CURING AND ADMIXTURES
technical Literature Series
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Ambuja technical Literature Series
CONTENT 1.0 2.0 3.0 4.0 5.0
1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0
CURING Curing methods and materials Defects in concrete due to inadequate curing Temperature for curing concrete Curing period Conclusion ADMIXTURES - PLASTICISERS Classification Purpose Purpose of the test Factors affecting admixture reaction Loss in workability (slump) Effects on properties of fresh concrete Effects on properties of hardened concrete Precautions Codes of practice Conclusion Ambuja technical Literature Series
curing
INTRODUCTION
The chemical action between cement and water, which results in the setting and hardening of concrete or mortar, is dependent on the presence of water. Although there is normally adequate quantity of water for full hydration when the concrete or mortar mix is prepared, it is important to ensure that the water is either retained or replenished to enable the chemical action to be continued till such time the required strength is gained.
A significant loss of water due to evaporation from the concrete or mortar surface may result in slowing down or stopping the hydration process and resulting in consequent reduction of strength and durability. To help the hydration process to continue, water in the capillaries should be prevented from evaporating. It is, therefore, necessary to maintain an environment of high humidity around the freshly placed concrete or mortar till it attains reasonably good strength. This process is called curing of concrete. Curing of concrete or mortar is the last step required to be taken in the process of concrete or masonry construction. Curing has a strong influence on various properties of concrete and therefore, it should not be taken lightly. Strength, durability watertightness, wear resistance, volume stability, chemical attacks and resistance to freeze-thaw cycle are much superior of a well cured concrete or mortar than that of a concrete wherein, curing was neglected, all other parameters being indentical. Ambuja technical Literature Series
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CURING METHODS AND MATERIALS
1.1 There are two general ways by which concrete can be kept moist and humid or kept at a favourable temperature. ●
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By maintaining the presence of mixing water in the concrete during the early hardening period. Methods generally deployed are ponding or immersion, spraying, sprinkling or fogging, wet covering using hessian cloth, gunny bags etc. By preventing loss of mixing water from the concrete by sealing the exposed surfaces of concrete. The exposed surfaces can be generally covered by membrane formed by a curing compound, impervious paper, plastic sheets or even by leaving formwork in place.
1.2 However at times, there is a need to accelerate curing or there
is necessity to expedite gain of strength due to adverse weather conditions or there is an intent to handle or put to use the concrete structure earlier. This is achieved by using live steam, heating coils or electrically heated forms or pads.
1.3 The curing method or combination of curing methods are generally selected depending on some of the following factors : ● ● ● ●
Specifications Availability of curing materials Economics Type of concrete structure (precast / cast-in-situ) Ambuja technical Literature Series
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Shape and size of concrete surface Aesthetic appearance
1.4 Brief descriptions of each of the curing methods are given below : 1.4.1 PONDING OR IMMERSION IN WATER
The best curing method is total immersion of concrete in water However, following precautions are necessary : ● Ponding water lost due to evaporation should be continuously replenished. ● Ponding water should also cover the corners and edges and should be able to cover the entire surface uniformly to avoid dry spots wherein curing would be deficient. ● Water used for curing should have identical properties as that of water used for manufacture of concrete. ● Water and the materials used for bunding should be free of substances that will stain or discolour the concrete surfaces. ● The difference in the curing water temperature and the concrete temperature should not be more that 11°C to prevent thermal stresses that could result in cracking. ● Adequate water should be available at site throughout the curing period. ● Bunds of impervious earth or cement mortar to retain water should be maintained throughout the curing period. ● The height of the bunds and area of concrete surface to be ponded must be so selected that there is at least about 25mm of water ponded on the highest surface.
1.4.2 FOGGING OR SPRAYING WATER
This method can be applied to both horizontal and vertical concrete / mortar surfaces. However, ample water is necessary throughout the curing period and round the clock. The following precautions are necessary : ● ●
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There should be continuous water supply available in adequate quantity. Covering the surfaces with gunny bags, hessian or plastic sheets may help in retaining the water for a longer period. However, there are chances that surfaces may not be uniformly moistened hence, care is to be taken to replenish the water before any part of the concrete surface starts drying. Continuous supervision is necessary, if this method is employed as there are good chances that during certain intervals of time, within the curing period, some surfaces will dry off and start cracking or crazing. Concrete or mortar will crack faster, if it is allowed to dry off between the alternate wetting applications. In this method, precautions have to be taken so that water sprayed on old concrete or mortar does not fall on to the freshly laid concrete or mortar causing erosion of the top surface, This system should be generally avoided when ambient temperatures are close to the freezing point.
