Indian Concrete Journal

Modifications suggested in IS: 456 (2000) with respect to maximum under-reinforced section with minimum tension reinforcement in beams Bijoy Kumar Jha, Santanu Bhanja

As per prescriptive method of design specified by our standards designers have the option of designing a reinforced concrete section in flexure within the range of two extreme conditions i.e. balanced and maximum under-reinforced. No specific recommendation or guideline is available regarding the choice of the section. Many designers are of the opinion that in balanced condition both the materials are optimally used and hence it is better. But the performance of an under-reinforced section is much better than that of a balanced one. The yielding of steel and plastic rotation of the section is maximum for maximum under-reinforced section and minimum for a balanced one. Again, a designer opts for a doubly reinforced section only when the external bending moment exceeds the limiting or balanced moment of resistance as a singly reinforced section. Thus, all doubly reinforced sections are designed as balanced in nature. The performance of a structural element should be properly assessed by the designer up to and beyond design loads i.e., under overloading and it needs to be ensured that failure occurs in flexure only as excepting this mode all other failure modes are brittle. For ensuring ductile failure the total strain at failure should be high and, in this context, ductility plays a very vital role. However, for achieving higher levels of ductility higher plastic rotations are necessary. In the present paper an attempt has been made to quantify the plastic rotations of maximum under-reinforced sections. It has been observed that with the minimum percentage of steel as per IS:456 the plastic rotations of sections are very high and seem to be virtually impossible to be actually mobilized especially for deeper Reinforced Concrete girders. These high rotations might result in actual collapse of the section. The codes do not furnish any information regarding the desirable values of plastic rotations of beams. An attempt has also been made to compare some important features of prescriptive method of design and performance-based design and their implication in design of earthquake resistant structures. In the present paper a number of relationships have been proposed for calculation of the minimum reinforcement in beams which will result in optimum plastic rotations that will not result in significant damage of the sections.

Shear strength of ternary blended ferrocement plates reinforced with welded wire mesh Battina Swathi, T. Chandrasekhar Rao

The shear study of reinforced ternary blended ferrocement was investigated by conducting flexural tests on simply supported rectangular plates underneath two symmetrical point loads. The essential variables were shear span-to-depth (a/d) ratio varying from 1 to 6, volume fraction reinforcement V_f, and quantity of reinforcement close to the compression face. Test results indicated that the diagonal cracking strength of ferrocement increases as the a/d ratio decreases, and V_f, close to the compression face was increased. An empirical equation for predicting diagonal cracking strength was proposed for the ferrocement elements that are prone to shear failure at small a/d ratios with high V_f.

Structural response of RC columns exposed to high temperatures Jagath Kumari Dungi, K. Srinivasa Rao

Concrete usually performs well in building fires. Concrete members undergo significant distress and retain no useful structural strength at high temperatures. In most of the structures, columns make up a common structural element. This paper presents the study of reinforced concrete (RC) columns subjected to high temperatures. A set of 1+16 RC columns made up of ordinary Portland cement (OPC) concrete mix and another set of 16 RC columns made with high volume fly ash (HVFA) concrete mix. 32 columns meant for temperature exposure from 100 to 800°C at an interval of 100°C for1 hour duration. One column used as companion column at room temperature. After heating, specimens were cooled to room temperature by two cooling methods one by air-cooling method and the other by water quenching method. The specimens have been tested to measure the ultimate load capacity of the columns and load-deformation behavior of the columns, and the results were analysed and presented in this paper. The OPC concrete columns performed better residual strengths and stiffness over HVFA concrete columns, when the specimens cooled by air-cooling method. Whereas, the HVFA concret columns shown slightly better performance over OPC concrete columns when they cooled by water quenching method.

Application of packaged dry concrete mixture for construction in remote areas Sonu Kumar Singh, Sanchit Gupta, Raheem Shaik, B. Bhattacharjee

Construction of infrastructure facilities requiring mass concrete such as transmission lines towers’ foundations, pedestals, and roadways in remote and inaccessible areas is a challenge in many parts of the Indian sub-continent, sub-Saharan Africa, and similar nations. The problem is especially prominent where there are no ready-mix concrete (RMC) facilities available in those farflung areas and the concreting work mainly relies upon on-site mixing and consequently, faces several problems with quality, durability, and overall performance. The work here signifies the importance of factory-prepared dry concrete mix which can be a suitable alternative to site mixing. The proposed dry concrete system only requires the addition of water i.e., wet mixing at the site with controlled quality of ingredients and thus, resulting in better performance. Thus, in this study, the applicability and feasibility of an improved variant in dry concrete named as packaged dry concrete (PDC) for mass concreting application have been studied. Experimental trials have been done to check the variability of PDC parameters to that of conventional concrete. The obtained results have shown the significance and feasibility of PDC and its potential as an alternative method of concreting in these remote areas.

Interaction diagrams for rectangular columns with high strengthconcrete R. K. Desai, N. Subramanian

High strength concrete (grades higher than M55) results in smaller sizes of columns leading to the availability of higher floor spaces. Compared to concrete grades of M55 and less, high strength concrete undergoes lesser strains for the same stress. Hence, use of high strength concrete in members will result in reduction in deflections and sway. The use of high strength concrete is increasing in the construction of highrise buildings, industrial buildings and long span bridges. Design aids in the form of interaction curves are an inevitable requirement for the design of columns subjected to axial load and bending moment. Availability of interaction diagrams for columns made of concrete grades beyond M55 is scarce. Considering this requirement, an attempt is made in this paper to suggest a systematic step by step procedure to formulate interaction curves for concrete grades above M55. In this paper, mathematical expressions have been derived for rectangular columns with reinforcement equally distributed on two sides. As the procedure suggested involves iterative computations using a systematic approach, it is well-suited to be developed as a computer program.The typical output of such a program has been validated and presented for M70 concrete, Fe-500 grade of steel and, d?/D = 0.05, in the form of interaction curves.