RHEOLOGICAL BEHAVIOR OF HIGH PERFORMANCE CONCRETE WITH MINERAL ADMIXTURES AND STEEL FIBERS
4.2. STUDY WITH MINERAL ADMIXTURES
4.2.4 Results and Discussion Use of Fly Ash
Fly ash was used as mass replacement for cement at rates 10%, 20%, 30%, and 50%. The 50% replacement level was incorporated to represent high volume fly ash concrete. The test results are shown in Fig 4.11 [bwc refers to by weight of cement in all the figures].
As expected, addition of increasing levels of PFA resulted in a reduction of yield stress up to 30% level. Beyond this value, there is a slight increase in yield stress up to 50%
level. The effect on plastic viscosity is peculiar for the mixes. Plastic viscosity increases
up to 10% and then gradually decreases up to 30%. The change in plastic viscosity beyond 30% is insignificant. Notably, the trend for yield stress and plastic viscosity are same for mixes containing PC and SN as HRWRA.
The spherical shape of PFA reduces frictional forces among the angular particles due to
“ball bearing” effect. Slight increase in yield stress at high volume replacement level may be due active adsorption of HRWRA molecules by un-burnt carbons. Un-burnt carbons in PFA are known responsible for loss of workability because of adsorption of HRWRA molecules. The reason for initial increase in plastic viscosity is not clear.
Use of Condensed Silica Fume
CSF was used as mass replacement of cement at rates 5%, 10%, 15% and 20%. The test results are shown in Fig 4.12. Plastic viscosity increases steeply up to 10% level and then decreases again showing an optimum value for maximum plastic viscosity. The effect of CSF on yield stress is variable. In Mix#C1:PC and Mix#C2:PC, yield stress decreases up to optimum values then increases again. In case of Mix#C3:SN, yield stress increases up to 5% level, remains same up to 15% and then again increases.
CSF has very high fineness and surface area. CSF particles are chemically highly reactive and adsorb HRWRA molecules with multi-layers. Consequently, as replacement level increases, yield stress increases in Mix#C3:SN. In Mix#C1:PC and Mix#C2:PC, possibly improved gradation due CSF and lubricating effect reduce the yield stress initially. The decrease in plastic viscosity at higher replacement levels is more complex, even reaching a value equal to corresponding mix without CSF. In view of these results, the simple adage that CSF reduces concrete workability cannot be wholly justified.
Use of Rice Husk Ash
RHA was used to replace cement on mass basis at rates 5%, 10%, 15% and 20%. Results are presented in Fig 4.13. Yield stress decreases almost linearly up to 10% level beyond that it still decreases at lower decrement rates. This behavior is somewhat unexpected because RHA particles are flaky, elongated and angular as evident from SEM photograph. Plastic viscosity increases tremendously with the increase in replacement level.
RHA particles have the highest surface area and fineness and lower reaction ability than cement. RHA particles fill into the spaces made by larger cement particle, decrease frictional forces of RHA-OPC system and improve packing thereby reducing yield stress.
The steep increase in plastic viscosity with the replacement levels suggests that fineness and shape of RHA play critical role. More the fineness more is the number of contacts among the particles and hence more is the resistance to flow. In addition, any deviation from a spherical shape implies an increase in plastic viscosity for the same phase volume.
Comparison of Rheological Parameters with Different Mineral Admixtures
Rheological parameters show different patterns with respect to different mineral admixtures. Yield stress shows that RHA and PFA act positively on workability whereas CSF acts negatively in this system i.e. yield value decreases with the increase in replacement level of RHA and PFA. With RHA as replacement material, workable mix beyond 20% replacement level is difficult to achieve. PFA keeps the mix workable over a very wide range of replacement level up to high volume replacement range. So, PFA may be a suitable option when low yield value is under consideration.
When low plastic viscosity values are desired, PFA seems to be the best option and RHA has the worst effect. In HPC, segregation of materials is an important factor since low plastic viscosity concretes are vulnerable to segregation. For designing HPC, therefore, moderate plastic viscosity is preferred. In view of this, CSF may be the suitable option.
(a)
0 50 100 150 200 250 300 350 400
0 10 20 30 40 50 60
PFA replacement level, % bwc
Yield Stress, Pa
Mix#C1:PC Mix#C2:PC Mix#C3:SN
(b)
05 1015 2025 3035 4045 50
0 10 20 30 40 50 60
PFA replacement level, % bwc
Plastic Viscosity, PaS
Mix#C1:PC Mix#C2:PC Mix#C3:SN
Fig 4.11 Effect of PFA replacement on rheological parameters (a) Effect on yield stress (b) Effect on plastic viscosity
(a)
0 100 200 300 400 500 600 700
0 5 10 15 20 25
CSF replacement level, % bwc
Yield Stress, Pa
Mix#1:PC Mix#2:PC Mix#3:SN
(b)
0 10 20 30 40 50 60
0 5 10 15 20 25
CSF replacement level, % bwc
Plastic Viscosity, PaS
Mix#C1:PC Mix#C2:PC Mix#C3:SN
Fig. 4.12 Effect of CSF replacement on rheological parameters (a) Effect on yield stress (b) Effect on plastic viscosity
0 50 100 150 200 250 300 350 400
0 5 10 15 20 25
RHA replacement level, % bwc
Yield Stress, Pa
Mix#C1:PC Mix#C2:PC Mix#C3:SN
0 50 100 150 200 250 300
0 5 10 15 20 25
RHA replacement level, % bwc
Plastic Viscosity, PaS
Mix#C1:PC Mix#C2:PC Mix#C3:SN
Fig. 4.13 Effect of RHA replacement on rheological parameters (a) Effect on yield stress (b) Effect on plastic viscosity