Issue 49

Y. Liu et alii, Frattura ed Integrità Strutturale, 49 (2019) 714-724; DOI: 10.3221/IGF-ESIS.49.64 722 There were a few initial micro-cracks and pores in the concrete, which were produced during the non-uniform shrinkage of coarse aggregate. These pores made CO 2 penetrate the concrete and cause the carbonization damages of the concrete. When the reinforced concrete was subject to the bending load, cracks first appeared in the tension zone as the load increased. Meanwhile, the steel bar began to bear part of the tension force. CO 2 mainly entered and eroded the inner part of the tension-zone of the concrete component along these cracks. With the increase of the bending load stress, the neutral axis of the block began to move towards the compression zone, and cracks appeared in the tension zone of the concrete. As the load further increased, the cracks became wider. This accelerated the erosion of the concrete by CO 2 and thus increased the carbonization rate. There were still cracks below the neutral axis, but the tension force in the tension- zone is mainly borne by the steel bars. At this time, the compression stress in the compression zone also increased. With the further increase of pressure, the cracks and pores in the concrete were closed, hindering the diffusion of CO 2 into the concrete and inhibiting the carbonization of concrete. However, the development of micro-cracks was very weak, and there was no macro change on the surface of the concrete. Therefore, the carbonization depth of concrete decreased as the load increased, but the change was relatively gentle. As the bending stress further increased, the cracks in the tension-zone developed faster. As shown in Fig. 6, the slope of the curve and the carbonization damage to the concrete tension zone increased significantly under 40%-60% of the ultimate bending load. Meanwhile, under this load, an obvious crack was found in the middle span of the test block. This significant crack damage in the tension zone aggravated the diffusion of CO 2 into the concrete to a greater extent and made the carbonization more serious. In addition, the curve declined more significantly under 40%-60% of the ultimate bending load, as shown in Fig. 5. When the strain in the compression zone increased, the concrete in this zone became more compact. However, when the load exceeded a threshold, the tension steel bar began to yield, the cracks in the tension-zone expanded and went upwards, and the curve of the carbonization depth in this area increased more sharply. The rapid development of cracks led to the crush of the concrete in the compression zone. This made the CO 2 erosion more rapid and aggravated the carbonization damage of the concrete. To study the relationship between the bending-compression load and the bending-tension load with respect to their effects on the carbonization damage of reinforced concrete blocks, the ratio between the carbonization depth of the bending-compression zone and that of the bending-tension zone in the same block was fitted with the bending stress level, but no significant correlation was found. However, a more stable and accurate relationship between these two parameters was noticed, so the carbonization influence coefficient of bending tension-compression load is defined as follows: ( ) t t c c X R Ln X   (4) where R t-c is the carbonization influence coefficient of bending tension-compression load; X t the carbonization depth of concrete under the bending-tension load; and X c , the carbonization depth of concrete under the bending-compression load. The relationship between the carbonization influence coefficient of bending tension-compression load K and the bending load stress level S of reinforced concrete with different fly ash contents is shown in Tab. 6, where R t-c is the carbonization influence coefficient of bending tension-compression load and S is the bending load stress level. Fly ash content Fitting equation Correlation coefficient R 0% 2 2.6657 4.6161 3.3387 t c R S S     1.000 20% 2 2.0945 4.155 3.3189 t c R S S     0.991 30% 2 0.5715 4.1445 3.3625 t c R S S     0.998 40% 2 2.0666 5.2414 3.606 t c R S S     0.988 Table 6: Equation of the carbonization influence coefficient of bending tension-compression load As shown in Tab. 6, there is a quadratic polynomial relationship between the carbonization influence coefficient of bending tension-compression load and the bending load stress level. In actual concrete engineering, some parts of the bending members are difficult to measure. By using the carbonization influence coefficient of bending tension-