Long term behavior of hygrothermaly conditioned concrete-filled fiber-reinforced polymetric tubes subjected to axial loads

Abstract

This study evaluated the behavior of concrete filled FRP tubes (CFFT) subjected to moisture absorption condition. Accelerated aging by means of hygrothermal process was conducted to predict long-term mechanical behavior of FRP material. FRP coupons were submerged in water at three different temperatures: 30°C, 40°C and 50°C. The CFFT and the concrete cylinders were submerged in water only at 30°C. FRP was characterized by means of following tests: axial tension, axial compression, and hoop tension. The CFFT and concrete cylinders were tested under compression load only. FRP coupons, concrete cylinders and the CFFT were tested for the following exposure times: 0 (unaged), 30, 100, 300, and 500 days. Strength degradation of aged FRP, concrete and the CFFT was detected over the exposure period. Results for samples of FRP aged at 40oC and 50oC showed reductions in strength with respect of time between 20% to 30%. In contrast, samples of FRP aged at 30oC showed a less reduction in strength with respect of time of approximately 15%. Based on the Arrhenius assumption and the methodology developed by Barbero and Damiani, predictions were made for residual strength of the FRP after 1000 days of aging. Arrhenius methodology predicts an axial compression strength reduction of 22% and a hoop tensile strength reduction of 19% after 1000 days of aging in water at 30°C. Similarly, Barbero and Damiani methodology predicts a compressive strength loss of 25% for the FRP. In order to predict the short and long-term response of concrete filled FRP tubes under axial compression load, 2D and 3D non-linear finite element models (FEM) were implemented. A surface contact was established between the concrete core and the FRP tube in both FE models to account for the possibility of separation. The non-linear concrete behavior was modeled using Drucker-Prager plasticity. The 3D FEM showed a better prediction with respect to the 2D model when compared with the experimental data. The FEM results for unaged specimens were obtained from a parametric study to evaluate the sensitivity of the response to the effect of the dilation angles. This procedure consists in using several dilation angles ( for the given friction angle in order to obtain the best axial and lateral response. The responses using three different dilation angles (δ) were practically the same for the 2D FEM, having all curves one on top of each other. The best friction angle (ß) and dilation angle (δ) combination found using the 3D FEM was 53° and 5° for the axial and lateral response. Similarly, the FEM results for concrete filled FRP tubes after being submerged for 500 days in water were obtained. For this case the best friction angle (ß) and dilation angle (δ) combination found was 53° and 50° respectively. Additionally, close form solution models were evaluated, using the stress-strain model developed by Fam and Rizkalla for concrete filled FRP tubes. The model was modified to include the effect of the confining effectiveness, allowing its application not only to FRP tubes but also to FRP jackets.