G Appa Rao

This paper reports on fracture properties of the interface in high strength concrete. The fracture properties included in this study are: strength, fracture toughness and ductility factors. Experiments were conducted on composite specimens in Mode-I and Mode-II loading effects. Two types of composite specimens were designed. In the first type, the mortar was cast against the hardened concrete on only one face, and in the second type, the mortar was cast on both the faces of the aggregate material. The latter type has been designated as sandwiched type. For Mode-I case, both the types of composite specimens were used, while for the Mode-II loading only sandwiched type of composite specimens were adopted. Two types of aggregate materials have been adopted: granite rock and hardened HSC. In the case of granite rock aggregates, rough and smooth surfaces were used, while in the case of HSC concrete, rough, smooth and casting surfaces have been adopted. Three types of mortar mixes were used at 0.40 water-binder ratio with 1:3, 1:2 and 1:4 cement-sand ratios. Silica fume was used 10% by weight of cement. Strong interface has been noticed with rough aggregate surface. The fracture toughness of the interface seems to be very high with rougher aggregate surface in Mode-I. The type of mortar matrix significantly alters the type of interface. The influence of size of interface area on the fracture properties has also been reported. Size effect seems to be clearly observed in Mode-II loading. The fracture toughness increases as the roughness and the inclination of the aggregate surface increases. Interestingly, the sandwiched composite specimens exhibited decreasing fracture toughness with increase in the interface area. Ductility factor, defined as the fracture energy per unit of peak load decreases as the surface area of the interface increases in Series II and III specimens. The CMODc seems to be slightly higher with rough aggregate surface. The interface exhibits significantly brittle behavior showing catastrophic failure, which indicates a possible application of LEFM to the interface.

Deep beams are commonly encountered in structural system as transfer girders, pile caps, and pier caps etc. The deep beams are shear critical rather than flexure. Shear strength of beam increases with decreasing shear span-to-depth ratio (a/d). Consistency of the existing shear strength models has been studied using 776 data points obtained on simply supported deep beams with low a/d ratio. It has been found that sectional method of shear design is inappropriate for deep beams. The Strut-and-Tie Model (STM) provision of ACI 318-14 produces conservative estimation of capacity compared to simple shear strength model and complicated softened STM. All the models estimated the capacity unconservatively, if overestimation percentage is limited to 5%.

The minimum flexural reinforcement required in reinforced concrete (RC) beams is affected by the compressive strength of concrete, yield strength of steel reinforcement, fracture toughness and size (depth) of member. This paper reports an analytical investigation carried out to predict the cracking behavior and minimum flexural reinforcement corresponding to limiting crack width. The minimum flexural reinforcement is defined corresponding to the stage where both the maximum surface crack width and the critical crack opening displacement (CODCT) are equal at the yielding of steel reinforcement. An analytical model has been proposed to determine the minimum flexural reinforcement through a parametric study involving several combinations of beam depth, fracture toughness, strength of concrete, and yield strength of steel reinforcement. The proposed model has been validated with the experimental observations reported in the literature on several RC beams of different depth and strength of concrete. The model predicts the minimum flexural reinforcement satisfactorily, agreeing with the experimental observations on various beam depths. The present model looks to be an improvement of several existing models incorporating all the influencing parameters.

Twelve reinforced concrete deep beams of three different depths and two different percentages of horizontal shear reinforcement with two types of distribution were studied for understanding the size effect and influence of the distribution of horizontal shear reinforcement on the shear strength and the cracking behaviour. Three beam depths namely 300mm, 600mm and 1200mm provided with two different percentages of horizontal shear reinforcement of 0.2 and 0.3, which were distributed (i). uniformly throughout the depth and (ii). uniformly only over the middle third of the depth. The shear span-to-depth ratio was 0.75. The flexural tensile reinforcement was 1.5% and provided with 25mm clear cover. The test results showed that the mode of failure was significantly altered by changing the beam depth. Sufficient ductility was achieved in small size beams, whereas relatively very high brittleness was observed in large size beams. As the quantity of horizontal shear reinforcement increases the shear strength and the deformability of the beams were found to be increased. The effect of the distribution of horizontal shear reinforcement along with the size effect seems to influence the strength and ductility of the beams. The uniform distribution of reinforcement over the middle third portion of the beam depth exhibited significant improvement on the shear strength and ductility. A strong size effect has been observed on the shear strength of RC deep beams. There exists a non-negligible size effect on the diagonal strength of RC beams. When the beam depth is increased from 600mm to 1200mm, there has been a decrease of diagonal strength ranging from 20% to 25%, while it ranges between 35% to 55% when the depth is increased from 300mm to 1200mm.

An important parameter that is being not accounted for in the design of concrete structures is the size of structure on strength. Size effect in shear has been a serious concern owing to sudden failure of large size members at lower stress levels compared to small size members. Currently, the design provisions for shear in reinforced concrete (RC) members by various codes of practice have not accounted for the size effect. In this paper some experimental investigations have been carried out in order to study the size effect on RC beams with a shear span-to-depth ratio 1.05. Geometrically similar beams were tested by varying depth of member and compressive strength of concrete under four-point loading. The shear reinforcement was varied maintaining balanced flexural reinforcement in all the beams. Sudden failures of beams have been observed in larger size beams cast from high strength concrete (HSC), which exhibited a strong size effect on strength. The observations from the study reveal that the effect of size of beam on shear strength in the design of large size RC members should be incorporated.

