R.V. Balendran

INFLUENCE OF STEEL FIBRES ON STRENGTH AND DUCTILITY OF NORMAL AND LIGHTWEIGHT HIGH STRENGTH CONCRETE

Abstract This paper presents the results of a series of experiments conducted to investigate the effectiveness of fibre inclusion in the improvement of mechanical performance of concrete with regard to concrete type and specimen size. Lightweight aggregate concrete and limestone aggregate concrete with and without steel fibres were used in the study. The compressive strength of the concrete mixes varied between 90 and 115 MPa and the fibre content was 1% by volume. Splitting tests on prisms and three-point bending test on notched beams were carried out on specimens of varying sizes to examine the size effect on splitting strength, flexural strength and toughness. The experimental findings indicate that the low volume of fibre has little effect on compressive strength but improve remarkably splitting tensile strength, flexural strength and toughness. The increase in splitting tensile strength, flexural strength and toughness index for lightweight concrete seems much higher than that of normal aggregate concrete. The size effect on prism splitting tensile strength is not significant beyond a critical (transition) size. There are apparent size effects on flexural strength and toughness index. As the specimen size increases, splitting and flexural strengths appear to decrease, and fracture behaviour tends to be more brittle.

Flexural behaviour of concrete beams internally reinforced with GFRP rods and steel rebars

Use of fibre‐reinforced polymer (FRP) composite rods, in lieu of steel rebars, as the main flexural reinforcements in reinforced concrete (RC) beams have recently been suggested by many researchers. However, the development of FRP RC beam design is still stagnant in the construction industry and this may be attributed to a number of reasons such as the high cost of FRP rods compared to steel rebars and the reduced member ductility due to the brittleness of FRP rods. To resolve these problems, one of the possible methods is to adopt both FRP rods and steel rebars to internally reinforce the concrete members. The effectiveness of this new reinforcing system remains problematic and continued research in this area is needed. An experimental study on the load‐deflection behaviour of concrete beams internally reinforced with glass fibre‐reinforced polymer (GFRP) rods and steel rebars was therefore conducted and some important findings are summarized in this paper.

The variation of radon exhalation rates from concrete surfaces of different ages

Abstract The radon exhalation rates from four concrete blocks cast with ordinary Portland cement (OPC) and four with substitution of pulverized fuel ash (PFA) were monitored for 34 months starting from November 1992. At the end of the 28th month, two OPC blocks and two PFA blocks were immersed in water, one from each group for 5 days the other one from each group for 10 days. These were then taken out of water and the radon exhalation rates monitored as usual. It was observed that the radon exhalation rate in general decreased with the age of the concrete blocks and the rate increased after the blocks were immersed in water. An explanation for the first result is that the gradual dehydration of concrete as it ages will reduce the water content in the pores of the concrete, thus reducing the probability of retaining radon within the pores and the probability of radon emanation from these pores. The second result gives strong support to this assertion.

Radon emanation from concrete surfaces and the effect of the curing period, pulverized fuel ash (PFA) substitution and age

Abstract Radon emanation rates from 48 concrete blocks have been monitored throughout a period in excess of 1 yr. These blocks have been produced for three distinct mixes of materials, i.e. mix A with 25% by weight substitution of type A pulverized fuel ash (PFA), mix B with 25% by weight substitution of type B PFA and mix C without PFA. Each mix is represented by four sets of concrete blocks with curing periods of 1, 3, 7 and 28 days. Every set comprises of four blocks each with the same composition and curing period. From the results, rates of change of radon emanation have been identified for two distinct periods, i.e. for the first month following curing and for the period following the first month. The different emanation rates are linked to the role of the superficial and inner pores of the concrete. It is also seen that the radon emanation rate tends to decrease with the age of the blocks for curing periods of 1, 3 and 7 days, but tends to increase for a curing period of 28 days, all of which can be rationalized in terms of the gradual dehydration of concrete as it ages. The patterns are similar for the different mixes, although the exact effect of PFA upon radon emanation rates remains unresolved.

