordinary concrete. This type of concrete is used for ordinary or common works such as columns and members subjected to medium loads, all general building RCC works, mass concrete work in culverts, retaining walls, compound walls, and ordinary machine bases. Ordinary concrete can also be used for long span arches with a mix of 1:1:2 and for heavy stressed members with a mix of 1:2:2.

In the second method, preliminary tests are conducted, the mix being designed by any one of the mix design methods to get the desired strength and durability. The concrete formed by this method is called controlled concrete. This type of concrete is used for all plain and reinforced concrete structures. The concrete mixes for controlled concrete are designed to have an average strength in preliminary strength test as shown in Table 8.11.

8.8.2 Batching of Materials

After fixing, the desired proportion of quantity of required ingredients, viz., cement, coarse-aggregate, fine-aggregate, cement and sand, has to be measured out in batches for mixing. This process of measuring out ingredients is called batching. Batching may be done by weight or by volume. Volume batching is inferior to weight batching as using the former is liable to change the volume of sand in bulking or aggregate constant void feasibility.

1. Weight Batching

In this batching method all the ingredients of concrete are directly weighed in kilograms. As the weight of cement bag is 50 kg, 20 bags are needed for 1 tonne of cement. For all important works the batching method should be used. This is a slow process.

2. Volume Batching

In this batching method, two units of measurements are employed: liquids are measured in litres and solids in cubic metres. That is all ingredients, viz., water, cement, sand and coarse aggregates are measured in litres, while the end-product concrete is measured in cubic metres.

In volume batching, other quantities are measured keeping cement as the base. Considering that 1 litre of cement equals 1.44 kg, a bag of 50 kg cement has a volume of 3.5 litres. Hence, for measuring aggregates wooden boxes of an inner volume of 3.5 litres has to be used. A size of box of 40 cm × 35 cm × 25 cm satisfies this 3.5 litre volume requirement. Handles are provided on the sides for handling.

As the density of water is 1 g/ml, it can be measured by weight or by volume. The quantity of water required depends on the water–cement ratio. Thus, for a water–cement ratio of 0.50 the quantity of water required is 25 litres (0.50 × 50 = 25 litres).

For accurate batching, the moisture content and absorption of aggregates and bulking of sand have to be ascertained.

8.8.3 Mixing of Concrete

Mixing of concrete may be done by hand or by a machine. Mixing should be done thoroughly so that the ingredients are uniformly distributed, and this can be judged by uniform colour and the consistency of concrete.

On a clean, hard and water-tight platform cement and sand are mixed dry using shovels until the mixture shows a uniform colour. Then aggregates are added and the ingredients are thoroughly mixed. Based on the water–cement ratio, the quantity of water required is calculated and added to the dry mix. The mass is then turned to obtain a workable mass and placed in the required area within 30 minutes. Hand mixing can be used for small quantities of concrete, or due to the non-availability of a machine or where the noise of the machine should be avoided. In general, extra cement of 10% is used to compensate the possible inadequacy.

Mixing by machine is always preferred. Concrete mixers are used for mixing concrete and are of two types, viz., (i) continuous mixers and (ii) batch mixers.

Continuous mixers are used for purposes where large quantities of concrete are needed such as dams, bridges, etc. Batch mixers are also called drum mixers, which consist of drums with blades or baffles inside them, and they are rotated. In the batch mixer, all required materials are fed into the hopper of the revolving drum in correct quantity. When the mix has attained the desirable consistency, the mix is discharged from the drum and conveyed to the concreting yard.

8.8.4 Conveyance of Concrete

The mixed concrete should be conveyed to the concreting yard as early as possible but within the initial setting time of the cement. The choice of conveyance depends on several factors, viz., nature of work, distance from the mixing place to the construction site, height to be lifted, type of cement, etc.

During the transit from the point of mixing to the point of placement, the following factors have to be borne in mind:

1. Care should be taken not to allow segregation of aggregates.
2. The containers of the drums should be tight such that there is a minimum loss of water.
3. The mixed concrete should be brought to the site before the initial setting time of the cement.

For ordinary simple works, a temporary ladder is erected to convey the concrete using baskets, or it is passed from hand to hand, i.e., by manual labour. For larger and important works, various mechanical devices such as vertical hoists, lift wells for tall structures, wheel barrows, etc., are used.

