Engineering properties and structural behaviour of high strength reinforced concrete beams
High Strength Concrete (HSC) has become a viable alternative to lower strength concrete. However, its utilisation is increasing faster than the development of suitable design recommendations. This is because limited and diverse investigations have been carried out concerning engineering properties and structural behaviour of HSC. The experimental investigation and theoretical considerations described in this thesis have been undertaken as an attempt to start to remedy this problem. As part of the investigation into the structural behaviour of HSC beams, a series of 18 different concrete mixes were tested in order to optimise HSC mixes using local aggregates. The effect of different factors such as w/c ratios, silica fume and superplasticizer dosages on compressive strength and splitting tensile strengths in the range of 80 to 120 N/mm2 (MPa) were studied. Additionally, new mathematical expressions were developed to replace some of the currently used relationships concerning HSC as a material, for a wide range of concrete mixes. The second part of this investigation concerned the structural behaviour of singly reinforced HSC beams in flexure. A total of 13 beams were manufactured using selected concrete mixes obtained from the first part of the investigation, i.e., 80, 100 and 120 MPa respectively. The size of the beam specimens was determined in order to make the beam fail in flexure, and also to be sufficiently large to simulate a real structural element of HSC. There were three beams in each of four groups, with the exception of group two which had four. For each group of beams the tensile reinforcement ratios were 1.03%, 1.42%, 1.94%, and 4.04% respectively. The test variables were thus the concrete strength and the longitudinal reinforcement ratio. The third part of this investigation concerned the structural behaviour of doubly reinforced HSC beams in flexure. Four beams were cast and tested using the same criteria of the singly reinforced beams. The variables considered were tensile and compressive reinforcement ratios. All beams in flexure were tested in a closed-loop testing machine under incrementally increased loading. Based on experimental evaluations and analytical considerations, the results are presented in terms of load-deflection characteristics, moment-rotation relationship, flexural strength capacity of high strength reinforced concrete (HSRC) beams, sectional ductility, crack patterns and surface crack width. Further, theoretical analyses were undertaken to idealise the nature of the concrete stress block developed in HSRC beams from elastic to ultimate load condition. In other words, proper idealisations of stress blocks were suggested in order to calculate the ultimate flexural capacity of HSRC members in good accuracy with experimental data obtained. The proposed equations have demonstrated rigorous outcomes on the flexural design of HSRC beams, providing a means of determining the internal forces in the context of compatibility of strains and equilibrium conditions of concrete. Additionally, the existing code of practice recommendations (BS 8110) were critically examined at ultimate strength capacity for HSRC beams having concrete strengths far beyond the limits normally considered in the code. The fourth part of this investigation concerned the shear capacity of HSRC beams. A total of 23 reinforced concrete beams, with and without shear reinforcement, were tested to determine their diagonal cracking and ultimate shear capacities. In beams without web reinforcement the shear span/ depth ratio was kept constant at 2 whilst concrete strength and flexural reinforcement ratio were varied. The concrete strength was varied from 40 to about 120 MPa while the steel reinforcement ratios were 1.94%, 2.92% and 4.04%. In beams with web reinforcement the variables were the same as beams without web reinforcement but in addition the a/d ratio was varied as 1.5, 1.75,2 and 2.6. One series of beams were cast with a preformed smooth inclined crack in order to remove aggregate interlock capacity. The effect of different parameters on shear capacity, structural behaviour in terms of load-deflection response, shear contribution carried by different mechanisms and shear ductility of HSRC beams were studied. In addition, an evaluation of some existing expressions from codes of practice used in calculating the shear stress were carried out to determine their applicability to HSRC beams. From this, new design equations have been developed to predict the ultimate shear stress of HSRC beams.
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