Experimental Study of Strenth of Latex Modified Fibre Reinforced Concrete

EXPERIMENTAL STUDY OF STRENTH OF LATEX MODIFIED FIBRE REINFORCED

CONCRETE

BY

A.DEVI PRASADH

MANUEL MARTIN

NANDAKUMAR

DEPARTMENT OF CIVIL ENGINEERINGHINDUSTAN COLLEGE OF ENGINEERING

(AFFILIATED TO ANNAUNIVERSITY)CHENNAI-603103

SYNOPSIS

Latex modified mortar and concrete provide a good workability, water retention over conventional cement mortar & concrete .In contrast to ordinary cement mortar and concrete which are apt to cause bleeding and segregation, the resistance of latex modified mortar & concrete to bleeding and segregation is excellent in spite of their larger flow ability characteristics. Setting time of latex modified mortar & concrete is delayed in some extent. In concrete the tensile & flexural strengths are improved over a normal concrete but in compressive strength there is no improvement.The polymer-cement ratio has more pronounced effect on the strength than the water cement ratio. When the sand-cement ratio increases, the flexural and compressive strength of latex-modified mortars are remarkably reduced, and the effect of the latex - cement ratio on the strengths gradually becomes smaller.

In the present work concrete has been modified using latex as the polymer .In addition steel fibres have been added to check combined properties of concrete.

In general there is increase in compressive, tensile & flextural strength with increase in fibre and latex.

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INTRODUCTION

Engineering achievements have always been closely associated with the availability of suitable materials for construction. Further progress of engineering will depend on continuous development of all forms of construction developed of all forms of construction.

Polymer concrete composites were developed during 1960’s in U.S.A. In INDIA it is widely used for rehabilitation of structures. The popularity gained by the materials is justified by its extra-ordinary high strength, lower unit weight, total water impermeability and unmatched chemical resistance.

Engineers are trying to improve its quality, strength, etc.against adverse condition. For satisfactory utilization of this alternative material, the various phases of examination to check its:

Technical feasibility Durability of processed concrete Economic feasibility

With the ongoing research being done to develop appropriate technology and field trials to monitor the performance ad assessment of economic feasibility, the use of this alternative material will become more viable.

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ADMIXTURES USED IN CONCRETE:

Admixture is defined as the material other than cement water and aggregates that is used as an ingredient of concrete and is added to batch immediately before or during mixing. Additive is a material which is added at the time of grinding cement clinker at the cement factory.

As per the report of ACI committee 212, admixtures have been classified into 15 groups according to type of materials constituting the admixture, or characteristic affect of the use. When ACI committee 212 submitted the report in 1954, plasticizers and super plasticizers, as we know them today, did not exist.

These days concrete is being used for wide varieties of purposes to make it suitable in different conditions. In these conditions ordinary concrete may fail to exhibit the required quality performance or durability. In such cases, admixture is used to modify the properties of ordinary concrete so as to make it more suitable for any situation.

Admixtures have been traditionally used to improve the properties of concrete. There are two types of admixtures: chemical admixtures and mineral admixtures. Examples of chemical admixtures are high range water-reducing admixtures such as super plasticizers which constituted a major break through in the development of High performance concrete (HPC).its use can drastically reduce the water cement ratio (w/c) from 0.5 or higher to 0.3 or low , while providing rheological control of the concrete , given proper mixture proportioning and materials selection.

The reduction in w/c yields denser paste matrix and strengthen paste aggregate bonding on the micro structural level. Mineral admixtures such as silica fume, fly ash, slag, rice-husk, ash also provide benefits in concrete.

The improved rheology and cohesiveness, lower heat of hydration, lesser thermal shrinkage, and higher resistance to sulphate attack emerged over the years on the use of different minerals admixtures. It is therefore true to say that the combined use of chemical and mineral admixtures has resulted in a new generation of concrete called HPC, which was already within the construction industry.

