Indian Concrete Journal

A case for the use of rectangular stress block in the revision of IS: 456 (2000) N. Subramanian

Most of the Indian concrete code [IS: 456 (2000)] provisions were developed during the 1970s and are based on experimental work on concrete having strength less than 40 MPa. The use of high-strength concrete (HSC), with strengths in the range of 60-130 MPa, has increased in recent years due to its superior performance and strength. High-strength concrete is used extensively in columns of multistory buildings, as it will reduce the size of columns and increase rentable space. The parabolicrectangular stress block specified in the IS: 456 (2000) was developed, however, on the basis of normal strength concrete column tests. Many international codes have specified Whitney’s equivalent rectangular stress block, which not only produces results comparable to that of the parabolic-rectangular stress block, but also has the advantage of reducing the computational effort. However, the applicability of the rectangular stress block to higher-strength concretes became questionable, especially for members under high levels of axial compression. Based on the experimental results on HSC columns, several modifications to the original rectangular stress block were proposed and different rectangular stress blocks are being specified in international codes, which can also tackle HSC. These different rectangular stress blocks are discussed and based on the comparison with several experimental data, a suitable one is recommended to be used in the future revision of IS: 456 (2000).

Seismic evaluation of RC frame designed with effective and gross section using FBD and DDBD approach Harshad Gajera, Jignesh A. Amin

Seismic performance evaluation of RC frame designed based on stiffness of effective section as per IS:1893-2016 and gross section as per IS:1893 (2002), respectively is carried out. For this purpose, 2, 4, 7, and 10 story RC frames are designed using two design methods namely, force based design (FBD) and displacement based design (DDBD) considering effective and gross section properties of RC elements, respectively. The designed RC frames are analyzed using nonlinear static pushover analysis (NSPA) and nonlinear response history analysis (NLRHA) to obtain the pushover curve and the inter-story drift ratio (IDR), respectively. Response reduction (R) factor of respective RC frames is evaluated using the information received from the bi-linear idealization of pushover curves. The evaluated R factor of RC frame designed based on the stiffness of the effective section is observed to be larger than that of a similar frame designed with gross section properties. The R value of RC frames designed using the DDBD approach is observed to be higher as compared to the similar frame designed using the FBD approach, suggesting higher energy dissipation ability of DDBD frames during nonlinear behavior. The results of NLRHA show that the IDR of all RC frames, irrespective of whether they are designed based on stiffness of either gross or effective section using the FBD or DDBD approach, respectively, is less than the maximum allowable IDR, suggesting that both FBD and DDBD methodology is capable of designing structures with controlled residual behavior, irrespective of gross or effective section used in the design. However, the IDR of the RC frame designed based on the stiffness of the effective section is observed to be lower as compared to the frame designed based on the stiffness of the gross section.

A simple methodology for prediction of concrete pumping through field-based study Sendhil Vigneshwar Dhanapal, Revanth Kumar Kandagaddala, Prakash Nanthagopalan

Considering the growing demand for placing the concrete to greater distances (vertical/horizontal) along with the development and use of different types of concretes, pumping is becoming a huge challenge in construction sites. More specifically, the prediction of pressure required to pump the concrete is of prime importance. In the present study, based on field measurements, an attempt was made to predict the pressure required for pumping the concrete using a modified Kaplan equation. The predicted and the actual pressure applied to concrete was compared. Based on results, a simple framework is proposed for the prediction of concrete pumping pressure to a reasonable extent.

Determination of the complete stress-strain response of concrete under uniaxial compression Stefie J. Stephen, Ernesto Zangelmi Júnior, Ravindra Gettu, Antonio Aguado, S. Vaishnav Kumar

The characterization of the complete uniaxial compressive stress-strain curve of concrete is crucial for a more rational and reliable design of concrete structures. However, without intrinsic knowledge of the influence of different experimental control variables and the failure mechanisms of concrete, it is impossible to obtain a stable softening response in a test. This paper discusses the various aspects to be considered while performing the characterization, provides a comprehensive description of the different steps involved in the post-processing of the obtained experimental data. The influence of different test variables such as the type of contact, specimen size, rate of loading, the grade of concrete and fibre dosage on the compressive response is also elucidated. The complete stressstrain curves for different concretes have been obtained using the proposed procedure, which can serve as guidelines for such characterization. The brittleness of higher strength concrete is clearly manifested by a stress-strain curve that descends more sharply than those of conventional concretes. Further, it is seen that the incorporation of steel fibres mitigates the brittleness by improving the compressive toughness through bridging of the cracks and internal confinement.