CE 2103 (Engineering Materialy CONCRETE REFERRENCES Properties of Concrete by A.M. Neville Advanced Concrete Technology-Constituent Materials ---~- by John Newman & Ban Seng Choo Significance of Tests and Properties of Concrete and Concrete-Making Materials ---~--by Joseph F. Lamond and James H. Pielert, Editors Concrete-Microstructure, Properties, and Materials -——-by P. Kumar Mehta & Paulo J. M. Monteiro A Text Book of Engineering Materials ——Dr. M.A. Aziz CONCRETE . Concrete is defined in ASTM Terminology Relating to Concrete and Concrete Aggregates (C 125) as ‘A composite material that consists essentially of a binding medium within which are embedded particles or fragments of aggregate. Concrete = Binding materials. + _ InertMaterials + Water (CementLime) _(Fine/Couirse* Aggregate) In hydraulic cement concrete, the binder is formed from a mixture of hydraulic cement and water. Hydraulic-cement concretes are those most widely used in the United States and world- wide. Hydraulic cement Hydraulic cement is defined in ASTM Terminology Related to Hydraulic Cement (C 219) as “a cement that sets and hardens by chemical interaction with water and that is capable of doing so under water.” Portland cement is the most important hydraulic cement Concrete may be considered as being composed of four basic separate ingredients: > Binding agent(cement/lime) > -coarse aggregates > fine aggregates > water Another way of looking at concrete is as a graded mixture of fine and coarse aggregates held together by wetted cement. Stil another way of viewing conerete is. that the coarse aggregates are held together by a mortar that is composed of cement, fine aggregates, and water. FUNCTION OF AGGREGATE IN CONCRETECE 2103 (Engineering Material) The aggregate gives volume to the concrete, around the surface of which the binding materials adheres in the form of a thin film. The voids in coarse aggregate are filled up with fine aggregate and again the voids in fine aggregate are filled up with the binding materials. Finally, the binding materials bind the individual units of aggregates into a solid mass with the help of water. FUNCTION OF WATER IN CONCRETE > To wet the surface of aggregates to develop adhesion because the cement paste adheres quickly and-satisfactorily to the wet surface of aggregates than to a dry surface. > To prepare a plastic mixture of the various ingredients and to impart workability to concrete to facilitate placing in the desired position > Water is needed for the hydration of the cementing to set and harden during the period of curing QUALITY OF WATER 4 Mixing: water should not ‘contain undesirable organic substances or inorganic constituents -in-excessive proportions. However, no standards explicitly prescribing the quality of mixing water are available, partly because quantitative limits of harmful constituents are-not known, but mainly because unnecessary restrictions could be economically damaging, + The water should be fit for drinking. Such water very rarely contains dissolve inorganic solids in excess of 2000 parts per million (ppm), and as a rule'less than - 1000- ppm: For ‘a waterlcement ratio of 0.5, the latter content, corresponds toa quantity. of -solids:-representing, 0,05% of the mass of cement and any effect of the common solids would be small. +4 Water with pH of 6.0 to 8.0.or possibly even 9, which does not taste salty is suitable for use, but dark color or bad smell do not necessarily mean that deleterious substances are present. 4 Natural water-that are slightly acid are harmless but water contain humic and other organic acids may adversely affect the hardening of concrete. This water should be tested before use. 4 The chloride content, SO3 content and alkali carbonates and bicarbonates of Brackish water should not exceed 500 ppm, 1000 ppm, 1000 ppm respectively. Water containing large quantities of chlorides tends to cause persistent dampness and surface efflorescence. Such water should, therefore, not be used where appearance of unreinforced concrete is of importance, or where a plaster finish is to be applied. e Much more importantly, the presence of chlorides in concrete containing embedded steel can lead to its corrosion. 