1.4.3 MOIST FABRIC COVERS
This method of curing is not quite observed in our country wherein different types of covers soaked with water are used on both vertical and horizontal concrete surfaces. Ambuja technical Literature Series
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The different types of fabrics or materials used are : ● Burlap ● Cotton mat ● Rugs ● Hessian cloth (multiple layers) ● Other moisture retaining fabrics / materials such as soil, sand, sawdust or hay. The precautions required in this curing process are as follows : ● The concrete should be covered with the fabric as soon as it has hardened sufficiently to prevent surface damage. ● The entire surface including the edges of slabs and joints should be kept covered so that parts of the structure are not inadequately cured. ● The fabric helps in retaining water for a greater period of time than otherwise. However, the fabric must be continuously kept wet or moist so that concrete surface is in contact with water or moisture throughout the curing period. ● If wet burlap is used, it should be free of sizing or any substance that is harmful to concrete and would cause discolouration. ● Burlap if used should be first rinsed with water to remove soluble substances and to make the burlap more absorbent. ● On flat surfaces a layer of earth, sand, sawdust (50mm thick) or hay (150mm thick) can be used as an effective medium to retain moisture for a longer period than the surfaces kept totally exposed. However, these materials will have to be kept continuously wet and prevented from blowing away or erosion.
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There are also good chances of discolouration of the concrete in this method and hence precautions must be taken while selecting the different cover materials.
1.4.4 PLASTIC SHEETS
Plastic sheets made from polyethylene film comes under the category of effective moisture barriers and can be used on horizontal, vertical surfaces as well as on surfaces of different shapes and sizes. The following precautions are necessary when this method is used : ● ● ● ● ● ● ●
All edges and corners must be kept well covered and sheets firmly placed on the concrete surface. Wind blowing away the sheets at the edges and corners and drying the concrete in these areas. Lap joints should be minimum 300mm between the sheets. The sheets should be in close contact with the concrete surface. Wind should not be allowed to blow between the concrete surface and the sheets, causing rapid drying of the moisture in the concrete top surface. The vertical surfaces must be firmly fixed with tapes or strings. Place and fix plastic sheets within half an hour of removal of formwork from vertical surfaces. On slabs the sheets should be placed as soon as concrete has hardened enough to prevent surface damage. Avoid plastic sheets from wrinkling while spreading on the horizontal surfaces, wrinkling may cause discolouration in patches. The polyethylene film should be of at least 0.10mm thickness and preferably be transparent or white opaque in colour when used in warm weather. Black sheets can be used during cool weather. Ambuja technical Literature Series
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1. Plastic sheet used for protective carring on horizontal surface of a RCC raft.
2. Plastic sheets used to protect block masonry, from rapid drying due to high ambient temperttrres and high wind velocities, during the curing period.
The polyethylene film / sheet can be ideally used in combination with other methods as stated below to give excellent curing to the concrete surface and totally eliminating any defects due to inadequate curing. â—? After placing the sheet firmly on the concrete surface water is allowed to flow between the concrete and the sheet. â—? After layers of soaked hessian cloth or burlaps are placed on the concrete surface they are covered with plastic sheets which help in retaining moisture for a very long time as evaporation is virtually prevented as condensation of water vapour takes place on the internal sheet surface. The above combinations are strongly recommended at site where drying shrinkage cracks and crazing are repeatedly observed.
1.4.5 CURING COMPOUNDS
Curing compounds are now easily available in our country. They generally consist of waxes, resins, chlorinate rubber and solvents of high volatility and form a thin liquid membrane on the concrete surfaces and result in preventing to a certain extent evaporation losses. The curing compounds are generally available in two types, clear or translucent and white pigmented. The clear or translucent curing compounds may contain a fugitive dye to assure complete coverage of the concrete surface by visual inspection. This dye fades away as soon as the application is few hours old. The following precautions are necessary when curing compounds are used : ● Pigmented compounds should be stirred in a container before use as there is a tendency for the dye to settle down. ● Curing compound should be applied soon after final finishing of the horizontal surface is completed or soon after the formwork of vertical surfaces is removed. ● Curing compound should be sprayed uniformly on the concrete surface. Brush application may not give the desired coverage or performance. ● Curing compound must be sprayed on moist concrete surface. If coating is sprayed evenly the curing compound will cover nearly 3.5 to 5sqm per litre. ● Curing compound must be preferably sprayed in two layers. The second layer must be sprayed at right angle to the first layer. ● Curing compounds can prevent proper bonding between hardened concrete and subsequent fresh concrete or bonding between steel and concrete. Ambuja technical Literature Series
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Proper precautions are necessary while spraying so that the curing compound is not accidentally sprayed onto reinforcement steel or construction joints. ● ● ● ● ● ● ●
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Curing compounds must be uniform and easy to maintain in a thoroughly mixed solution. The layer must not sag, run off, peak or collect in grooves. The film formed by curing compounds should be tough to withstand early construction traffic without wear or tear. The curing compound film should be non yellowing and should not stain the concrete surface. Above all, the curing film should have good moisture retention property. The spray nozzle must be held about 300 to 500mm from the surface especially during windy conditions. Make sure that the cleaning solvent is available for cleaning the spraying equipment, the nozzle and the hose whenever there is break in usage for more than half an hour. Do not use curing compound on surfaces which are to receive renderings, screeds or paints. The resin film generally breaks down and flakes off after 28 days exposure to bright sunlight but if any is left on the surface, it will prevent bonding of any surface applications which may come on the concrete surface subsequently. As a safety precaution, the person spraying the curing compound must wear goggles.
