رفتار سازه بتن نوع پودری خود تراکم: عملکرد باند و ظرفیت برشی

An experimental test program was carried out to investigate the bond and shear performance of powder-type self-compacting concrete (SCC). In order to examine the bond strength of reinforcement in concrete, pull-out tests (according to RILEM recommendation RC6 part 2) were performed. In total, 72 pull-out specimens were tested, cast with different concrete mixtures and rebar diameters (8, 12, 16, and 20 mm). It was found that SCC shows normalized characteristic bond strength values as high as or higher than vibrated concrete (VC). In addition, as the bar diameter increases, larger bond strengths are measured, with the highest values for bars with diameter 12 or 16 mm. When larger diameters up to 20 mm are used, a decrease in bond performance is noticed. To study the shear behaviour, four-point bending tests were executed. Small SCC and VC beams were cast with different reinforcement ratios (1.0%, 1.5%, and 2.0%) and tested with different shear span-to-depth ratios (from 1.5 to 3.0), with a total of 102 beams. A slightly decreased shear capacity is observed for SCC. Also, higher ultimate shear stresses are recorded when higher reinforcement ratios or smaller shear span-to-depth ratios are applied.

Self-compacting concrete (SCC) is becoming increasingly used in civil engineering. Powder-type SCC differs from vibrated concrete mixtures (VC) by the increased amount of fine aggregates and fillers, and the addition of a superplasticizer which increases the workability. As a result, SCC is capable of flowing under its own weight and completely filling the formwork. Also a dense and adequate homogeneous material is achieved without the need of compaction. Therefore, it can be used to cast narrow, complex formworks, even in the presence of dense reinforcement [1] and [2]. Today, the fresh properties and durability behaviour of SCC are thoroughly investigated in literature, while the mechanical properties – such as bond and shear behaviour – are less reported. Because of this lack of information regarding structural performance of SCC members, this material is still not confidently used by designers and engineers in the construction industry, despite the many advantages, such as increased productivity, reduced labour and higher quality of the structure [3]. Reinforced concrete (RC) is a composite material, designed to resist compressive stresses (concrete) and tensile stresses (reinforcement steel). To achieve an effective RC structure, a good bond between concrete and reinforcement steel is necessary to enable force transfer between both materials. In literature, many test results of pull-out tests show that the bond strength of SCC is as high as or higher than VC [4], [5], [6], [7], [8], [9], [10], [11] and [12]. Depending on the quality and the compressive strength of the concrete, the bond strength of SCC is about 5–40% higher [6], [7] and [8]. This increased bond performance can be attributed to a reduced formation of bleed water under the reinforcement bars due to the absence of compacting equipment [7], [8] and [9]. In addition, previous tests with bar diameters ranging from 12 to 40 mm showed a significant size effect on the bond strength: for smaller bar diameters, higher bond stresses are found [10] and [11]. As opposed to pull-out test, similar results regarding bond performance are achieved when beam tests are conducted to examine the bond behaviour between concrete and reinforcement steel [12]. Only smaller slip values are measured when the compressive strength of the concrete increases when beam tests are applied. Concerning the bearing capacity of SCC beams, there is some concern among researchers and designers that they may not be strong enough in shear. Because of the use of fine aggregates and a smaller amount of coarse aggregates, a weak interlock mechanism is expected. Kim et al. [13] already confirmed this statement after experimentally determining lower fracture reduction factors – and thus less aggregate interlock – using SCC. In the research by Hassan et al. [14], an up to 17% reduction in shear strength was found in SCC beams subjected to shear failure. Additionally, the ultimate shear load grew with the increase of longitudinal reinforcement and/or decrease in beam depth. However, Lachemi et al. [15] indicated a difference in shear capacity of only 5% between SCC and VC. Taking into account a large scatter on the test results, this makes it hardly convincing that SCC has a poor shear resistance compared to VC. Desnerck [16] also pointed out that any potential reduction in the shear strength of hardened SCC due to reduced aggregate interlock may in reality be outweighed by the overall gain in matrix quality, including the interfacial transition zone (ITZ). By means of push-off tests, normalized ultimate shear strengths of SCC up to 15–20% higher than for VC were measured in some cases. However, when SCC is modified to become a mixture requiring compaction by reducing the content of superplasticizer, a decrease in ultimate shear strength of 8% is found due to vibration. From these results, it is not possible to conclude that SCC shows worse shear behaviour than VC. In order to validate or elucidate the findings mentioned above, this paper evaluates the bond and shear performance of SCC. Pull-out tests are performed to examine the bond-slip behaviour between concrete and reinforcement steel with different diameters (8, 12, 16, and 20 mm). To investigate the shear capacity of SCC, four-point bending tests are carried out on small concrete beams without stirrups. The influence of different parameters such as concrete type, shear span-to-depth ratio, and reinforcement ratio is examined.

The bond and shear behaviour of self-compacting concrete (SCC) has been studied by pull-out and four-point bending tests. Four different SCC and two vibrated concrete (VC) mixtures were used to cast the test members. For the bond behaviour, the influence of concrete type and bar diameter (8, 12, 16, and 20 mm) was investigated. During shear capacity tests, concrete type, shear span-to-depth ratio, and reinforcement ratio were varied. The results of the experiments have led to the following conclusions: (a) The normalized bond strength of SCC is as high as or higher than the normalized bond strength for VC. An increase up to 68% for the normalized characteristic bond stress and 39% for the normalized ultimate bond stress is measured for SCC. This improvement in bond behaviour of SCC can be attributed to the higher amount of fine aggregates and higher workability, which results in a better containment of the reinforcement bars. (b) For small diameters the bond strength increases with increasing bar diameter up to a diameter of 12 or 16 mm. For larger diameters, the bond strength shows a decreasing trend. A modified equation for EC-1992-1-1 is proposed, covering diameters 8–20 mm. In order to cover a broader range, more experimental research is required. (c) SCC beams showed equal or lower ultimate shear strength than VC test members. A maximum reduction of 6.9% was observed in SCC beams. The recorded difference is not significant enough to prove any inferior shear behaviour of SCC, although this could be expected due to the decreased amount of coarse aggregates and thus reduced aggregate interlock, and follows literature on this matter. (d) Lower shear capacities were measured for increasing shear span-to-depth ratios a/d. An increase of 27.3% is observed, when a/d decreases from 3.0 to 2.0. This improvement can be partially attributed to the influence of the direct strut action into the supports, encouraged by the use of small test beams. A modified equation for the shear resistance according to EN 1992-1-1 is proposed, based on the experimental results of one SCC mixture, taking into account the influence of the a/d-ratio. (e) Beams with a higher steel reinforcement ratio generally showed better shear behaviour than those with low longitudinal steel ratio, due to an improved dowel action of the reinforcement. (f) The equations from fib MC2010 and ACI 318-11 (with safety factors equal to 1) are underestimating the experimental bond and shear strength values. The ultimate shear bond stress according to MC2010 is between 0.18 and 0.36 times the experimental values. For the shear capacity, theoretical values of 52% and 67% of the experimental results are calculated, according to MC2010 and ACI 318 respectively.