The effect of environmental loading on restrained reinforced concrete T-Beams

Abstract

Environmental conditions have the potential to cause stresses within reinforced concrete structures which equal or exceed the stresses caused by typical design dead or live loading. Concrete bridges are particularly susceptible to these effects as result of exposure to harsh environments throughout construction and service periods. The stresses caused by environmental loading are influenced by the material composition as well as the geometric properties of the structural element. Consequently, the selected type of cross-section used in bridge construction will influence the bridge’s response to environmental conditions. The main objective of this study was to compare the response of two scale model T-beams with different cross-sections to environmental loading, with specific focus on temperature and shrinkage effects independently and in combination. The T-beams were designed to be solid and voided to emulate the configurations of solid spine beam and box girder bridge cross sections, respectively. The scale size of the T-beams had two major advantages. One advantage was the practicality of constructing moulds and casting smaller sections compared to full scale sections. The second advantage was the ability to analyse the response of beams, commonly employed in construction and of similar size, to environmental loading. This expands the scope of considerations beyond bridge design applications.

The stress profiles through the T-beams were of particular interest since they provided clear insights into the conditions which caused tensile stresses which exceeded the tensile capacity of the concrete and should therefore be considered within design. In order to calculate the stress profiles in both vertical and transverse directions, the temperatures and strains throughout the T-beams had to be recorded. Thermocouples and Vibrating Wire Strain Gauges were embedded into the concrete and measurements of temperature and strain were logged at 15min intervals, from the time of casting for the duration of the study. To allow accurate strain measurement, the T-beams were designed and cast in a manner which allowed unrestricted free movement as far as possible. This was achieved by minimising the constraints imposed by the supports. Typical daily thermal stresses were considered and factors such as heat of hydration and drying shrinkage over time were analysed to identify conditions which caused tensile stresses greater than the tensile strength of the concrete. This study showed that cross-section type and restraint conditions had significant effects on T beam behaviour in response to environmental conditions. The solid T-beam developed smaller longitudinal stresses caused by shrinkage and temperature but highlighted the need to consider the stress differentials in the transverse direction as well as the web to flange thickness ratio. The voided T-beam was more prone to curvature caused by temperature, shrinkage and swell, but developed more uniform stresses in the transverse direction of the section. Furthermore, the largest stress component was caused by the restraint of average longitudinal elongation and contraction of both T-beams, which emphasized the need to consider the timing at which restraint is applied to concrete elements.