4 Sea water has a total salinity of about 3.5% (78% of dissolved solid is NaCl and 15 % of dissolved solid is MgCl. and MgSO,) and produces a slightly higher early strength but a lower long-term strength; the loss of strength is usually no more than 15 % and can therefore often be tolerated. Some tests suggest that sea water slightly accelerates the setting time of cement; others show a substantial reduction in the initial setting time but not necessarily in the final set. Generally, the effects on settingCE 2103 (Engineering Material) aro unimportant if water is acceptable from strength considerations. The appendix to BS 3148: 1980 suggests a tolerance of 30 minutes in the initial setting time + Tho water brought In by the aggregate (aggregate usually contains surface moisture) should be free from harmful material +. Impurities in water * may interfere with the setting of the cement, * may adversely affect the strength of the concrete or cause staining of its surface, * may lead to corrosion of the reinforcement. In the construction of unreinforced concrete, water containing very high percentages of Salts of sodium, potassium, calcium and magnesium used in making concrete containing Portland coment blended with fly ash did not adversely affect the strength of concrete. However, no information on long-term behavior is available. Biologically treated domestic waste water has also been investigated for use as mixing water but much more information about the variability of such water, health hazards and long-term behavior is required. For these reasons, the suitability of water for mixing and curing purposes should be considered. Clear distinction must be made between the quality of mixing water and the altack on hardened concrete by aggressive waters. Indeed, some waters- which adversely affect hardened concrete may be harmless or even beneficial **** One ppm is equivalent to 1 milligram of something per liter of water (mg/l) or 1 milligram of something per kilogram soil (mg/kg). TYPES OF CONCRETE 4 Based.on unit weight, concrete can be classified into three broad categories. Normal-weight concrete Concrete containing natural sand and gravel or crushed-rock aggregates, generally weighing about'2400 kglm3 (4000 Ib/yd3), is called normal-weight concrete, and it is the most commonly used concrete for structural purposes. Lightwoight concrete For applications Where a higher strength-to-weight ratio is desired, it is possible’ to reduce the unit weight of concrete by using natural or pyro-processed aggregates with lower bulk density, The term lightweight concrete is used for concrete that weighs less than about 1800 kg/m3 (3000 Ib/yd3). Heavyweight concrete Heavyweight concrete, used for radiation shielding, is a concrete produced from high- density aggregates and generally weighs more than 3200 kg/m3 (5300 Ib/yd3).. “"* Pyro processing is used (o increase. the economic value of ores, minerals,-waste and related materials by changing their mechanical and/or chemical properties through the addition or removal of heat. 4 Based on compressive strength, concrete can be classified into three broad categories, > Low-strength conoreto: less than 20 MPa (3000 psi) Moderate-strangth concrete: 20 to 40 MPa (3000 to 6000 psi) > High-strength concrete: more than 40 MPa (6000 psi) -CE 2103 (Engineering Material) Moderate-strength concrete also referred to as ordinary or normal concrete is used for most structural work. High-strength concrete is used for special applications. It is not possible here to list all concrete types. There are numerous modified concretes which are appropriately named: for example, fiber reinforced concrete, expansive-cement concrete, and latex-modified concrete. Typical proportions of materials for producing low-strength, moderate strength, and high- strength concrete mixtures with normal-weight aggregate are shown in Table below Cement 336 510 Water 178 173 Fine aggregate 43. 390 Coarse aggregate 1032 872 (Cement paste proportion, percent by mass a4 percent by volume Water/cement by masi rength, MPa WORKABILITY The ASTM C 125-93 definition . "Property. determining the effort required to manipulate-a freshly mixed quantity’ of concrete with minimum loss homogeneity" is called workability, : The ACI definition of workability, given in ‘ACI.116R-9 : Property of freshly mixed concrete or mortar which determines 'the ease and homogeneity with which it-can be mixed, placed, consolidated, and finished is called workability Workability of a concrete is a composite property with at least two main components, as follows: fee 2 : Consistency- indicates the mobility or flow ability of freshly mixed concrete “Cohesiveness’ indicates the water-holding capacity (the opposite of bleeding) and the coarse aggregate-holding capacity (the opposite of segregation) Necessity for Sufficient Workability Concrete must have workability such that compaction to maximum density is possible with a reasonable amount of work or with the amount that are prepared to put in under given conditions. The work done is used to overcome the friction between the individual particles in the concrete and also between the concrete and the surface of the mould or of the reinforcement. These two can be called internal friction and surface friction, respectively. In addition, some of the work done is used in vibrating the mould or in shock and, indeed, in vibrating those parts of the concrete which have already been fully consolidated. Thus the work done consists ‘wasted part’ and ‘useful’ work, the latter as mentioned before, comprising