1.4.6 FORMS LEFT IN PLACE
Forms provide good protection against loss of moisture, if the top exposed surface of concrete is kept moist. It is only after stricking the formwork that further curing may be necessary. The following precautions are necessary for form curing : ● Balance exposed surfaces should be kept continuously wet using a soaker hose. ● Wood forms left in place should be kept moist by sprinkling especially during hot and dry weather. ● Steel formwork left in place should be kept covered with soaked hessian and kept wet. ● If formwork cannot be kept moist than it is preferable to remove them and use alternative curing methods.
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2.0
DEFECTS IN CONCRETE DUE TO INADEQUATE CURING
The present generation of cements are finer than those available earlier, due to which they gain strength quite rapidly and, therefore, need good curing at an early period of time. Adequate strength when curing can be started is often developed within less than a couple of hours of compaction and finishing operations. It is often observed that at high ambient temperatures and windy conditions freshly placed concrete develops plastic shrinkage cracks (Figure No. 3) and crazing within few hours of its finishing. These cracks, are often through the entire slab thickness, result in permanent defects in the concrete structure causing leakages and subsequent loss of durability.
2.1 Concrete placed with pumps have much higher workability than
the conventional concrete hence there is a tendency of over vibration
3. Concrete stirfnce displaying plastic 4. Concrete surface displaying crawng. shringkage cracks. One of the reasons could One of the rewon could be improper or be improper or delayed curing. delayed curing. Ambuja technical Literature Series
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of the placed concrete and resultant segregation causes fines to surface at the top and coarser materials to settle below. The top layer of finer fines have tendency to shrink and crack. To avoid this it is advisable to plough up the coarse aggregates using a fork (punja) prior to trowelling or finishing.
2.2 Concrete slab surfaces, exposed in the preliminary stages (at the
time when initial setting or hardening is taking place) to high velocity winds and high temperatures, shrink due to rapid drying of the surface moisture. It is, therefore, very essential to protect such surfaces from wind with a plastic sheet or other types of covers and prevent the drying of concrete surface. Drying due to high ambient temperatures is prevented by placing moist (not very wet or soaked) hessian or allowing the water to flow gradually on the surface between the concrete and the plastic sheet after concrete has gained the initial set. In certain cases drying is so rapid that before the slab at one end is placed and finished, the concrete placed at the earlier end starts showing signs of shrinkage cracks. Trowelling over the cracked concrete surface when it is in plastic stage can also help reduce cracking of the surface.
2.3 Depending on concrete materials, their temperatures and ambient
conditions, plastic shrinkage cracks often appear within 30mins to 6hrs of concrete placing, compacting and finishing. Generally, these cracks appear at random locations in slabs or over slab reinforcement. The cause for such cracking in slabs is due to rapid-early-drying or due to location of steel near the top slab surface where settlement of concrete mix due
to bleeding (non-cohesiveness) has taken place. Early improvement in concrete as suggested above can help to reduce such cracking.
2.4 Crazing or cracking of concrete in a close crazy pattern on the surface is often observed within one to seven days, and at times even later.
Rich concrete mix, cast in impermeable formwork to give fair faced smooth finish, often displays crazing which can be avoided by improving curing. Floated concrete in slabs, due to poor curing and over trowelling, also display crazing. This too can be improved by proper curing and finishing.
2.5 At some sites the following procedures were followed successfully
to avoid plastic shrinkage cracks : â—? The concrete of high workability after placing is first compacted properly in the deeper areas (beams) and lightly in the shallow areas (slabs) and left without finishing for sometime. After about 30 minutes lapse of time (depending on the concrete mix and ambient conditions), the stiffer concrete is once again compacted this time perfectly and finished thereafter. â—? The top surface, if not required to be finished trowel smooth, should be broomed, so that the surface tension on the thin finished layer is reduced, thereby reducing the chances of cracking at the surface.
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3.0
TEMPERATURE FOR CURING CONCRETE
It is essential to relate temperature of concrete and curing. During curing, adequate warmth is necessary. It is observed that temperature around 15°C is considered optimum for normal curing. However, for ambient temperatures in the range of 10°C to 40°C, normal curing methods as explained earlier are suitable. Additional precautions for curing are necessary, if the temperatures are outside the normal curing temperature range as stated above.
