Behaviour of cracked reinforced concrete beams under repeated and sustained load types

This paper compares the behaviour of reinforced concrete beams subjected to sustained and repeated load types in bending. Mid-span deflections and surface strain profiles were monitored over an 80 day period so that comparisons could be made between the structural response of RC (reinforced concrete) beams to the different load types tested. The load level at which beams under a sustained load were held represented the mean value of the upper and lower load of the repeated load types. The two different repeated load amplitudes investigated corresponded to 24 and 12% with respect to the mean load. In addition, beams were subjected to two different loading frequencies throughout testing (0.2Hz and 1Hz). Measured surface strains were considerably higher in both the tension and compression zones of beams subjected to repeated load types. This was thought to be primarily due to the effects of cyclic creep in the compression zone and a higher degree of cracking within the tension zone. The progressive long-term increase in deflection is shown to be a result of strain development primarily in the compression zone. In contrast, the strain increase at tensile reinforcement level after 10 days under load is limited, regardless of the applied load type. The repeated load types are shown to affect the section stiffness most significantly within the initial period of loading (0-10 days). Results also show that the beam’s response is sensitive to increases in both the loading frequency and load amplitude. This study aims to highlight the importance of considering not only the current loading conditions, but also the load history of a reinforced concrete section (particularly any temporary load peaks, which would lead to a permanent deterioration of section stiffness,) when attempting to determine the current state of an RC section. Using experimental results, the validity of Expressions 7.19 and 7.9 of Eurocode 2 (EC2) are examined. The additional damage within the tension zone caused by the repeated load types is not accounted for in either expression, meaning that the tension stiffening effect is overestimated. Consequences of this are that design code SLS (serviceability limit state) checks for beams with complex load histories may be inaccurate, and that the average steel stress within the reinforced concrete sections may be underestimated.