رفتار سازه و تجزیه و تحلیل حرارتی ساده شده تیرهای بتنی با مقاومت معمولی و با مقاومت بالا زیر آتش

The objective of this study is to investigate the effects of concrete compressive strength and cover thickness on the structural behavior of reinforced concrete (RC) beams under fire. For this purpose, four normal-strength and high-strength concrete test beams were fabricated and tested under the ISO 834 standard fire curve to point of the failure. The test set-up was designed to evaluate the heat distribution and displacement changes of simply supported beams subjected to sustained loads under fire. Test results for normal-strength and high-strength concrete beams were compared for each of the test variables. The test results show that the relationships between time and temperature distributions in the beam sections are very similar and are unrelated to the strength of the concrete, with the exception of the upper part of the beam section. They also showed that the rates of deflection increase for both normal-strength and high-strength concrete beams is very similar before spalling but becomes remarkably high for high-strength concrete beams after spalling. A simplified model was proposed to determine the effect of spalling on the temperature gradient of a high-strength concrete beam. The results of finite difference method (FDM) analysis using this proposed model showed a section temperature gradient that was similar to that of the test results.

Structures have to be able to maintain their stability and strength for a certain time to ensure life safety and property protection under fire. Concrete has been a leading structural material for a long time. Generally, concrete has been known to have good fire resistance. High temperatures induce severe microstructural changes and internal stresses that alter the mechanical properties of Portland cement concrete and result in a decrease in load capacity and an increase in the deformation of concrete members. However, from previous research, high-strength concrete (HSC) is known to have less fire resistance than normal-strength concrete (NSC), and HSC, especially, has been found to be prone to spalling under high temperature. HSC is defined as a concrete whose strength is over 40 MPa (in Korea) or over 42 MPa (in ACI committee 363). Spalling of concrete subjected to fire is related to pore pressure in consequence of the vaporization of water in the concrete and thermal stress induced from restrained thermal dilatation. Many researches have been done on the effect of spalling of HSC by many researchers, including Kalifa et al. [1], [2], [3] and [4]. Some studies on the mechanical properties and fire performance of HSC have also been conducted by Phan, Chan, and others [5], [6], [7], [8] and [9]. Recent research on HSC under fire has focused on the changes in mechanical properties and spalling only for the material level, and on HSC columns for the structural level [10], [11], [12], [13], [14] and [15] and slabs [16] and [17]. Little research has been done on the structural behavior of HSC beams subjected to high temperature in comparison with NSC beams under the same conditions [18], [19], [20] and [21]. To ensure that HSC can be used safely in high-rise buildings, where HSC is commonly used in structural members, it is also important to examine whether HSC members suffer from fire damage to a greater degree than NSC members. The different types of fire damage to HSC would be generated in structural members based on the type of cement, aggregate property, concrete mixture, percentage of moisture content, age, etc. [1], [2], [3], [4], [5], [6], [7], [8] and [9]. These different types of fire damage to HSC include mechanical strength reduction, spalling, cracking, and deforming. An experimental comparison of the influences of high temperature on HSC and NSC flexural members is very important in order to ensure appropriate fire protection and to investigate different aspects of the fire damage to HSC and NSC beams. This study was intended to investigate the effects of compressive strength, cover thickness, and fire exposure time on the structural behavior of HSC and NSC beams under fire. Test results were investigated by the temperature distribution in the beam section layer and the deflection caused by elevated temperature. A simplified analysis model was proposed to determine the effect of spalling on the temperature gradients of the HSC beams.

To investigate the effects of concrete compressive strength and cover thickness on the temperature distribution and structural behavior of RC beams, four beams were tested under the ISO 834 standard fire curve. The beams were all simply supported and loaded by D+0.4LD+0.4L. The temperatures were measured to failure at three points (Low, Mid, and Up), and vertical displacements were measured at the center of the beam span. The time of failure was 151, 223, 140 and 161 min for N4, N5, H4, and H5 respectively. The cover thickness had an effect on the failure time of NSC beams, but had no effect on HSC beams. This might be because of spalling. The temperature difference at the upper point in a section between N4 and N5 is greater than in the HSC series. This is also induced by spalling of the upper part in HSC. The cover thickness in HSC needs to be reconsidered separately from that for NSC. In all cases of comparison regarding the strength of concrete, the temperatures of HSC beams are higher than those of NSC beams. The retention range of the NSC series at around 100 °C is longer than for HSC at all measured temperatures. This might be caused by NSC containing much more moisture than HSC. It has to be noted that the temperature profile of an HSC element level has to be considered for practical application separately from NSC elements. The vertical displacement of N4 was slightly greater than that of N5, and failure was in a stiff manner. The displacement of specimen H4 was reduced between 25 and 50 min and then it increased again sharply. There was explosive spalling in H4, so the cross-sectional area was reduced because of losing cover. This induced a reduction of dead load and displacement for H4. From the test results, the loss of H4 section area was greater than that for H5. Hence, the displacement may be independent of the cover thickness and dependent on the amount of spalling in HSC. The explosive spalling that occurred in the HSC beams before 60 min reduces their loading capacity because of the loss of cross-sectional area and moment of inertia and also decreases their elastic modulus. Hence, the displacement of an HSC beam is smaller that or similar to that of an NSC beam before spalling, but it becomes much greater after spalling. In this respect, explosive spalling in HSC beams has to be prevented to resist fire effectively, so a method to reduce spalling could be applied, such as mixing polypropylene fiber, when HSC beams are designed for resisting fire. A simplified model was proposed to determine the effect of spalling on the temperature gradient of an HSC beam. The results of an FDM analysis for the proposed model showed temperature gradients for sections similar to the test results. The proposed idealized model is applicable for heat transfer analysis with a very simple method. However, additional models are required for adequate application to complex conditions, considering aspects such as concrete strength, water–cement ratio, type of aggregate, etc.