Special Concretes and Concreting Methods

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Special Concretes and

Concreting MethodsUNIT 7 SPECIAL CONCRETES AND CONCRETING METHODS

Structure 7.1 Introduction Objectives

7.2 Lightweight Concrete 7.3 Heavy Concrete 7.4 Mass Concrete 7.5 Pre-cast Concrete 7.6 High Early Strength Concrete 7.7 Vacuum Concrete 7.8 Pre-stressed Concrete 7.9 Ultra-light-weight Concrete 7.10 Colcrete 7.11 Ready Mix Concrete 7.12 Gunite 7.13 Ferro-cement 7.14 Roller Compacted Concrete 7.15 Fibre Reinforced Concrete 7.16 Hot Weather Concreting 7.17 Cold Weather Concreting 7.18 Underwater Concreting 7.19 Quality Control of Concrete 7.20 Summary 7.21 Answers to SAQs

7.1 INTRODUCTION

In the previous unit, you have studied different concreting operations, concreting formwork, types of joints in concrete. In this unit, you will study special types of concrete and concreting methods under extreme environmental conditions.

Objectives After studying this unit, you should be able to

• classify the types of special concrete,

• appreciate the importance and purposes of special concretes,

• discuss the problems encountered in hot and cold weather concreting,

• describe the precautions to be taken in hot and cold weather concreting, and

• discuss the methods of underwater concreting.

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Concrete Technology 7.2 LIGHTWEIGHT CONCRETE

Lightweight concrete is manufactured not only on account of its lightweight but also due to high thermal insulation compared with normal concrete.

The self weight of conventional concrete is high. Density of normal concrete varies from 2200 to 2600 kg/m3. Due to heavy weight, conventional concrete becomes an uneconomical structural material to some extent particularly for high rise buildings. The density of lightweight concrete varies from 300 to 1850 kg/m3.

Lightweight concrete is prepared from the following materials.

Binding Material

Different types of cements can be used as binding material. The materials such as lime-slag, lime-cinder, etc. can also be used as binding material.

Aggregates

The loose porous materials are used as aggregates for lightweight concrete. The natural porous aggregates can be obtained by crushing light-weight rocks. The artificial porous aggregates can be obtained from industrial wastes.

Steel

As lightweight concrete is highly porous, the corrosion of reinforcement takes place, if not properly protected. Hence, the lightweight concrete should be made sufficiently dense when used for RCC work. Sometimes, the reinforcement is coated with anti-corrosive treatments, when lightweight concrete is used.

Water

The strength of lightweight concrete mainly depends on the amount of water in the concrete mix. The use of potable water is necessary to prepare light weight concrete. The water-cement ratio should be carefully calculated to achieve optimum strength of lightweight concrete. When water content reaches its optimum value, there is corresponding increase in the strength of lightweight concrete.

Advantages

Following are the advantages of lightweight concrete :

(a) The use of lightweight concrete results in the reduction of cost to the extent of about 30 to 40%.

(b) The reduction in weight of concrete helps easy removal, transport and erection of pre-cast products.

(c) The lightweight concrete can be prepared by using the local industrial waste, if found suitable.

(d) The lightweight concrete has greater fire resistance as compared to the ordinary concrete.

(e) The sound absorption coefficient of the lightweight concrete is about twice that of the ordinary concrete. The sound absorption of lightweight concrete is good because the air-borne sound energy is converted into heat in the small channels of the concrete.

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Concreting Methods(f) The lightweight concrete has generally a lower thermal

expansion than ordinary concrete.

(g) The lightweight concrete increases the progress of construction work.

Limitations

Following are the limitations of lightweight concrete :

(a) The depth of carbonation, i.e. the depth within which corrosion can occur under suitable conditions, is nearly twice than that of normal concrete. Hence, special care will have to be taken to provide sufficient cover to the reinforcement of the lightweight structures to take protection against corrosion.

(b) The lightweight concrete has less strength as compared to the ordinary concrete.

(c) The lightweight concrete produces a harsh mix, therefore it is of low workability.

(d) Mixed design procedures are not well established for the lightweight concrete.

Production The various methods of producing lightweight concrete depend on :

(a) The formation of air voids in the concrete by omitting fine aggregates called as No-fine concrete.

(b) The formation of air voids in a cement paste by the addition of some substance which causes a foam called as Aerated or Cellular or Gas or Foamed concrete.

(c) The presence of air voids in the aggregate called as Lightweight aggregate concrete.

Types

Following are the types of lightweight concrete: (a) Lightweight aggregate concrete

(b) No-fines concrete

(c) Air-entrained concrete

7.2.1 Lightweight Aggregate Concrete Lightweight aggregates can be classified into two categories namely natural lightweight aggregates and artificial lightweight aggregates.

Following are the examples of natural lightweight aggregates.

(a) Rice husk (b) Pumice

(c) Volcanic cinders (d) Diatomite

(e) Sawdust (f) Scoria

Following are the examples of artificial light weight aggregates.

(a) Thermocole beads (b) Artificial cinders

(c) Expanded perlite (d) Coke breeze

(e) Foamed slag (f) Exfoliated vermiculite

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(g) Sintered fly ash (h) Expanded shales and slate

(i) Bloated clay. Natural lightweight aggregates are not found in many places. They are also not of uniform quality. Therefore, they are not used widely in making lightweight concrete. Pumice is the only natural lightweight aggregate which is used widely.

Lightweight aggregate concrete is made by the use of lightweight aggregates. Different lightweight aggregates have different densities. Strength of lightweight concrete depends on the density of concrete. Less porous aggregate which is heavier in weight produces stronger concrete particularly with higher cement content. The grading of aggregate, the water-cement ratio and the degree of compaction also affect the strength of concrete. Lightweight aggregate concrete exhibits higher moisture movement than the normal concrete. Concrete while wetting swells more and while drying shrinks more. The coefficient of thermal expansion of concrete made with lightweight aggregate is generally much lower than ordinary concrete.

7.2.2 No-fines Concrete This concrete is obtained by omitting fine aggregate from the mix so that there is an agglomeration of nominally one-size coarse aggregate particles, each surrounded by a coating of cement paste up to about 1.3 mm thick. There exists, therefore, large pores within the body of the concrete, their large size means that no capillary movement of water can take place and consequently the rate of water penetration is low.