POLYMER BONDING AGENTS:

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It is one of the well known facts that there will not be perfect bond between the old concrete and the new one. Quite often new concrete or mortar is required to be laid on old concrete surface. For example, for providing an overlay on existing pavement, in providing a screed over the roof for waterproofing or repair work etc... The bonding characteristics can be greatly improved by providing a bond between the old concrete and the new concrete surface or mixing the bonding agent with the new concrete or mortar. The use of bonding agent distinctly improves the adhesion of new concrete or mortar to old surface. The mixing of bonding agents with concrete or mortar improves the workability also at lower water cement ratio and thereby reduces the shrinkage characteristic. It also helps in water retention in concrete to redcap the risk of early drying. It further improves the water proofing quality of treated surface.

POLYMER MODIFIED MORTAR FOR REPAIR AND MAINTAINTENCE:

Sometimes concrete surfaces require repair. The edge of a concrete column may get chipped off; or ceiling of concrete roof may get peeled off, or a concrete floor may get pitted in course of time. Hydraulic structures often require repairing. Prefabricated members such as pipes, poles posts and roofing elements often get chipped off while stripping formwork, handling and transportation. In the past cement mortar was used for any kind of repair and as universal repair materials. Cement mortar is not the right kind of material for repair. Now there are many kinds of repair materials, mostly polymer modified, available for effective repair. They adhere very firmly to the old concrete surface on account of greatly improved bond characteristics. These materials are often stronger than the parent materials. They are also admixed with some other materials which make them set and harden very rapidly.

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ROLE OF FIBRES:

When the loads imposed on concrete approach that for failure cracks will propagate, sometimes rapidly; fibres in concrete provide a means of arresting the crack growth. Reinforcing steel bars in concrete have the same beneficial effect because they act as long continuous fibres. Short discontinuous fibres have the advantage, however, of being uniformly mixed and dispersed throughout the concrete. Fibres are added to a concrete mix which normally contains cement, water and fine coarse aggregate. Among the more common fibres used steel, glass, asbestos and polypropylene.

ENVIRONMENTAL FACTORS:

Resistance of fibre-reinforced concrete to environmental factors such as frost action depends on the quality of the concrete. Fibres can be effective, however in reducing frost damage because of their crack-arresting properties. Care should be taken to ensure that an adequate amount of entrained air is incorporated in the mix additional resistance to freezing and salt corrosion.Other environmental problems such as acid attack, sulphate attack and alkali-aggregate reaction are generally not augmented by the presence of fibres unless there is a chemical reaction between the fibre and the concrete.

OBJECTIVE OF THE EXPERIMENT:

To experimentally study compressive, tensile and flexural strength of latex-modified fibre reinforced concrete of M40 grade.And these results are compared with conventional concrete of M40 grade.

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LITERATURE REVIEW:

Attempts to increase Compressive strength of concrete have been successful. But tensile strength and ductility not modified. Modification of concrete with latex is the answer today with improved ductility.Special applications are in the seismic areas and structures subjected to dynamic loads.

Latex modified concrete improves durability of concrete.

Latex also reduces the permeability of concrete.

Latex modified mortar and concrete provide a good workability, water retention over conventional cement mortar & concrete. Latex used here is manufactured by FOSROC chemicals (India) pvt, Ltd under the chemical name NITOBOND SBR.In contrast to ordinary cement mortar and concrete which are apt to cause bleeding and segregation, the resistance of latex modified mortar & concrete to bleeding and segregation is excellent in spite of their larger flow ability characteristics. Setting time of latex modified mortar & concrete is delayed in some extent. In concrete the tensile & flexural strengths are improved over a normal concrete but in compressive strength there is no improvement.

MATERIALS USED:

The ingredients used in the test are as follows:1. 53 grade ordinary Portland cement (IS 12269-1987) passing

through IS90 microns sieve.2. Fine aggregate passing through 2 mm sieve3. Coarse aggregate passing 20 mm sieve4. Latex solution manufactured by FOSROC chemicals under the

brand name Nitobond SBR.5. Corrugated Steel fibres with aspect ratio of 0.8

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MIXTURE PROPERTIES:

WORKABILITY: Generally , latex-modified mortar and concrete provide a good workability over unmodified cement mortar and concrete .This is mainly interpreted in terms of improved consistency due to the ball bearing action of polymer particles and entrained air and the dispersing effect of surfactants in the latexes .This tendency is more significant at smaller sand-aggregate ratios at large unit cement content.

WATER RETENTION:Latex-modified mortar and concrete have a markedly improve water retention over unmodified cement mortar and concrete. The water retention is dependant on the polymer-cement ratio.

