In engineering, reinforced concrete beams are subjected to natural erosion and the working stress state. However, previous studies regarding structural fire have focused on new concrete beams, and most results are obtained from no-stress-state tests. Hence, research that considers the natural erosion and working stress states of beam members in fire is lacking. In this study, fire experiments were performed to investigate the differences between long-term service concrete beams and new elements under loads. A technique using ARAMIS three-dimensional (3D) optical strain measurement was adopted; it provides increased sample throughput and 3D measurements that indicate the change in strain more intuitively. Results show that the moisture content of concrete determines the spalling degree of concrete under the influence of working stress. With the increase of concrete moisture content, the range of temperature of heated and backfire surface increases and the spalling phenomenon is more obvious, especially when the moisture content exceeds 2.5%. In addition, the long-term service specimens are affected by fire more easily, thereby accelerating the destruction process owing to the degree of carbonation.

在工程中，钢筋混凝土梁承受自然侵蚀和工作应力状态。然而，以前关于结构火灾的研究主要集中在新的混凝土梁上，并且大多数结果都是从无应力状态测试中获得的。因此，缺乏考虑梁构件在火灾中的自然侵蚀和工作应力状态的研究。在这项研究中，进行了火灾实验以研究长期使用的混凝土梁与荷载作用下的新构件之间的差异。采用了使用 ARAMIS 三维 (3D) 光学应变测量的技术；它提供了更高的样品通量和 3D 测量，可以更直观地指示应变的变化。结果表明，混凝土的含水率决定了混凝土在工作应力影响下的剥落程度。随着混凝土含水量的增加，受热面和回火面的温度范围增大，剥落现象更加明显，尤其是当含水量超过2.5%时。此外，长期服役的标本更容易受到火灾的影响，从而因碳化程度而加速破坏过程。