8.8.5 Compaction of Concrete

Compaction of concrete has to be done as early as possible after placing the concrete in place. The purpose of compaction is to expel air and bring the particles closer so as to reduce the void and make the concrete denser. This increased density will give higher strength and make the concrete impermeable. Over-compaction leads to segregation while under-compaction makes the concrete lean. To check for correct compaction, the compaction should be stopped as soon as the cement paste starts appearing on the upper surface of the concrete.

Compaction by hand may be performed by rodding, tamping, ramming or hammering. Wooden or steel hammers are used for ramming massive concrete works. Tampering is adopted for compacting slabs or other horizontal surfaces. Rodding is followed for compacting vertical sections. In all cases of hand compaction high water–cement ratio should be adopted.

Compaction by machines is performed using mechanical vibrators. This method of compaction has several advantages as detailed below:

1. The concrete produced is dense and impermeable.
2. A lesser water–cement ratio results in about 15% reduction in the use of cement.
3. A better bond exists between steel and concrete.
4. The surface of the concrete is uniform because of machine compaction.
5. Because of a high aggregate–cement ratio, there is a possibility for the reduction in creep and shrinkage.
6. Filling small openings is feasible because of good consistency in concrete.
7. It is relatively fast in placing concrete.
8. It consumes comparatively less time, materials and labour and is hence economical.

8.8.6 Curing of Concrete

The following methods of curing are adopted depending on the type of work:

1. Direct Curing

In this method water is directly applied to the surface of curing. In this process, the surface is continuously cured by stagnating water, or using moist gunny bags, straws, etc. These methods are used for horizontal surfaces. Vertical surfaces can be cured by covering moist gunny bags or straws.

2. Membrane Curing

In this method, steps are taken to prevent water evaporation from finished concrete surfaces. This is done by covering the surfaces with water-proof papers, polythene papers or by spraying with patented compounds or covering with a bituminous layer to form an impervious film on the concrete surface.

3. Steam Curing

This approach is widely used in precast concrete units. Here the precast units are kept under the warm and damp atmosphere of a steam chamber.

4. Surface Application by Chemicals

Chemicals such as calcium chloride are spread as a layer on the finished concrete. The chemical absorbs moisture from the atmosphere and prevents evaporation of the moisture from the concrete surface.

8.9 FINISHING

Finishing is the last stage in concrete construction. After casting of a concrete, the concrete does not offer a pleasant architectural appearance. In some cases like beams, finishing may not be needed. For a residential building, airport or road pavement and culvert and bridges, finishing is a must. Now-a-days, the centering materials are so made such that the concrete exhibit a pleasant surface finish. Many of the prefabricated concrete units are made in such a way to give an attractive architectural effect.

Different types of finishes have been in use now-a-days. Surface finishes may be grouped as under.

1. Formwork finishes
2. Surface treatment
3. Applied finishes

8.9.1 Formwork Finishes

Concrete maintains the shape of formwork, i.e., centering work. Thus, keeping the required shape through formwork, viz., undulated fashionable shapes, V-shaped finishes, plain surfaces, etc., any pleasing surface can be obtained. The imaginative ideas of architects may be implemented by a careful formation of concrete surface.

A properly made out formwork can give a very smooth surface using the right proportioning of materials better than that made by the best mason. Because of increasing cost of labour, self-finishing concrete surfaces are preferred.

8.9.2 Surface Treatment

This is a commonly used method of surface finishing. It is important for a residential floor to be smooth. To obtain a smooth finish, first the proportioning of mix should be appropriate. The finishing of surface should be at the same rate as that of placing of concrete. Attention must be paid to the extent and time of transportation. Careful attention should be paid to the non-formation of laitance, that excess mortar does not remain and excess water is not accumulated on the surface. A poor and dissatisfactory surface is formed due to hurried completion.

Rough finishes are required on concrete pavement slabs, air-field pavements, on roads, etc. In such cases, the concrete is brought to the plane level surface, and then lightly raked, broomed, textured or scratched to create a rough surface. Other finishes are to give good look like exposed aggregate finish.

8.9.3 Applied Finish

Applied finish is the exterior application of rendering made on concrete structures. In this case the concrete surface is finished and kept wet after which mortar (1:3) is applied. The desired finish is then given to the mortar.