The reasons for this can probably be explained in terms of the hydrophilic colloidal properties of latex themselves and the filling and sealing effects of impermeable polymer films formed. Accordingly, a sufficient amount of water required for cement hydration is held in the mortar/concrete, hence dry cure is preferable rather than wet or water cure. The water retention generally increases with rising polymer-cement ratio, and becomes nearly constant at a polymer cement ratio of 5 to10%.Such excellent water-retention of the latex modified mortars is most helpful to inhibit dry-out phenomena in thin layer linings or coatings on highly water – absorbable substrates such as dried cement mortars.

BLEEDING AND SEGREGATION:

In contrast to unmodified cement mortar and concrete, which are apt to cause bleeding and segregation, the resistance of latex-modified mortar and concrete to bleeding and segregation is excellent in spite of their larger flow ability characteristics. This is due to the hydrophilic colloidal properties of latexes themselves and the air-entraining and water-reducing effects of the surfactants contained in the latex.

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SETTING BEHAVIOUR:In general the setting of latex-modified mortar and concrete is delayed to some extent in the comparison with unmodified cement mortar and concrete, and this trend is dependant ion the polymer type and cement ratio. The slower setting does not cause inconvenience in practical applications. Natural rubber modified mortar (NR) causes the most delay in setting. Usually, the reasons for the setting delay are the surfactants such as alklylbenzene, sulfonates and caseinates contained in the latexes inhibit the hydration of cement. Rheological studies on polyvinyl acetate - modified concrete is that the hydration of cement is inhibited by the adsorption of the surfactants on the binder surface.

STRENGTH:

The strength properties of the latex-modified mortar and concrete are influenced by various factors, which tend to interact with each other. The main factors are the nature of materials used such as latexes, cement, aggregates and controlling factors for mix properties (e.g.: polymer-cement ratio, water cement ratio, binder-voids ratio, cutting methods and testing methods etc).

Latex –modified mortar and concrete show a noticeable increase in the tensile and flexural strengths but no improvement in the compressive strengths. Thus in this investigation steel fibre is added in addition to latex to increase certain amount of compressive strength and to improve crack resistance.

EFFECTS OF CONTROL FACTORS FOR MIX PROPORTIONS:

The binder of latex-modified mortar and concrete consists of polymer latex and inorganic cement, and their strength is developed as a result of an interaction between them. Low polymer-cement ratio of 5 % or less also not effective because of little improvement in the strength. Consequently the polymer cement ratio range of 5 to 20% is used in practice. Most latex-modified mortars and concretes cured under favorable conditions have effective strength properties at polymer cement ratios up to 20% and the strength may be reduced at polymer-cement ratios exceeding 20%.

EFFECTS OF SAND-CEMENT RATIO:

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When the sand-cement ratio increases, the flexural and compressive strengths of latex-modified mortars are remarkably reduced, and the effect of the polymer-cement ratio on the strengths gradually becomes smaller.

EFFECTS OF CURING CONDITIONS:

Favorable curing condition requirements for latex-modified mortar and concrete differ from those for ordinary cement mortar and concrete, because their binder consists of two ordinary cement phases of latex and hydraulic cement with different properties. Optimum strength in the cement phase is developed under wet conditions such as water immersion and high humidifies , where strength development in the latex phase is attained under dry conditions .It is evident that optimum strength in most latex modified mortars and concrete is obtained by achieving the reasonable extent of cement hydration under wet conditions at early ages, followed by dry conditions , such curing conditions are most suitable sensitive for the mortars than for the concretes because of a difference in the retention due to their specimen sizes.

RELATION BETWEEN SURFACE HARDNESS AND COMPRESSIVE STRENGTH:

The surface hardness of latex-modified systems is generally improved to some extent over ordinary cement systems, depending on the polymer type and the polymer-cement ratio. A definite correlation between the surface hardness and compressive strength of most latex-modified systems is recognized.

SRESS-STRAIN RELATIONSHIP, MODULUS OF ELASTICITY AND DUCTILITY:

Most latex-modified mortars and concretes provide a higher deformation, ductility and elasticity than ordinary cement mortar and concrete, their magnitude depending on polymer type and polymer-cement ratio. The maximum compressive strain at failure increases with rising polymer-cement ratio, even though there is no pronounced change in the modulus of elasticity in compression. The maximum compressive strain at a polymer-cement ratio of 20% increases to 2 to 3 times that of unmodified mortar.

