To meet the needs of rehabilitation or strengthening of existing historic concrete buildings and bridges, 12 low strength reinforced concrete beams, with the cubic compressive strength of concrete less than 15 MPa, were strengthened by gluing carbon fiber composite sheets at the bottom of the beams, and monotonic loading tests for strengthened beams were conducted. Test results show that the strengthened beams without additional anchorages failed due to debonding, plane cross sections cannot always remian plane because the strain of carbon fiber sheets became larger than that of concrete at the bottom of the beam after yielding of the rebar, with the help of U type strips, not only the ultimate bearing capacity but also the deformation ability of strengthened beams can be improved significantly, the flexural ultimate bearing capacity of a strengthened beam may decrease significantly if the sustaining load on the beam before strengthening exceeds 40% of its bearing capacity. According to the failure modes of strengthened beams, a brace-arch model based method was proposed to calculate the ultimate flexural bearing capacity of strengthened RC beams. Comparison between calculation and the test results verified that the influence of debonding of longitudinal carbon fiber sheets, the influences of additional anchoring of U type carbon fiber strips and the influence of sustaining load on the ultimate bearing capacity of strengthened beams can be taken into account by using the proposed method.%针对既有历史建筑和中小型桥梁维修改造工程的实际需要，采用梁底粘贴单向碳纤维布的方法对12根低强度等级（混凝土立方体抗压强度低于15MPa）混凝土梁进行加固，并对加固梁进行单调加载试验。结果显示，由于混凝土强度较低，加固梁易发生剥离破坏，纵筋屈服后梁底纤维应变大于梁底混凝土应变，平截面假定无法保持；设置U形压条既能提高加固梁的承载力，又能改善加固梁的变形性能；当加固前梁上持荷值超过未加固梁承载力40％时，加固效果会明显降低。根据试验梁的破坏特征，提出基于拉杆一拱模型的加固梁抗弯承载力的计算方法。理论计算和试验结果比较表明所提方法能够全面考虑纤维剥离、U形压条以及持荷对加固梁抗弯承载力的影响。
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