The density of no-fines concrete depends primarily on the grading of the aggregate for a given type of the aggregate. With one-size aggregate, the density is about 10% lower than when well-graded aggregate of the same specific gravity is used. The usual aggregate size is 10 to 20 mm. No-fines concrete compacts very little as compared to ordinary concrete. The compressive strength of no-fines concrete varies generally from 1.4 MPa to 14 MPa.

7.2.3 Air-entrained Concrete The cement concrete, prepared by mixing aluminium in it, is called air entrained or cellular or aerated or gas or foamed concrete.

Following are the types of air entrained concrete : Gas Concrete

It is obtained by mixing finely divided aluminium powder in cement matrix. A chemical reaction takes place with hydroxide or alkali or calcium. It liberates hydrogen which forms the bubbles. Hydrogen peroxide can also be used which generates oxygen.

Foamed Concrete It is produced by adding foaming agent to the mix. It introduces and stabilizes air bubbles during mixing at high speed. Air entrained concrete may not contain the aggregate. Powdered zinc can also be used in this concrete. Depending upon density, strength and thermal conductivity also vary. Mixes have densities between 480 kg/m3 to 1120 kg/m3. Cellular concrete has a high thermal shrinkage and moisture movement. These can be minimized by high pressure steam curing. It also increases the compressive strength of concrete. Cellular concrete offers

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Concreting Methodsbetter fire resistance than ordinary concrete. It has good resistance to frost. Cellular concrete is mostly suited for partitions for heat insulation purposes due to its low thermal conductivity and for fire proofing.

7.3 HEAVY CONCRETE

Heavy concrete is also called as heavy-weight concrete or high density concrete. Heavy concrete can be produced by using specially heavy weight aggregates and compacting well by mechanical means. Both natural and artificial heavy aggregates are available. The high density aggregates are heavy iron ores such as magnesite or haematite, etc. Fine natural sand is used as fine aggregate. The density of such concrete is around 3400 kg/m3. The heavy aggregate is crushed to produce a fine material and this is used as a fine aggregate instead of sand to produce extra heavy concrete. Steel shots and iron punching are the artificial aggregates. Artificial aggregate used should be free from oil, which prevents bond. The artificial aggregates are very costly compared to natural aggregates. Heavy concrete produced from artificial aggregates may have a density of about 5500 kg/m3 .The density of heavy weight concrete varies from 3360 to 5280 kg/m3. Heavy weight concretes can be suitably used for gravity dams, retaining walls or special atomic power plants, vessels, etc. It is also suited for preparing counter-balance weights for lift bridges and ballast blocks for ships, where the high density of heavy concrete reduces the volume of concrete required to produce the same dead weight, leading to economy.

SAQ 1

(a) Fill in the blanks. (i) The weight of lightweight concrete is _______________ that of

dense concrete. (ii) The cement requirements of lightweight are ____________ than

that of gravel concrete. (iii) Air-entrained concrete is also called _______________. (iv) Concrete for atomic reactors must of _______________.

(b) State True or False.

(i) The lightweight concrete aggregates have high values of absorption.

(ii) Bond strengths of lightweight as well as dense concrete are same.

(iii) Cellular concrete has a high thermal shrinkage and moisture movement.

(iv) In heavy density concrete, the aggregates of higher specific gravity are used.

(c) What do you mean by lightweight concrete?

(d) What are the properties of lightweight concrete?

(e) Why lightweight concrete is preferable than dense concrete?

(f) What are the advantages of lightweight concrete?

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(g) Write notes on gas concrete and foamed concrete.

(h) What do you understand by heavy density concrete?

(i) Where heavy density concrete is used?

7.4 MASS CONCRETE

Mass concrete can be defined as concrete which is placed in massive structures like dams, canal locks, bridge piers, etc. A large size aggregate (up to 150mm maximum size) and a low slump are adopted to reduce the quantity of cement in the mix to about 5 bags per cubic meter of mass concrete. The mix is relatively harsh and dry and hence requires power vibrators of the immersion type for compaction. This concrete is generally placed in open forms. The heat of hydration may lead to a considerable rise of temperature because of the large mass of the concrete. It results in extensive and serious shrinkage cracks. These shrinkage cracks can be prevented by using low cements and placed by continuous proper curing of concrete. Placing the concrete in shorter lifts and allowing several days before the placement of the next lift of concrete can help in the dissipation of heat. Circulation of cold water through the pipes buried in the concrete mass may prove useful. Alternatively, where possible, concreting can be done in the winter season such that the peak temperature in concrete can be lowered, or the aggregates may be cooled before use. Mass concrete develops relatively high strength during the first month because of the temperature rise due to heat of hydration. At later periods, the strength will be less than that of a continuously cured concrete at normal temperatures. The volume changes of mass concrete during setting and hardening are very small but large creep can occur at later ages.

7.5 PRE-CAST CONCRETE

Pre-cast concrete means the members or blocks are pre-casted at some factory and are transported to the place where these are required. It is possible to prepare well-made pre-cast products by keeping a high standard of finishing. The pre-cast products are fencing posts, pipes, paving slabs, concrete blocks, etc.

Following is the procedure for preparing pre-cast products :

(a) The moulds of timber or steel are prepared to the shape of the product.

(b) The reinforcement, if any, is put up in the moulds as per design.

(c) The concrete is mixed in the desired proportion and placed in the moulds.

(d) The finishing of the products is then carried out.

(e) The products are then sufficiently cured in specially constructed tanks.

(f) The products are then dispatched for use at site of work.

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Concreting MethodsAdvantages

Following are the advantages of pre-cast concrete :

(a) As production takes place in a factory, quality control is excellent.

(b) As mass production takes place, it is economical.

(c) The labour required in the manufacturing process of the pre-cast units can easily be trained.

(d) The pre-cast products may be given the desired shape and finish with accuracy.

(e) The pre-cast structures can be dismantled, when required and they can be suitably used elsewhere.

(f) The work can be completed in a short time, when pre-cast units are adopted.

(g) When pre-cast structures are to be installed, the amount of scaffolding and formwork is considerably reduced.

(h) The moulds used for preparing the pre-cast units are of steel with exact dimensions in all directions. These moulds are more durable and they can be used several times.

Disadvantages

Following are the disadvantages of pre-cast concrete :

(a) It requires good roads for transporting their products to distant sites without delay and damage.

(b) It becomes difficult to produce satisfactory connections between the pre-cast members.

(c) It requires special and heavy equipment to handle their products at site as well as at factory.