work done to overcome the internal friction and the surface friction, Because only the internal friction is an intrinsic property of the mix, workability can be best defined as the amount of useful internal work necessary to produce full compaction. The need for compaction becomes apparent from a study of the relation between the degree of Compaction and the resulting strength. The relation between the strength ratio and the density ratio as shown in figure below(CE 2103 (Engineering Material) ty Ret o The above figure shows that the presence of voids in concrete greatly reduces its strength. ‘5% of voids can lower strength by as much as 30 per cent, and even 2 per cent voids can result in a drop of strength of more than 10 % Voids in concrete are in fact either bubbles of entrapped air or spaces left after excess water has been removed. The volume of the latter depends primarily on the water/cement ratio of the mix; to a lesser extent, there may be spaces arising from water trapped underneath large particles of aggregate or undemeath reinforcement. The air bubbles, which represent ‘accidental air i.e. voids within an originally loose granular material, are governed by the grading of the finest particles in the mix and are more easily expelled from a wetter mix than from a dry one. For any given method of compaction there may be optimum water content of the mix at which the sum of the volumes of air bubbles and water space will be a bined At this optimum water content the highest density ratio of the concrete would be obtained. Factors affecting Workability Workable concrete is the one which exhibits very little internal friction between particle and particle or which overcomes the frictional resistance offered by the formwork surface or reinforcement contained in the. concrete with just the amount of compacting efforts forthcoming. The factors helping concrete to have more. lubricating effect to reduce internal fiction for helping easy compaction are given below: Water Content . Mix Proportions * Size of Aggregates ‘Shape of Aggregates Surface Texture of Aggregate Grading of Aggregate Use of Admixtures. vvvvvVY Water Content The higher the water content per cubic meter of concrete, the higher will be the fluidity of concrete, which is one of the important factors affecting workability. At the work site, supervisors who. are not well versed with the practice of making good concrete, resort to adding. more water for increasing workability. This. practice is.often resorted: to because this is one of the easiest corrective measures that can be taken at site. It should be noted that from the desirability point of view, increase of water content is the last recourse to be taken for improving the workability even in the case of uncontrolled concrete. For controlled concrete one cannot arbitrarily increase the water content. In case, all other steps to improve workabilty fail, only as last recourse the addition of more water can be considered. More water can be added, provided a correspondingly higher quantity of cement is also added to keep the water/cement ratio constant, so that the strength remains the same. Mix ProportionsCE 2103 (Engineering Material) +” The higher the aggregate/cement ratio, the leaner is the concrete. In lean concrete, 1655 quantity of paste is available for providing lubrication, per unit surface area of aggregate and hence the mobility of aggregate is restrained. On the other hand, in case of rich concrete with lower aggregate/cemont ratio, more paste Is available to make the mix cohesive and fatty to give better workability. Size of Aggregate The bigger the size of the aggregate, the less is the surface area and hence less amount of water is required for wetting the surface and less matrix or paste is required for lubricating the surface to reduce internal friction. For a given quantity of water and paste, bigger size of aggregates will give higher workability within certain limits. Shape of Aggregates ‘The shape of aggregates influences workability in good measure. Angular, elongated or flaky aggregate makes the concrete very harsh when compared to rounded aggregates or cubical shaped aggregates. Contribution to better workability of rounded aggregate will come from the fact that for the given volume or weight it will have less surface area and less voids than angular or flaky aggregate. Not only that, being round in shape, the frictional resistance is also greatly reduced. This explains the reason why river sand and gravel provide greater workability to concrete than crushed sand and aggregate. , ‘The importance’ of shape of the aggregate will be of great significance in the case-of present-day high-strength and high: performance concrete when we use very low wic in the order-of about 0:25: We have already:talked about that in the years to come natural sand will be exhausted or costly, ‘One