Shear Strength Of Hollow-core Slabs Research Articles

Shear strength of prestressed 160 mm deep hollow core slabs

Prestressed hollow core (HC) slabs have been used all over the world due to their technical and economic advantages for diverse applications. Therefore, knowledge of the structural behaviour of this precast element is required for successful floor design, especially in relation to its actual shear failure mode. For this purpose, many analytical and empirical equations have been proposed to predict the shear strength of HC slabs; however, the depth of the slab influences its failure mode. Shear tests on HC slabs with 160 mm depth were carried out to describe the failure mode of these slabs. The failure loads were compared with the equations available in the literature, to identify which of the equations better represent the shear strength obtained from the shear tests. Two series of tests were carried out for slab elements with the same transversal section, but with different numbers of strands. Some HC slabs had cores filled with concrete in order to analyse the influence of the concrete filling on the shear strength of these slabs. In this case, an expansive additive was used to compensate the shrinkage of the concrete cast in the cores. Additionally, some slabs were tested under pure flexure in order to determine the effective prestress losses at the strands. Additional tests were carried out to estimate the actual transmission length of prestress to the concrete. The results show that the equations based on the flexural shear failure mode suggested by Brazilian Standard NBR 14861 and by European standard EN 1168 predicted the shear strength of the 160 mm deep HC slabs with good accuracy. Furthermore, equations based on the tension shear failure mode with an adequate reduction in flexural stiffness can be used to predict the shear strength of these slabs. The filled cores did not contribute to an increase of the shear strength of the slabs, which is overestimated by the equations reported by some standards. Good prediction accuracy of the shear strength of these slabs can be achieved by equations based on the tension shear failure mode with an adequate reduction in flexural stiffness.

Effect of polypropylene and steel fibers on web-shear resistance of deep concrete hollow-core slabs

Twelve shear tests were conducted on fiber-reinforced precast/prestressed concrete hollow-core (PCHC) slabs to quantify the effectiveness of polypropylene (pp) and steel fibers (hooked and high-strength/straight types) on shear resistance. Different volume fractions of pp fibers (0.11 and 0.22%) and steel fibers (0.51 and 0.89%) were investigated. The experimental results showed that there were moderate increases in shear strength with the use of pp fibers while substantial increases in shear resistance were observed with the use of steel fibers. In addition, shear strength increased with steel-fiber content. Moreover, with the same steel fiber content of 0.89%, specimens with high-strength/straight fibers exhibited superior performance in shear resistance compared to specimens using hooked steel fibers. In addition, web-shear failure occurred in the majority of specimens. However, as steel fiber content was increased up to 0.89%, in some instances, failure mode shifted from web-shear to flexural-shear failure. In addition, the 2010 fib model code formula for shear strength of steel-fiber-reinforced-concrete (SFRC) members was used to estimate shear capacity of the tested slabs using steel fibers. The calculations showed that the fib model code was overly conservative for shear-strength predictions of SFRC hollow-core slabs reported in this investigation. A semi-empirical formula to calculate shear strength of hollow-core slabs with steel fibers was proposed. It was verified using a database including the 6 shear-test results from this study and another 39 shear-tests on SFRC hollow-core slabs compiled from literature. The formula had a mean value of 1.12 for the ratio of experimental shear strength to calculated shear strength with a coefficient of variation of 0.21.