(d) Pre-cast concrete requires standardized designs and massive repetitive type of construction to achieve economy.

7.6 HIGH EARLY STRENGTH CONCRETE

High early strength concrete sets and hardens quickly as compared to ordinary concrete. It is used for construction work especially in cold weather which enables the formwork to be reused quickly. Sometimes it is desired to obtain a concrete which can be put to use as early as possible. The concrete, under such situations, must be capable of gaining sufficient strength at an early age. This can be achieved either with normal Portland cement or with special cements, e.g. High early strength Portland cement. There is difference between properties of raw materials between the two cements. The early strength cement is more finely ground. The strength of concrete is governed by time of mixing, water-cement ratio, compaction, temperature, etc.

SAQ 2

(a) What are the advantages of pre-cast concrete?

(b) What are the disadvantages of pre-cast concrete?

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(c) What do you understand by pre-cast concrete?

(d) Write a short note on high early strength concrete.

7.7 VACUUM CONCRETE

High water-cement ratio is harmful to the quality of concrete and low water-cement ratio does not give sufficient workability. In concreting thin sections like slab and walls a fluid mix with water-cement ratio of 0.5 to 0.65 is required to facilitate the placing and compaction. Such a mix will have relatively low strength and poor abrasion resistance. In such situations, the vacuum treatment of concrete, involving the removal of excess water and air by using suction can be helpful. The process, when properly applied, produces concrete of good quality. It also permits removal of formwork at an early age to be used in other repetitive work. The equipment consists of a vacuum pump, water separator and filtering mat. The duration of treatment depends upon the water-cement ratio and the quantity of water to be removed. It generally ranges from 1 to 15 minutes for slabs varying in thickness from 25 mm to 125 mm. The vacuum treatment is not very effective for water-cement ratios below 0.4. The vibration of concrete before vacuum treatment can assist the process. The application of vibration simultaneously with vacuum treatment after initial vibration is very defective. Vibration beyond 90 s may damage the structure of concrete and hence the vibrations should be stopped beyond this period and only vacuum needs to be applied for the remaining duration of the treatment.

Vacuum-treated concrete provides a good bond with the underlying concrete. The vacuum treatment has been found to considerably reduce the time of final finishing of floor and stripping of wall forms. The strength of concrete and its resistance to wear and abrasion increases and total shrinkage is reduced. Vacuum treatment can be effectively used in the resurfacing and repair of road pavements, Vacuum concrete has been extensively used for factory production of plain and reinforced concrete units. It is also used in construction of horizontal and sloping concrete slabs, such as floor slabs, road and air pavements, thin load-bearing and partition walls.

7.8 PRE-STRESSED CONCRETE

The concrete in which internal stresses are intentionally induced in a systematic planned manner such that the stresses resulting from the superimposed loads get counter balanced to a desire degree is called a pre-stress concrete.

The principle of pre-stressing concrete consists of inducing sufficient compressive stress in concrete before a member is subjected to loads, in the zones which develop tensile stress due to applied load. The pre-induced compressive stress in concrete neutralizes the tensile stress developed due to external loads. Hence, zone ultimately will be free from any stress. In a pre-stressed member, the entire cross-section becomes effective for resisting bending and danger of cracking is minimized.

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Concreting MethodsAdvantages

(a) Pre-stressing eliminates the cracks under all stages of loading. The entire cross-section becomes effective.

(b) There is considerable reduction in dead weight of superstructure. Due to this, cost of foundation also considerably reduces.

(c) It is possible to take full advantage of high compressive strength of concrete and high tensile strength of steel used.

(d) Because of higher strength, pre-stressed concrete can be safely used for structures having longer spans and which are subjected to heavy loads, impact and vibrations.

(e) In the pre-stressed concrete structures, deflection of beams is considerably reduced.

(f) Pre-stressed concrete requires only 1/3 of the concrete required for reinforced concrete but of superior quality. The amount of steel required is about only 1/4 of steel in reinforced cement concrete. Thus, there is always a considerable saving in material cost in the case of pre-stressed concrete.

(g) There is considerable saving in cost of shuttering and centering in large structures, because pre-stressed concrete members are manufactured in factories.

Limitations (a) Higher grades of concrete and steel are required. (b) Skilled labours are required. (c) Pre-stressed concrete is generally limited to members which have

long spans and carry heavy loads. (d) Special pre-stressing equipment are required. (e) Handling and transportation of heavy pre-stressed member is difficult. (f) Cost of the materials is higher due to the higher grade. (g) More labours are required.

SAQ 3

(a) Write short note on vacuum concrete?

(b) What do you understand by pre-stressed concrete?

(c) What are the advantages of pre-stressed concrete?

(d) What are the limitations of pre-stressed concrete?

7.9 ULTRA-LIGHT-WEIGHT CONCRETE

The density of ultra-light-weight concrete varies from 600 kg/m3 to 1000 kg/m3. It is made up from a mixture of cement, sand and expanded-polystyrene beads (1 mm to 6 mm in diameter). This concrete is mainly used for pre-fabricated

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non-load bearing panels, hollow and solid blocks, and in highway construction as a part of sub-base where frost could endanger the stability of the sub-grade. This concrete has high thermal insulation efficiency. The expanded bead products may be treated with bromine solutions for improving the fire resistance and self-extinguishing characteristics. The expanded beads have density about 12 kg/m3 to 25 kg/m3. The commonly used size of expanded beads is about 1 mm to 3 mm. The beads deteriorate producing a characteristic yellowing when exposed to sunlight. The expanded-polystyrene concrete mixes can be designed to have compressive strengths up to 15 MPa to 20 MPa. The expanded-polystyrene beads become electro-statically charged during processing which makes them difficult to wet during mixing. Proneness to segregation can be overcome by using a bonding agent controlling the fluidity of the paste. Thermal insulation and compressive strength properties of expanded-polystyrene concrete increase with its density. The setting and hardening rates can be controlled by selecting the suitable cement and using water-reducing admixtures. Conventional methods can be used for casting and placing the expanded-polystyrene concrete. The elastic and shrinkage deformations are considerably greater than for normal-weight concrete. The conventional workability tests, i.e. slump test, Vee-Bee test, flow table test and compacting factor test are unsuitable in case of expanded polystyrene concrete.

