Bond performance prediction model of defective grout in post-fire sleeve connections under cyclic loading

Abstract

Sleeve grouting defects are one of the most common issues for the prefab structure and may cause the significant safety hazards for the assembled buildings. In seismic zones, especially for the defective assembled buildings have experienced fires, the post-fire seismic performance of connection joints needs to be adequately assessed to guide post-disaster rehabilitation work. To this end, the prediction model of a bonding performance of the defective grout in sleeve under cycling loading after a fire is essential. In this paper, the defective grouted sleeve connection specimens were fabricated and studied under cycling loading tests after the standard fires with consideration of the effect of the protective layer of concrete. The results show that the defect-induced instability in the sleeve restraint system is a significant cause of the grout bonding failure after fires. With a defect rate of 10%, the qualified fire resistance time for the seismic zone joints under the instable condition of restraint system was shortened by at least 30 min compared to the stable restraint system, and the qualified fire resistance temperature was reduced by at least 240 ℃, accordingly. Once the defect rate is not less than 30% and the fire time is less than 30 min, no matter what the defect type of the grout sleeves is, the code requirements for seismic zones would not be applicable and some certain implementations should be taken to remedy the situation. Furthermore, the degradation of ultimate bond strength of grout under repeated post-fire loading shows exponential relationship with the defect rate, while the bond stiffness decreases linearly with defect rate. Under the cyclic loading condition, the influence of high temperature and defect on the degradation of the grouting material is not simply combined; while, the high temperature and defect will be coupled to intensify the degradation of the grouting material. A unified prediction model for the bond strength of defective grouting material under cycling loading at the ambient temperature and after fire was developed. The predicted results agreed with the experimental results; and the prediction model can provide the guidance for the performance assessment of defective grouted sleeve joints at ambient temperature and after fire in seismic zones.