High levels of sustained load can lead to time-dependent failure of reinforced concrete (RC) members. This in turn may lead to collapse of all or part of a building. Design errors, construction errors, and material deterioration may lead to concrete elements being subjected to high levels of sustained loads well exceeding typical service loads. Plain concrete can experience compressive failure when subjected to a high sustained stress (over 75% of its short-term strength). However, there is a lack of knowledge about the time-dependent strength and stiffness characteristics of RC members under high sustained loads. This paper presents the results of experimental testing of simply supported shear-controlled RC beams under high sustained loads. Two series of beams, consisting of 4 and 5 beams, were tested at concrete ages of 67 to 543 days to represent in-service concrete structures. The applied sustained loads ranged from 82% to 98% of the short-term capacity and lasted for 24 to 52 days. Test results indicated that high sustained load may eventually lead to failure (collapse); however, the level of load needs to be very close (~98%) to the short-term capacity. Under sustained load, all specimens experienced increased deflection with over half of the deflection increase occurring in the first 24 h. The sustained load increased the deflection at shear failure by 190% on average. The increase in the beam deflection may allow for load redistribution in redundant structural systems. A sharp increase in deflection due to tertiary creep occurred in a short time (~2 min) before failure, indicating little warning of the impending failure.
Fracture behavior of high strength concrete (HSC) with different types of short fiber (steel, polypropylene (PP), and steel + pp) was investigated in the present work. The fracture behavior of edge-notched beam was determined in three-point bending condition. The crack length to depth ratio, a/d, was equal to 0.2, 0.3, 0.4 and 0.5. The fracture parameters were determined using linear elastic fracture mechanics (LEFM) and the Hillerborg model. The results in the present paper indicated that, adding short fibers to HSC improved its compressive strength in addition to the obvious enhancement in ductility. The mode of failure for various fiber reinforced concrete (FRC) types under compression was varied compared to that of plain concrete. All these cubes failed due to multiple tensile vertical cracks. In general, a small effect of short fibers in improving the indirect tensile strength and flexural strength of HSC. HSC with Steel and PP Hybrid Fiber (SPPFRC) showed superior compressive, tensile, and flexural strengths over the others FRCs. Fracture toughness based on LEFM (K IC) has a limited variation with increasing a/w for HSC and all FRCs. Therefore, the mean value of K IC is calculated and trusted. The predicted values of undamaged defect based on LEFM are comparable to the maximum aggregate size. Therefore, the values of K IC calculated based on LEFM were reasonable.
The discussers appreciate the effort made by the authors in proposing an analytical model to predict the mechanical behaviour of hybrid fibre-reinforced concrete (HyFRC or HFRC) taking into consideration the synergy concept (Abadel et al., 2016). However, their experimental results contradict this concept. For example, all the properties of mix M4 (0·2% PPF + 1·2% SF) are lower than those of steel fibre-reinforced concrete (SFRC) having only 1·2% SF, mix M1, that is, negative synergy. Generally, for the same fibre volume fraction (F Vf ), all the properties of HyFRC mixes are lower than those of SFRC mixes, as shown in Figure 10. Furthermore, the modulus of rupture, splitting tensile strength and flexural toughness of SFRC with a lower F Vf , mix 1, are greater than those of mix 3 (SFRC with a higher F Vf ). Although the authors mentioned that 'The fibres were added to plain concrete in parts to prevent fibre balling and to ensure the homogeneity of the concrete mixture. The mixing was done for about 3 min to ensure the proper distribution of fibres in the concrete mass', they did not show the readers any validation for this, such as images of the fracture surfaces of the different types of specimens showing the uniform dispersion of the three different types of fibres, or any statistical analysis of the experimental results, such as the standard deviation or the coefficient of variation. Casting and compaction affect the fibre orientation throughout the specimen and could potentially lead to fibre segregation. Furthermore, different types of fibres require different optimum ranges of rheology of the cementitious mortar to achieve good fibre dispersion. This can lead to a significant challenge in processing hybrid fibres in the same matrix; see, for example, Abou El-Mal et al. (2015). Authors' replyThe authors would like to thank the discussers for their interest in this paper and their insightful comments on the work. The authors' responses to the individual comments are given below under two main headings. Synergy in HFRC mixesIn the study under discussion, the synergy in different HFRC mixes was only observed to some extent for mix M7, as mentioned in the section of the original paper entitled 'Discussion of test results'. As the main objective of adding fibres to concrete is to improve the tensile characteristics of the concrete, such as modulus of rupture, splitting tensile strength and direct tensile strength, these properties of HFRC mixes having 1·4% volume fraction of fibres are compared with mix M3 (containing only steel fibres) and shown in Figure 11. It is to be noted here that the tensile characteristics presented in the figure are in terms of the compressive strength of concrete, that is, the ratio of tensile characteristics to the compressive strength. The figure shows the presence of synergy in mix M7. As the ratio of M7 to M3 is slightly greater than unity, the synergy in mix M7 was not strongly claimed in the paper. Other researchers (e.g. Banthia and Gupta, 2004) have also reported synergy eff...
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