steel fibres

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  • 1.PERFORMANCE OF CONCRETE BLENDED WITH STEEL FIBERSOBJECTIVETo increase the compressive strength, tensile strength and ductility of concrete byadding steel fibers to ordinary concrete

2. WORKABILITYAccording to IS:6461-1973 workability is defined as that property of freshly mixedconcrete or mortar which determine the ease and homogeneity with which concrete canbe mixed, placed, compacted and finished. Workability is broadly defined as the ease with which concrete can becompacted 100% having regard to made of compaction and place of deposition.Every job requires a particular workability. A concrete which is consideredworkable for mass concrete foundation is not at all workable for concrete to be used inroof construction. Similarly, a concrete considered workable for thick sections is notworkable required to be used in thin sections. Therefore, the word workability assumesfull significance of the type of work, thickness of sections, extent of reinforcement andmode of compaction.Factors affecting WorkabilityWorkable concrete is the one which exhibits very little internal friction betweenparticle and particle or which overcomes the frictional resistance offered by theformwork surface or reinforcement with just the amount of compacting efforts. Thefactors helping concrete to have more lubricating effect to reduce internal friction forhelping easy compaction are as follows:- 1. Water content 2. Mix proportions 3. Size of aggregates 4. Shape of aggregates 5. Surface texture of aggregates 6. Grading of aggregates 7. Use of admixturesWater contentWater content in a given volume of concrete will have significant influence onworkability. The higher the water content per cubic meter of concrete, the higher will bethe fluidity of concrete, which is one of the most important factors affecting workability.The damaging effect of more water in concrete is reduction in compressive strength. In 3. case where all steps to improve workability fail, only as a loss recourse the addition ofmore water should be considered. More water can be added, provided acorrespondingly higher quantity of cement is also added to keep the w/c ratio content,so that the strength remains the same.Mix proportionsAggregate/cement ratio is an important factor influencing workability. The higherAggregate/cement ratio, the leaner is the concrete. In lean concrete, less quantity ofpaste is available for providing lubrication per unit surface area of aggregate and hencethe mobility of rounded aggregate of some volume. A reduction of inter particlesfrictional resistance offered by smooth aggregates also contributes to higher workability.Size of aggregatesThe bigger the Size of aggregates, the less is the surface area and hence lessamount of is required for wetting the surface and less matrix or paste is required forlubricating the surface to reduce internal friction. For a given quantity of water andpaste, bigger the Size of aggregates will give higher workability. The above, of coursewill be true within certain limits.Shape of aggregates Shape of aggregates influences workability in good measures. Angular,elongated or flaky aggregate makes the concrete very harsh when compared torounded aggregate or cubical shaped aggregates. Contribution to better workability ofrounded aggregate will come from the fact that for the given volume of concrete orweight it will have less surface area and less voids than angular or flaky aggregate. Thisexplains the reasons why river sand and gravel provide greater workability to concretethan crushed sand and aggregate.Surface textureThe influence of surface texture on workability is again due to the fact that thetotal surface area of rough texture aggregate is more than the surface area of smoothrounded aggregate of same volume from the earlier discussion it can be inferred thatrough texture aggregate will show the poor workability and smooth or glassy textureaggregate will give better workability the reduction of inter particle frictional resistanceoffered by smooth resistance also contribute to the higher workability .Grading of aggregateThis is one of the factors which will have maximum influence on workability. Awell aggregate is the one has least amount of voids in a given volume. Other factors 4. being constant, when the total voids are less, excess paste is available to givelubricating effect. Hence better the grading, less is the void content and higher theworkability.Slump testSlump test is the most commonly used method of measuring consistency ofconcrete which can be employed either in laboratory or at site of the work. It is not asuitable method for very wet or very dry concrete. It does not measure all factorscontributing to workability nor is it always representative of the placability of theconcrete. However it is used conveniently as a control test and gives or indication of theuniformity of concrete from batch to batch. Repeated batches of the same mix, broughtto the same slump, will the