has to.go for manufactured sand. Shape of crushed’sand as available today is unsuitable but the’ modern crushers are-designed to yield-well shaped and well graded aggregates. : Surface Texture The influence-of surface texture on workability is again due to the fact that the total surface area of-rough textured: aggregate is more than the surface area of smooth rounded aggregate of same volume. Front the earlier discussions’ it’ can be inferred that’ rough textured: aggregate will. show -poor’ workability and’smooth or: glassy textured aggregate will give better. workability: A reduction of inter particle frictional resistance offered by smooth aggregates also-contributes to higher workability Grading of Aggregates i This is one of the factors which will have maximum influence on workability. A well graded aggregate is the one which has least amount of voids in a given volume, Other factors being constant, when the total voids are less, excess paste is available to give better lubricating effect. With excess amount of paste, the mixture becomes cohesive and fatty which prevents segregation of particles. Aggregate particles will slide past each other with the least amount of compacting efforts. The better the grading, the less is the void content and higher the workability. The above is true for the given amount of paste volume. Use of Admixtures Of all the factors mentioned above, the most import factor which affects the workability is the use of admixtures. In Chapter 6, it is amply described that the plasticizers and super plasticizers greatly improve the workability many folds. It is to be noted that intial slump ‘of concrete mix or what is called the slump of reference mix should be about 2 to 3 cm to enhance the slump many fold at a minimum doze. One should manipulate other factors to obtain initial slump of 2 to 3 cm in the reference mix. Without initial slump of 2-3 cm, the workability can be increased to higher level but it requires higher dosage - hence uneconomicalCE 2103 (Engineering Matera!) Use of air-entraining agent being surface-active, reduces the internal friction between fhe particles. They also act as artificial fine aggregates of very smooth surface. It can be viehCe that air bubbles act as a sort of ball bearing between the particles to slide past each oft and give easy mobilty to the particles. Similarly, the fine glassy pozzolanic materials, insp! of increasing the surface area, offer better lubricating effects for giving better workability MEASUREMENT OF WORKABILITY None of these methods below are satisfactory for precisely measuring or expressing Workability. Some of the tests, measure the parameters very close to workability and provide useful information. The following tests are commonly employed to measure workability. Slump Test Compagting Factor Test Remolding Test ‘ Flow Test Ball penetration Test Nesser's K-tester Two-point Test vvvvvVY 4 Slump Test ‘Slump test is the most commonly used method of measuring consistency of concrete which can be employed either in laboratory or at site of work. It is not a suitable method for very wet or very dry concrete. It does not measure all factors contributing to workability, nor is it always representative of the placability of the concrete. However, it is used conveniently as a control test and gives an indication of the uniformity of concrete from batch to batch. Repeated batches of the same. mix, brought to the same slump,;will have the same water content and water cement ratio, provided the weights of aggregate, cement and admixtures are uniform and aggregate grading is within acceptable limits. Additional information on workability and quality: of conerete ican be obtained by observing the manner in which concrete slumps. Quality of concrete can also be further assessed by giving a few tapings or blows by tamping rod to the base plate. The deformation shows the characteristics of concrete with respect to tendency for segregation, The apparatus for coriducting the slump test essentially consists of a metallic mould in the form of a frustum of. cone having the internal dimensions as under: Bottom diameter: 8°(20cm) Top diameter: 4" (10cm) Height: 12" (30cm)CE 2103 (Engineering Matenal) + True slum shear collapse The thickness of the metallic sheet for the mould should not be thinner than 1.6 mm, ‘Sometimes the mould is provided with suitable guides for lifting vertically up. For tamping the cpncrete asteel — -tamping —_—_rod 16 mm(5/8") dia, 06 meter along with bullet end is used. The intemal surface of the mould is thoroughly cleaned and freed from superfluous moisture and adherence of any old set concrete before commencing the test. The mould is placed on a smooth, horizontal, rigid and non-absorbent surface. The mould is then filled in four layers, each approximately 1/ 4 of the height of the mould. Each layer is tamped 25 times by the tamping