Experimental Investigations on Strength Characteristics of High Performance Concrete Using Silica Fume and Superplasticizer

For several decades, concrete has been the most widely used construction material. Though conventional concrete (CC) performs very well under normal conditions, in special situations a very high compressive strength of concrete is necessary together with sustainability to aggressive environments. Hence, higher compressive strength of range 60-140 MPa is essential. The concrete mixes so far were designed only for strength and workability requirements. For the performance of long satisfactory life, the designed mixes should be checked and proved for their durability properties such as low permeability, high corrosion resistance, freezing and thawing resistance, fire existence etc. This necessitates a detailed study on High performance concrete (HPC). This paper formulates a simplified mix design procedure for HPC by combining BIS and ACI code methods of mix design and available literatures on HPC. Based on the above procedure M80 and M100 mixes are arrived at. These HPC mixes are tested experimentally for compression, split tension, flexure and workability. The performance of the designed mixes is very good and the results are reported in this paper. Keyword: low permeability, resistance, fire existence, HPC


INTRODUCTION
HPC is a construction material which is being used in increasing volumes in recent years due to its longterm performance and better rheological, mechanical and durability properties than CC. HPC possesses invariably high strength, reasonable workability and negligible permeability. Compared to CC, preparation of HPC requires lower water-binder (w/b) ratio and higher cement content. The durability properties of concrete are given importance, which makes High Strength Concrete (HSC) into HPC. HSC refers to concretes of grade above M60. High strength and better durability properties become reality for CC by reducing porosity, in homogeneity, micro cracks in concrete and the transition zone. This is how HPC is evolved.
Incorporation of mineral admixtures like Silica Fume (SF), Fly ash, Granulated ground blast furnace slag, Rice husk ash act as pozzolanic materials as well as micro fillers, thereby the microstructure of hardened concrete becomes denser and improves the strength and durability properties. Addition of chemical admixtures such as super plasticizer improves the properties of plastic concrete with regard to workability, segregation etc.
The HPC permits use of reduced sizes of structural members, increased building height in congested areas and early removal of formwork. The use of HPC in pre-stressed concrete construction makes greater span-depth ratio, early transfer of pre-stress and early application of service loads. Low permeability characteristic of HP Creduces risk of corrosion of steel and attack of aggressive chemicals. This permits the use of HPC in marine/offshore structures, nuclear power plants, bridges and places of extreme and adverse climatic conditions. Eventually, HPC reduces maintenance and repair costs.
used. But to understand the behavior of concrete and will, to produce a concrete mix within closely controlled tolerances". Concrete is a three-phase composite material, the first two phases being aggregates and bulk hydrated cement paste (hcp) and the third being the "transition zone". The transition zone is the interfacial region between the aggregate particles and the bulk "hcp". It is the weakest link and if this is strengthened, then the strength and impermeability (durability characteristics) of concrete are improved to a greater extent. This is made possible by reducing w/b ratio and use of SF. SF improves the above properties by pozzolanic action and by reactive filler effect. SF contains a very high percentage of amorphous silicon dioxide which reacts with large quantity of Ca(OH)2 produced during hydration of cement to form calcium-silicate-hydrate (C-S-H) gel. This gives strength as well as improves impermeability. This is known as pozzolanic action (chemical mechanism). Another action, a physical mechanism called "filler effect" in which the small spherical shaped SF particles disperse in the presence of a super plasticizer to fill the voids between cement particles and accelerates the hydration of C3S, since SF is fine reactive filler. These results in well packed concrete mix. Due to pozzolanic action between SF and Ca (OH) 2, the larger size crystals of CA(OH)2 converts to crystal of C-S-H gel which is dense, leading to reduction of pore size. This effect along with the improved particle distribution results in reduction of the thickness of transition zone and leads to densely packed stronger and less permeable concrete.

III. SIGNIFICANCE AND OBJECTIVES
The objectives of the present investigation are to develop a simplified mix design procedure, specially for HPC by varying the percentage replacement of cement by SF (0-15%) at a constant dosage of super plasticizer, based on BIS & ACI code methods of mix design procedure and available literatures on HPC. Investigations were carried out on the above procedure to produce HPC mixes for M80 and M100 grades using 12.5mm maximum size of aggregates to ascertain the workability and the mechanical properties of the designed mixes and to find an optimum cement replacement by SF.
Hence in the present investigation more emphasis is given to study the HPC using SF and super plasticizer so as to achieve better concrete composite and also to encourage the increased use of SF to maintain ecology.

IV.
EXPERIMENTAL PROGRAMME Experimental investigations have been carried out on the HPC specimens to ascertain the workability and strength related properties such as compressive strength, split tensile strength, flexural strength and Elastic modulus of the designed trial mixes and also Non-Destructive Test (NDT) -Ultrasonic Pulse Velocity (UPV) has been carried out to check the quality of concrete.

Mix Design for HPC
Since there is no specific method of mix design found suitable for HPC, a simplified mix design procedure is formulated by combining the BIS method, ACI methods for concrete mix design and the available literatures on HPC using SF.

Formulation of mix design procedure Target mean strength
The target mean strength ( fck ) is calculated as follows: (fck ) = fck + (t x S) with usual BIS notations. When adequate data are not available to establish `S', the ( fck ) value can be determined from the following table 1 as given by ACI Report 318.
International Journal of Trend in Scientific Research and Development (IJTSRD) ISSN: 2456-6470 Page: 384

Selection of maximum size of coarse aggregate (CA)
The maximum size of the coarse aggregate is selected from the following table 2 as given by ACI Report 211.4R.93.