7.10 COLCRETE

Colcrete is grouted and is generally suitable for underwater. It is made by introducing colgrout into the voids of pre-placed coarse aggregates by injection method. The surface area of coarse aggregate in 1 m3 of concrete amounts to about 68800 m2 in the conventional concrete of 1 : 2 : 4 mix. Practically, it is difficult to wet and activate the above area.

Properties

(a) The separation of smaller particles of cement is achieved efficiently.

(b) It makes it nearly immiscible in water. It prevent aggregation of sand and reduces bleeding to a minimum.

(c) Its fluidity permits it to be pumped to considerable distances to the point of placement.

(d) Every particle of cement in the mix is completely wetted. The cement is thoroughly mixed with other constituents.

Materials for Colcrete

The materials for colcrete are :

(a) Water,

(b) Cement,

(c) Sand,

(d) Coarse aggregate

(e) Admixtures.

Water

Water should have the same properties as the mixing water for conventional concrete.

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Concreting MethodsCement

All cements as per IS specifications are suitable for colcrete work. Sand

The grading of sand should be continuous without gap. It should not have excess of larger, medium or smaller particles as these have predominant effect on the strength of colcrete. The percentage of oversize should be less than 5%. The fineness modulus of sand should not vary more than plus or minus 0.10.

Coarse Aggregate The size of coarse aggregate should not be less than 40 mm and more than 5 % by weight of the particles smaller than this specified size. The coarse aggregate acceptable for concrete may be used for colcrete.

Admixtures Admixtures like surkhi, flyash, pumice, pozzolanas can be incorporated in the mix to substitute cement. On ignition, flyash should not show 8 % loss.

Properties Following are the properties of colcrete :

(a) Colcrete has higher strength. (b) The compressive strength of cube increases with the

increase in mixing time. It becomes unworkable after a total mixing time of 90 seconds.

(c) The strength decreases by about 20% when dirty aggregates are used.

(d) The use of very coarse sand is not beneficial in colgrout works although compressive strength increases with fineness modulus.

(e) When fineness modulus of sand is between 1.9 to 2.2, bleeding is minimum. It increases both ways with increase or decrease of fineness modulus.

(f) The compressive strength of colcrete reduces by the use of elongated and flat aggregates.

(g) Sand-cement ratio increases with the increase in water-cement ratio. It is more true in case of fine sand than coarse sand.

SAQ 4

(a) Write short note on ultra-light-weight concrete. (b) Fill in the blanks :

Colcrete means _________________. (c) What do you mean by colcrete? (d) Describe the properties of colcrete. (e) State true or false :

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Colcrete is stable and pumpable.

7.11 READY MIX CONCRETE

Really this is not a special concrete but is an ordinary concrete of required grade produced in bulk quantity by RMC automatic or semi-automatic plants situated at a distance from the place of construction and, therefore, transportation facilities are required. Ready Mix Concrete (RMC) is mainly associated with infra-structural developments of medium and large size projects. In ready mix concrete production, every ingredient of concrete in correct proportion is mixed by the machines and so there are no chances of errors and better quality control is achieved. Due to the higher speed one can produce large amount of concrete within a short duration and then produced concrete either can be pumped to the site or can be transported by transit mixers or by trucks.

Following are the limitations of RMC :

(a) Skilled operators are required.

(b) It is suitable for bulk production and for mass concreting operations.

(c) Not suitable for small consumers.

(d) Cement is required in bulk quantity.

(e) Good roads are required for transportation of concrete.

(f) Profit margin is low.

(g) Storage place for aggregate is required.

7.12 GUNITE

Gunite is referred as air blown mortar and concrete, gunned concrete, spraycrete, sprayed concrete, shotcrete, pneumatically applied mortar or concrete. It is a mortar or concrete conveyed through a hose and pneumatically projected at high velocity onto a surface. The jet force impacting on the surface compacts the material. Generally a dry mixture is used. The material is capable of supporting itself without sloughing or sagging even for vertical as well as overhead applications. Gunite is best adopted from the quality and cost point to thin lightly reinforced sections. Gunite is also advantageous to shoot certain heavy structural members in new construction and to bond columns, girders or walls to existing construction. Gunite should not be used for spirally reinforced columns or pilings. Uses

Gunite is used in the following works. (a) Refractory linings in furnace walls, stacks and boilers. (b) Coatings over masonry, concrete, rock and steel. (c) Encasement of structural steel for reinforcing and fireproofing. (d) Repairs of deteriorated concrete in structures like dams, tunnels,

reservoir linings, water front structures, etc. (e) New structures like roofs, walls, tunnels, sewers, shafts,

pre-stressed tanks, reservoir linings, swimming pools, canal linings.

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Concreting Methods(f) Repair of earthquake and fire damaged to concrete and masonry

structures. Properties

Following are the properties of gunite : (a) Gunite is stronger than conventional concrete. (b) Gunite requires low water-cement ratio which is lower than for

most concrete mixes. (c) Gunite is structurally adequate and durable material. (d) It is capable of excellent bond with masonry, concrete and steel. (e) The drying shrinkage of gunite is somewhat higher than for

most low slump conventional concrete and generally falls within the range of 0.06 to 0.10 %. Low slump conventional concrete is placed in heavier sections using larger aggregates and leaner mixes. It will tend to give more shrinkage cracking and, therefore, may require a closer joint spacing.

SAQ 5

(a) What do you understand by RMC? (b) What are the advantages of RMC? (c) What are the disadvantages of RMC? (d) What is gunite? (e) Where gunite is used? (f) State the properties of gunite.

7.13 FERRO-CEMENT

Ferro-cement is a relatively new material consisting of wire meshes and cement mortar. It consists of closely spaced wire meshes which are impregnated with rich cement mortar mix. The ferro-cement members are usually of 20 mm to 30 mm thickness with 2 mm to 3 mm external covers to the reinforcement. The reinforcement or wire mesh is usually of 0.5 mm to 1 mm in diameter at 5 mm to 10 mm spacing and cement mortar is of cement sand of 1 : 2 or 1 : 3 with water-cement ratio 0.4 to 0.5. The steel quantities vary between 300 to 500 kg per m3 of mortar. The basic idea behind this material is that concrete can undergo large stains without cracks in the neighborhood of the reinforcement throughout the mass of the concrete.

Advantages

(a) It has lesser dead weight due to small thickness.

(b) It is simple in construction.

(c) It has high tensile strength.

(d) It has non-corrosive nature.

(e) It is easy to repair.

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(f) It is suitable for special structures like shells, hanging roofs, silos, water tanks and pipelines.

