Microstructure Of High Strength Concrete Research Articles

Effect of Silica Fume on Permeability and Microstructure of High Strength Concrete

The important concrete structure in the vicinity of industry, thermal power plant suffers deterioration by the acid rain cause due to combination of CO2, SOx and NOx with rain water. A combined attack that is from acid as well as sulphate can be observed under impact of sulphuric acid. It attacks on Calcium hydroxide and form Calcium sulphate, which can be leached out easily and make Interfacial Transition Zone (ITZ) poor. The water retaining structure such as dam, weir should be impermeable and that can be achieved by binary cementitious blends, using Silica fume (SF). Silica fume a by product of silicon industry, proves very effective in improving the microstructure of concrete due to their finer particle size, approximately 100 times finer than cement particles. The SEM image of binary blended high strength concrete (HSC) with Silica fume shows the condensed packing of cement hydration product and a dense microstructure as compare to control mix. The water permeability test result reveals that there is about 87 percent reduction in the coefficient of permeability achieved by inclusion of 10% Silica fume (SF) by weight of cement. Rapid chloride penetration test (RCPT) has been performed to investigate the ingress of chloride ions into the concrete. There was significant reduction in chloride ions penetration recorded due to SF inclusion.

The influence of elevated temperature on strength and microstructure of high strength concrete containing ground pumice and metakaolin

A laboratory study is performed to evaluate the influence of elevated temperature on the strength and microstructural properties of high strength concretes (HSCs) containing ground pumice (GP), and blend of ground pumice and metakaolin (MK) mixture. Twelve different mixtures of HSCs containing GP and MK were produced, water-to-binder ratio was kept constant as 0.20. Hardened concrete specimens were exposed to 250°C, 500°C and 750°C elevated temperatures increased with a heating rate of 5°C/min. Ultrasound pulse velocity (Upv), compressive strength (fc), flexural strength (ffs) and splitting tensile strength (fsts) values of concrete samples were measured on unheated control concrete and after air-cooling period of heated concrete. The crack formation and alterations in the matrix, interface and aggregate of HSCs were examined by X-ray diffraction (XRD), scanning electron microscope (SEM) and polarized light microscope (PLM) analyses. XRD, SEM and PLM analyses have shown that, increasing target temperature result with decrease in mechanical properties i.e. Upv, fc, ffs and fsts values. Elevated temperature also results with crack formation, and increasing target temperature caused more cracks. Alterations in the matrix, interface and aggregate were, also observed by these analyses. The experimental results indicate that concrete made with MK+GP blend together as a replacement of cement in mass basis behaved better than control concrete made with cement only, and concrete containing only GP as a cement replacement.

Influence of Metakaolin on Strength and Microstructure of High-Strength Concrete

Concrete with a compressive strength of 100MPa was produced with metakaolin(MK), and the effect of MK replacement levels on compressive strength of concrete with a water to binder ratio of 0.25 was studied in this paper. XRD, microhardness tests, SEM and MIP were used to investigate the influences of MK on the phase composition, microhardness of interfacial transition zone(ITZ), microstructural morphology and pore structure of the high-strength concrete (HSC). The results showed that the incorporation of MK promoted the hydration process and decreased the amount of Ca(OH)2 crystals. Furthermore, MK increased the ITZ microhardness of HSC, enhanced the hydrates and improved the bond performance of aggregate and paste. The weak zone between aggregate and paste became almost disappeared when 10% MK was blended. Meanwhile, refinement of the concrete pore structure was obtained in the presence of MK. The improvement of strength and microstructure became more obvious as the MK replacement level increased to 15%.

Effects of Nano-CaCO<sub>3</sub> on the Compressive Strength and Microstructure of High Strength Concrete in Different Curing Temperature

In this paper, effects of nano-CaCO3 on compressive strength and Microstructure of high strength concrete in standard curing temperature(21±1°C) and low curing temperature(6.5±1°C) was studied. In order to improve the early strength of the concrete in low temperature, the early strength agent calcium nitrite was added into. Test results indicated that 0.5% dosage of nano-CaCO3 could inhibit the effect of calcium nitrite as early strength agent, but 1% and 2% dosage of nano-CaCO3 could improve the strength of the concrete by 13% and 18% in standard curing temperature and by 17% and 14% in low curing temperature at the age of 3days. According to the XRD spectrum, with the dosage up to 1% to 2%, nano-CaCO3 can change the orientation index significantly, leading to the improvement of strength of concrete both in standard curing temperature and low curing temperature.

Creep Behavior of High-Strength Concrete with Polypropylene Fibers at Elevated Temperatures

This paper will explain how experimental studies have been conducted to examine the creep behavior of high-strength concrete (HSC) with polypropylene (PP) fibers of 2 kg/m3 (0.125 lb/ft3) when exposed to elevated temperature. Various types of heating and loading schemes are considered, including transient temperature and stepwise loading paths. Tests are carried out at temperatures up to 700°C (1292°F) and a stress ratio up to 0.6. HSC with PP fibers performs better than normalstrength concrete (NSC) at temperatures below 500°C (932°F) and shows an obvious increase in creep behavior at 700°C (1292°F). At the same stress ratio and duration of 120 minutes, the creep strain developed at 700°C (1292°F) was approximately 10 times more than that developed at 500°C (932°F). There was no abrupt change in creep behavior at or approximately 2200°C (428°F) when the PP fibers evaporated. It is likely that the markedly different response at 7000°C (1292°F) was related to the change in the microstructure of HSC at high temperatures. Increasing the stress ratio at constant temperature will lead to an abrupt increase in creep strain rate. HSC with PP fibers showed a large transient creep strain, and this phenomenon becomes obvious at higher temperatures. Finally, fitting equations are proposed to represent the creep behavior of HSC with PP fibers.

Mechanical properties and microstructure of high strength concrete containing polypropylene fibres exposed to temperatures up to 200 °C

High strength concrete has been used in situations where it may be exposed to elevated temperatures. Numerous authors have shown the significant contribution of polypropylene fibre to the spalling resistance of high strength concrete. This investigation develops some important data on the mechanical properties and microstructure of high strength concrete incorporating polypropylene fibre exposed to elevated temperature up to 200 °C. When polypropylene fibre high strength concrete is heated up to 170 °C, fibres readily melt and volatilise, creating additional porosity and small channels in the concrete. DSC and TG analysis showed the temperature ranges of the decomposition reactions in the high strength concrete. SEM analysis showed supplementary pores and small channels created in the concrete due to fibre melting. Mechanical tests showed small changes in compressive strength, modulus of elasticity and splitting tensile strength that could be due to polypropylene fibre melting.

Transient heating effect on high strength concrete

This study shows some differences in the properties and the behaviour at high temperature of two concretes (ordinary and high strength) made with the same calcareous aggregates. During heating tests at 1 °C min −1, cylindrical samples of diameter 160 × 320 mm of high strength concrete, more dense, may explode in a critical temperature zone between 250 and 300 °C. Differences in behaviour between OC and HSC appeared at high temperatures: there were disparities especially in thermo-hydric transfer, porosity and thermal stability. The dense microstructure of high strength concrete was found to slow up the escape of vaporized water.