The strengthening intervention of RC structures often involves already cracked concrete. To evaluate the effect of the level of damage prior to the strengthening (pre-cracks) on the behavior of the flexurally strengthened RC slabs with prestressed NSM CFRP laminates, an experimental research was carried out. Two pre-cracking levels of damage were analyzed and, for each one, three levels of prestress were tested (0%, 20% and 40%). The obtained results showed that the strengthening of damaged RC slabs with prestressed NSM CFRP laminates results in a significant increase on the load carrying capacity at serviceability limit states. Pre-cracked RC slabs strengthened with prestressed NSM CFRP laminates presented a load carrying capacity almost similar to the corresponding uncracked strengthened slabs. To determine the effective prestress level in CFRP laminates, the variation of strain over the length of the CFRP and over time was experimentally recorded. The prestress transfer length was also evaluated. The experimental results revealed that the transfer length of CFRP laminates was less than 150 mm, and the maximum value of strain loss out of transfer length (around 14%) was measured close to the cracked section of the damaged RC slabs. Significant part of strain loss in CFRP laminates occurred during 24 h after releasing the prestress load.

钢筋混凝土结构的加固干预通常涉及已经开裂的混凝土。为了评估加固前的损坏程度（预裂纹）对具有预应力 NSM CFRP 层压板的抗弯加固 RC 板性能的影响，进行了一项实验研究。分析了两种预开裂水平的损坏，并且对每一种都测试了三种预应力水平（0%、20% 和 40%）。获得的结果表明，用预应力 NSM CFRP 层压板加固损坏的 RC 板导致在使用极限状态下的承载能力显着增加。用预应力 NSM CFRP 层压板加固的预开裂 RC 板的承载能力几乎与相应的未开裂加固板相似。为了确定 CFRP 层压板的有效预应力水平，通过实验记录了 CFRP 长度和时间上的应变变化。还评估了预应力传递长度。实验结果表明，CFRP 层合板的传递长度小于 150 mm，并且在接近损坏 RC 板的裂纹部分测量了传递长度之外的应变损失最大值（约 14%）。CFRP 层压板的大部分应变损失发生在释放预应力载荷后的 24 小时内。应变损失的最大值（约 14%）是在靠近损坏 RC 板的裂纹部分测量的。CFRP 层压板的大部分应变损失发生在释放预应力载荷后的 24 小时内。应变损失的最大值（约 14%）是在靠近损坏 RC 板的裂纹部分测量的。CFRP 层压板的大部分应变损失发生在释放预应力载荷后的 24 小时内。