Effect of Elevated Temperature on Bond Strength in Different Types of Reinforced Concrete

Abstract

Reinforced concrete structures are exposed to different levels of temperatures due to fire accidents, explosions, etc. These cases require evaluations for all mechanical properties which are affected by high temperatures. One of the most important properties in reinforced concrete is bond strength between steel reinforcement and concrete which is affected seriously by high temperatures specially when embedded in certain types of concrete. The main purpose of the this research is to evaluate residual bond strength for steel reinforcement when embedded in different types of concrete (light weight concrete and high strength concrete) after exposing it to different levels of temperature. The experimental program consists of fabrication and testing of 120 pull-out cylinder specimens (60 pull-out specimens of HSC and 60 pull-out specimens of LWAC), where every 60 pull-out specimens are classified into four groups with three variables: compressive strength, steel bar diameter and the effect of of type cooling (i.e. air or water), under four levels of temperature (150, 250, 400, 500Co) and at the room temperature. After casting and curing, the test is done after 28 days, the pull-out specimens are tested in a specially fabricated frame. In addition to the pull-out specimens, 120 cylinders were tested in compression (60 cylinders of HSC and 60 cylinders of LWAC) and other 120 cylinders were tested in tension (60 cylinders of HSC and 60 cylinders of LWAC). The compressive strength results are compared with the CEB strength reduction curve and CEN one, it was noticed that these results were in better agreement with the CEN design curve than the CEB one. The test results for all types of concrete showed that bond deteriorates at high temperatures. The residual bond strength-temperature relationship was similar to that of concrete compressive strength-temperature relationship. The results show that the bond strength increased with increasing compressive strength of unheated specimens, but for heated specimens the residual bond strength decreased with increase of compressive strength , where the residual bond strength at 500Co of nominal compressive strengths 60 and 70MPa were about 54% and 52% respectively for HSC. For the LWAC, the residual bond strength at 500Co of nominal compressive strengths 25 and 30MPa were about 65% and 63% respectively. The result shows also that the bond strength decreases with the increase in diameter of bar. Another result shows, that cooling in water after heating causes more reduction in strength than cooling in air which is about 21% for HSC and 12% for LWAC at temperature of 500Co. The test results were compared with previous study on (NWC). They show, that the bond strength at temperature 500Co for LWAC is better than NWC and HSC of about13% and 18% respectively at temperature 500Co, and bond strength at elevated temperature for NWC is better than HSC of about 10% at temperature 500Co. An equation for calculation of percentage of the residual bond strength has been proposed for temperature range of (28-500Co) based on linear regression analysis with R equal to 0.86and 0.72 for HSC and LWAC, respectively. The computed results are compared with experimental data and with other investigations. Another equation is proposed to estimate the bond and residual bond strength as a function of parameters used in the study at temperature range of (28-500Co) with R equals to 0.98 and 0.99 of HSC and LWAC, respectively.