Current practice related to design of concrete structures for deflection control is reviewed. The paper discusses the limitations of the current code procedures based on minimum thickness rules and deflection calculations. Results are presented to demonstrate the sensitivity of deflections to span to depth ratio, sustained live load, and extent of cracking.

With the introduction of high strength concrete and larger diameter strands, more emphasis is placed on developing new types of prestressing alternatives that would take advantage of these new developments. Moreover, the use of replaceable unbonded tendons allows future upgrade, maintenance, and rehabilitation of concrete structural elements. However, few researchers have addressed the behavior of beams with unbonded tendons under service loads. There is a need to adopt simplified and rigorous procedures for the prediction of deflection as well as the cracking behavior. This paper presents a new rigorous procedure for calculating the deflection of high strength concrete girders prestressed with unbonded tendons. The analysis is based on the “trussed-beam” analogy used to model the prestressed concrete girder and the unbonded tendons as trussed beam system. To validate the analysis, a total of twenty-two beams with various parameters were tested to failure. Results are presented in terms of load versus deflection as well as the stress in tendon.

Composite materials and high strength adhesives are becoming popular for various structural repair jobs. These materials provide good solutions for repairing, and retrofitting concrete structures. In this system repair work can be done within a short time, without using much labor, and the repair materials generally do not alter the geometric shapes of the original structural member. However, the interactive behavior (both short and long term) of un-bonded post-tensioned structural components, when repaired and retrofitted with composite materials is not yet properly understood. The current research was conducted to understand this. Eleven 12’-0" long beams B1 thru B11, were loaded close to their ultimate values. The tests were stopped based on three criteria: excessive cracking, or pre-stressing load reaching over 80%-85% fpu, or deflection reaching twice the allowable deflection. The cracked beams were repaired with varying amount of composite materials adhesives, and tested again. The beams were named B1CR, B2CR, B3EgR and so on, where CR represents repair with carbon FRP and EgR represents repair with e-glass FRP. The tests were stopped again based on the above three criteria. Using Carbon and E-glass FRP and adhesives, a very effective repairing and retrofitting system has been developed. The load carrying capacities and deflections of these beams and that of the original un-cracked beams were compared. Various repairing schemes were adopted. Attempts were made to find the minimum usage of the composite materials, prevent diagonal shear cracking, and prevent sudden compression failures. The changes in post-tensioning forces, the effect of having mild steel in the tension zone, and crack propagation at ultimate stage were recorded and discussed here.

Current code provisions for deflection calculations in reinforced concrete beams and slabs are discussed. A comprehensive series of laboratory tests to investigate long term tension stiffening effects is summarised, results from which indicated that tension stiffening decays much more rapidly than was previously understood. Revised code provisions are proposed based on this finding.

This paper presents the results of a novel technique in remediating cracks and fissures in concrete by utilizing microbiologically induced calcite (CaCo3). Bacillus Pasteruii, a common soil bacterium was used to induce calcite precipitation. This technique is highly desirable because the mineral precipitation induced as a result of microbial activities, is pollution free and natural. The effectiveness of this technique was evaluated by comparing the compressive strength and stiffness of cracked specimens remediated with bacteria and those of the control specimens (without bacteria). Experimental investigation was also conducted to determine the strength regaining capacity (modulus of rupture) of cracked beams remediated with different concentrations of bacteria. This paper also presents the results of a durability study on cement mortar beams treated with bacteria, exposed to alkaline, sulfate and freeze-thaw environments. Different concentrations of bacteria were used for the investigation. It was found that the use of bacteria improved the stiffness, compressive strength, modulus of rupture and durability of concrete. Scanning electron microscope (SEM) was used to document the role of microbiologically induced mineral precipitation in improving the strength and durability aspects of concrete. Energy Dispersive X-ray (EDX) spectrometer analysis of the precipitated crystals indicated abundance of calcium and the precipitation was confirmed to be calcite.