3.1 PRECAUTIONS ABOVE 40°C The following precautions are necessary, if the ambient temperatures are above 40° C: ● Higher the ambient temperature, the greater is the chance for concrete to dry out if curing and surface protection are not done. It is therefore, necessary, that curing and protection of concrete surface from loss of water is done immediately after compaction and finishing. ● All concrete surfaces are required to be covered with wet (moist not soaked) hessian or other types of covers as soon as finishing is over. The covers should be kept moist for at least few hours until the concrete can take wetter curing without surface damage. ● Cold water for ponding or sprinkling on the covers will help in cooling the concrete surface to temperature below 40°C. However, precautions are necessary that after the concrete surface has hardened, cold water used for curing should not have temperature which is far less than temperature of the concrete at its surface. Ambuja technical Literature Series
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Ponded curing water should be adequate enough and replenished before the surface dries up. Alternate wetting and drying is undesirable, because it cracks the concrete surface. In very hot climate, form curing is undesirable. The formwork should therefore, be removed and wet curing commenced immediately thereafter. In hot weather, continuous wet curing is a must in the first 24 hours. Further, curing must be continued depending on availability of water or other curing materials / methods. Curing must be uninterrupted and continued for as long a time as practicable. The duration of curing should definitely be more than, in-case of normal temperature range. Curing compounds should be applied only after wet curing for 24 hours is completed. Concrete surface should be allowed to dryout gradually at the end of the curing period to avoid chance of cracking or caking.
3.2 PRECAUTIONS BELOW 10°C ●
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Generally precautions are necessary in cold climate to protect the concrete adequately so that the concrete temperatures due to the exothermic reaction are retained significantly within the concrete mass. The lower the ambient temperature, slower is the rate of hydration which results in slower rate of gain of strength. It is observed that very little hydration occurs below a temperature of 4° C. Adequate care has to be taken so that water in freshly placed concrete does not freeze at any time till adequate strength
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gain has occured. It is, therefore, essential to keep the temperature around and within the concrete to as high as possible (at least 10° C) so that, at no stage, the water mixed in concrete gets converted to ice. In ambient temperatures below 0°C, chemical additives (antifreeze) are used to lower the freezing temperature of water in the concrete. Due to this, the water remains in a fluid form and reacts with cement without creating problems in the hydration process. As the hydration process continues, the temperature within the concrete mass rises, due to which freezing of the water in concrete is further prevented till such time concrete has sufficiently hardened. Once water freezes in fresh concrete, concrete can lose about half of its potential design strength. In cold climate, temperature of concrete at the time of placing is kept as high as possible, so that concrete after it is cast does not suffer volume changes or lack of hydration due to freezing water within the mix. Formwork and concrete, in cold climate, are covered with thick blankets during the curing period, so that due to the heat of hydration the concrete temperature rises and hydration process improves which in turn improves the rate of gain of strength of concrete.
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4.0
CURING PERIOD
As a thumb rule curing period should be continuous and as long as it is practical, as all the desired properties of concrete are improved by curing. The curing period is dependent upon the following factors : ● The type and quantity of cement in the concrete / mortar mix. ● The quantum of strength required. ● The shape of the concrete structure i.e. exposed surface area to volume of the concrete. ● Weather (Ambient temperature, humidity, windy conditions). ● Environmental conditions. ● Future exposure conditions. The curing period should be such that concrete attains at least 70% of the specified compressive or flexural strength. However, in warm or hot climate, concrete may achieve this strength within 3 days. Concrete or mortar made by using cement of slow strength gain characteristics will need longer period of curing. It is recommended to cure heavily reinforced massive concrete structures for at least a minimum of 7 days. It is ideal to have curing temperatures between 10° C to 27° C. In cold weather curing temperatures are required to be maintained between 10° C to 21° C by giving additional heat by vented gas, oil fired heaters, heating coils or steam. In hot weather curing must commence as soon as possible and the concrete needs to be protected from loss of moisture as early as possible sometimes even within one hour of its finishing. Ambuja technical Literature Series
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5.0
CONCLUSION
Concrete in its early age is tender and weak. It can easily develop defects which can never get rectified. It, therefore, requires proper protection and curing. Under no condition curing should be neglected. If good quality of concrete is desired, then right from selection of raw materials to the curing stage, all steps have to be planned and supervised properly.
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Admixtures Plasticisers
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INTRODUCTION
There are essentially four ingredients in concrete, i.e. the coarse
aggregates, fine aggregates, cement and water. However, in order to improve certain properties of concrete, both in plastic and hardened state, it is necessary to add the fifth ingredient in the concrete mix. This fifth ingredient can be a mineral or chemical admixture. If concrete mix is properly designed, most concrete do not need any admixture. However, there may be a need to produce concrete with a particular result and using admixture could be the most convenient way. It is very important to note that any type of admixture should be used only when there is a valid reason to use it. There are several types of admixtures available but the most popular and most often used is the plasticiser. Plasticiser is a chemical admixture and this publication covers in details its classification, interaction, useful effects on concrete properties and precautions to be exercised in its usage. Plasticisers are extensively used in concrete mixes for the past three decades the world over. However, in India this usage has considerably increased in the last five years. Plasticisers or any type of admixtures are not substitutes for badly designed concrete mixes or bad construction practices. They are aids to modify good mixes and good construction practices to achieve certain specific requirements which can be conveniently achieved. There are two types of plasticisers. The ordinary plasticiser or plasticiser is one type and the other is a superior plasticiser or superplasticiser. Ambuja technical Literature Series