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The polymer-cement ratio is raised, the modulus of elasticity in tension decreases, and the elongation increase and is 2 to 3 times grater than that on unmodified concrete. This is explained by considering that the polymer films formed in the concrete effectively halt propagating micro cracks through their high tensile strength and elongation. The modulus of elasticity tends o decrease with the rise in the polymer-cement ratio.

SHRINKAGE, CREEP AND THERMAL EXPANSION:

The drying shrinkage increases with additional dry curing period , and becomes nearly constant at a dry curing period of 28 days regardless to polymer type and polymer cement ratio generally , the 28 th day drying shrinkage tends to decrease with increasing polymer cement ratio PVAC , NR and Chloroprene rubber(CR) modified mortars have a large shrinkage compared to that of unmodified mortars evaporation of the large amount of water absorbed in polymer phase due to the low water resistance of the polyvinyl acetate itself.

WATER PROOFNESS AND WATER RESISTANCE:

Latex-modified mortars and concrete have a structure in which the large pores can be filled with polymer with continuous polymer films. These features are referred in reduced water absorption water permeability and water vapour transmission as a result; latex-modified mortars and concrete have improved water proofness over ordinary mortars and concrete.

ADHESION OR BOND STRENGTH:

A very useful accepts of latex-modified mortars and concrete is their improved adhesion or bond strength to various substrates compared to conventional mortars and concrete. The development of adhesion is attributed to the high adhesion of polymers. The adhesion is usually affected by polymer-cement ratio and the properties of the substrates used. The data of adhesion often shows considerable scatter, and many vary depending on the testing methods, service conditions or porosity of substrates. The adhesion of most latex-modified mortars tend to increase with rising polymer cement ratio; although for a few types there is optimum polymer-cement ratios.

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The mix proportions also influence the adhesion, namely, the strength of the mortar substrates in 1:2 mix substrates through rather than through the interface. However it appears that the adhesion than the flexural strength

IMPACT RESISTANCE:Latex-modified mortars or concrete has an excellent impact resistance in compression with conventional mortars and concrete this is because of polymer they have high impact resistance. The impact resistance generally increase with rising polymer-cement ratio. The data of the impact resistance vary markedly between the testing methods .the impact resistance of the latex-modified mortars with elastomers is superior to the mortars with thermo plastic resins. The impact resistance SBR –modified mortars with polymer cement ratio of 20% is about 10 times greater than that of the unmodified mortars.

CHEMICAL RESISTANCE:

Most latex modified mortars and concrete are attacked by inorganic or organic acid and sulphates since they contain hydrated cement that is no –resistance to these chemical resistance is generally rated as good to fats and oils, but to organic solvents.

PROPERTIES OF FIBRES:

Concrete lends itself to a variety of innovative designs as a result of its many desirable properties. Not only can it be cast in diverse shapes; but it also posse’s high compressive strength, stiffness, low thermal and electrical conductivity and low combustibility and toxicity.Two characteristics, however, have limited its use it is brittle and weak tension. Recently, however the development of fibre-reinforced composites in the plastics and aerospace fields has provided a technical basis for improving these deficiencies.

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PHYSICAL AND MECHANICAL PROPERTIES OF SELECTED FIBRES

TYPE OFFIBRE

DIAMETER µm

SPECIFIC GRAVITY

FAILURE STRAIN%

MODULUDS OF ELASTICITY, GPa

TENSILE STRENGTH, GPa

Steel 5-500 7.8 3-4 200 1-3

Glass 9-15 2.6 2-3.5 80 2-3

COMPOSITE PROPERTIES:

Fibres can improve the toughness, the flexural strength, or and are chosen on the basis of their availability, cost and fibre properties.

