Issue 49

Y. Liu et alii, Frattura ed Integrità Strutturale, 49 (2019) 714-724; DOI: 10.3221/IGF-ESIS.49.64 716 Number Fly ash content Cement (Kg) Fly ash (Kg) Sand (Kg) Stone (Kg) Sand rate Water (Kg) W/B Water-reducer A 0% 373.63 0 682.87 1114.16 38% 179.34 0.48 0.40% B 20% 298.90 74.73 682.87 1114.16 38% 179.34 0.48 0.46% C 30% 261.54 112.09 682.87 1114.16 38% 179.34 0.48 0.52% D 40% 224.18 149.45 682.87 1114.16 38% 179.34 0.48 0.56% Table 1: Concrete mix proportions Experimental method Preparation of test blocks: 100mm×100mm×300mm rectangular concrete test blocks were used in the experiment. The reinforcement diagram of a test block is shown in Fig. 1. Figure 1: Reinforcement diagram of the reinforced concrete block Load device: A long-term bending-load device was developed in the experiment by reference to the other load devices [16-17, 19-24]. As shown in Fig. 2, the device consists of two screws with a diameter of 12mm, eight nuts, four shims and a steel column support. If two torque wrenches are used to twist the nuts at the same time and with the same speed to produce a tightening force, the screw will bear tension, forming pressure on both sides of the block, thereby exerting bending load on the test block. The bending tension load generated on the side of the block away from the support is defined as the tension surface, while the bending compression load generated on the side of the block near the support is called the compression surface. In this experiment, the compression load is directly applied on the pouring surface. The bending load stress of the block can be adjusted by using the torque wrench to apply the torque. This loading strategy is highly reliable, as evidenced by its extensive application in the research of concrete durability under bending stress. Figure 2: Long-term bending-load device Experimental procedure The three-point bend loading test was first performed on the concrete blocks using a universal testing machine. The ultimate bending load F (i.e. the pulling force of the screws at both ends of each block) of the three blocks in each mix ratio were measured separately on the universal testing machine. To ensure the reliability of measured results, the average value was taken as the final F value of reinforced concrete under this mix ratio. The corresponding F was 37.88 kN, 33.16 kN, 34.18 kN and 32.04 kN when the fly ash content of the test blocks was 0%, 20%, 30% and 40%. Each group of test blocks were loaded with 0%, 20%, 40% and 60% of the ultimate bending load based on the formula T = K • F • D [25] ( T is the torque magnitude; K is the torque coefficient, which was set to 0.19; and D is the screw diameter,