rod taking care to distribute the strokes evenly over the cross section. After the top layer has been rodded, the concrete is struck off level with a trowel and taping rod, The mould is removed from the concrete immediately by raising it slowly and carefully in a vertical direction. This allows the concrete to subside. This subsidence is referred as SLUMP of concrete. The difference in level between the height of the mould and that of the highest point of the subsided concrete is measured. This difference in height in mm. is taken as Slump of Concrete. ASTM measures the centre of the slumped concrete as the difference in height. ASTM also specifies 3 layers, The pattern of slump is shown in Figure. It indicates the characteristic of concrete in addition to the slump value. If the concrete slumps evenly itis called true slump. If one half of the cone slides down, itis called shear slump. In case of a shear slump, the slump value is measured as the difference in height between the height of the mould and the average value of the subsidence. Shear slump also indicates that the concrete is non-cohesive and shows the characteristic of segregation Itis seen that the slump test gives fairy good consistent results for a plastic-mix. This test is not sensitive for a stiff-mix. In case of dry-mix, no variation can be detected between mixes of different workability. In the case of rich mixes, the value is often satisfactory, their slump being sensitive to variations in workabilityCE 2108 (Engineering Material) ‘Compacting Factor Test Such dry concrete are insensitive to slump test. ‘The compacting factor test has been developed at the Road Research Laboratory U.K. and it is claimed that it is one of the most efficient tests for measuring the workability of concrete. It is more precise and sensitive than the slump test and is particularly useful for concrete mixes of very low workability as are normally used when concrete is to be compacted by vibration. This test works on the principle of determining the degree of, compaction achieved by a standard amount of work done by allowing the concrete to fall through a standard height. The degree of compaction, called the compacting factor is measured by the density ratio i.e., the ratio of the density actually achieved in the test to density of same concrete fully compacted. > The sample of concrete to be tested is placed in the upper hopper up to the brim. > The trap-door is opened so that the concrete falls into the lower hopper. > Then the trap-door of the lower hopper is opened and the concrete is allowed to fall into the cylinder. > In the case of a dry-mix, itis likely that the concrete may not fall on opening the trap- door. In such a case, a slight poking by a rod may be required to set the concrete in motion, The excess concrete remaining above the top level of the cylinder is then cut off. with the. help of plane blades supplied with the apparatus. > The outside of the cylinder is wiped clean. The concrete is filled up exactly up to the top level-of the cylinder. It is. weighed to the nearest 10 grams. This.weight is known as ‘Weight of partially compacted concrete" Bo be > The cylinder is emptied and then refilled with the concrete from the same sample in layers approximately5 cm deep. The layers..are heavily rammed. or preferably vibrated $0.as to. obtain full compaction. eee eon ntye s > The top surface of the fully compacted concrete is then carefully struck off level.with the top of the cylinder. and weighed to the nearest 10 gin. This weight is known:as "Weight of fully compacted concrete”. 1 1 ay Trp op ; ewan bef Doar Hopaen A Dimension (cm) Top internal diameter 25.4CE 2103 (Engineering Material) Bottom. internal diameter 127 internal height 279 " Lower Hopper, B Top internal diameter | 22.9 Bottom internal diameter 427 intemal height 22.8 [tyler onal Tnternal diameter 162 , Tatemnal height 30:5 Disfance between bottom of upper hopper and 203 top of lower hopper ; Distance between bottom of lower hopper and top of cylinder : 203 ght of partially compacted concrete hr of f' Ce The Compacting Factor= if compaccad colctere ‘The weight of fully compacted concrete can also be calculated by knowing the proportion of materials, their respective specific gravities, and the volume of the cylinder. It is seen from experience, that it makes very little difference in compacting factor value, whether the weight of fully compacted concrete is calculated theoretically or found out actually after 100 per cent compaction. It canbe realized that the compacting factor. test -measures. the inherent characteristics of the concrete which relates very close to the workability requirements of concrete and as such itis one of the good tests to depict the workability of concrete. Table: Workability, Slump and Compacting Factor of Coneretes with 20 mm or 40 mm Maximum Size of Aggregate Description