Sometimes the rendering applied on a wall is pressed with a sponge. On repeating this process, the sand is exposed and the surface obtains a finish which is known as sand facing. Another type of finish known as Rough Cast Finish is also done. In this type, a wet plastic mix (three parts of cement, one part of lime, six parts of sand and four parts of about 5-mm-size peagravel aggregate) is dashed on the wall surface using a scoop or plasterer’s trowel.

Other finishes under this category are non-slip finish, coloured finish, wear-resistant floor finish, craziness finish, etc.

8.10 TESTING OF FRESH AND HARDENED CONCRETE

Testing of fresh and hardened concrete is important in concrete construction. Tests are conducted on fresh concrete to check the workability of concrete, and on hardened concrete to determine the strength, creep effects, durability, etc.

8.10.1 Testing of Fresh Concrete

The following tests are commonly employed to measure workability of fresh concrete:

1. Slump Test
2. Compaction factor Test
3. Flow Test
4. Kelly Ball Test
5. Vee Bee Consistometer Test

1. Slump Test

It is the most commonly used method of measuring consistency of concrete. This test can be conducted in the field or in a laboratory. However, this test is not suitable for very wet or very dry concrete.

The apparatus for conducting the slump test consists of a metallic mould in the form of a frustum of a cone with a 20 cm diameter at the bottom, 10 cm diameter at the top and 30 cm in height. A steep tamping rod of 16 mm diameter, 0.6 m long with a bullet end is used for tamping.

The internal surface of the mould is thoroughly cleaned and placed on a smooth non-absorbent horizontal surface. The mould is filled in four layers of equal height. Each layer is compacted by giving 25 blows with the tamping rod uniformly. After filling the mould and rodded, the excess concrete is shaken off and levelled.

The mould is lifted upwards from the concrete immediately by raising it slowly. This allows the concrete to subside. This subsidence is referred to as the slump of concrete. The difference in height of the mould and that of the subsided concrete is measured and reported in mm, which is considered to be the slump up of concrete.

The pattern of slump also represents the characteristics of concrete (Fig. 8.2). If the slump of the concrete is even, it is called a true slump. If one-half of the cone slides down, it is called a shear slump. In this case the average value of the slump is considered. The shear slump also indicates that the concrete is not cohesive and reflects segregation. Concrete mixes are classified based on the slump as given in Table 8.12.

Figure 8.2 Types of slumps

Slumps recommended for various works of concrete construction are presented in Table 8.13.

The slump test can be conducted both in the laboratory and at the work site. The slump test results grant the facility to easily detect the difference in water content of successive batches of the identical mix.

2. Compacting Factor Test

This is a more refined test than the slump test. The test measures the degree of compaction obtained by using certain energy in overcoming the internal friction of the concrete. This property is a measure of workability.

The test apparatus consists of two conical hoppers with bottom doors and a separate cylinder kept at the bottom. The concrete is filled in the top hopper fully without compaction and released successively through the two hoppers and into the bottom cylinder (Fig. 8.3). After striking off the level in the cylinder the weight of the concrete (W₁) in the cylinder is determined. The same cylinder is filled with the same batch of concrete and compacted to get the maximum weight (W₂). The ratio of the observed weight, W₁, to the theoretical weight, W₂, i.e., W₁/W₂ is the compacting factor. The workability, compacting factor and the corresponding slump are given in Table 8.14.

Figure 8.3 Compacting factor test apparatus (dimensions shown are in mm)

The compacting factor test measures the quality of concrete, which relates very close to the workability. This test clearly depicts the workability of concrete.

3. Flow Test

This test gives an indication of the quality of concrete with respect to consistency, cohesiveness and non-segregation. In this test a mass of concrete is subjected to floating, and the flow or spread of the concrete is measured. The flow is related to workability.

The test apparatus consists of a flow table of 76 mm diameter on which concentric circles are marked (Fig. 8.4). A mould similar to that used in the Slump Test with a base diameter of 25 cm, an upper diameter of 17 cm and a height of 12 cm is used. The mould is kept on a clean table, and concrete is filled in two layers with each layer being rodded 25 times with a tamping rod of 1.6 cm diameter and 61 cm long with a rounded end. The excess concrete at the top of the mould is levelled. The mould is lifted vertically upwards completely. The concrete stands on its own without support.