Use of scanning electron microscopy in concrete studies

This paper offers a brief review of the present use of scanning electron microscopy (SEM) in concrete studies, from the perspective of how research in materials science is translated into applications in construction engineering. It describes the scope of present use of the method, and attempts a prospective for the near future in areas where more work could make productive use of the technology. Selected case studies have also been discussed. The electron microscope has been used as a research tool in understanding the root cause of the differing performance of various types of concrete under various conditions, a development tool in making better concrete, and a diagnosis tool on problems like cracking of concrete. The paper also explains how sample preparation affects the type and quality of information which the SEM can produce.

Fibre reinforced polymer materials for prestressed concrete structures

Fibre reinforced polymer (FRP) materials are currently used for concrete structures in areas where corrosion problems are serious. Recent applications of FRP rebars in normal reinforced concrete structures in fact cannot fully utilise the strength of FRP. A more rational use of FRP would be in the area of prestressed concrete (PC) structures. In spite of the superb strength provision of FRP tendons over steel tendons, use of FRP PC members is often questioned by practising design engineers. This is largely due to the brittleness of FRP tendons and lack of ductility in FRP RC structures. Recent research has demonstrated some important findings in promoting the confidence of adopting FRP RC beams. This paper reviews some recent work on the use of FRP in PC structures. Future possible research areas are also highlighted.

Estimating the elastic modulus of concrete made with artificially manufactured lightweight aggregates

Discusses the results of a study of the moduli of elasticity of concretes made with artificially manufactured lightweight aggregates subjected to uniaxial compression and uniaxial tension. Two artificially manufactured lightweight aggregates and one normal weight aggregate (for comparison) were used in the investigation. Concrete mixes designed to have compressive strengths varying from 20 MPa to 60 MPa were used in this study. Presents the results of static and dynamic moduli of elasticity, Poisson′s ratio, ultrasonic pulse velocity, compressive strength and tensile strength tests. Observes that the static modulus of elasticity in tension is nearly equal to the static modulus of elasticity in compression at a stress level of one‐third the ultimate stress. Compressive modulus values are shown to be dependent on the stress level and type of modulus, i.e. either secant or tangent. On the other hand, the tensile modulus is not affected by the stress level. The modulus of elasticity of lightweight aggregate c...

Strength development, deformation properties and mix design of pulverized fuel ash concrete

Discusses the properties of pulverized fuel ash (PFA) concrete both in its fresh and hardened states, with emphasis on aspects which are relevant to warmer localities such as Hong Kong. Sufficient research evidence, including surveys of existing structures, have been reviewed to conclude that PFA, when used correctly and properly, has the potential to enhance the performance of good concrete. Analyses the contribution of PFA in the enhanced performance of concrete. Cautions that much of the enhanced performance depends on the microstructure of the concrete, which in turn depends significantly on a proper mix design, with or without PFA. Proposes that water reduction is a desirable property of PFA. However, considers it possible to accommodate PFA characteristics in the mix design and by the use of plasticizer to produce good quality PFA concrete. Briefly discusses the characteristics of Hong Kong locally produced PFA.

Analytical behaviour of concrete beams reinforced with steel rebars and FRP rods

Publisher Summary Reinforced concrete (RC) beam embedded with conventional steel reinforcement usually gives a ductile behavior but low strength, whereas use of novel fiber-reinforced-plastic (FRP) rods provides a higher ultimate strength but limited ductility. This chapter presents an analytical model generated to compute the flexural behavior of concrete section reinforced with conventional steel reinforcing bars and novel FRP rods. The model computes the flexural behavior of concrete members reinforced with conventional steel reinforcing bars and novel FRP rods. The model implementation is based on the use of the true stress-strain relationships for plain concrete, steel rebar, and FRP rod. The main variable considered is arrangement/depth of steel rebars and FRP rods. The chapter concludes with some particular findings from the model and the results are also compared with that of concrete beam reinforced only with steel rebars. One of the findings is that the failure of steel-reinforced section with addition of FRP rods is usually governed by concrete crushing or FRP snapping. Another finding, when steel rebars and FRP rods are employed, concrete crushing is usually accompanied by small value of FRP depth whereas breaking of FRP is normally the case for large value of FRP depth.