G.Appa Rao

Investigations on the performance of silica fume-incorporated cement pastes and mortars

Studies on the performance of cementitious products with silica fume (SF) are very important, as it is one of the inevitable additives to produce high-performance concrete (HPC). In this study, some experimental investigations on the influence of SF on various preliminary properties of cement pastes and mortars are reported. The properties included specific gravity and normal consistency (NC) of cement and air content and workability of mortar with different SF contents. Pozzolanic and chemical reactions of SF have been studied on setting times, soundness and shrinkage of cement pastes. Further, strength developments in compression and tension in cement mortars have also been studied at various SF contents. SF was varied from 0% to 30% at a constant increment 2.5/5% by weight of cement. Test results show that the SF changes the behavior of cement pastes and mortars significantly. It has been observed that the water–binder (w/b) (cement+SF) ratio seemed to play an important role for the performance of the products with higher SF contents. NC, soundness and drying shrinkage of cement pastes and the strength of mortar increase as the SF content increases, while the initial setting times of cement pastes and the air content and workability of mortar decrease as the SF content increases. However, hardly any influence has been observed on the final setting times of cement pastes. The early age hydration reactions of C3A and C3S increase with the addition of SF. The optimum SF content ranges between 15% and 22%.

Fracture energy and softening behavior of high-strength concrete

An experimental investigation on the fracture properties of high-strength concrete (HSC) is reported. Three-point bend beam specimens of size 100 x 100 x 500 mm were used as per RILEM-FMC 50 recommendations. The influence of maximum size of coarse aggregate on fracture energy, fracture toughness, and characteristic length of concrete has been studied. The compressive strength of concrete ranged between 40 and 75 MPa. Relatively brittle fracture behavior was observed with the increase in compressive strength. The load-CMOD relationship is linear in the ascending portion and gradually drops off after the peak value in the descending portion. The length of the tail end portion of the softening curve increases as the size of coarse aggregate increases. The fracture energy increases as the maximum size of coarse aggregate and compressive strength of concrete increase. The characteristic length of concrete increases with the maximum size of coarse aggregate and decreases as the compressive strength increases, (C) 2002 Elsevier Science Ltd. All rights reserved.

Influence of the roughness of aggregate surface on the interface bond strength

An experimental investigation on the bond strength of the interface between mortar and aggregate is reported. Composite compact specimens were used for applying Mode I and Mode II loading effects. The influence of the type of mortar and type of aggregate and its roughness on the bond strength of the interface has been studied. It has been observed that the bond strength of the interface in tension is significantly low, though the mortars exhibited higher strength. The highest tensile bond strength values have been observed with rough concrete surface with M-13 mortar. The bond strength of the interface in Mode I load depends on the type of aggregate surface and its roughness, and the type of mortar. The bond strength of the interface between mortar M-13 cast against rough concrete in direct tension seems to be about one third of the strength of the mortar. However, it is about 1/20th to 1/10th with the mortar M-12 in sandwiched composite specimens. The bond strength of the interface in shear (Mode II) significantly increases as the roughness and the phase angle of the aggregate surface increase. The strength of mortar on the interface bond strength has been very significant. The sandwiched composite specimens show relatively low bond strength in Mode I loading. The behavior of the interface in both Mode I and Mode II loading effects has been brittle, indicating catastrophic failure.

Long-term drying shrinkage of mortar - influence of silica fume and size of fine aggregate

In this paper, an experimental investigation on the effect of silica fume and size of aggregate on the long-term drying shrinkage of mortar is reported. Silica fume was used as a partial replacement by weight of cement from 0% to 30%. The maximum size of fine aggregate was 1.18 and 2.36 mm, respectively, in series I and II mortar mixes. The water-cementitious material ratio and the cementitious material-sand ratio were 0.50 and 1:3. The ultimate drying shrinkage was measured up to the age of 1095 days. From the experimental test results, it was observed that the addition of silica fume has a significant influence on the drying shrinkage at early ages of mortar. It increases with increase in silica fume content. This increase in early days drying shrinkage is due mainly to very high pozzolanic reaction and pore size refinement mechanism of silica fume. The drying shrinkage in mortars with higher contents of silica fume was observed to be as high as 7-10 times greater than that observed in mortars without silica fume at early days. No significant influence of silica fume was found on the long-term drying shrinkage of mortar. Furthermore, it has been dramatically observed that the long-term drying shrinkage of mortar decreases with the increase in the size of fine aggregate.

Development of strength with age of mortars containing silica fume

Some experimental investigations on the development of compressive strength with age of mortar incorporated with silica fume with different water/binder (w/b) ratios have been reported. The silica fume content varied from 0% to 30% by weight of cement. Four w/b ratios, 0.35, 0.40, 0.45, and 0.50, were used. At every w/b ratio, the strength developments at 3, 7, 28, and 90 days have been observed. The highest development rate of compressive strength was observed at early ages (3 and 7 days). At w/b ratio 0.35, the highest value has been observed at 3 days with a silica fume content of 22.5%. At w/b ratios 0.35, 0.40, and 0.45, the strength increases up to an optimum content of silica fume, beyond which it decreases as the silica fume content increases. After 7 days, the strength development has been slowed down significantly at these w/b ratios. However, at w/b ratio 0.50, consistent strength development has been observed at the ages of 3, 7, 28, and 90 days. This has been dependent on the continuous availability of water content at all the ages. However, the ratios of strength of mortar at 3 and 7 days to strength at 28 days, and at 3, 7, and 28 days to strength at 90 days have been observed to be higher at w/b ratio 0.35 than those with other w/b ratios.

Influence of silica fume on long-term strength of mortars containing different aggregate fractions

An experimental investigation on the influence of high-strength cement with silica fume on long-term strength of mortar is reported. The effect of aggregate size and specific surface of aggregate on the variation of compressive strength has been studied. It has been observed that the early strength development was very significant with the addition of silica fume in mortars. It was also found that the size of the aggregate and its specific surface play a very significant role on the strength of the mortar. The strength of mortar increases initially and then gradually decreases as the grain size and the specific surface of aggregate increases. The modulus of elasticity increases as the compressive strength of the mortar increases. Significant strength losses have been observed in both silica fume and non-silica fume mortars at the age of 180 days.

Influence of interface properties on fracture behaviour of concrete

Hardened concrete is a three-phase composite consisting of cement paste, aggregate and interface between cement paste and aggregate. The interface in concrete plays a key role on the overall performance of concrete. The interface properties such as deformation, strength, fracture energy, stress intensity and its influence on stiffness and ductility of concrete have been investigated. The effect of composition of cement, surface characteristics of aggregate and type of loading have been studied. The load-deflection response is linear showing that the linear elastic fracture mechanics (LEFM) is applicable to characterize interface. The crack deformation increases with large rough aggregate surfaces. The strength of interface increases with the richness of concrete mix. The interface fracture energy increases as the roughness of the aggregate surface increases. The interface energy under mode II loading increases with the orientation of aggregate surface with the direction of loading. The chemical reaction between smooth aggregate surface and the cement paste seems to improve the interface energy. The ductility of concrete decreases as the surface area of the strong interface increases. The fracture toughness (stress intensity factor) of the interface seems to be very low, compared with hardened cement paste, mortar and concrete.