Strengthening of Shear Deficient Reinforced Concrete Beams Using Polymer Modified Ferrocement (PMF)

Abstract

There is a common phenomenon of shear failure in RCC beams, especially in old buildings and bridges. Any possible strengthening of such beams needs to be explored that could strengthen and make them fit to fulfil the strengthcriteria. The present research has been carried out to determine the performance of polymer modified ferrocement (PMF) used to strengthen the pre-damaged RCC beams. Ferrocement has become a universally acceptable thin composite material in the field of construction technology. This is primarily due to its better strength to thickness ratio. In the present research work, the polymer modified ferrocement has been used to strengthen the beams as it had been come of age in the last two decades or so and has found that the conventional ferrocement made with cement mortar matrix does not exhibit significant durability. Hence, two types of polymers were chosen toadd in the mortar matrix to improve the properties of traditional ferrocement. As a pilot study, the effects of addition of Vinyl-Acetate-Ethylene (VAE) polymer and Styrene-Butadiene-Rubber latex (SBR) polymer on the properties of mortar and ferrocement have been studied. The polymer-modified mortar (PMM) matrices were prepared by adding different percentages of polymers varied from 0% to 20%. Cement-sand ratio for mortar was fixed as 1:2 and water-cement ratios have been also worked out for a specified flow value of 105±5%. The compressive strength, workability, flexural strength,air content and Young’s modulus of elasticityof PMMs was worked out by conducting the various tests. Test results showed that the mortars modified with polymers showed better performance performed as compared to un-modified mortars. Further, the polymer modified ferrocement (PMF) elements were cast with different volume fraction of reinforcement consisting of 2 and 3 layers of wire mesh. Same polymer modified mortar composition as of pilot study, was used to cast the PMF specimens. The overall test results reflect that the performance of SBR modified ferrocement is better as compared to VAE modified ferrocement. Flexural strength, tensile strength and elongation of three-layered PMF on addition of 15% SBR, showed an enhancement by 48.04%, 39.41% and 17.59% respectively, as compared to conventional ferrocement. This is attributable to the development of a polymeric layer, in which the polymer elements merge, a polymeric compound is propagated and an interlocked mesh composition is formed. Hence, based upon these test results, the 15% SBR modified mortarin three-layered wire mesh ferrocement elements was used to strengthen the RCC beamspecimens for better durability and strength. Forty-eight shear designed and shear deficient real size beams have been cast and tested to achieve the objective of present work. Ultimate shear load carrying capacity of control beams were found forshear-span (a/d) ratios of 1.0 and 3.0. The shear designed beams contained stirrups spacing of 150mm c/c and to make the beams deficient in shear, these stirrups were spaced at 450mm c/c. Based upon the test results of control beams, three zones (elastic, elasto-plastic and plastic zones) were located on the load v/s deflection curve. The pre-damaging load for beamshas been worked out corresponding to the load level of 45%, 75%, and 95% of ultimate load, respectively. After that the sets of remaining beams were loaded with different pre-determined damage levels and then strengthened with 20mm thick PMF. U-shaped jacketing methodology was made to strengthen the beams for a length equal to ‘2d’, irrespective of the a/d ratio, with distance ‘d’ on each side from load point. The ultimate load-carrying capacity of the strengthened beams was found out by conducting the re-load test for a/d ratio 1and 3. The results indicate that the PMF strengthened beams show full restoration and also enhancement of ultimate shear load carrying capacity by 5.90% to 12.03%. The ductility and stiffness of the strengthened beams were improved; hence the corresponding deflections were prolonged. On the other hand, the cracking pattern of PMF strengthened beams was also improved remarkably. All the test results have been discussed and analysed in term of loads, deflections, effect of spacing of stirrups,effect of shear span ratios,modes of failure and cracking behaviour. Finite element modelling of control as well as strengthened beams was made on ATENA-3D software to validate the test results. The FE model also shows similar type of behaviour to experimentally tested control and strengthened beams with an error of ±8% for both a/d ratios of 1 and 3.