Fibres also generally reduce creep strain, which is defined as the time-dependant deformation of concrete under a constant stress. For instance, steel-fibre-reinforced concrete can have tensile creep values 50 to 60 percent of those for normal concrete. Compressive creep values, however, may be only 10 to 20 % of those for normal concrete.Shrinkage of concrete, which is caused by the withdrawal of water from concrete during drying, is lessened by fibres. Shrinkage of glass-fibre-reinforced concrete is decreased by up to 35% with the addition of 1.5% y volume of fibres.Other properties of concrete, such as compressive strength and modulus of elasticity, are not included in the tables since they are affected to a much lesser degree by the presence of fibres.Innovations in engineering design, which often establish the need for few building materials, have made fibre-reinforced cements very popular. The possibility of increased tensile strength and impact resistance offers potential reactions in the weight and thickness of building components and should also cut down resulting from shipping and handling.Although ASTM C440-74a describes the use of asbestos-cement and related products, there are, at this, no general ASTM standards for fibre-reinforced cement, cement, mortar and concrete. Until these standards become available, it will be necessary to rely on the experience and judgment of both the designer and the fibre manufacturer.

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EXPERIMENTAL INVESTIGATIONS

INTRODUCTIONThe aim of this experimental work is to compare the strength of conventional concrete with concrete with steel fibres and also to compare the first crack load, ultimate load, and crack pattern and deflection response of plain concrete beam and with latex-modified fibre reinforced concrete beam. Test for finding out the compressive strength, tensile strength, flexural strength, impact strength was conducted. In order to find out the compressive strength, concrete cubes having a size of 150x150x150mm were cast and tested using UTM. For finding out the spilt tensile strength concrete cylinders having 150 mm diameter and 300 mm height were cast and tested with UTM with the diameter horizontal. In order to find out the flexural strength concrete prism having size 100x100x750mm were casted and tested in UTM

MATERIALS USED AND THEIR SPECIFICATIONS:The materials used and their specifications are as follows CEMENT:

The type of cement used was ordinary Portland cement and its specific gravity is 3.15. The cement was confirming to IS 269-1976

FINE AGGREGATE:

Locally available sand without debris was used, tests were conducted as per IS2386 (PART I). Specific gravity of fine aggregate is 2.64

COARSE AGGREGATE:

Crushed granite stone aggregates of maximum size of 20 mm was used tests were conducted as per IS 2386(part III) of 1963.Specific gravity of coarse aggregate is 2.69

WATER:

As per IS 456-2000 recommendations, potable water was used for mixing of concrete

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CONCRETE MIX PROPOTION:

Concrete was designed as per IS 10262-1982. The target strength of the mix was 40N/mm² of cube at the age of 28 days. The mix adopted is

1: 0.85: 2.24 by weight with water cement ratio of 0.35

STEEL FIBRES:

The corrugated steel fibres were used with aspect ratio 60 (36/0.6)Length of the fibre 36 mmThickness of fibre 0.6mmThe tensile strength of fibre is in the range of 1-3 Gpa

THE MIX: Latex added is 5% of weight of cement Steel 1.5% of volume of concrete

CASTING AND CURING OF SPECIMENS:

The materials were weighed carefully using the balance for the ordinary concrete fine aggregate and cement were weighed and mixed thoroughly, the coarse aggregate was then added and mixed with above. Steel fibres were then added following latex and water are added and mixed thoroughly to get a good mix.

For preparing the specimen for determining the compressive, tensile, flexural strength permanent steel moulds of standard size 150x150x150mm, 150mm diameter 300 mm height , 150x150x750 mm respectively.The sides and bottom of all the moulds were properly oiled for ease demoulding. Then the fresh was filled layer by layer and then compaction was done by table vibrator.

Before mixing the concrete the mould and other materials were kept ready. The fresh concrete was filled in the mould. Care should be taken to see that the concrete was compacted perfectly. The compaction was carried out manually and the top surface was leveled and finished. All the moulds were demoulded 24 hrs after casting, cured in water for another 27 days. They were tested on 28th day as per IS 456-1978.

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TESTING OF SPECIMENS

CUBE COMPRESION TEST:

The test was conducted as per IS 516-1959. the cube of standard size 150x150x150mm were uses to find the compressive strength of concrete specimens were placed on the bearing surface of UTM, of capacity 1000 tonnes without eccentricity and a uniform of loading of 140 kg per cm^2 per minute was applied till the failure of the cube at failure, the failure of the maximum load was noted and the compressive strength was calculated. Cube compressive strength (σcc) in Mpa = Pf/Ab

Where Pf= failure load (N)Ab = bearing area of the cube (mm²).