The table is raised and dropped 12.5 mm with the cam arrangement, 15 times in about 15 seconds. The diameter of the spread concrete is measured in 6 directions and the average value is taken. The flow of the concrete is defined as the percentage increase in the average diameter of the spread concrete to the base diameter of the spread concrete to the base diameter of the mould, i.e.,

The value varies from 0 to 150%.

The spread pattern of the concrete also reflects the tendency of the segregation. The flow test is a laboratory test.

Figure 8.4 Flow table apparatus

4. Kelly Ball Test

It consists of a metal hemisphere of 15 cm diameter weighting 13.6 kg (Fig. 8.5). The concrete base should be 20 cm depth, and the minimum distance from the centre of the ball to the nearest edge of the concrete is 23 cm. The ball is gradually lowered to the surface of the concrete. The depth of the penetration is read immediately on the stem to the nearest 6 mm. The test can be done in a shorter periods of about 15 seconds. This test gives more consistent results than slump tests.

The test can be performed in the field and it can be performed on the concrete placed on the site.

Figure 8.5 Kelly ball

5. Vee Bee Consistometer Test

This test consists of a vibrating table, a metal pot, a sheet metal cone and a standard iron rod (Fig. 8.6). A slump cone with concrete is placed inside the sheet metal cylindrical pot of the consistometer. The glass disc is turned and placed on the top of the concrete in the pot. The vibrator is switched on, and the stop watch is started simultaneously. The vibrator is kept on till the concrete in the cone assumes a cylindrical shape. The time is noted. The time required in seconds for the concrete to change from the shape of a cone to the shape of a cylinder is known as the Vee Bee Degree.

This is a good laboratory method and is more suitable for very dry concrete. This test measures the workability indirectly.

Figure 8.6 Vee Bee consistometer-type VBR

8.10.2 Testing of Hardened Concrete

The following tests are conducted for hardened concrete:

1. Compressive Strength Test
2. Flexural Strength Test
3. Split-tension test

1. Compressive Strength Test

This is an important test as most of the properties of concrete are qualitatively related to it. It is an easy and most common test. The tests are conducted on cubical or cylindrical specimens.

The cube specimen is of size 15 cm × 15 cm × 15 cm, and the cylinder is of 15 cm diameter and is 30 cm long. The largest nominal size of the aggregate does not exceed 20 mm. The moulds must be of metal moulds, preferably of steel or cast iron. The moulds are made in such a way that the specimens are taken out without damage. A tamping steel bar of 16 mm diameter and 0.6 m long with a bullet end is used for compacting.

The test cube specimens are made as soon as practicable. The concrete is filled into the mould up to approximately 5 cm. Each layer is compacted by the tamping rod (25–35 strokes depending on 10–15 cm depth) or by vibration. The top layer is compacted using a trowel. It is covered with a glass or metal plate to prevent evaporation. The specimens are demoulded after 24 hours and submerged in fresh clean water or saturated lime solution and kept there and taken out just prior to the test. The water should be maintained approximately at 27º ± 2 ºC and on no account must the specimens be allowed to dry out.

The specimens are tested in a compression testing machine on completion of 7 and 28 days. Compression on the cube or cylinder undergoes lateral expansion owing to Poisson’s ratio effect.

Cylindrical specimens are less affected by end restraints caused by plaster, and hence it is believed to give more uniform results than the cube. Further, the cylinder simulates the real condition in the field in respect of the direction of the load. Normally, the strength of the cylindrical specimen is taken as 0.8 times the strength of cubical specimens.

2. Flexural Strength Test

Concrete is relatively strong in compression and weak in tension. Tensile stresses can develop in concrete due to drying, shrinkage, rusting of steel reinforcement, temperature gradient and many other reasons. Hence, the tensile strength of concrete gains importance.

Direct measurement of tensile strength is not feasible. Hence, beam tests are found to be dependable to measure the flexural strength property of concrete. The Modulus of Rupture is taken to be the extreme fibre stress in bending.

The value of the Modulus of Rupture depends on the dimension of the beam and the type of loading. The loading adopted is central or two-third point loading. In the central point loading, the maximum fibre stress occurs below the point of loading where the bending moment is at the maximum. In the two-point loading, a critical crack may appear at any section, where the bending moment is at the maximum, or the resistance is weak. In general, the two-point loading yields a lower value of the Modulus of Rupture than the centre point loading.