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1.0
CLASSIFICATION
1.1 Ordinary water reducing plasticisers which enable upto 15 percent of water reduction.
1.2 High range water reducing superplasticisers which enable upto 30 percent of water reduction.
The water reducing admixtures are further classified as normal, retarding or accelerating. Some plasticisers also cause air entrainment in con¬crete. The Superplasticisers are classified based on their chemical composition: ● Sulphonated melamine - Formaldehyde condensates ● Sulphonated napthalene - Formaldehyde condensates ● Modified lignosulphonates ● Carboxylated acrylic esters copolymers ● Others like sulphuric-acid esters, amide polysaccharide mixtures, carbohydrate esters etc. The sulphonated melamine based plasticisers are generally preferable if ambient temperatures are low, while sulphonated napthalene based plasticisers are preferable in high ambient temperatures due to their good workability retention quality. Polycarboxylate admixtures are high performance plasticisers which also help in slump (workability) retention for a longer period. For the sake of convenience word “Plasticiser” used in this booklet shall henceforth mean both plasticiser as well as superplasticiser. Ambuja technical Literature Series
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2.0
PURPOSE
Concrete must be placeable and durable. Durability includes considerations for appropriate strength for local conditions. Initially concrete should have qualities like cohesiveness and workability so that it can be placed and compacted with ease. Once concrete hardens, strength and durability considerations are of prime importance to concrete. While designing a concrete mix all aspects of concrete requirements both in fresh (plastic) and hardened (set) form should be considered. Purpose for using a plasticiser to improve certain requirements of concrete should be clearly defined and understood. The plasticisers are generally used to achieve the following:
2.1 In Fresh Concrete ● ● ●
Increase workability and /or pumpability without increasing the water/cement ratio Improve cohesiveness and thereby reducing segregation or bleeding. Improve to some extent set retardation.
2.2 In Hardened Concrete ● ● ●
Increase strength by reducing the water/cement ratio, maintaining same workability. Reduce permeability and improve durability by reducing water/ cement ratio. Reduce heat of hydration and drying shrinkage by reducing cement content. Ambuja technical Literature Series
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3.0
PURPOSE OF THE TEST
The plasticisers interact both physically and chemically with cement particles.
3.1 Physical Interaction
Plasticisers are principally surface active (surfactants). They induce a negative charge on the individual cement particles such that the fine cement particles are dispersed due to interparticle repulsion. Fine cement particles being very small clump together and flocculate when water is added to concrete. This ionic attraction between the particles trap considerable volume of water and hence water required for workability of concrete mix is not fully utilised. Negative charges are induced on the fine cement particles causing floes to disperse and release the entrapped water. Water reducing admixtures or plasticisers, therefore, help to increase the flow of the concrete mix considerably.
3.2 Chemical Interaction
Essentially water reducing admixtures perform to plasticize concrete more by physical interaction than by chemical interaction. The chemical interaction is generally responsible for slump retention or retardation of concrete. When admixtures are dosed, the initial hydration of aluminate and silicates is delayed since some chemicals strongly affect the formation and growth of cystals of the hydrated phases. The chemical admixture first gets absorbed on the hydrated phases containing alumina and depending on the quality of the admixture and C3A content of cement it is further absorbed on other hydrated phases of silicates. This inhibition of the surface hydration reaction of the cement particles leaves more Ambuja technical Literature Series
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1. Disperse mechanism of cement particles in concrete before and after addition of plasticiser.
water to fluidise the mix and thereby increase the workability of concrete and also retain the workability for a longer period. Numerous organic compounds such as lignosulphonates, hydroxy carboxylic and carboxylic acids and their salts etc. cause retardation.
3.3 SUMMARY
The concrete gets fluidised or attains high workability when plasticisers or superplasticisers are used. To achieve a homogeneous distribution of water and optimal utilisation of the cement particles, water get dispersed and flocculation is prevented due to the following effects : ● Reduction of surface tension of water and lubrication of film between the cement particles. ● The induced electrostatic repulsion due to absorption of charged polymer on the cement particles causes dispersion of cement grains and releases the trapped water from cement flocs. ● Steric hindrance preventing particle-to-particle contact. ● Change in morphology of hydration product. ● Retardation due to inhibition of the hydration reaction of cement particles leaving more water to plasticize the mix.
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4.0
FACTORS AFFECTING ADMIXTURE REACTION
The following factors influence the plasticizing effect of water reducing chemicals:
4.1 Types of Superplasticisers
It is difficult to compare the efficiency of one plasticiser from another. There are many inherent properties of an admixture which influences its efficiency. The prime factor which increases the efficiency is molecular weight. Higher the molecular weight, higher is the efficiency of plasticiser in concrete. However, beyond optimum value of molecular weight efficiency is expected to decrease.
