High strength steel reinforcement in ordinary reinforced and fibre reinforced cement composite lightweight concrete beams
When high strength steel is used as reinforcement in lightweight concrete members, great economies can be achieved. However, because lightweight concrete has low tensile strength and modulus of elasticity, the working steel stresses hoped for may not be fully utilised due to the limit states of serviceability (cracking and deflection) not being satisfied. To control the amount of cracking and deflection in flexural concrete members, a new type of construction has been employed, whereby precast fibre reinforced cement (f. r. c) units in the form of thin channels are used as a surface reinforcement at the flexural tensile zone of the concrete members. The concrete in the tensile zone, confined by the f. r. c channel, will have a greater resistance to formation and extension of crack; consequently the rate of reduction in the flexural rigidity of the member will be decreased. A total of 27 ordinary reinforced and fibre reinforced cement composite lightweight concrete beams, 150mm wide, 300mm deep and 5m long were tested, 18 under static load test, 5 under fatigue load test and 4 under sustained load test. The composite beams were similar to the ordinary beams in every respect, except that f. r. c. channels (150mm width, 60mm length of upstands and 6mm thickness) were incorporated as integral parts on their flexural tensile sides. The main parameters employed for both beams were the type and amount of steel provided for the tension reinforcement. The various types of reinforcement with the corresponding nominal yield, or 0.2% proof stress employed were mild steel (275 N/mm2), Unisteel 410 (410 N/mm2), Unisteel 550 (550 N/mm2), "Kam 60" (590 N/mm2) and "Kam 90" (875 N/mm2). The flexural behaviour of both types of beams under static, fatigue and sustained types of loading has been studied, great emphasis being placed upon the limit states of ultimate strength, cracking and deflection with particular reference to the contribution of the f. r. c. channels in the composite beams. From the results, it is concluded that a considerable reduction in the amount of deflection and cracking can be achieved by using f. r. c. channels at the flexural tensile zone of concrete members, thus allowing a more efficient use of the highstrength steel.