The various types of loading are shown in Fig. 8.7. The size of the specimen is 15 cm × 15 cm × 70 cm. In case of concrete with an aggregate of size less than 20 mm, a beam size of 10 cm × 10 cm × 50 cm may be used. The mould may be of metal or steel or cast iron. The tamping bar may be of steel weighing 2 kg, 40 cm long and should have a ramming face of 25 mm².

The testing machine should have sufficient loading capacity with a specific rate of loading such that the permissible errors on the applied load should not be greater than ± 0.50%.

The flexural strength of the specimen is expressed as the Modulus of Rupture, f_b, as

(8.3)

where P = maximum load in kg applied to the specimen

a = 17–20 cm for a 15.0-cm specimen or >13.3 cm for a 10.0-cm specimen

b = measured width in cm of the specimen

d = measured depth in cm of the specimen at the point of failure

Figure 8.7 Loading arrangement in the flexural beam test

If a is less than 17.0 cm for a 15.0-cm specimen or less than 11.0 cm for a 10.0 -cm specimen, the results of the test may be discarded.

3. Split-tension Test

This is an indirect tension test. This is also referred to as the Brazilian test. In this test a cylindrical specimen is placed horizontally between the loading surfaces of a compression testing machine. The load is applied until failure of the cylinder along the vertical diameter. The test specimen is shown in Fig. 8.8.

Figure 8.8 Split-tension test

When the load is applied along the diameter, compressive stresses develop immediately below the two generators to which the load is applied. But a larger portion about 5/6^th of the depth is subjected to tensile stress.

The main advantage of the test is that the same compression testing machine and the same cylindrical specimen used for the compression test may be used. Narrow packing strips of suitable material such as plywood are used to reduce the high compressive stresses.

The Split-tension Test is simple to perform and generally gives more uniform results. The tensile strength from the Split-tension Test is almost near its true tensile strength than the Modulus of Rupture. The Split-tension Test gives 5–12% higher value than the direct tensile strength.

8.11 CRACKS IN CONCRETE

Factors that contribute to the formation of cracks in cement concrete are discussed below.

8.11.1 Plastic Shrinkage Cracks

Evaporation of water from fresh concrete is due to the absorption by formwork and during the hydration process. The surface of concrete dries up when the loss of water from the surface of the concrete is faster than the migration of water from the interior to the surface. This creates a moisture gradient which results in surface cracking while the concrete is stiff in a plastic condition. The magnitude of the crack depends upon the rate of evaporation of water from the surface of the concrete.

8.11.2 Settlement Cracks

Plastic concrete settles when vibrated. There will be no cracks if the concrete settles uniformly. Some cracks are bound to occur if there is any obstruction to uniform settlement due to reinforcement or larger pieces of aggregates. Such cracks are called settlement cracks. Such cracks are formed generally in deep beams.

8.11.3 Cracks Due to Bleeding

Water being light in weight moves upward in concrete, which leads to bleeding. This water evaporates to make the top surface porous having very little abrasion resistance. Masons often float the concrete when the bleeding water is still standing on the surface. Too much of working on the exposed surface leads to pressing the coarse aggregate down and bringing up fine particles of cement and water. Such a condition develops cracks on the surface.

8.11.4 Cracks Due to Delayed Concrete Curing

During the initial stages of curing, uninterrupted hydration is a must. Due to the hot sun, winds and lower relative humidity, young concrete dries faster leading to plastic shrinkage cracks.

8.11.5 Cracks Due to Construction Effects

Improper formwork with less rigidity may lead to sinking, bending, etc., and the wet concrete may cause cracks or deformation after compaction which may go unnoticed.

8.12 QUALITY CONTROL OF CONCRETE

Quality control implies that the assigned work is done according to the specifications agreed in the contract. Major civil engineering works such as multistoreyed buildings, dams, harbours, etc., have to be constructed with utmost care as they have to last and be used for decades. Specifications of work should be framed based on cost or standard processors so that they serve effectively as a guide to complete the work with high quality. The specifications are as important as the design of the project.

To make a quality concrete construction at a site, fieldwork has to be organized with the three divisions with mutual coordination, viz., the engineering division, the manufacturing division and the placing division. The engineering division looks after all forms, reinforcements details and installation of all embedded parts. The manufacturing division takes care of the control of materials, batching and mixing of concrete. The placing division takes care of placing, curing and other subsequent works.