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Limitations

(a) Skilled labours are required.

(b) Strict quality control is required.

(c) Proper pre-fabrication techniques are required.

7.14 ROLLER COMPACTED CONCRETE

Roller compacted concretes are used in construction of dams, multiple layer placements for foundations and rapid placement of single layer paving for highways and runways. It is a dry concrete, i.e. no slump, which has been consolidated by vibrating rollers. The roller compacted concrete must be dry enough to support the mass of the vibrating equipment. It must be sufficient wet to allow the mixing and consolidation processes. This concrete can be used for continuous placements without cold joints. To achieve adequate bonding of the roller compacted concrete to hardened concrete at the bottom of the cold joints, segregation must be prevented and a high plasticity bedding mix must be used at the start of placement.

7.15 FIBRE REINFORCED CONCRETE

Fibre reinforced concrete is defined as concrete made with hydraulic cement, containing fine or fine and coarse aggregate and discontinuous discrete fibres. The fibres can be made from natural material like cellulose, sisal or from artificial material like glass, polymers, carbon and steel. The quantity of fibres used is about 1 % to 5 % by volume. The reason of reinforcing the cement-based matrix with fibres is to increase the tensile strength by delaying the growth of cracks. Steel fibre is one of the most commonly used fibre. The diameter may vary from 0.25 mm to 0.75 mm. Nylon fibres are found suitable to increase the impact strength of concrete. They possess very high tensile strength. Fibres may be circular or flat. Glass fibre is a recent introduction in making fibre concrete.

SAQ 6

(a) Explain what is mean by ferro-cement.

(b) State the advantages and limitations of ferro-cement.

(c) Write short note on roller compacted concrete.

(d) Write short note on fibre reinforced concrete.

7.16 HOT WEATHER CONCRETING

The concreting done at atmospheric temperature above 40ºC or where the temperature of concrete at the time of placement is expected to be beyond 40ºC may be called as hot weather concreting. Special precautions should be taken to place concrete at a temperature above 40ºC. High ambient temperature, reduced

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Concreting Methodsrelative humidity and high wind velocity are the climatic factors affecting concrete in hot weather.

7.16.1 Problems Encountered in Hot Weather Concreting Following are the problems encounter in hot weather concreting :

Reduction in Strength

Concrete produced and cured at higher temperature generally develops higher early strength than normally produced concrete but the eventual strengths are lower. High temperature causes greater evaporation and hence requires increase of mixing water, consequently reducing the strength. The effect of simultaneous reduction in the relative humidity, it is seen that specimens moulded and cured in air at 23ºC and 60% relative humidity and at 38ºC and 25% relative humidity attained strengths of only 73% and 62% respectively, in comparison with the specimens which are moist-cured at 23ºC for 28 days.

Accelerated Setting

Rapid hydration takes place due to higher temperature causing accelerated setting. This results in reducing the handling time of concrete and also lowering the strength of hardened concrete. The workability of concrete decreases and hence water requirement increases with the increase in the temperature of concrete. It has been found that an approximately 25mm decrease in slump has resulted from 11ºC increase in concrete temperature. The addition of water without proper adjustments in mix proportions adversely affects the quality of concrete.

Rapid Evaporation During Curing

A rapid initial hydration results in a poor microstructure of gel, which is probably more porous, resulting in a large proportion of the pores remaining unfilled. This reduces the strength of concrete. The hydration of cement takes place only in water-filled capillaries so it is imperative that a loss of water by evaporation from the capillaries be prevented. Water lost internally by self-desiccation has to be replaced by water from outside.

Difficulty in Controlling the Air Content

It is very difficult to control the air content in air-entrained concrete at higher temperature. Due to this control of workability is difficult. For a given amount of air-entraining agent, hot concrete entrains less air than does concrete at normal temperature.

Increased Tendency to Cracking

The rate of evaporation depends on the ambient temperature, relative humidity, wind speed and concrete temperature. More rapid evaporation leads to plastic shrinkage cracking, and subsequent cooling of hardened concrete introduces tensile stresses.

7.16.2 Precautions to be Taken During Hot Weather Concreting Proportioning of Concrete Mix

As far as possible, cement with lower heat of hydration should be preferred to those having greater fineness and heat of hydration. The mix should be designed to have minimum cement content consistent with other functional requirements. Accelerators should not be used in hot weather concreting.

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Water-reducing or set-retarding admixtures may be used in hot weather concreting.

Temperature Control of Concrete Ingredients Controlling the temperature of the ingredients of concrete results in lowering the temperature of concrete. The ingredients may be protected from direct sunrays by using sheds. Water can be sprinkled on to the aggregate before using them in concrete. The temperature of water is easier to control than that of other ingredients of concrete. The mixing water has the greatest effect on lowering the temperature of concrete, because the specific heat of water is nearly five times that of common aggregate. The pre-cooling of aggregates can be achieved at the mixing stage by adding calculated quantities of broken ice pieces as a part of mixing water, provided the ice is completely melted by the time mixing is completed.

Production and Delivery The period between mixing and delivery should be kept to an absolute minimum by coordinating the delivery of concrete with its rate of placement. The temperature of aggregates, water and cement should be maintained at the lowest practical levels so that the temperature of concrete is below 40ºC at the time of placement. A suitable metal clad thermometer should be used to measure the temperature of concrete at the time of leaving the batching plant.

Placement and Curing of Concrete The sub-grade, formwork and reinforcement should be sprinkled with cool water just before the placement of concrete. The area around the work should be kept wet to the extent possible to cool the surrounding air and increase its humidity. Speed of placement and finishing helps minimize problems in hot weather concreting. After compaction the concrete should be protected to prevent the evaporation of moisture by means of wet gunny bags. After the concrete has attained a degree of hardening sufficient to withstand surface damage, moist-curing should start. Continuous curing is important because the volume changes due to alternate wetting and drying promote the development of surface cracking. On the hardened concrete, the curing shall not be much cool than the concrete because of the possibilities of thermal stresses and resultant cracking. High velocity winds cause higher rate of evaporation and hence windbreakers should be provided as far as possible. Concreting can be done during night shifts as far as possible.

