Optimum depth of a lower concrete grade at the tension zone in a two-layer reinforced concrete beam

This paper presents the results of an investigation into the optimum depth of the lower concrete grade (LCG) at the tension region in a two-layer reinforced concrete beam. A total of nine (9) simply supported two-layer RC beams (1100 x100 x150 mm) were studied. Two 8 mm and two 6 mm diameter rods were used as reinforcement at the bottom and top of each two-layer beam, respectively. The beam samples were grouped into two: the first group comprises two-layer RC beams produced with 1:2:4 as the higher grade and 1:3:6 as the lower grade; the second group comprises two-layer RC beams cast with 1:2:4 as the higher grade and 1:4:8 as the lower grade. The depth of LCG adopted for each group is 25 mm to 100 mm at a step of 25 mm out of the total beam depth of 150 mm. The beams were subjected to two-point static loading to evaluate the load resistance and deflection. The results show that the greater the depth of the layer under compression, the stiffer the beam. The two-layer RC beam has an equal loading carrying capacity as the beam made entirely of higher grade. The depth of the layer of RC beam under tension in two-layer beams should be kept between 40 and 50% of the overall beam depth, which would be desirable structurally.