of | Slump(mm) | Compacting factor | Uses for which concrete is workability suitable No O = Roads vibrated by power- operated machines. At the more workable end of this Very low 5-10 078 group, concrete may be compacted in certain cases with hand-operated machines oa 70 hae Roads vibrated by _hand- operated machines. At the2103 (Engineering Material) more workable end of this group, concrete may be manually compacted in roads using aggregate of rounded or irregular shape, ~— Mass concrete foundations without vibration or lightly reinforced sections with vibration, Medium ‘AL the less workable end of this group, manually ‘compacted flat slabs using 36-75 ne crushed aggregates, Normal reinforced concrete manually compacted and heavily reinforced sections with vibration High “|For sections with congested reinforcement, Not normally wiles Cd suitable for vibration. For _| pumping and tremie placing _- | Very High 160- = Flow table test is more Collapse suitable. Effect of Time and Temperature on workability ‘Water Loss in concrete due to ' > absorption by the aggregate if not saturated, > evaporation, particularly if the concrete is exposed to'sun or wind, > the initial chemical reactions ' Hat Sketbes ae centage af inital np Tne-mire Figure: Loss of slump with time since mixing (based on ret 4 60)CE 2103 (Engineering Material) The exact value of the loss in workability depends on several factors, Tho highor the initial workability the greater the slump loss > Tho rato of loss of slump is highor in rich mixes, The rato of loss depends on the proparties of tho coment used: the rate is higher when the alkali contant is high and when the sulfate content is too low. } the moisture condition of aggregate (at a given total water content): the loss is Greater with dry aggregate due to the absorption of water by aggregate with time » Water-reducing admixtures delay the initial stiffening of concrete but often lead to a increased rate of loss of slump with time, > The workability of a mix is also affected by the ambient temperature woe Ye | © 0 wo Influence of temperature on slump of concrotes with diferent maximum aggregate size Infiuenceof temperature onthe amount of water required to change slumped Itis apparent that on a hot day the water content of the mix would have to be increased for a constant early workability to be maintained, The loss of slump in stiff mixes is less influenced by temperature because such mixes are less affected by changes in water content. Figure shows that as the concrete temperature increases the percentage increase in water required to effect a 25 mm (1 in.) change in slump also increases as the loss of slump with time is also affected by the temperature.CE 2103 (Engineering Material) sa} f / [ el pf | Fad / foe We iL 7 / | ast | © eve a Because workability decreases with time, itis important to measure, say, slump after a predetermined time lapse since mixing. There is value in determining slump immediately after the discharge of the concrete from the mixer for the purpose of control of batching. There is also value in determining slump at the time of placing the concrete in the formwork for the purpose of ensuring that the workability is appropriate for the means of compaction to be used PROPERTIES OF CONCRETE Physical properties of concrete The requirements of concrete are complex, but the ultimate aim is to produce the most economical combinations of concrete materials that will satisfy the performance requirements and specifications. A properly designed concrete mixture should possess the following physical properties: 1. When still in the plastic state, it must be adequately workable. 2. Itmust fulfill the required strength parameters. 3. Durability to be able to withstand imposed forces and elements such as traffic ~ abrasion for a concrete pavement, 4. Other properties that may vary in importance with the location of the. concrete in a structure are permeability and appearance. SEGREGATION Segregation can be defined as the separation of the constituent materials of concrete. A good concrete is one. in. which all the ingredients are properly distributed to make a homogeneous mixture. If a sample of concrete exhibits a tendency for separation of say, coarse aggregate from the rest of the ingredients, then, that sample is said to be showing the tendency for segregation. Such concrete is not only going to be weak; lack of homogeneity is also going to induce all undesirable properties in the hardened concrete. There are considerable differences in the sizes and specific gravities of the constituent ingredients of concrete. Therefore, it is natural that the materials show a tendency to fall apart. Segregation may be of three types > the coarse aggregate separating out or settling down from the rest of the matrix,, CE 2103 (Engineering Material) >. the paste or matrix separating away from coarse