4.2 Dosage
Optimal dosage of a superplasticiser is important. Dosage beyond the optimal dosage may be detrimental and can cause segregation or excessive retardation which can be harmful to concrete. The dosage optimisation can be done by using Marsh Cone apparatus or using minislump test or by the flow table methods described in ASTM C 109. The Marsh Cone test gives the measure of the flow time which is directly related to the viscosity of the grout. In this test water, cement and superplasticiser are thoroughly blended and a measured quantity of this grout is poured into a conical vessel with a standard opening at the bottom. The time taken for a given volume of the grout to pass through the standard opening is measured.
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It is generally observed that viscosity decreases as the superplasticiser dosage increases until a certain optimal superplasticiser dosage is reached. Beyond this dosage any further addition of superplasticiser does not significantly reduce viscosity. This limiting dosage is called a saturation point beyond which higher dosages are not added or recommended.
4.3 Compatibility with cement
All plasticisers/superplasticisers do not produce the same extent of concrete mix improvement with all types /brands/grades of cement. It is therefore essential to ensure that cement and admixtures are compatible with each other. The chemical and physical interaction can depend on their individual chemical and physical properties. Fineness, chemical composition, C3A content, SO3 (gypsum) content of cement and chemical composition and molecular weight of the plasticiser are mainly responsible for their interaction with each other. It is therefore, preferable to study different combinations before finalising an optimum combination.
4.4 Concrete Mix Design and Mix Composition
Every ingredient in the concrete mix is responsible for the performance of the plasticiser. The properties which influence the performance or efficiency of the plasticiser are :
Water
Wetter the mix, better is the physical interaction of the plasticiser and hence, better is dispersion in the mix. Coarse Aggregate Porosity, proportion and grading influences the performance of the plasticiser. Fine Aggregates Porosity, proportion, grading and silt content influence the performance of the plasticiser. Cement Fineness, gypsum content and C3A content. Out of the above three properties C3A content plays a major role on the efficiency of the plasticiser. Higher C3A content can reduce the efficiency of the plasticiser. Other mixtures The efficiency of the plasticiser may also get influenced, if other chemical or mineral admixtures like air-entraining agent, flyash, slag or silica fume are used. It can therefore, be concluded that proper trials are necessary before a particular dosage of plasticiser is fixed to achieve the desired efficiency of dispersion and workability.
4.5 Other Factors
There are various other factors which can influence the performance of the admixture and influence the dosage of the admixture or its performance. Ambuja technical Literature Series
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4.5.1 High Ambient Temperature and Low Relative Humidity
Loss of water from the mix due to above factors will also cause reduction in workability and thereby loss of wetness of the mix. This ultimately results in lowering the efficiency of the plasticiser.
4.5.2 Batching of Plasticiser
Accuracy of dosage of plasticiser for every batch of concrete produced has to be done with care and accuracy. Manual batching, if done, has to be carefully monitored. Batching mixing plants having admixture dispensing arrangements are most suitable for accurate batching. While underdosing of plasticiser will cause lower workability, over dosage of plasticiser is also harmful. Overdosage can cause severe concrete segregation as well as excessive retardation specially if the admixture is lignosulphonate based and if unÂŹdesirable components in comÂŹmercial lignosulphonate are not removed.
4.5.3 Mixing of Concrete Materials
It is very important that the plasticiser is thoroughly mixed with various concrete ingredients. Mixing sequence and method, play an important role and if done correctly can enhance the performance of plasticiser in concrete. In conventional concrete, produced by tilting or reversible drum mixers, plasticiser is likely to give poor performance as these mixers
are not very efficient mixing devices. In any case, manual or hand mixing of concrete materials when admixture is used is ruled out. Efficient mixing devices such as pan mixers and compulsive or shaft mixers or other specialised types of mixers will enhance the performance of the plasticiser as it will be very uniformly and properly mixed with other ingredients.
The most recommended method of dosing admixture in a conventional concrete mixers is as follows: ● ● ● ● ● ●
Add all concrete materials, except the admixture and last two litres of water, into the mixer. Mix the materials completely (2 to 3mins) Add the required dosage of plasticiser in a container which has two litres of water which is yet to be added to the mixed materials. Thoroughly mix the plasticiser in water. Add this plasticiser and water solution into the mixer. Continue mixing operation for one more minute.
It must be noted that performance of plasticiser is generally dependent on the workability of concrete.
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5.0
LOSS IN WORKABILITY (SLUMP)
Concrete with or without a plasticiser can lose workability due to several factors from the time concrete materials are mixed to the time plastic concrete is placed and compacted at the desired location. The drop in workability is attributable to the following factors: ● High ambient temperature, low humidity and high wind velocity causes the water to evaporate ● Water absorption by aggregates due to their porosity or dryness In a plasticized concrete the above factors can cause a more rapid drop of workability as there is less quantity of water in the concrete mix containing the plasticiser. The low quantity of water available for workability due to use of plasticiser may further reduce on account of the above factors. The following methods can therefore, be adopted : ● ●
Add plasticiser few minutes before placing the concrete Add plasticiser using smaller successive dosages at certain time inter¬vals so that at the time of placing, required workability is obtained
However, in warm or hot ambient weather conditions the best option would be to use a retarder cum plasticiser or use a plasticiser with a retarder. Usually single admixtures are available which can serve both as plasticiser as well as workability retention or retarding admixtures.
