## 3 Experimental Results and Discussions
### The Calculation Formula of Recycled Concrete Hollow Block Masonry Strength  
It was significant to research the formula for calculating the compressive strength of RAC hollow block masonry for its application. It can be found from the above results that the evolution law and internal mechanism of compressive strength were similar between NAC and RAC hollow block masonry. The calculation formula of NAC block masonry applies to this study [28], as follows:  
\[f_{\mathrm{m}}=k_{1}f_{1}^{\alpha}(1+0.07f_{2})k_{2} \tag{1}\]  
where \(f_{\mathrm{m}}\) was the calculated value of masonry compressive strength; \(f_{1}\) and \(f_{2}\) were the average compressive strength of block and masonry respectively; \(\alpha\) and \(k_{1}\) were the effect coefficients of block shape, size, masonry method, and other factors of different types of masonry; \(\alpha\) and \(k_{1}\) were equal to 0.9 and 0.46 for concrete block; \(k_{2}\) was the influence coefficient of mortar strength on the compressive strength of masonry, and \(k_{2}\) was 1.0 in this study. \(\alpha\) was still 0.9 in the study due to there being the same size between NAC and RAC blocks. Regression analysis was carried out on the strength test value of RAC block masonry, and \(k_{1}\) = 0.538 was obtained, which was higher than 0.46 in the specification. [PERSON] et al. [29] reported similar results. For safety reasons, the parameter values in the specification should be adopted because the strength of RAC blocks was discrete relatively. The results were calculated according to formula 1 and are listed in Table 5.  
It can be found that the measured value of RAC block masonry compressive strength was higher than the calculated value by formula in specification. The ratio of the measured value to the calculated value was concentrated between 1.1 and 1.2, which can be indicated by the formula for the compressive strength of NAC block masonry being suitable for RAC block masonry. The standard value and design value of material strength were significant in the design of masonry structure. The probability distribution of material strength should conform to the normal distribution,  
Figure 8: Effect of mortar strength on the compressive strength of RAC blocks masonry. and the standard value of material strength could be determined by the 0.05 quantile of probability distribution. According to the literature [28; 30], the standard value \(f_{\rm k}\) of its compressive strength can be calculated as follows:  
\[f_{\rm k}=f_{\rm m}-1.645\alpha=f_{\rm m}(1-1.645\delta) \tag{2}\]  
where \(\alpha\) was the standard deviation of compressive strength, \(\delta\) was the variation coefficient of compressive strength, and the value of \(\delta\) was 0.21 (the average variation coefficient of specimen with 50% recycled aggregate substitution rate) in this paper. The ratio of the standard value to the component coefficient was defined as the design value of material performance. The strength design value \(f\) of RAC block masonry structure was the strength representative value adopted by masonry structural members when they were designed according to the ultimate state of bearing capacity. Considering the influence of geometric parameter variation, calculation mode uncertainty, and other factors on reliability, the calculation formula of \(f\) is as follows:  
\[f=\frac{f_{\rm k}}{\gamma_{f}} \tag{3}\]  
where \(\gamma_{f}\) was the partial factor for material performance of RAC block masonry structure, and \(\gamma_{f}\) was taken as 1.612 when the construction quality was considered to be B. The construction quality was difficult to guarantee due to the RAC block masonry being prepared by RAs with high water absorption and porosity. It was necessary to increase the \(\gamma_{f}\) to 1.7 in order to ensure a reliable strength design value.