aggregate > Water separating out from the rest of the material being a material of lowest specific ; gravity. ‘A well made concrete, taking into consideration various parameters such as grading, size, shape and surface texture of aggregate with optimum quantity of waters makes 2 cohesive mix. Such concrete will not exhibit any tendency for segregation. The cohesive and fatty characteristics of matrix do not allow the aggregate to fall apart, at the same time; the matrix itself is sufficiently contained by the aggregate. Similarly, water also does not find it easy to move out freely from the rest of the ingredients. The conditions favorable for segregation are the badly proportioned mix where sufficient matrix is not there to bind and contain the aggregates. Insufficient\y mixed concrete with excess water content shows a higher tendency for segregation. Dropping of concrete from heights.as in the case of placing concrete in column conereting will result in segregation. When concrete is discharged from a badly designed mixer, or from a mixer with worn out blades, concrete shows a tendency for segregation. Conveyance of concrete by conveyor belts, wheel barrow, long distance haul by dumper, long lift by skip and hoist are the other situations promoting segregation of concrete. : ~ Vibration of concrete-is one of the important methods of compaction. It should ‘be remembered that only comparatively dry mix should -be vibrated, It too wet a mix is expessively vibrated; it is likely that the concrete gets segregated. It should also be remembered ‘that vibration is, continued just for required'time for optimum results. If the vibration is continued for a long time, particularly, in too wet a:mix, it is-likely to result in segregation of concrete.due to settlement of coarse aggregate in-matrix. yi“ ‘in the-recent time we-use concrete with very:high slump particularly in-RMC. The slump vale required at the batching point may be in the-order of.150-mm and at the pumping point the slump.may be-around'100-mm: At both: these points: cubes:are cast: One has-to take cage to compact: the cube mould with these high slump concrete. If sufficient care and understanding of concrete is:not exercised, the concrete in the cube .mould ‘may ‘get segregated and show low strength. Similarly care must be taken: in the compaction of such concrete in actual structures to avoid segregation While finishing concrete floors or pavement, with a view to achieve a smooth surface, masons ‘are likely to work too much with the trowel, float or tamping rule immediately on placing concrete. This immediate working on the concrete on placing, without any time interval is likely to-press the :coarse aggregate down,:which’ results in the movement of excess of matrix or paste to the surface, Segregation caused on:this account, impairs the homogeneity and serviceability of concrete. The excess mortar at the top causes plastic shrinkage cracks, From the foregoing discussion, it can be gathered that the tendency for segregation can be remedied by correctly proportioning the mix, by proper handiing, transporting, placing, ‘compacting and finishing. At any stage, if segregation is observed, remixing for a short time would make the concrete again homogeneous. As mentioned earlier, a cohesive mix would reduce the tendency for segregation. For this reason, use of certain workability agents and pozzolanic materials greatly help in reducing segregation. The use of air-entraining agent appreciably reduces segregation. ‘Segregation is difficult to measure quantitatively, but it can be easily observed at the time of concreting operation. The pattern of subsidence of concrete in slump test or the pattern of spread in the flow test gives a fair idea of the quality of concrete with respect to segregation. BLEEDING Bleeding is sometimes referred as water gain. It is a particular form of segregation, in which some of the water from the concrete comes out to the surface of the concrete, being of the lowest specific gravity among all the ingredients of concrete. Bleeding is predominantly observed in a highly wet mix, badly proportioned and insufficiently mixed concrete. In thinCE 2103 (Engineering Material) members like roof slab or road slabs an; is crcassiie Bese id when concrete is placed in sunny weather show ene to bleeding, water Comes up and accumulates at the surface. Sometimes, along ah this water, carta quantity of cement also comes to the surface. When the surface is z p wih the trowel and floats, the aggregate goes down and the cement and water me up to the top surface. This formation of cement paste at the surface is known as In such a case, the top surface of slabs and pavements will not have good wearing gual. This laitance formed on roads produces dust in summer and mud in rainy season. Ing to the fact that the top surface has a higher content of water and is also devoid of