7.16.3 Admixtures Admixtures are the substances, other than the cement, water and aggregate, which are used to incorporate or modify certain properties of concrete for certain particular jobs and are added to the batch immediately before or during the mixing of ingredients of concrete. Concrete is used for different purposes under different conditions. Therefore, Ordinary Concrete may not serve the required qualities. In such cases, admixtures are used to modify the properties of Ordinary Concrete so as to make it suitable for any conditions. Admixtures are classified as follows : Accelerators

Accelerators are the materials which increase the rate of development of strength of concrete and may be used where early strength of concrete is required. Calcium chloride reduces the time of initial and final setting. With

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Concreting Methods2% of CaCl2 in cement, the setting time is reduced one-third of the normal time. 2% mixture results in 170% more strength after one day and 30% more strength after seven days. After 28 days increase in strength is 10%. Beyond 4% it has been found to be harmful.

Retarders

Retarders are the substances which are used to retard the setting of cement thereby increasing the setting time of cement. When pumping of concrete is required for long distances, retarders are used so that concrete does not get stiffened during pumping process. Calcium lingo sulfonate is one compound, which is extensively being used for these purposes.

Water-repelling Agents

These are the materials which when mixed with concrete, repel the water from concrete and to make the concrete more waterproof. These agents are used when the concreting is going on in a very wet condition, to avoid the entry of water into the concrete.

Workability Agents

The workability of concrete increases by using workability agents. Workability agents give smoothness and reduce the bleeding of concrete. These materials are useful in cases when the aggregate is deficient in fines and to achieve workability requirement even with low water content ratio. Commonly used workability agents are fly ash, bentonite clay, finely divided clay, talc, hydrates lime, etc.

Gas Forming Agents

A gas forming agent is a chemical admixture such as aluminium powder which produces minute bubbles of hydrogen gas throughout the matrix. These agents are used to make concrete lightweight, to stop bleeding in concrete, to make it more dense, etc. These are used in very small quantities.

Air-entraining Agents

This admixture is used for inducing minute air bubbles uniformly throughout the concrete mass. These are used to increase the workability of concrete and also to increase resistance to freezing and thawing. These agents also reduces the bleeding of concrete. These agents entrap more air during the preparation of concrete and that air acts as a sort of ball bearing between the ingredients of concrete.

Natural Cement Materials

Some materials have got cementing properties naturally and when these materials are used with cement, then these have been found to give cement of more strength. They increase the workability and decrease bleeding, segregation and heat of hydration. Natural cement materials are hydraulic lime, fly ash, surkhi, etc. As more water is required with natural cement materials, the shrinkage may increase.

7.17 COLD WEATHER CONCRETING

Concreting done at a temperature below 5ºC is called as cold weather concreting. Special precautions should be taken to place concrete at a temperature below 5ºC. Due to low temperature, the problems are mainly due to the slower development

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of concrete strength, the concrete in the plastic stage can be damaged if it is exposed to low temperatures which cause ice lenses to form and expansion to occur within the pore structure and subsequent damage may occur due to alternate freezing and thawing when the concrete has hardened.

7.17.1 Problems Encountered in Cold Weather Concreting Following problems are encountered during concreting in cold weather conditions : Delayed Setting

Temperature affects the rate of hydration of cement. If the temperature is low, concrete will take long time to set and harden. At low temperatures, the development of concrete strength is retarded as compared to the strength development at normal temperatures. The setting period necessary before removal of formwork is, thus, increased. Although the initial strength of concrete is lower, the ultimate strength will not be severely affected provided the concrete has been prevented from freezing during its early life. The rate of progress of work will be very slow and the economy will be affected in case of cold weather concreting.

Early Freezing of Concrete When the temperature is sub-zero the free water in the plastic concrete freezes. This freezing prevents the hydration of cement and also makes the concrete expand. This expansion affects the strength of concrete. Plastic concrete may suffer permanent damage if it is exposed to freezing temperature. If the concrete is allowed to freeze before a certain pre-hardening period, it may suffer irreparable loss in its properties so much so that even one cycle of freezing and thawing during the pre-hardening period may reduce compressive strength to 50% of what would be expected for normal temperature concrete. The pre-hardening period depends upon the type of cement and environmental conditions. It may be specified in terms of time required to attain a compressive strength of the order of 3.5 to 7 MPa; alternatively it can be specified in terms of period varying from 24 hours to even three days depending upon the degree of saturation and water-cement ratio.

Stresses Due to Temperature Differential A cracking of concrete may take place due to a large temperature differential. It has a harmful effect on durability. Such situations are likely to occur in cold weather at the time of removal of formwork.

7.17.2 Precautions to be Taken in Cold Weather Concreting Proportioning of Concrete Ingredients

The quantity of cement in the mix affects the rate of increase in temperature, an additional quantity of cement may be used. It would be preferable to use high alumina cement for concreting during frost conditions. The main advantage is that a higher heat of hydration is generated during the first 24 hours. During this period, sufficient strength (approximately 10 to 15 MPa) is developed to make the concrete safe against frost action. No accelerator should be used if high alumina cement is used. Alternatively,

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Concreting Methodsthe rapid hardening Portland cement or accelerating admixtures used with proper precautions can help in getting the required strength in short period. Air-entraining agents are generally used in cold weather. Air-entrainment increases the resistance of hardened concrete to freezing and thawing and normally, at the same time, improves the workability of fresh concrete. The calcium chloride used as accelerating admixture may cause corrosion of reinforcing steel. In any case, calcium chloride should not be used in pre-stressed concrete construction. The important factor for cold-weather concreting is the attainment of suitable temperature for fresh concrete.

Temperature Control of Ingredients The temperature at the time of setting of concrete can be raised by heating the ingredients of the concrete mix. Heating of mixing water is easy. The temperature of the water should not exceed 65ºC as the flash set of cement will occur when the hot water and cement come in contact in the mixers. Therefore, the heated water should come in direct contact with the aggregate and the sand first. The aggregates may be heated by passing steam through pipes embedded in aggregate storage bins. Another precaution taken along with the heating of ingredients is to construct a temporary shelter around the construction site. The air inside is heated by electric or steam heating or central heating with circulating water. The temperature of ingredients should be so decided that the resulting concrete sets at a temperature of 10ºC to 20ºC.

Use of Insulating Formwork The heat generated during hydration of cement can be gainfully conserved by having insulating formwork covers capable of maintaining concrete temperature above the desirable limit up to the first 3 days even though the ambient temperatures are lower. Timber, clean straw, blankets, tarpaulins, plastic sheeting, etc. can be used as a formwork covers. Timber formwork may be sufficient for moderately cold weather. The efficiency of the covers depends upon the thermal conductivity of the medium as well as ambient temperature conditions.