Estimation of free water content
The water content to obtain the desired workability depends upon the amount of water and amount of super plasticizer and its characteristics. However, the saturation point of the super plasticizer is known, and then the water dosage is obtained from the following table 3. If the saturation point is not known, it is suggested that a water content of 145 litres/m 3 shall be taken to start with.

Super plasticizer dosage
The super plasticizer dosage is obtained from the dosage at the saturation point. If the saturation point is not known, it is suggested that a trial dosage of 1.0% shall be taken to start with.

Estimation of air content
The air content (approximate amount of entrapped air) to be expected in HPC is obtained from the following table 4 as given by ACI Report 211.4R.93 for the maximum size of CA used. However, it is suggested that an initial estimate of entrapped air content shall be taken as 1.5% or less since it is HPC, and then adjusting it on the basis of the result obtained with the trial mix.

Selection of coarse aggregate (CA) content
The coarse aggregate content is obtained from the following table 5 as a function of the typical particle shape. If there is any doubt about the shape of the CA or if its shape is not known, it is suggested that a CA content of 1000 kg/m 3 of concrete shall be taken to start with. The CA so selected should satisfy the requirements of grading and other requirements of BIS: 383 -1970.

Selection of water -binder (w/b) ratio
The water-binder ratio for the target mean compressive strength is chosen from figure 1, the proposed w/b ratio Vs compressive strength relationship. The w/b ratio so chosen is checked against the limiting w/c ratio for the requirements of durability as per table 5 of BIS: 456 -2000, and the lower of the two values is adopted. The binder or cementitious contents per m 3 of concrete is calculated from the w/b ratio and the quantity of water content per m 3 of concrete.
Assuming the percentage replacement of cement by SF (0-15%), the SF content is obtained from the total binder contents. The remaining binder content is composed of cement. The cement content so calculated is checked against the minimum cement content for the requirements of durability as per table 5 and 6 of BIS: 456 -2000 and the greater of the two values is adopted.

Super plasticizer content:
The mass of solids in the super plasticizer (Msol) in kg, the volume of liquid super plasticizer (Vliq), the volume of water in the liquid super plasticizer (Vw) and the volume of solids in the liquid super plasticizer (Vsol) are calculated from the following equations: Sca, Ssf = specific gravities of saturated surface dry coarse aggregate and silica fume respectively, and Vsol, Vea = volume of solids in the super plasticizer and entrapped air (litres) per m 3 of concrete respectively.
The fine aggregate content per unit volume of concrete is obtained by multiplying the absolute volume of fine aggregate and the specific gravity of the fine aggregate.

Moisture adjustments
The actual quantities of CA, FA and water content are calculated after allowing necessary corrections for water absorption and free (surface) moisture content of aggregates. The volume of water included in the liquid super plasticizer is calculated and subtracted from the initial mixing water.

Unit mass of concrete
The mass of concrete per unit volume is calculated by adding the masses of the concrete ingredients.

Trail mix proportion
Because of many assumptions underlying the foregoing theoretical calculations, the trial mix proportions must be checked. If necessary, the mix proportion should be modified to meet the desired workability and strength criteria, by adjusting the percentage replacement of cement by SF, percentage dosage of super plasticizer solid content of binder, air content and unit weight by means of laboratory trial batches to optimize the mix proportion. Fresh concrete should be tested for workability, unit weight and air content. Specimens of hardened concrete should be tested at the specified age.

V. TESTS ON FRESH AND HARDENED CONCRETE Workability tests such as Slump test, Compaction
Factor test and Vee-Bee consistometer test were carried out for fresh concrete as per BIS specifications, keeping the dosage of super plasticizer as constant at 3 % by weight of binder. For hardened concrete, cube compression strength test on 150 mm size cubes at the age of 1 day, 3 days,7 days, 14 days, 28 days & 56 days of curing were carried out using 3000 KN capacity compression testing machine as per BIS : 516-1959. Also compression strength and split tensile strength tests on 150 mm x 300 mm cylinders and flexure test on 100 mm x 100 mm x 500 mm beams were carried out on 28 days cured specimens as per BIS specifications. The stressstrain graph for HPC is obtained using compressor meter fitted to cylinders during cylinder compressive strength test. UPV measurements were taken using NDT method on 150 mm size cubes for assessing the quality of concrete as per BIS : 13311 (Part 1) -1992.

VI. RESULTS AND DISCUSSION Tests on fresh concrete
The test results of workability are listed in table 8 and 9 and also shown in figures 2, 3 & 4. It was observed that the workability of concrete decreased as the percentage of SF content was increased.

Tests on hardened concrete
The results of cube compression strength, cylinder compression strength, spilt tensile strength, flexural strength and Modulus of Elasticity are also listed in table 8 and 9.
The optimum percentage of cement replacement by SF is 10% for the above tests for M80 and M100 grades of concrete. This may be due to the fact that the increase of strength characteristics in concrete is due to the pozzolanic reaction and filler effects of SF. The variations of average compressive strengths with respect to % of SF at different ages are shown in figures 5&6. The ratio of cylinder to cube compression strength was found to be 0.81. The flexure strengths obtained experimentally are higher than the value calculated by the expression 0.