SPLIT TENSILE STRENGTH OF CONCRETE:

This test was conducted as per IS 5816-1970. The cylinders of standard size 150mm diameter and 300 mm height was placed on the UTM, with the diameter horizontal at the top and bottom two strips of wood where placed to avoid the crushing of concrete specimen at the points where the bearing surface of the compression testing machine and the cylinder specimen meets. The maximum load was noted down. The spilt tensile strength (Tsp) = 2P/пdl (Mpa)Where P is maximum load (N)d = measured diameter of specimen (mm)l = measured length of specimen (mm)

FLEXTURAL STRENTH TEST:

This test is conducted as per IS516-1959.prisms of standard size 150x150x750mm were used. Tests were carried in UTM the loads were applied at 190mm from either ends. Uniform loading was applied and maximum loading was noted.The modulus of rupture was calculated The modulus of rupture (fb) =3Pa/bd²Where P = load (N)d=depth of the prism mmb= breath mma= distance between support and the point load

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DISCUSSIONS AND COMPARSION OF TEST RESULTS

COMPARSION OF COMPERSSIVE STRENGTH:

Percentage of Fibre by weight of cement

Percentage of latex by volume of concrete

Compressive strength 3days (N/mm²)

Compressive strength 7 days(N/mm²)


Percentage increase in 3 days



0 0 23 31.4 57.5 78.5

Percentage of Fibre








0 1.5 23.33 34.23 58 85

Percentage of Fibre








5 1.5 23.76 34.88 59 87

COMPARSION OF SPLIT TENSILE STRENGTH

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Percentage of Fibre






0 0 3.00

Percentage of Fibre






0 5 3.35

Percentage of Fibre






1.5 5 3.50

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CONCRETE DESIGN MIX

INTRODUCTION:Concrete Mix Design is a process by which we determine the relative proportion of the various materials of concrete with an aim to achieve a certain minimum strength and durability, as economically as possible. Basically two factors are involved in concrete design mix. We have to achieve a certain minimum strength, and we have to do it as economically as possible. Two kinds of costs are involved in the making of concrete; namely cost of materials and cost of labor. The labor cost, which comprises of formwork, batching, mixing, transporting and curing is nearly the same for good concrete as well bad concrete. Among the material costs in conventional concrete, the cost of cement, which binds the aggregate together, is far higher than the costs of the other ingredients. Therefore the mix design aims at selecting as little cement as possible, consistent with the requirement of strength and durability.The ingredients of concrete can be broadly classified into (1) aggregate and (2) paste. The paste lubricates the concrete and is responsible for its workability. The lubricating effect of the paste is directly proportional to the dilution of the paste. But more dilute the paste, less strong it will be. It is be noted that the strength of concrete is limited by the strength of the paste, because the mineral aggregate, with rare exceptions are for stronger than the paste, because compound. Also the permeability of concrete is determined by the quality and continuity of the paste, since little water flows through the aggregate either under capillarity. Further, the predominant contribution to drying shrinkage of concrete is that of paste.

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DETERMINATION OF SPECIFIC GRAVITY FOR FINE AGGREGATE:

This test is used to determine the specific gravity of the sand. Specific gravity test is conducted by using Balance & Pyconometer. The pyconometer is cleaned for presence of dust , or moisture inside and its empty weight is taken A small quantity of dry sand is put inside the pyconometer so as to fill about one fourth of the pyconometer and the weight of pyconometer with sand is taken The pyconometer is then filled, completely with distilled water. Any entrapped air shall be eliminated by rotating the pyconometer in its side The pyconometer shall be topped up with distilled water to remove any forth from the surface, dried on the outside and weighed. The pyconometer is refilled with distilled water to the same level as before, dried on the outside and weighed.

OBSERVATIONS AND CALCULOATIONS;

For 100% riverbed sand:Weight of empty pyconometer (w1) =Weight of pyconometer and dry sand (w2) = Weight of pyconometer, sand and water (w3) =Weight pyconometer and water (w4) =Specific gravity, G = (w2-w1)/ [(w2-w1)-(w3-w2)]

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Documents

Experimental Study of Strenth of Latex Modified Fibre Reinforced Concrete