This article presents an experimental investigation on 11 reinforced concrete (RC) deep beams without web reinforcement. Shear spandepth ratio (a/d), beam depth, and percentage of tension reinforcement are the variables. Beams with a/d less than 1.5 could carry loads beyond the diagonal cracking of concrete due to the formation of a diagonal concrete strut. Diagonal cracking of deep beams with an a/d of 1.0 was observed at a load of approximately 20% of ultimate capacity. Further, 607 more RC deep beam experimental results were collected from the literature. Strut-and-tie model provisions of ACI 318-14 and ACI 318-19 were validated with a database of 618 RC deep beam results. The capacity of beams with a/d higher than 1.5, and concrete strengths higher than 60 MPa was overestimated by ACI 318-19 even after the reduction of strut efficiency factor from 0.6 to 0.4. However, the percentage overestimation by ACI 318-19 is within 5.0%. ACI 318-14 predicts the capacity more accurately than ACI 318-19 with a strut efficiency factor of 0.6, with only 6.0% of overestimation, if the maximum shear strength limit is considered. The maximum shear strength limit of ACI 318-14 is found to be highly conservative. The reduction of strut efficiency factor in ACI 318-19 without modifying maximum shear strength limit results in highly conservative estimations.

The shear span-to-depth ratio (a/d) is the most important factor influencing the shear strength of reinforced concrete beam and makes the failure mechanism a complex one. As the a/d ratio decreases, the shear strength of beam increases, by altering the mode of failure from diagonal tension to diagonal compression. IS: 456 (2000) and Euro code 2 developed shear strength expression empirically from the experimental results of beams with a/d ratio of more than 2.0, and the same has been adopted with an enhancement factor for the design of beams with a/d ratio of less than 2.0, ignoring the mode of failure. The validity of these modified expressions has been reviewed using experimental database of 227 beam test results with a/d ratio of less than 2.0. In addition, the ACI 318-14 provisions based on the strut-and-tie model (STM) have been reviewed. The STM based approach seems to be more appropriate for the design of beams with a/d ratio less than 2.0. Due to its rationality, the strut-and-tie model for the design of structures with complex failure mechanism need to be recommended in the IS code of practice.

Some experimental investigations on the bond strength and analytical modeling of bond stress-slip response in reinforced concrete (RC) have been reported. The effect of strength of concrete, bar diameter, embedment length and type of confinement has been studied. Tests were conducted on pull-out specimens. The parameters of the study include; two embedment lengths (50 mm and 150 mm), two bar diameters (16 mm and 20 mm) and two concretes (40 MPa and 50 MPa). The effect of confinement was also studied by providing different types of confinement such as lateral ties and spirals along with control specimens without confinement The test results show that the bond strength decreases as the length of embedment increases. The effect of bar diameter is negligible in the range of diameters adopted. As the compressive strength of concrete increases, the bond strength increases and the slip at failure decreases. The bond strength increases with lateral confinement in general. The failure in concrete with spiral reinforcement was relatively ductile. The measured bond strength of concrete ranges between 9.0 MPa and 19.0 MPa in confined concrete. In unconfined concrete, it ranges between 60 to 70% of the confined strength. Splitting failure of concrete was predominant in unconfined concrete, which was altered to pullout type by providing with lateral confinement The bond stress-slip responses have been idealized in concrete with and without confinement The bond stress-slip response in the post-peak region has been observed to be relatively steep as the strength of concrete increases.

The behaviour of reinforced concrete deep beams is complex due to small shear span-todepth ratios, which deviates its behaviour from the classical Bernoulli's beam behaviour. Such behaviour is predominant in cases where members are supported over small spans carrying heavy concentrated or distributed loads. Such is the case in the structural members like pile cap, transfer girder, panel beam, strap beam in foundation, walls of rectangular water tank, shear wall etc. This paper reports on the influence of Poly propylene fibers combined with and without steel fibers on the stiffness, spall resistance and shear strength of RC deep beams. A total of 21 beams were tested to failure under two-point loading, which were compared with the ACI code provisions. The shear span-to-depth ratios adopted were 0.7 to 0.9 incorporating three steel fiber volume fractions of 0%, 1%, 1.25% along with two different fibers of Steel and Poly propylene with volume fractions of (1.0 + 0.0) %, and (1.0 + 1.0) %. The beams with shear span-to-depth ratios 0.7, 0.8 and 0.9 showed an increase of 21.9%, 23.43% and 23.9% in the ultimate load carrying capacity with combined steel and poly propylene fibers as replacement of web reinforcement with reference to that of the beam without web reinforcement. With the above combinations, the shear strength and stiffness of the beams have been found to be improved. When the horizontal shear reinforcement was increased, the shear strength was found to increase. © (2013) Trans Tech Publications, Switzerland.

The toughness indices of fiber reinforced concrete under Mode II loading effects are rarely reported due to lack of information on standard testing procedures. However, the direct shear test with improvement over JSCE-SF6 method is generally accepted to study Mode II fracture parameters. In this paper, experimental investigations to determine the fracture properties and toughness indices of steel fiber reinforced concrete (FRC) under Mode II loading are reported. Straight steel fibers of length 25 mm with an aspect ratio of 44.6 were randomly distributed in concrete with varying fiber volume fractions of 0, 0.5, 1.0 and 1.5%. A symmetrical Mode II loading set up was designed to achieve an ideal shear failure. It has been observed that the failure was due essentially to shear (Mode II) fracture without secondary flexural cracking. Plain concrete failed at a low equivalent shear strain of 0.5%, while the addition of steel fibers improved the shear strains up to as much as 8.0%. The shear strength and the shear toughness of concrete with the addition of steel fibers have been improved very significantly. As the volume fraction of fibers increases, the shear strength increases up to an optimum volume fraction, beyond which there has been no improvement on the shear strength. However, the toughness indices determined in Mode II loading (shear) have been observed to be about 15 times as high as that under Mode I loading (flexure). © 2009 RILEM.