# A Durability Prediction Method for Historical Square Rebar Reinforced Concrete Buildings
## 4 The Durability Prediction Calculation Methods for Square Rebar Components
### Corrosion-Induced Cover Cracking Experiments for Square Rebar Component  
The corrosion induced cover cracking experiment was performed using accelerated square rebar corrosion, and the experiment included ten specimens. The experimental square rebars were collected from abandoned components of historical RC buildings in China, and the age of the experimental square rebars are comparable to the age of the analyzed structures. The rust of the square rebars was removed before the experiment. Figure 5 shows the vintage square rebars. Specimens with dimensions of 150 mm \(\times\) 200 mm \(\times\) 150 mm and their schematic drawings are shown in Figure 6. According to the historical literature [31; 32; 33], the ratios of content and volume were Cement:Sand:Stone (Coarse aggregate) = 1:2:4, and water accounted for 10-13% weight of all dry contents. The average compressive strength (three specimens) of the standard 150 mm cube was 15.6 MPa after 28 days of curing in the standard condition. The results obtained were qualified as the compressive strength of concrete in history [31; 32; 33]. The detailed information used in the experiment is listed in Table 2. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**Items**} \\ \cline{2-7}
**1 Cases** & \multicolumn{2}{c}{**2 Concrete Average**} & \multicolumn{2}{c}{**2 Average Thickness of**} & \multicolumn{2}{c}{**2 Concrete Average Measured**} \\  & \multicolumn{2}{c}{**Compressive Strength/MPa**} & \multicolumn{2}{c}{**Concrete Cover/mm**} & \multicolumn{2}{c}{**Carbonation Depth/mm**} \\ \cline{2-7}  & **Column** & **Beam** & **Column** & **Beam** & **Column** & **Beam** \\ \hline Main Hall of the Tomb of Yu (1933) & 15.7 & 15.4 & 40 & 41 & 33 & 40 \\ \hline No.1 Building of Zhongshan East & & & & & & \\ Road (1935) & 12.4 & 11.7 & 34 & 27 & 62 & 65 \\ \hline Dahua Cinema (1931) & 15.6 & 17.8 & 33 & 34 & 63 & 40 \\ \hline Dacheng Factory (1935) & 17.0 & 22.9 & 35 & 34 & 45 & 40 \\ \hline Main Hall of Nanjing Museum & & & & & & \\ (1937) & 18.0 & 23.2 & 34 & 35 & 43 & 49 \\ \hline Huangpu Hall of Jiangsu & & & & & & \\ Conference Center (1931) & 14.7 & 17.6 & 35 & 33 & 45 & 54 \\ \hline Former site of overseas Chinese & & & & & & \\ Hostel (1933) & 16.5 & 14.8 & 30 & 28 & 58 & 62 \\ \hline Lingyuan Post Office (1947) & 14.0 & 17.4 & 40 & 38 & 55 & 45 \\ \hline Former site of National Art & & & & & & \\ Museum (1925) & 14.4 & 17.1 & 35 & 33 & 60 & 71 \\ \hline Former site of the Ordnance Special & & & & & & \\ School of the National & - & 17.2 & - & 25 & - & 55 \\ Revolutionary Army (1932) & & & & & & \\ \hline Former site of Young Men’s & & & & & & \\ Christian Association (1946) & 10.4 & - & 38 & - & 47 & - \\ \hline Former site of Central Radio & & & & & & \\ Equipment Co., Ltd. (1946) & - & 12.2 & - & 32 & - & 40 \\ \hline \hline \end{tabular} \({}^{1}\) The figure in bracket is the time of the completion of the building. \({}^{2}\) Each of the average data was obtained by testing 6–10 structural components, for both beam and column cases respectively.  
\end{table} 
Table 2: The measured concrete material properties of some typical historical square rebar reinforced concrete buildings. Many studies have adopted the electrochemical acceleration of rebar corrosion to obtain the critical corrosion depth of the rebar simultaneously when cover cracking occurs. The direct current was used to accelerate the corrosion of the square rebars, and the current value was 100 \(\mu\)A/cm\({}^{2}\)[3,5,7]. The electrolyte solution consisted of a 5% NaCl solution [5,7], and half the height of the specimens was immersed in the solution upon the wooden heel blocks. After the electrical wires were welded on the reinforcement, the side surfaces and the external rebars were coated with approximately 2 mm epoxy resin. After the solidification of the epoxy resin and before the impression of the direct current, the test specimens were immersed in the solution for 72 h, to ensure that the specimens were in the wet state before electrification.