Placement and Curing The surface on which the concrete is to be placed should be sufficient warm. All ice, snow and frost should be completely removed before placing of the concrete. Water curing is not to be used during the periods of freezing or in near freezing conditions.

Removal of Formwork Formworks are a sort of protection against cold water to the concrete. It is advantageous not to remove formworks until the end of a minimum period of protection. As the rate of gain of strength is slow during the cold weather, the formwork and props have to be kept in place for a longer time than the normal concreting.

SAQ 7

(a) What do you understand by hot weather concreting? (b) State the problems encountered in hot weather concreting. (c) What are the precautions required to be taken during hot weather

concreting?

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(d) What do you understand by cold weather concreting? (e) State the problems encountered in cold weather concreting. (f) What are the precautions required to be taken during cold weather

concreting?

7.18 UNDERWATER CONCRETING

When the concrete is to be placed underwater, special precautions need to be taken. Following are the recommendations of the Portland Cement Association in regard to the quality of concrete :

The concrete should be plastic and cohesive but should have good flowability. This requires a fairly high slump, usually 150 to 180 mm. A richer mix then generally used for placing under normal conditions is required; usually the cement requirement is not less than eight bags per cubic meter of concrete. The proportions of fine and coarse aggregates should be adjusted to produce the desired workability with a somewhat higher proportion of fine aggregate than used for normal conditions. The fine aggregate proportion can often be from 45 to 50% of the total aggregate, depending on the grading. It is also important that the aggregate contain sufficient fine material passing in the 300 micron and 150 micron sieves to produce a plastic and cohesive mixture. ASTM standard specifications for concrete aggregate require that not less than 2% pass the 150 micron sieve. The fine aggregate should meet the minimum requirements and somewhat higher percentage of fines would be better in many cases. For most works, coarse aggregate should be graded up to 20 mm or 40 mm. The coarse aggregate should not contain loam which may cause laitance while being worked. The formwork not only has to impart the required shape to the structure or to fits elements, it must also protect the concrete mix during placing until it matures, from the direct action of current and waves. The formwork also serves as a temporary protective casing which during concreting prevents possible washing out of cement and the leakage of cement mortar from the concrete mix. After completion of concreting, it will protect the soft concrete from the impact and abrasive action of the water currents. The cofferdams may be constructed to reduce the velocity of flow.

Methods of Concreting

Following are the principal methods of placing concrete underwater :

(a) Tremie method,

(b) Placing in dewatered caissons or cofferdams,

(c) Bucket placing,

(d) Placing in bags, and

(e) Pre-packed concrete.

7.18.1 Tremie Method It is a watertight pipe having a funnel shaped hoper at its upper end and loose plug at the bottom or discharge end. Its diameter is about 250 mm. To dewater the tremie and control the distribution of concrete the valve at the discharge end is used. The tremie is built up in 1 to 3.5 m sections. It is supported on a working

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Concreting Methodsplatform above water level. The air and water must be excluded from the tremie by keeping the pipe full of concrete all the time during concrete. The capacity of the hopper should be at least equal to that of tremie pipe. A plug formed of paper is first inserted into the top of the pipe in charging the tremie. As the hopper is filled, the pressure of fresh concrete forces the plug down the pipe, and the water in the tremie is displaced by concrete. For concreting the tremie pipe is lowered into position and the discharge end is kept as deeply submerged beneath the surface of freshly placed concrete as the head of concrete in tremie permits. The pipe is raised slightly as concreting proceeds and the concrete flows outward. Care should be taken to maintain continuity of concreting without breaking the seal provided by the concrete cover over the discharge end. The tremie should never be moved laterally through freshly placed concrete. The tremie should be lifted vertically above the surface of concrete and shifted to its new position. When large quantities of concrete are to be placed continuously, place the concrete simultaneously and uniformly through a battery of tremies, rather than shift a single tremie from point to point. The segregation and non-uniform stiffening can be minimized by maintaining the surface of concrete in the forms as level as possible and by providing a continuous and rapid flow of concrete. The spacing of tremies recommended between 3.5 m and 5 m and that the end tremies should be about 2.5 m from the formwork.

Figure 7.1 : Typical Arrangement for a Tremie Pipe

7.18.2 Placing in Dewatered Caissons or Cofferdams The placing in dewatered caissons or cofferdams follows the normal in-the-dry practice.

7.18.3 Bucket Placing The concreting can be carried out at considerable depths by using this method. The buckets are usually fitted with drop-bottom or bottom-roller gates which open freely outward when tripped. The bucket is completely filled with concrete and its top covered with a canvas cloth or a gunny bag to prevent the disturbance of concrete as the bucket is lowered into water. The bucket is lowered by a crane up to the bottom surface of concrete and then opened by a suitable arrangement provided at the top. The concrete should be discharged directly against the surface on which it is deposited. Early discharge of bucket, which permits the fresh concrete to drop through water, must be avoided. This method permits the use of slightly stiffer concrete than does tremie method. In this method, some

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buckets are provided with a special base which limits the agitation of the concrete during discharge and also while the empty bucket is hoisted away from the fresh concrete. The disadvantage of this method is the difficulty in keeping the top surface of the placed concrete reasonably level.

Figure 7.2 : Typical Arrangement for a Bottom Opening Bucket

7.18.4 Placing in Bags In this method, properly concrete filled bags are lowered into water and placed carefully in a header and stretcher fashion as in brick masonry with the help of divers. The method has advantages in that, in many cases, no formwork is necessary and comparatively lean mixes may be used provided sufficient plasticity is retained. The work is slow and laborious, as the accurate positioning of the bags in place can be only accomplished by the divers. The bags and labour necessary to fill and tie them are relatively expensive. This method is only suited for placing the concrete in shallow water. Voids between adjacent bags are difficult to fill. There is little bonding other than that achieved by mechanical interlock between bags.