Enhancing the shear strength of hollow-core slabs by using polypropylene fibres

Experimental and numerical research studies demonstrated that the addition of fibres in correct proportions, enhances the shear behaviour of Reinforced Concrete (RC) elements, allowing to totally or partially replace the conventional web reinforcement. However, despite of this increase of knowledge about Fibre Reinforced Concrete (FRC), there is still a gap in the applicability of fibres as a shear reinforcement in certain prestressed structural elements where the use of conventional transverse reinforcement is difficult due to their manufacturing process, e.g. extruded elements in dry concrete. In this context, the present paper evaluates the possibility of enhancing the shear strength of Hollow-Core Slabs (HCS) by using Polypropylene Fibre Reinforced Concrete (PFRC). HCS can be critical in shear at their end zones, since these zones are disturbed regions (already stressed in tension by splitting forces) in which the beneficial effects of the prestressing actions on the shear strength are not active yet. Five full-scale HCS (42 cm high, 120 cm wide and 600 cm long) were tested under shear loading (1 in RC and 4 in PFRC). Two tests were performed on each slab varying the shear-span-to-effective depth ratio (a/d = 2.8 according to EN1168 and a/d = 3.5). Results show that macro-synthetic fibres are able to improve the shear strength of hollow-core slabs of about 25%; furthermore tests according to EN1168 are more affected by arch actions as compared to a/d = 3.5. Finally, the comparison between experimental results and predictions of four international codes (Eurocode 2, ACI 318-14, Model Code 2010 and EN1168), highlighted the need of improving the actual shear formulations.

프리스트레싱을 하지 않은 철근콘크리트 일방향 중공슬래브의 전단강도 평가

국내에서는 프리스트레싱을 하지 않은 RC 중공슬래브 공법이 개발되어 현장에 사용되고 있다. 현행 설계기준에 따라 복부 유효폭(<TEX>$b_w$</TEX>)을 바탕으로 계산된 RC중공슬래브의 전단강도는 지나치게 보수적이고, 중공재 특성의 영향을 합리적으로 고려하지 못한다. 이 연구에서는 1방향 RC중공슬래브의 전단성능을 연구하였다. 휨파괴 또는 전단파괴가 발생하도록 설계된 두께 300 mm, 폭 600 mm의 중공슬래브 실험체 10개를 제작하여 2점 가력 단순보 실험을 수행하였다. 실험결과, RC중공슬래브의 전단강도는 중공률이 클수록 전단강도가 감소하였지만, 복부 유효폭(<TEX>$b_w$</TEX>)을 포함한 다른 요인들은 거의 영향을 미치지 못하였다. KCI 2012, ACI 318-11, Eurocode 2, CSA A23.05 등 현행 설계기준에 따라 예측된 중공슬래브 실험체의 전단강도를 실험강도와 비교하였다. 또한 실험 관찰을 바탕으로 이축평면응력이 작용하는 콘크리트 압축대의 전단-인장 파괴이론을 사용하여 RC중공슬래브 실험체의 전단강도를 평가하였다. In Korea, non-prestressed reinforced concrete hollow-core slab construction method has been developed and used in construction fields. Provisions for the shear strength of hollow-core slabs based on the effective width of web (<TEX>$b_w$</TEX>), which are specified in current design codes, are too conservative and cannot consider accurately the effects of properties of hollow cores. In this study, the shear performance of one-way RC hollow-core slabs was investigated. Two-point loading simple beam tests were performed for ten hollow-core slabs with a thickness of 300 mm and a width of 600 mm, designed to fail in shear or flexure. Results showed that the shear strength of hollow-core slab was degraded as the void ratio increased but hardly affected by other factors including the effective width of web. Shear strengths of the hollow-core slabs estimated by KCI 2012, ACI 318-11, Eurocode 2, and CSA A23.05 were compared with the test strengths. In addition, based on the test observations, the shear resistance of the hollow-core slabs was estimated by using the shear-tension failure theory for the concrete compression zone subjected to biaxial plane stresses.

Anchorage failure and shear design of hollow‐core slabs

AbstractA number of studies in recent years have attempted to understand and calculate the shear strength of hollow‐core slabs, but no consensus has been reached on this issue. The current design methods for hollow‐core shear resistance are derived from experimental results and elastic theories that are not usually directly related to the behaviour at the ultimate limit state. Moreover, some manuals on this subject do not discuss anchorage failures, which although not common in this type of slab, may influence the shear strength. This paper considers the anchorage failure of strands using the concepts of Eurocode 2 and presents an analytical methodology for shear design based on the modified compression field theory (MCFT). Furthermore, the safety concepts of Eurocode 2 are properly presented and evaluated using the experimental data available in the literature. The proposed methodology is proved to be accurate and is simple enough for use in design. Comparisons with CSA A23.3 and Eurocode 2 are also shown.