The whole aim is to produce inexpensive high-quality concrete. The general requirements to produce high-quality concrete, i.e., densest, more workable and high strength, are as follows:

1. Air bubble should be completely removed from the concrete.
2. Compaction of concrete should be such that a minimum void is present.
3. Adequate curing for 28 days has to be effected.

Resources such as supervisors, engineers, etc., who are involved in the production of high-quality concrete should be aware of all the factors affecting good-quality concrete. Accordingly, the execution of quality control of concrete should be carried out at every stage. Carefully constructed high-quality concrete work has the following advantages:

1. The possibility of failure is minimized.
2. Lower cost of construction with long life.
3. Low-maintenance cost.
4. Possibility of using low-grade materials for some other purposes.

In summary, to obtain high-quality concrete all suitable precautions must be taken to ensure proper inspection of the ingredients, proper batching and mixing, proper transportation and careful placing, adequate curing and careful renewal of formwork and necessary finishing.

8.13 NON-DESTRUCTIVE TESTING

Non-destructive testing is done on hardened concrete. In non-destructive testing methods, some properties of concrete are used to estimate strength, durability, elastic parameters, crack depth, micro-cracks and progressive deterioration of concrete.

Such properties of concrete are hardness, resistance to penetration of projectiles, rebound number, resonant frequency, ability to allow ultrasonic pulse velocity, ability to scatter and transmit X-rays and gamma rays, its response to nuclear activation and acoustic emission. Various non-destructive methods have been developed using one or more of the above properties. Some of the important methods in use are explained below.

8.13.1 Schmidt’s Rebound Hammer

This is a commonly adopted equipment for measuring surface hardness. It consists of a spring control hammer which slides on a plunger and is housed in a tube. Once the plunger is pressed against the surface of the concrete, the mass behind the spring rebounds. After impact, the spring control mass rebounds and takes the rider along the guide scale. Based on the position, the reading is taken. Considering the reading and the calibration, the actual strength can be determined.

8.13.2 Frequency Method

It is another important non-destructive method used to determine the compressive strength and other properties. The fundamental principle on which the method is based is the velocity of material through a material. A mathematical relation could be made between the resonant frequency of the material to the Modulus of Elasticity of the material. The property of homogeneous material can be made use of heterogeneous materials like concrete with judgement.

8.13.3 Nuclear Method

This is a new technique which is used to determine the moisture content and the cement content. This method employs the scattering of neutrons directed towards the concrete and the number of neutrons returned. With a standard relationship number of neutron and water content/cement content, the required water or cement content can be obtained.

8.13.4 Radioactive Method

Here X-rays and gamma rays are used. When X-rays and gamma rays are passed through concrete, the electromagnetic spectrum penetrates concrete but undergoes attenuation in the process. The degree of attenuation is a function of the kind of matter traversed, its thickness and the wavelength of the radiation. Further, the intensity of the incident gamma-rays and the emerging gamma-rays after passing through the specimens is measured. These two values are used to calculate the density of concrete.

The gamma-rays transmission method is particularly used to measure the thickness of concrete slabs of known density. This is achieved by passing gamma rays of known intensity to penetrate through the concrete. The thickness of the concrete is measured based on the intensity of gamma rays measured on the other end.

8.13.5 Pullout Test

Here a rod is embedded in concrete blocks. These are pulled out and the strength of the concrete is determined. The ideal way to use the Pullout test in the field is to incorporate assemblies for pull out in the structure itself. These could be pulled out and the strength determined.

8.13.6 Pulse Velocity Method

It consists of two parts, viz., the mechanical ionic pulse velocity method and the ultrasonic pulse velocity method.

The mechanical sonic pulse velocity method consists of measuring the time of travel of longitudinal or compressive waves generated by a single impact hammer blow or repeated blows. The ultrasonic pulse velocity method consists of measuring the time of travel of electronically generated mechanical pulses through the concrete. Of these two, the ultrasonic pulse velocity has gained popularity throughout the world.

The pull velocity methods have been used to evaluate the quality of concrete, concrete strength, durability, Modulus of Elasticity, detection of water, etc.

High pulse velocity readings in concrete are indicative of concrete of good quality. Table 8.15 gives the pulse velocity range of quality of concrete (Leslie and Chessman, 1949, reported by Shetty, 2006).

