Issue 49

Y. Liu et alii, Frattura ed Integrità Strutturale, 49 (2019) 714-724; DOI: 10.3221/IGF-ESIS.49.64 718 the bottom. This suggests that the compactness at the top is worse than that at the bottom. Through measurement of the electric flux at the tops and bottoms of other test blocks, it was also noticed that the electric flux at the top was much larger than that at the bottom [26], further proving the widespread phenomenon of higher permeability at the tops of the concrete blocks after setting and hardening. As a result, CO 2 can more easily and rapidly enter and erode the inner part of the block from the top. Furthermore, the carbonization depth at the top was greater than that at the bottom, so the coefficient K was greater than 1. However, when the carbonization depth reached a certain value, the closer the central carbonization zone of the cross section was, the smaller the difference in the concrete compactness would be. Thus, the carbonization depths at the top and bottom would be closer, making the curve change more gently. Fly ash content Coefficient Carbonization depth/ mm 7d 14d 28d 0 X p 4.74 5.48 7.94 X b 4.50 5.34 7.93 K 1.05 1.03 1.00 20% X p 7.13 8.71 9.58 X b 5.40 7.20 8.26 K 1.32 1.21 1.16 30% X p 7.61 8.76 9.98 X b 5.90 7.30 8.60 K 1.29 1.20 1.16 40% X p 8.00 9.25 10.63 X b 6.20 7.84 9.75 K 1.29 1.18 1.09 Table 2: Carbonization depths of the pouring and bottom surfaces of concrete/mm Figure 3: Curve of the carbonization influence coefficient K of the pouring surface at different fly ash contents Figure 4: Comparison between the top and bottom surfaces of the concrete test blocks As shown in Fig. 3, the K values of the blocks without fly ash were always lower than those of the blocks with fly ash. When the blocks are vibrated, the fluidity and segregation of the cement paste will increase under the action of shaking