SUMMARY AND CONCLUSIONS
7.2 MAJOR FINDINGS
Some major findings from the present study are given below:
• Contribution of friction between concrete and side of the wall has been found to be significant in evaluating yield stress and plastic viscosity of fresh concrete.
Neglecting such effect causes lower estimate of these parameters.
• Coarse and fine aggregate gradation and size have significant influence on yield stress and plastic viscosity. Optimum values exist for percentage sand, mean sand size, and HRWRA dosage. It was observed that in addition to water cement ratio, ratio of cement paste volume to aggregate volume affect yield stress and plastic viscosity significantly.
• Good correlation between yield stress and slump, yield stress and sump flow was observed in high performance concrete. Yield stress decreases linearly as either slump or slump flow increases.
• A relationship also exists between plastic viscosity and slump, plastic viscosity and slump flow, plastic viscosity and slump time. The optimum plastic viscosity is 60- 80 Pa.s for maximum slump, maximum slump flow and maximum slump time. No correlation exists between yield stress and slump flow time for the concrete studied.
• Optimum value of condensed silica fume (CSF) replacing cement has been found to exist for minimum yield stress and maximum plastic viscosity. However, the optimum values for yield stress and plastic viscosity are not necessarily the same.
In concrete mix with SN as high range water reducer, yield stress is found to increase continuously.
• There is a decrease in yield stress and plastic viscosity of concrete when PFA replaces cement. Yield stress, however, slightly increases at higher replacement levels up to high volume level. The change in plastic viscosity is found to be insignificant.
• Experimental study using Rice Husk Ash (RHA) reveals that yield stress decreases due to increase in RHA replacement level. Plastic viscosity increases very steeply and the percentage increase in plastic viscosity is the highest among all the additives
• For two-component binder system such as cement-RHA, cement-CSF, cement- PFA, RHA gives the lowest value of yield stress whereas CSF produces concrete with the highest value of yield stress. When low value of plastic viscosity is under consideration, PFA shows the best effect while RHA shows the worst rheological performance of concrete.
• In case of ternary blends with equal masses, rheological properties are found to be lower compared to those of single mineral additives. CSF-RHA is found to yield the most suitable rheological performance with moderate plastic viscosity and low yield stress.
• Yield stress and plastic viscosity increase with the increase in fiber volume concentration. In some cases, rheological parameters may also decrease at low fiber volume concentration that can be explained with the coupling phenomena between improved packing density and mechanical interlocking. Mechanical interlocking dominates at higher volume concentrations. Only low and medium volume fraction of fibers was investigated because high volume fraction FRC is not workable and may not follow Bingham’s equation.
• Rheological parameters increase with increasing fiber aspect ratio. The effect of this parameter on plastic viscosity is less significant. The change in yield stress at low aspect ratio and ratio greater than 100 is also less.
• The effect of fiber diameter on rheological parameters is less pronounced over the range of diameters studied. Among all the three fiber parameters investigated, volume fraction has the highest impact on the rheological behavior and diameter has the least.
• Vebe and flow tests results on FRC have been used to investigate the relationship between theses tests results and rheological parameters. It was concluded that flow test may be a better test for FRC compared to Vebe test as it is found sensitive over all ranges of workability of concrete.
• Quantitative characterization of workability of fresh HPC has been outlined using rheological parameters. The quantitative scale has been correlated to the various classes of workability such as medium, high and very high workability as determined by slump test.
• Rheology box was used based on slump values to categorize the workability.
Upper and lower limits of rheology boxes were utilized to construct a power based scale of workability of HPC. This scale combines three parameters namely yield stress, plastic viscosity and shear strain rate into a single parameter. It is possible to assess the workability category with this new scale of energy dissipation rate of concrete flow.
• Shear rate plays a critical role in workability characterization of high performance concrete in addition to yield stress and plastic viscosity.
• Rheological parameters have been used to outline a mix design procedure of HPC. Dependence of compressive strength on yield stress and plastic viscosity was studied. Study reveals that with the increase in yield stress, the compressive strength increases with non uniform rate. Compressive strength also increases up to certain level of plastic viscosity.
• It is found that superplasticizer type plays an important role in displaying the variation of compressive strength with yield stress and plastic viscosity. However, two types of superplasticizer viz. SN and PC shows an overlapping zone of plastic viscosity to attain the maximum compressive strength whereas compressive strength is the highest for larger yield stress in mixtures contain SN compared to the mixtures containing PC.
• From various experimental results, chart for finding aggregate volume to paste volume ratio corresponding to yield stress and plastic viscosity has been prepared for various water cement ratio. This was used to find cement content in the mix which avoids the water-cement ratio versus compressing strength relationship as used in design of conventional and high strength concrete mix.
• Correction factors to be used in yield stress and plastic viscosity for different sand zone and maximum size of aggregate have been suggested for working out the mix proportions.
• The mix design procedure is valid for yields stress value 40-820 Pa, Plastic viscosity 15-120 Pa.s and compressive strength 40-90 MPa.
• Laboratory trials have been given for three mixes of target strength 45 MPa, 60 MPa and 70 MPa and found successful with the worked out proportions adopting proposed method.