دانلود رایگان مقاله کاهش تحکیم تیرهای فلنجد ریخته گری با بتن خودمتراکم سبک

Abstract

Prestress loss estimation is a necessary and important procedure in design of pretensioned concrete girders. The current specifications were developed from the experimental results of normal-weight concrete that are possibly inaccurate in estimating prestress losses for members cast with lightweight self-consolidating concrete (SCC). This study measures prestress losses for two full-scale double-tee girders cast with sand-lightweight SCC. Expanded clay, which had a specific gravity of 1.25 and an absorption capacity of 15%, was used as the lightweight coarse aggregate for the designed concrete mixture. The prestress losses were measured for 26 days and at 83 days using vibrating wire strain gauges attached to prestressing strands, after which the tested girders were then used in the construction of a parking garage. The experimental results indicated that the modulus of elasticity of lightweight SCC can be predicted using a correction factor of 0.99. The measured elastic-shortening loss was slightly lower than the predicted values. The predicted time-dependent losses, however, significantly over-estimated the measured results, which yielded the over-estimation of total prestress losses that varied from 86% to 153%.

7. Recommendation

The experimental results of this study confirm the over-estimation of current AASHTO specifications for the time-dependent losses of pretensioned members using sand-lightweight concrete. In addition, the predicted prestress losses are almost two times greater than the measured values. In this study, the experimental results are not adequate to propose new coefficients for enhancing the accuracy of existing methods. Therefore, more research is needed to evaluate prestress losses for pretensioned concrete girders cast with lightweight concrete. 1. The measurement of prestress losses for full-scale pretensioned concrete girders, which are cast with normal-strength and highstrength lightweight SCC, would provide useful experimental results for evaluating the adequacy of current AASHTO specifi- cations. The experimental data can be additionally employed to revise the existing coefficients used in estimating time-dependent losses to enhance the accuracy in predicting the total losses. 2. Lightweight aggregates significantly affect the concrete properties and consequently influence the prestress losses of pretensioned concrete girders. Future research should develop concrete mixtures that contain various types of lightweight coarse aggregates, which may include expanded shales, clays, or slates. It is necessary to develop particular models to predict creep and shrinkage of lightweight SCC. The proposed models can be calibrated and validated that based on the prestress losses measured on pretensioned concrete girders. 3. A statistical study, which synthesizes experimental results collected in the literature, would be a useful tool to develop an extended specification for estimating prestress losses of pretensioned concrete girders cast with lightweight SCC.