Experimental Study On Bond Behavior Between Geopolymer Concrete And Steel Bars

The bonding effect between reinforcement and concrete is the fundamental of the two kinds of material working together. Serious consequence will be caused once bond failure occurred between steel bars and concrete. Geopolymer concrete is regarded as a kind of promising construction material, due to its environmental-friendliness and excellent mechanical properties. Although there are a lot of researches on the basic mechanical properties of geopolymer concrete in recent years, there are few studies on the bond behavior between steel bars and geopolymer concrete, especially on the high temperature bond behavior of geopolymer concrete. To investigate the bond behavior between steel bars and geopolymer concrete at ambient temperature and after exposure to elevated temperatures, pull-out tests and beam tests were conducted on steel bar-geopolymer concrete composite specimens in this paper. The main research contents and conclusions are listed as follows:1. Pull-out tests were carried out on 24 geopolymer concrete prisms with steel bars embedded. The bond mechanism was analyzed,and the effect of compressive and split tensile strength of geopolymer concrete, steel bar type, steel bar diameter, concrete cover and steel bar anchorage length on bond properties between steel bars and geopolymer concrete was explored. Also a comparison on the bond behavior of steel bars with geopolymer concrete and ordinary cement concrete(OPC) was conducted. The test results show that, for geopolymer concrete specimens with steel bar anchorage length of 5d(d is the diameter of steel bar), splitting failure of geopolymer concrete tends to occur if the relative cover thickness(c/d) is lower than 3.67. An anchorage length of 9d is long enough for specimens with steel bar diameter of 14 mm, to make the bar yield before the bar was pulled out. Moreover, the bond strength of steel bars with geopolymer concrete is not lower than that of OPC with same strength grade. Based on the test results, a bond-slip constitutive model between deformed bars and geopolymer concrete was established. And the calculated bond-slip curves by using this model are close to that obtained from pull-out tests.2. Pull-out tests were conducted on 75 geopolymer concrete specimens and 15 ordinary cement concrete specimens with steel bars embeded after exposure to elevated temperatures. The influence of steel bar type, steel bar diameter on bond properties between steel bars and geopolymer concrete after exposure to elevated temperatures was analyzed. The experimental results indicate that no significant degradation occurred on bond strength between geopolymer concrete and steel bars when exposure temperature is lower than 300℃. However, when temperature exceeds 300℃, bond strength degrades quickly, and even at a higher rate than that of compressive strength of geopolymer concrete. After exposure to elevated temperatures, geopolymer concrete also exhibits comparable bond strength with the same strength grade OPC. Based on the test results, a regression model on bond strength degradation of geopolymer concrete specimens, as a function of exposure temperatures, was developed. The calculated curves using the regression model on bond strength-temperature relationships shows a good agreement with that from test results.3. Ten geopolymer concrete beam specimens and two OPC beam specimens with steel bars embedded were tested on static load. The influence of stirrup spacing and steel bar anchorage length on bond properties between geopolymer concrete and steel bars was studied. A comparison between bond behavior of geopolymer concrete and OPC in a real mechanical state, and a comparison between beam tests and pull-out tests were carried out. The test results show that a decrease of stirrup spacing improves the ductility of specimens greatly, but has no significant effect on bond strength, when the stirrup spacing is larger than steel bar anchorage length. For the beam specimens with a bar diameter of 18 mm, an anchorage length of 7d is long enough to make the bar yield before bond failure. Under the similar conditions, bond strength between geopolymer concrete and steel bars obtained from beam tests is higher than that from pull-out tests.