Indian Concrete Journal

Experimental Study on Stress Strain Behavior of Normal and High Strength Unconfined Concrete Brijesh Singh, V. V. Arora, Vikas Patel

Stress-strain parameters of concrete is essentially required during the design phases of structural members. With the evolution of normal concrete to High Strength Concrete (HSC); various studies on stress-strain behavior of High Strength Concrete (HSC) are available in the literature. Such studies done by various researchers are differing to each other because of the different mix proportions and material properties. The High Strength Concrete is generally defined as concrete above M50. Bureau of Indian Standard (BIS) code IS: 456-2000 in clause 6.1 (Table-2) has identified concrete grade M10 to M80. However, note: 2 under Table 2 of IS: 456-2000 states that for concrete of grades higher than M55, design parameters given in the code of practice may not be applicable and values may be obtained from specialized literatures and experimental results. In the absence of design parameters in the codes, designers are not able to use high strength concrete, even though the laboratories and RMC plants in the country have the expertise to design and produce high strength concrete. Therefore, this research was intended to develop design parameters for high strength concrete so that designers can use high strength concrete in design of structures with confidence. In many design standards, the conventional rectangular stress block developed for Normal Strength Concrete is still being used for design of HSC beams. The aim of this study was to develop design parameters for flexural design of High Strength Concrete for incorporation in Indian Standard for Design (IS: 456-2000). For study, the different mixes ranging from w/c ratio 0.47 to 0.20 using granite and calc-granulite aggregate were used. The aim of study is to understand the behavior of normal and high strength concrete including comparison of empirical formulae proposed for calculation of ultimate strain and strain at peak stress for High Strength unconfined concrete in European design standard EC: 02-2004 with the experimental results and proposal for revision in IS:456-2000. The study also includes the cube compressive strength to cylinder compressive strength ratio for high strength concrete.

Development of IS:456 Shear Design Provisions-I: Members without Shear Reinforcement Dipti Ranjan Sahoo

One-way shear design provisions of Indian Standard IS:456-2000 were first introduced in its 1978 version. The empirical design equations were developed based on the limited test results available more than 40 years ago. Since then, significant advancement on the shear strength models has been done through extensive numerical and experimental studies. As a result, the shear design provisions of many building codes worldwide have been updated considering the influence of many critical parameters on the shear strength of concrete members. Current shear design provisions of IS:456-2000 do not explicitly consider the influence of high compressive strengths of concrete, member sizes, aggregate types and other parameters. Therefore, there is a need to assess the adequacy and safety of the shear design provisions of IS:456-2000. In this study, one-way shear design provisions of various codes of practice have been reviewed. Various critical parameters influencing the shear strength of concrete members have been discussed. A simplified expression has been developed in a more rationalized manner to predict the concrete shear stress of members without shear reinforcements. Moreover, guidelines to compute the maximum value of concrete shear stress have been proposed in order to avoid the crushing failure of diagonal compression strut in the concrete members.

Development of IS:456 Shear Design Provisions-II: Members with Shear Reinforcement Dipti Ranjan Sahoo

This paper presents the development and validation of proposed shear design provisions for concrete members. The design expressions for members with shear reinforcement as presented in this paper and for members without shear reinforcements presented in the companion paper are validated by comparing the predicted shear strengths with the test results. More than 1000 shear test results available in ACI-DAfStb database have been considered for the validation. Additional expressions are also proposed to compute the design concrete shear stress in the presence of axial compressive and tensile forces. The recommended design provisions resulted in the conservative estimates of shear strengths of members with and without shear reinforcements as compared to the test results. In addition, the modifications to the current IS:456-2000 provisions have been proposed for the minimum area of shear reinforcement in members of normal and high-strength concrete not requiring shear stirrups. It is noted that the expected (unfactored) shear strengths of concrete members predicted using the proposed design expressions matched very well with the test results.

Effect of Recycled Aggregate on Shear Behavior of Steel Fiber Reinforced Self Compacting Concrete Kannam Praveen, S. Venkateswara Rao, P. Rathish Kumar

Self-Compacting Concrete (SCC) is flowable and highly viscous which does not require any external compaction during casting and placing. Use of recycled aggregates as replacement up to 50% of natural coarse and fine aggregates is been widely used by many researches in past few years. In the present study an attempt is made to study the behavior of steel fiber reinforced self-compacting concrete under shear by using 100% recycled concrete aggregates as coarse and fine aggregates. The experimental programme consisted of 32 beams of which 16 beams each were cast with 100% natural and 100 % recycled aggregate. The size of the beam was fixed at 100 x 200 x 1200 mm. Due to the use of recycled concrete aggregates as coarse and fine aggregates, compressive strength is reduced by 7.8% and 8% for SCC30 (30 MPa) & SCC70 (70 MPa) Concrete. Ultimate shear strength is reduced by 14% and 12% due to use of recycled concrete aggregates for SCC30 and SCC70 beams respectively. It was observed from the experimental results that Addition of Steel fibers has increased the mechanical properties for both NASCC and RASCC and also, combination of stirrups and steel fibers has shown better performance on SCC beams. An equation to predict ultimate shear strength of NASCC and RASCC is proposed based on nonlinear regression analysis.

Effect of Synthetic Fibres on Compressive Strength of Concrete Shailja Gupta, V. K. Verma

In the present work, the effect of synthetic fibres on the mechanical properties of concrete are investigated. Total fifteen samples of M30 grade concrete with and without fibres were cast. The M30 grade concrete was designed as per IS 10262-2009 and Conplast WL was added as a superplasticizer. The water cement ratio of the mix was 0.45. The fibre was added in the proportion of 0%, 0.25%, 0.50%, 0.75% and 1.00% by volume of concrete. At 0.75% addition of synthetic fibre the compressive strength test gives best result in concrete respectively.

Study on Fracture Behavior of High Strength Concrete Including Effect of Steel Fibre Vikas Patel, Brijesh Singh, V. V. Arora

With increase in use of High Strength Concrete (HSC) and its known brittle behavior it is very important to understand the fracture behavior of HSC. This paper presents an experimental investigation on the fracture behavior of normal strength concrete and high strength concrete. The effect of addition of the steel fibres on the fracture behavior of the concrete has been also studied. Fracture study was carried out by conducting three-point bending tests on notched beams. The paper covers the study on mainly two grades of concrete with w/c ratio 0.47 and 0.20 with and without steel fibres of 0.25% and 0.50%. Various fracture parameters like the fracture energy, length of fracture process zone, critical crack tip opening displacement and the fracture toughness were determined as per RILEM laid procedure. The test results showed that the fracture parameters are sensitive to the fibre addition. The study indicated that the addition of steel fibres mostly influences the response of the concrete after the onset of initial cracks thereby bridging of cracks, which aids in increasing the ductility of the concrete after the post cracking stage. The results indicate that with increase in steel fibre content, the fracture energy increases and characteristics length decreases. Therefore, the brittle fracture behavior of the HSC can be overcome by incorporation of even small amounts of steel fibers and thus having behavior similar to normal strength concrete.

Point of View : A Need to Revise Minimum Tensile Steel Area Provision for RC Beams given in Clause 26.5.1.1 of IS 456:2000 (Plain and RC Code of Practice) Dada S. Patil

The point-of-view presents the need to revise the IS:456-2000 provisions on the minimum percentage of longitudinal steel in a concrete beam, particularly with high-strength concrete.