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6.0
EFFECTS ON PROPERTIES OF FRESH CONCRETE
6.1 Workability
The main reason why plasticisers are used in concrete is to improve the workability of concrete. The slump of concrete mix can be greatly increased if plasticiser is used. Table below shows the increase of workability of concrete mix from 25mm to 100mm using an admixture dosage of 0.4% by weight of cement. Table No. 2 Concrete Mix
Cement Content (kg/m3)
w/c
Reference mix without Plasticiser
440
0.37
Mix with Plasticiser
440
0.37
Slump (mm)
Strength (kg/cm2) at 7 days
28 days
25
390
540
100
411
541
Cohesiveness
It is essential to design a concrete mix which has proper cohesiveness This helps in reducing the chances of segregation and thereby reduces chances of large porosity in concrete. It can also reduce bleeding of concrete and this in turn helps in reduction of plastic shrinkage cracks and improvement in strength and durability of the concrete top surface.
6.3. Retardation / Retention of Slump
Retardation is generally measured by the time taken for concrete to reach a particular value of penetration resistance. If an admixture is Ambuja technical Literature Series
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used to achieve retardation then the improvement in retardation is measured as a difference between the stiffening rate of a reference (without admixture) mix and the mix containing the admixture. Retardation of concrete mix using a suitable admixture could come in very useful when large concrete pours are carried out in high ambient temperatures. Retardation of concrete mix using an admixture prevents formation of cold joints as lower layer of concrete will not stiffen till such time subsequent layer is placed and compacted over it. Plasticisers having retardation characteristics, if used in a concrete mix, can help in extension of setting time of concrete and thereby help in placing and compacting concrete in massive pour in warm/hot weather conditions. Workability retention, unlike retardation, is not easily defined. Retardation slows down cement hydration which in one way also help in retention of workability as less water is consumed due to cement hydration during the initial stages and therefore, more water is available for workability. However, retardation has little effect on workability loss. Workability of concrete with retarding admixture will drop at the same rate as reference or control mix until a slump of 50 to 75mm is reached. The best way to obtain the required workability at the time of placing and compacting is by starting at a higher workability. There are
several ways to achieve it. Additional water if added will improve the workability but the strength and durability of the mix will be seriously effected. It is, therefore, very important that workability improvement should not be done at the sacrifice of the strength and durability requirements of concrete. This can be achieved by using a plasticiser which improves the workability significantly without change in water cement ratio or cement content.
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7.0
EFFECTS ON PROPERTIES OF HARDENED CONCRETE
In case higher strength is desired, the plasticiser can be added to a concrete mix reducing its water requirement and at the same time keeping the workability unchanged. Table No. 2 below shows the increase in strength without reduction in workability of concrete due to plasticiser dosage of 0.4% by weight of cement in reference mix. Table No. 2 Concrete Mix
Cement Content (Kg/M3)
w/c
Reference mix without Plasticiser
315
0.60
Mix with Plasticiser
315
0.53
Slump (mm)
Strength (Kg/cm2) at 7 days
28 days
95
218
291
90
285
375
7.1 Reduction of Permeability of Concrete and Improvement in Durability
From above table, it is also observed, that W/C reduces if plasticiser is added to a concrete mix without change in workability of the mix. This results in less ingress of moisture, chlorides and other aggressive chemicals (Refer our booklet on Durability for details) as the capillary pores are smaller and get blocked faster when W/C is low. Besides, due to reduction in W/C, the strength is increased. Concretes with higher strength are more resistant to carbonation (Refer our booklet on Durability for details) Ambuja technical Literature Series
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7.2 Reduction of Heat of Hydration
The chemical reaction between cement and water creates heat and increases the concrete temperature. This temperature depends on the quantity and properties of cement in the concrete mix and degree of cooling done at the time of concrete production. At the centre of large blocks of concrete such as foundations, rafts, the temperature rise can be around 13°C for every 100kg of cement per cubic meter of concrete. Large temperature rise caused due to use of rich mixes in large pours cause a temperature differential between the interior and the exposed surface of concrete. This causes thermal stress due to temperature difference between the concrete’s interior and the surface. This can lead to severe thermal cracking. Reduction of cement content results in reduction of heat of hydration and thereby problems associated with it are reduced. If plasticiser is used in concrete, cement consumption can be reduced without change in W/C or reduction in strength. Table No. 3 below shows reduction in cement per cum of concrete without change in workability and strength when plasticiser dosage was 0.2% by weight of cement.