Pulse velocity techniques have been used successfully for the detection of cracks. This is possible only when the width of the crack is of considerable depth and of appreciable width. The basic principle in such a situation to detect the crack of the depth is that no signal will be received at the receiving transducers, the pulse will pass around the end of the crack and signal is received at the transducers. However, the pulse would have travelled a distance longer than the straight line path upon which pulse velocity computations are made. The difference in the velocity of pulse is used to estimate the path length and therefore the crack depth. Figure 8.9 illustrates the principle of crack detection.

Figure 8.9 Pulse velocity technique

8.14 LIME CONCRETE

8.14.1 Ingredients of Lime Concrete

Lime concrete is made of lime, fine aggregate and coarse aggregate and mixed in suitable proportions in addition to water. Hydraulic lime is generally used as a binding material, sand, surkhi and cinder are used as fine aggregates, and broken bricks or broken stones are used as coarse aggregates.

8.14.2 Properties of Lime Concrete

1. It provides a good base and is capable of taking loads.
2. It has a certain degree of flexibility and adjusts very well with the surface in contact.
3. It exhibits a certain degree of water proofing property.
4. It has adequate volumetric stability when matured lime is used.
5. It resists weathering effects and is quite durable.
6. It can be prepared easily with less cost.

8.14.3 Preparation of Lime Concrete

Good lime concrete is prepared using hydraulic lime. Fat lime is not used as it is not suitable to use in large masses. The sand to be used should be free from impurities. The coarse aggregates used for lime concrete are broken bricks or broken stones. The water used should be clean.

The mix proportion adopted for various works are as follows:

1. Under floors – 1:2:3
2. Foundation work – 1:2:4

The following procedure is adopted while preparing lime concrete:

1. Based on the mix, the quantity of coarse aggregate required is measured, slaked and soaked adequately with water, and a measured quantity of sand is added.
2. Dry mixing is done first, and then a sufficient quantity of water is added. The whole mass is again mixed thoroughly to obtain the correct consistency.
3. The wet concrete is laid in uniform layers in such a manner that after compaction, the thickness should not be more than 15 cm. The compaction is done with light hammers.
4. The concrete is allowed to set for 24 hours.
5. The surface of the concrete is watered and rammed with heavy hammers till the concrete is thoroughly compacted.

8.14.4 Uses of Lime Concrete

Lime concrete is used for

1. Foundation bases of load-bearing walls, columns and floors.
2. Filling haunches over masonry arches.
3. Terrace finishing as it is the quality of volumetric stability.

8.14.5 Precautions

The following precautions have to be observed:

1. The person working should wear rubber gloves and rubber gumboots, otherwise there is a possibility of formation of rashes on the skin due to lime.
2. Workers also should apply oil over their skins to protect themselves from rashes and skin cracking.

1. Concrete is an important construction material consisting of ingredients which are inert materials, viz., coarse and fine aggregates and binding materials, viz., cement or lime.
2. Plain cement concrete is the simplest form of concrete made with cement, sand, pebbles or crushed rock and water.
3. Cement concrete provided with steel reinforcement for bearing tension, bending, torsion, fatigue, etc., is referred to as reinforced cement concrete.
4. Pre-stressed concrete is one in which high tensile steel wires are stretched initially to the desired level and concrete is placed. After setting and completion of curing, concrete is used.
5. Light-weight concrete has less density compared to the conventional concrete. Light-weight concrete may be made (i) by omitting the fine aggregate, (ii) by using light-weight aggregate and (iii) by aerating concrete.
6. Concrete used for the casting of various structural components with or without reinforcement is called precast cement concrete.
7. Cement concrete is made to a plastic state by adding water and hardens on curing. Plastic concrete is called fresh concrete. The concrete formed to a required shape and cured is said to be hardened concrete.
8. Aggregates are the major and important constituents of concrete. Aggregates may be classified as (i) normal weight aggregates, (ii) light-weight aggregates and (iii) heavy-weight aggregates.
9. Concrete can be considered to be a two-phase material, viz., the paste phase and the aggregate phase. The paste phase is connected with cement, and the aggregate phase is connected with aggregates.
10. The Fineness Modulus is an empirical factor; the larger the figure, coarser is the material. For good concrete, the value of the Fineness Modulus of coarse aggregate should be between 6 and 8.5, and fine aggregate should be between 2 and 3.2.
11. As a general guidance, water fit for drinking is fit for preparing concrete. If the pH is between 6 and 8, the water is considered to be suitable.
12. Admixtures and construction chemicals are those added to the ingredients of the concrete or at a later stage to obtain the required mix for the desired strength and suitability.
13. Plasticizers are added to improve the desired qualities that are required for plastic concrete.
14. A retarder is an admixture which slows down the process of hydration because of which the concrete remains plastic and workable.
15. An accelerator is a type of admixture which is added to obtain early strength.
16. Air-entrained concrete is made using air-entraining cement or by the addition of an air-entraining agent.
17. Pozzolanic or mineral admixtures modify certain properties of fresh and hardened concrete, such as lowering the heat of hydration, increasing the water tightness, reducing alkali–aggregation reaction, resisting a sulphate attack and improving workability.
18. Damp-proofing admixtures have the property of filling pores or repelling water.
19. Mix design of concrete is the process of selecting the required ingredients of concrete and finding their relative proportions with the aim of producing an economical concrete of certain strength and durability.
20. For a given set of materials, the four factors to be considered in the design of a concrete mix are:

    (i) Water–cement ratio or cement content

    (ii) Cement–aggregate ratio

    (iii) Gradation of aggregates

    (iv) Consistency

21. Two popular methods of mix design are: the American Concrete Institute (ACI) Method of Mix Design and the Bureau of Indian Standards (BIS) Method of Mix Design.
22. The following steps are followed for the manufacture of cement:

    (i) Proportioning of concrete

    (ii) Batching of materials

    (iii) Mixing of concrete

    (iv) Conveyance of concrete

    (v) Placing of concrete

    (vi) Compaction of concrete

    (vii) Curing of concrete

23. The concrete formed based on the mix-design is called controlled concrete, whereas concrete formed by an adopting ratio (e.g., 1:2:4) is called ordinary concrete.
24. Batching of materials may be weight batching or volume batching.
25. The following tests are employed to measure the workability of fresh concrete:

    (i) Slump Test

    (ii) Compaction Factor Test

    (iii) Flow Test

    (iv) Kelly Ball Test

    (v) Vee Bee Consistometer

26. The tests conducted for hardened concrete are as follows:

    (i) Compressive Strength Test

    (ii) Flexural Strength Test

    (iii) Split-tension Test

27. The cracks formed in concrete are as follows:

    (i) Plastic Shrinkage Crack

    (ii) Settlement Cracks

    (iii) Cracks due to delayed concrete curing

    (iv) Cracks due to bleeding

    (v) Cracks due to construction effects

28. Quality control implies that assigned work is done according to the specifications agreed in the contract.
29. Non-destructive testing methods on hardened concrete use some properties of concrete to estimate the strength, durability, elastic parameters, crack depth, micro-cracks, and progressive deterioration of concrete.
30. Non-destructive testing methods are: the Schmidt’s Rebound Hammer Method, the Frequency Method, the Nuclear Method, the Radioactive Method, the Pull out Test and the Pulse Velocity Method.
31. Lime concrete consists of lime, a fine aggregate and a coarse aggregate, mixed in proportion in addition to water. It is cheaper and has less strength than cement concrete.

1. Distinguish between lime and cement concrete.
2. What is meant by proportioning of concrete?
3. Why proportioning of concrete is necessary?
4. Can sea water be used for making concrete? Explain.
5. What is meant by curing of concrete?
6. What are the quality requirements of water?
7. Discuss the importance of water–cement ratio.
8. How is compaction of concrete achieved?
9. What are the types of vibrators available for compaction of concrete and where they are used.
10. Explain the factors affecting evaporation of water from concrete.
11. Discuss in detail the various methods of curing.
12. What are admixtures? Explain any two.
13. What is mix design? Explain the ACL method of mix design.
14. How batching of materials is done?
15. Distinguish between fresh and hardened concrete.
16. What is meant by controlled concrete?
17. What do you understand by controlled concrete? How it is different from ordinary concrete.
18. Define and explain the workability of concrete.
19. Briefly explain the types of finishing.
20. List the tests conducted on fresh concrete. Explain any one test.
21. Explain the Compression Strength Test on hardened concrete.
22. How is quality control of concrete done?
23. Explain any one non-destructive method of testing concrete.
24. Explain crack detection in RCC structures with a sketch using ultrasonic non-destructive test equipment.