International Concrete Abstracts Portal

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.

Cracking caused by both shrinkage and external loads in reinforced concrete members is examined both experimentally and analytically. The mechanisms of cracking and the factors affecting the time-varying width and spacing of flexural cracks in beams and slabs and direct tension cracks in longitudinally restrained members are examined. Laboratory tests on twelve reinforced concrete beams and slabs subjected to sustained service loads were conducted in order to measure and quantify the effects of steel area, steel stress, bar diameter, bar spacing, concrete cover, concrete strength and concrete shrinkage on the extent of flexural cracking and the width of flexural cracks both immediately after loading and in the long-term after almost 400 days under load. In addition, an analytical procedure is presented that models time-dependent cracking. Use is made of the tension chord model developed by Marti et al.1 which is here modified to study the tensile zone of a flexural member and the time-dependent effects of creep and shrinkage. A second series of tests on longitudinally restrained slab specimens is also reported and the analytical procedure is extended to model the time-dependent development of direct tension cracking.

A number of different crack width prediction equations have been proposed for use with partially prestressed concrete structures, but laboratory and field results for members with varying levels of post-tensioning are limited. A durability study at The University of Texas at Austin involved beams with varying combinations of bonded post-tensioning and non-prestressed reinforcement. This work included extensive crack width measurements at varied load levels. A follow-up study is now on-going at Penn State University to consider beams with a combination of pretensioning and post-tensioning (such as found in spliced girder applications). This paper presents and discusses the measured crack data from these studies, and compares the data to selected existing crack width prediction models. Comparison of measured and predicted crack widths did not reveal a single comprehensive crack width prediction formula for the range of variables considered.

Current structural concrete design standards require that post-tensioned slab systems be provided with mild steel or post-tensioning tendons to control potential transverse cracking due to shrinkage and temperature effects that may occur soon after concrete is cast and over the life of the structure. For the case of prestressed concrete structures, the ACI 318 Building Code requires that the tendons should be designed such that they provide a minimum average compressive stress of 100 psi (0.7 MPa) on a gross concrete area as an effective prestress. This level of prestress is expected to be sufficient to control cracking caused by shrinkage and temperature effects. The Code further requires that the effects of restraint be considered. Field evidence suggests that this minimum value may not be adequate for some post-tensioned one-way slab systems. Relevant case studies are presented and discussed. In these cases, the structural design included the minimum prestress as required, yet significant transverse cracking was observed on some areas of the slab despite the presence of the shrinkage and temperature tendons. This paper presents a design procedure for controlling shrinkage and temperature cracking in post-tensioned one-way slab systems consistent with current code requirements. A method is proposed by which an equivalent shrinkage stress is calculated. This stress is then used to design the shrinkage and temperature tendons. The equivalent shrinkage stress is calculated considering the restraint on the slab provided by elements such as columns and shear walls. It is shown that this restraint, which is a function of the respective stiffness of the restraining elements, is very significant and its effects should be considered if shrinkage and temperature cracking is to be effectively controlled by either tendons or mild steel reinforcement.

Epoxy coated reinforcement is often used as a means of improving the durability of structures. The use of epoxy coating on reinforcement, however, has been shown to decrease bond strength resulting in increased crack spacing and crack widths relative to uncoated reinforcement. While the general influence of epoxy coating on cracking is recognized, there is scare data available to substantiate the magnitude of the effect. The objective of this research was to evaluate the influence of epoxy coating on crack spacing and crack widths. Ten specimens were subjected to a constant moment region, and the crack spacings and widths were measured over a range of reinforcement stress levels. Primary variables included the epoxy coating thickness, reinforcement spacing, and reinforcement stress. The results from this study showed that epoxy coating thickness affects both crack spacing and width. In general, as coating thickness is increased, crack width increased and crack spacing decreased. While crack control aimed at minimizing corrosion of reinforcing steel may not be of concern when epoxy coated steel is used, crack control remains an important design consideration for aesthetic reasons, for increasing structural durability, and for the design of water-retaining or other specialized structures.