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.

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.

Inverted “T” bent caps are used extensively in concrete highway bridges because they are aesthetically pleasing and offer a practical means to increase vertical clearance. The problem is that at service load unacceptable diagonal cracks frequently occur at the re-entrant corners between the cantilever ledges and the web. In order to control the diagonal cracks, an extensive three-phase investigation was carried out. The first phase was to predict the diagonal crack widths at the interior portions of the bent caps. A 2-D analytical model, called Compatibility-Aided Strut-and-Tie Model (CASTM), was developed. This model was calibrated by testing seven full-size 2-D test specimens. The second phase was to predict the diagonal crack widths at the end faces of the exterior portions of bent caps. In this phase the CASTM was extended to 3-D analysis, which was calibrated by testing ten 3-D specimens. In the third phase, two whole-wholebent-cap specimens were tested to determine the effective distribution width in the vicinity of an interior applied load on the ledge. Crack control methods for the interior spans and the exterior end faces were recommended.