تیرهای Tشکل به هم پیوسته RC آسیب دیده در اثر آتش سوزی و تقویت شده با ورق های CFRP
Abstract
1- Introduction
2- Experimental programme
3- Observations during fire exposure
4- Discussion of results and analysis
5- Conclusions
References
Abstract
This paper presents experimental results of the structural performance of fire-damaged continuous reinforced concrete (RC) T-beams subsequently strengthened with externally bonded carbon fiber reinforced polymer (EB-CFRP) sheets. A series of seven specimens were tested with different fire exposure time and subsequent strengthening techniques. Experimental results showed that both the unexposed control beams and unstrengthened fire-damaged continuous RC T-beams exhibited flexure failure modes and significant redistribution of moment between hogging and sagging regions. The fire-exposed beams had notably degraded strength and stiffness attributable to the internal temperatures exceeding 500 °C for some portion of the exposure. The EB-CFRP strengthening of fire damaged beams was shown to mostly mitigate the effects of fire exposure. The EB-CFRP retrofit measures successfully restored the virgin capacity of the beam and were sufficient to also restore most of the lost initial stiffness. Simple prediction using plane sections analysis and the assumptions of the 500 °C isotherm method [1] were shown to accurately predict the behaviour of the fire-damaged specimens.
Introduction
Elevated temperatures associated with fire exposure cause severe damage to reinforced concrete (RC) structures, resulting in loss of strength and stiffness and the development of relatively large permanent deformations during and following exposure. The behaviours are attributed to the degradation of mechanical properties of both concrete and steel reinforcement and the redistribution of stresses within the beam after fire exposure [2]. Externally bonded carbon fibre reinforced polymer (EB-CFRP) composites are one proposed technique for repairing fire-damaged RC beams. In addition to their acknowledged advantages of having a high strength to weight ratio and good durability [3], EB-CFRP sheets are also easily applied to structures having varying geometry. There exist a number of guide standards for design and installation of EB-CFRP for RC structures – perhaps the most commonly cited is ACI 440.2R-17 [4] which will be referred to in this paper. Countless studies have investigated the flexural behaviour of RC beams strengthened with EB-CFRP sheets under ambient conditions. Results show that EB-CFRP sheets are able to enhance the flexural capacity of beams but decrease the available ductility compared with unstrengthened control beams. Both effects result from the increased effective longitudinal reinforcing ratio afforded by the EB-CFRP. The majority of study on EB-CFRP as a flexural retrofit measure has been conducted on simple span beams and slabs. In experimental studies of continuous RC beams, improved flexural capacity as a result of the presence of EB-CFRP is reported. However, in continuous beams having discrete EB-CFRP applied in regions of high flexural stress, a primary observed mode of failure is the brittle peeling of the concrete cover at the terminations of the CFRP sheets (so-called “end peel” failure) [5,6]. This failure mode, while simple to mitigate in determinate structures [4], is difficult to address in continuous structures in which there is a stress reversal at the girder faces, the point of inflection is not fixed, and moment redistribution is likely. Aiello et al. [7,8] investigated the ability of RC continuous beams strengthened with CFRP to redistribute moment and found that the percentage of moment redistribution of the analysed beams reaches 20%. ACI 440.2R17, for instance, permits redistribution up to 20% but also cautions that the redistribution that can be relied upon falls with increasing EB-CFRP reinforcement. The adoption of hybrid (carbon/glass) FRP sheets has been shown to improve the ductility that may be achieved in FRP-repaired continuous RC beams [9].