7.18.5 Pre-packed Concrete In this method, the aggregate should be wetted before placing in position. This method is also called as grouted concrete. It consists of placing the coarse aggregate only in the forms and thoroughly compacting it to form a pre-packed mass. This mass is then grouted with the cement mortar of the required proportions. The mortar that grouts the concrete displaces water and fills the voids. Dense and compact concrete should be prepared by using well graded aggregates. The maximum size of aggregate conveniently used is 80 mm. The coarse aggregate may also be allowed to fall from heights of up to 4 m, without causing any appreciable segregation. Only shutter vibrations can be used for compacting the coarse aggregate. The mortar consists of fine sand, pozzlanic filler material and as chemical agent, which serves the penetration, early setting of cement, the dispersion of particles and to increase the fluidity of mortar. An air-entraining agent is also added to the mortar to entrain about 4% of air. A small variation of the procedure of preparation of the cement mortar for grouting leads to a process called colcrete. In this process, the mortar grout is prepared in a

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Concreting Methodsspecial high-speed mixer. No admixtures are used in this process. The high speed mixing produces a very fluid grout, which is immiscible with water. The maximum size of sand used is 5 mm and the sand should be well graded. Rich cement mortar is used for underwater construction and grouting of pre-stressing cables in post-tensioned bonded construction. The mix ratio ranges from 1 : 1.5 to 1 : 4 with a water-cement ratio of about 0.45. The pre-packed aggregates can be grouted by the following methods :

(a) The mould can be filled with grout and the coarse aggregate can be deposited in the grout.

(b) The grout can be poured on the top surface of aggregate and allowed to penetrate to the bottom. The method is particularly useful for grouting thin sections.

(c) Pumping the grout into aggregate mass from bottom at carefully designed positions through a network of pipes. The formwork should be constructed at the top of the coarse aggregate in this method.

The grout pressure will be about 0.2 to 0.3 MPa. The quantity of grout should be estimated from the void contents of the coarse aggregates. The pre-packed concrete has lower drying shrinkage and higher durability, especially freezing and thawing resistance compared to ordinary concrete for the same proportions. The rate of development of strength is comparatively slow for the first two months and the eventual strengths are about the same as for normal concrete. This method is very much useful for underwater construction and repair work of mass concrete structures, such as dams, spillways, etc.

SAQ 8 SAQ 8

(a) Enlist methods of underwater concreting.

(b) Explain with sketch bucket placing method of underwater concreting.

(c) Write short notes on ‘Placing in bags’ and ‘Pre-packed concrete’.

7.19 QUALITY CONTROL OF CONCRETE

The quality control of concrete is important to achieve the intended requirements of the structures. The true success of the quality control system is in timely detections of errors in the concreting process. Efficient, reliable and maintenance free service given by a structure during its life span by itself a testimony of strong and durable concrete work. Following are some of the factors, which affect the quality of concrete :

(a) Storage of cement,

(b) Measurement of ingredients of concrete,

(c) Batching operation,

(d) Admixtures,

(e) Mixing of concrete,

(f) Compaction of concrete,

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(g) Placing of concrete,

(h) Curing of concrete,

(i) Formwork,

(j) Finishing of concrete, and

(k) Joining or repairs of concrete members.

There are three stages of Quality Control in concrete making, which are as follows :

7.19.1 Stages of Quality Control Quality Control Prior to Construction

In this stage, properties and testing of constituent materials of concrete are done. Mainly cement and aggregates are tested for required physical and chemical properties and their suitability of a particular concrete is ascertained. Properties of aggregates like size, shape, texture, strength, specific gravity, bulk density, water absorption, soundness, bulking of sand and durability are found and tested in laboratory. Physical properties of cement and chemical composition of cement are found out and cements suitability is decided after matching these properties with standard specifications. Effect of aggregate properties on strength of concrete is also studied.

Quality Control During Construction

In this stage, water-cement ratio and its effect on concrete workability for different conditions and concrete mix design are studied. Also how best the ingredients of concrete can be stored, batched and mixed is decided. Method of curing and duration of curing are decided and method of finishing of concrete is also decided.

Quality Control After Construction

As such quality of concrete cannot be controlled after construction but certain tests definitely can be carried out to check whether quality control in concrete making has been ensured or not. There are different tests, which can be done on hardened concrete to check the strength of concrete, which will give the indication of the beginning of Quality Control.

7.19.2 Tests to Check Quality of Concrete in Various Stages First Stage Test

(a) Fineness of cement

(b) Consistency of cement

(c) Initial and final setting time of cement

(d) Adulteration in cement

(e) Fineness modulus and grading of aggregates

(f) Bulking of sand

(g) Specific gravity of sand

(h) Impact value of aggregate

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Concreting Methods(i) Abrasion value of aggregate

(j) Soundness of cement

(k) Crushing value of aggregate

Second Stage Tests

(a) Water-cement ratio

(b) Slump test

(c) Compressive strength test

(d) Tensile strength test

7.19.3 Functions of Quality Control System Following are the functions of a Quality Control System :

(a) Ensure that the equipment and testing instruments are calibrated and properly maintained.

(b) Define the activities affecting quality through instructions and procedures.

(c) Carry out unannounced spot checks on the quality of materials.

(d) Identify the problems and take the action to get the solution.

(e) Verify the implementation of solutions and corrections.

(f) Report regularly on the effectiveness of the system.

7.19.4 Guidelines of Quality Control Following are the guidelines of quality control :

(a) The quality control department must be under the control of the top man, i.e. Highest authority.

(b) The highest authority in the organisation must believe in quality control.

(c) Every person in organisation must be aware of quality control system and inspection procedures.

(d) Quality must always be given preference over economy and speed.

(e) Timely inspection and remedial measures are essential.

(f) Good quality construction can only be done by using good quality material and good workmanship.

(g) Detected mistakes should not be repeated.

(h) Quality Control inspector should be honest, alert and of high integrity.

(i) Carelessness and ignorance should be avoided.

(j) No compromise should be done at any cost. Unacceptable material or work must be rejected.

(k) Required material testing and concrete testing must be carried out at site only.

SAQ 9

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(a) State the importance of quality control of concrete.

(b) State and explain the stages of quality control of concrete.

(c) Enlist the tests to check quality of concrete.

(d) State guidelines for quality control of concrete.

(e) State the functions of quality control system.

7.20 SUMMARY

In this unit, you have studied special types of concrete and concreting methods under extreme environmental conditions. Concrete is a widely used material containing a binder and a mineral filler. The proportioning of ingredients of concrete plays very important role in strength of concrete. In the next unit, you will study objectives and methods of mix design, yield of concrete and cement factor.

7.21 ANSWERS TO SAQs

Refer the relevant preceding text in the unit or other useful books on the topic listed in the section ‘Further Reading’ given at the end of the booklet to get the answers of SAQs.

Documents

Special Concretes and Concreting Methods