7.3 Shrinkage and Creep
Drying shrinkage cracks are generally caused due to loss of water from the capillary pores smaller than 0.5 micron, loss of absorbed water from close to solid surface and/or loss of water present between the layers of calcium-silicate hydrated structure.
Table-3 Concrete Mix
Cement Content (kg/m3)
w/c
Reference mix without plasticiser
460
0.43
Mix with plasticiser
395
0.43
Slump (mm)
Strength (kg/cm2) at 7 days
28 days
100
320
420
100
336
435
The loss of water as stated above causes strain and the resultant stress due to volume reduction. If this stress is greater than the concrete strength drying shrinkage cracks are caused. Higher cement and water content in a concrete mix can cause more drying shrinkage cracks. Concrete capillary pore refinement due to use of mineral or chemical admixtures can also cause more drying shrinkage cracking problems. In general, these properties of plasticized concrete are not significantly influenced as compared to identical concrete made without their use. To a certain extent, plasticisers help in reduction of cement or water and therefore reduction of hydrated paste and hence reduction of problems related to shrinkage and creep.
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8.0
PRECAUTIONS
There are some precautions necessary and required to be taken when using plasticisers in concrete. ● ● ● ●
● ● ● ●
●
All plasticisers should be tested adequately for their desired performance in full scale before being used. It is important to do trial mixes using the plasticiser with same concrete materials as proposed to be used at site. This is necessary to judge the performance and to achieve the desired results. Make sure right admixture is used. All admixtures should be properly stored and labelled. Make sure that plasticiser is added to the concrete mix in correctly measured proportion. Adding more or less could be harmful to the concrete mix. It is recommended to use dispensers for dosing liquid plasticisers. Ensure that admixture is uniformly dispersed on the concrete materials and therefore, it is essential to mix the admixture with water before dosing into the concrete mix. Plasticisers should be free from chlorides and sulphates. If plasticisers are used after lapse of time, it is essential to stir the contents in the container before using it. If aggregates are having excessive moisture they can be mixed with cement 30 seconds to 1 minute before mixing water with the plasticiser partial hydration of cement particles is allowed before beneficial to dispensive action and slump retention. It is most important to remember that plasticisers are not substitutes for good concrete practices. Ambuja technical Literature Series
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9.0
CODES OF PRACTICE
Property
Requirement
Water Content BS - reduction ASTM - maximum AS - maximum BIS - maximum Stiffening time - BS 0.5 N/mm2 3.5 N/mm2 Setting time - ASTM Initial 3.5 N/mm2 Not more than Final 27.6 N/mm2 Not more than Setting time - AS Deviation from Control: Initial 3.5 N/mm2 Maximum Final 27.6 N/mm2 Maximum Setting Time - BIS Allowable deviation from control sample Initial maximum Final maximum Compressive strength % of control minimum age 3 daysASTM AS BIS
Atleast 5% 95% of control 95% of control 95% of control Within ± 1h of control Within ± 1h of control
1h earlier nor 1½h later 1h earlier nor 1½h later
± 1h ± 1h
± 1h ± 1h 110 110 110
7 days-
BS ASTM AS BIS
110 110 110 110
28 days-
BS ASTM AS BIS
110 110 110 110 Ambuja technical Literature Series
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11.0
CONCLUSION
Durability requirements should be incorporated while designing concrete mixes. Developed countries have come to a definite conclusion that to improve durability of reinforced concrete structures it is essential to go for high strength, most often above 40N/mm2 concrete. This is due to the fact that all high strength concrete have to be produced with very low water-to-cement ratio, and low water-to-cement ratio makes concrete less permeable, than concrete made from higher water-to-cement ratio. If concrete is impermeable it prevents ingress of aggressive chemicals which attack mainly the steel reinforcement in concrete or at times the concrete itself. Preparation of high performance concrete too is inevitable to use plasticisers due to their dispersing and water reducing action. Once the conventional concrete mixes in form of 1:2:4 or 1:1½: 3 are abandoned in favour of correctly designed concrete mixes more extensive use of plasticisers will come into existence.
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NOTES:
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AMBUJA TECHNICAL SERIES INDEX No.
Name
01
Aggregates for Mortar And Concrete
02
Batching and Mixing
03
Concrete Compaction
04
Concrete Mix Design
05
Concrete Test Cubes
06
Concrete Transport
07
Cover to Reinforced Concrete
08
Curing + Admixtures
09
Durability of conccrete
10
Flyash blended pottland pozzolana cement & concrete
11
Formwork Inspection & Safety
12
Ordinary Portland Cement
13
Reinforcement Steel
14
Slump Test + Quality control
15
Water to cement ratio
Gujarat Ambuja Cements Ltd. Technical Services Division Elegant Business Park B-MIDC, Cross Road, Behind Kohinoor Continental off Andheri-Kurla Road, Andheri (E), Mumbai - 400 059 Tel.: 6616 7158 / 4066 7154. Fax: 4066 7714