same water content and water cement ratio provided theweights of aggregate, cement and admixture or uniform and aggregate grading is wit inacceptable limit. Additional information of workability and quality of concrete can beobtained by observing the manner in which in concrete slumps. Quality of the concretecan also be further assessed by giving a few tamping or blows by tamping rod to thebase plate. The deformation shows the characteristics of the concrete with respect totendency for segregation. The apparatus for conducting the slump test essentially consist of a metallicmould in the form of frustum of a cone having the internal dimension as under.Bottom diameter : 20 cm Top diameter: 10 cm Height : 30 cTrue slump In this case mix subsides uniformly and cohesively. This type is obtained forrich mixtures which have proportion of fine aggregates.Shear slump In this half of the cone shear of along on inclined plane. This is so for leanermixtures such as 1:6 or 1:8 and where slump requirement is higher. The slump ismeasured from level of the cone to center of sheared plane. 5. Collapse slump In this type of slump the concrete just collapse and spread over a large area.This occurs in very wet mixes.Lamination of slump testThe slump test has no relation to the useful internal work. For very low degreeworkability the mix gives zero slumps and the text is in effective. It is also ineffective forlow mixes. For low workability concrete compaction factor test is useful.Compaction factor test The compaction factor test was developed by the road research laboratoryUK; the test was works on the principle of determining the degreeof the compaction achieved by standard amount of work done by following the concreteto fall through a standard height. The degree of compaction is measured by density ratiothat i.e. radio of actual density achieved in test to density of some concrete fullycompacted.The compaction factor test apparatus consist of to hopper vessels providedwith hinged doors at bottom. A cylindrical vessel is placed below the hopper. In anexperimental procedure the concrete after mixing is placed in an upper hopper than itshinged bottom door is opened to allow the concrete to fall in to second hopper. Thesecond hopper is opened next and concrete falls in to the cylinder. Excess concrete incylinder is struck of and cylinder is then weighted. The cylinder is then emptied andrefilled in three layers of concrete. Each layer being compacted by giving 25 blows. Theexcess concrete is then struck of and cylinder is weighted again. The radio of uncompacted concrete and hand compacted concrete gives the compaction factor.Workability of different condition Workability is dependent on the proportions of the ingredient materials aswell as on their individual characteristics. The degree of workability required for properplacement and consolidation of concrete is governed by dimension and shape of thestructure and by spacing and size of reinforcement. Small complicated section withmuch reinforcement steel or either embedded parts require concrete of high. 6. Si.no.Type of constructionSlump Max(cm)Min(cm)1 Reinforced foundation walls and footings 10.0 5.02 Unreinforced footings caissons & substructure7.52.5walls3 Reinforced slabs beams and columns 12.57.54 Building columns 12.55.05 Bridge decks 7.5 5.06 Pavements5.0 2.57 Side walls drive ways & slabs on ground10.05.08 Heavy mass construction5.0 5.0Table 4.3.1Significance of compressive strength Test to determine are undoubtedly the most common type made to evaluate the properties of hardened concrete. The reasons are mainly 1. The strength of concrete in compression has in most cases a direct influence on the load carrying capacity of both plain and reinforced concrete structure 2. Of all the properties of hardened concrete, those concerning strength can usually by determine early. 3. By means of correlation with other more complicated test, the result of the strength can be used as qualitative indication of other important properties of hardened concrete.Factor affecting compressive strength The factors affecting the compressive strength of the hardened concrete areas follows 1. The ratio of cement to mixing water 2. The ratio of cement to aggregate 3. Grading, surface structure, shape, strength and stiffness of aggregateparticles 4. Degree of compaction 5. Maximum size of aggregateIn the above factors it can be further inferred that water cement ratio primarilyaffect the strength directly, where as other factor indirectly affect the strength byaffecting the water cement ratio. 7. Strength of the concrete primarily depends upon on the strength of the cementpaste. The strength of the paste increases with cement content and decreases with aircontent. The relation between water cement ratio and strength of the concrete shown infigIt can be seen from the graph that low water cement ratio could be used whenthe concrete is vibrated to achieve higher strength, where as comparati

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