aggregate matter; it also develops higher shrinkage cracks. If laltance is formed on a Particular lift, a plane of weakness would form and the bond with the next lift would be poor This could be avoided by removing the laitance fully before the next lft is poured. Water while traversing from bottom to top, makes continuous channels. If the water cement ratio used is more than 0.7, the bleeding channels will-remain continuous and unsegmented by the development of gel. These continuous bleeding channels are often responsible for causing permeability of the concrete structures. While the mixing water is in the process of coming up,:it may be ‘intercepted by aggregates. The bleeding water is likely to accumulate below the aggregate. This accumulation of water.creates water voids and reduces the bond.between the aggregates and the paste. The above aspect is more pronounced in. the case’ of flaky aggregate. Similarly, the: water. that accumulates below the. reinforcing bars,. particularly. below .the cranked bars, reduces the bond between the reinforcement and the:concrete. The poor bond between the aggregate and the paste or the reinforcement and the paste due to bleeding can. be remedied by revibration of concrete. The formation of laitance and.the consequent bad effect can be reduced by delayed finishing operations. Bleeding rate increases with time up to about one hour or so. and thereafter the rate decreases but continues more or less till the final setting time of cement. Bleeding is an inherent phenomenon in concrete. All the same, it can be reduced by proper proportioning and uniform and complete mixing. Use of finely divided pozzolanic materials reduces bleeding by creating a longer path for the water to traverse. It has been already discussed that the use of air-entraining agent is Very effective in reducing the bleeding. It is also reported that the bleeding can be reduced by the use of finer cement or cement with low alkali content. Rich mixes are less susceptible to bleeding than lean mixes. The bleeding is not completely harmful if the rate of evaporation of water from.the surface. is equal.to. the rate of bleeding. Removal of water, after it had played its role in providing workability, from the body of concrete by way of bleeding will do well to the conerete. Early bleeding when the concrete. mass is fully plastic, may. not cause much. harm, because concrete being in a fully plastic condition at that stage will get subsided and compacted: It is the delayed bleeding, when the conorete has lost its plasticity that causes undue harm.to thé concrete. Controlled revibration may be adopted to overcome the bad effect of bleeding, Bleeding presents a very serious problem when Slip Form.Paver.is used for construction of concrete pavements. If two much of bleeding water accumulates on the surface of pavement slab, the bleeding. water flows out over the unsupported :sides which cause collapsing of sides. Bleeding becomes a major consideration in such situations... In.the: pavement construction finishing is done.by texturing or. brooming. Bleeding water delays the texturing and application of curing compounds. Method of Test for Bleeding of Concrete This method covers determination of relative quantity of mixing water that will bleed from a sample of freshly mixed concrete. ; ; ‘A cylindrical container of approximately 0.01 m? capacity, having an inside diameter of 250 mm and inside height of 280 mm is used. A tamping bar similar to the one used for slump test is used. A pipette for drawing off free water from the surface, a graduated jar of 100 cm? capacities is required for test.CE 2103 (Engineering Material) ‘A sample of freshly mixed concrete is obtained. The concrete is filed in 60 mm layer for a depth of 250 + 3 mm (5 layers) and each layer is tamped by giving strokes, and the top surface is made smooth by trowelling. The lest specimen is weighed and the woight of the concrete is noted. Knowing the total water content in 1 m3 of concrete quantity of water in the cylindrical container is also calculated, The cylindrical container is kept in a level surface free from vibration at a temperature of 27°C + 2°C, It is covered with a lid. Water accumulated at the top is drawn by means of pipette at 10 minutes interval for the first 40 minutes and at 30 minutes interval subsequentl tll bleeding ceases. To faciitate collection of bleeding water the container may be slighty tited All the bléeding water collected in a jar. Total quantity of bleeding water Bleeding water percentage = Total quantity of water in the sample of conorete x 100 STRENGTH CREEP OF CONCRETE FACTORS INFLUENCING THE PROPERTIES OF CONCRETE CHEMICAL ATTACK OF CONCRETE DESIGN OF CONCRETE MIXES ADMIXTURES USED IN CONCRETE PROPERTIES OF ADMIXTURE IN CONCRETE WeCE 2103 (Engineering Material) CURINGDi, Md. Rashedul Haque kan Depa,tnent of Ci Engineering sullen aed