The behaviour of reinforced concrete beams under cyclic temperature profiles

Abstract

On a hot summer day, high levels of solar radiation are incident on concrete bridge decks, causing non-linear variations in temperature through the depths of the structures. The changes in temperature lead to axial expansion and curvature of decks. In addition, self-equilibrating stresses arise from the non-linearity of the temperature profiles. In continuous structures, where axial expansion and curvature may be restrained, secondary thermal stresses are induced also. Tests are described in which reinforced concrete beams were subjected to daily cycles of heating. Applied temperature profiles were similar to maximum positive profiles for the United Kingdom. It was found that a linear elastic analysis, based on the uncracked section, generally gave an adequate prediction of thermal strains of non-loaded and loaded single span beams during heating cycles. Changes in curvature, during heating, were substantially modified in the early stages of loading, due to the existence of creep curvatures. Two span beams were tested also, with special regard to changes in support reactions during heating. No evidence was found in any of the tests of increases in the extent of cracking as a result of heating. Daily heating cycles were imposed over several weeks and evidence was found of a steady increase in the coefficient of thermal expansion, attributed to drying. Changes in curvature resulting from non-uniform shrinkage were observed, these being greatest in non-heated beams. Creep curvatures were found to increase roughly in proportion to the time-averaged beam temperature. Creep results were compared with prediction methods, including an incremental rate of creep analysis. Additional tests are described in which variations in moisture content and in the coefficient of thermal expansion within members were identified, using a segmentation technique.