International Concrete Abstracts Portal

The objective of this study is to revise the resistance model for calibration of the ACI 318 Code, using the new material test data. The research focused on the development of statistical parameters of the load carrying capacity for reinforced concrete beams, slabs and columns. The considered materials include ordinary concrete and high strength concrete, both cast-in-place and plant-cast. Resistance is considered as a product of three random variables representing the uncertainty in material properties, dimensions and geometry (fabrication factor) and analytical model (professional factor). Material test data is presented in form of the cumulative distribution functions (CDF) plotted on the normal probability paper for an easier interpretation of the results. In addition, the statistical parameters are determined for a normal distribution that is fitted to the lower tail of the CDF. The most important parameters are the mean value, bias factor and the coefficient of variation. It was observed that the quality of materials and workmanship have been improved over the last 30 years and this is reflected in reduced coefficients of variation. The two other parameters, i.e. fabrication and professional factors, are also considered and summarized based on the available literature. The developed resistance models were obtained by Monte Carlo simulations. They can serve as a basis for the selection of resistance factors (strength reduction factors).

This paper covers the state of knowledge on the cracking development and mitigation of cracks in two-way action slabs and plates as a measure of the health state of structural floors. In addition, the present trend is to construct green concrete structures for long-term behavior in structural and environmental serviceability performance. As supported two-way concrete slabs are an integral component of most structures, crack control is a major factor in their design. This subject has been of major interest to the author since the 1960s when extensive research on cracking in two-way action slabs and plates was started at Rutgers University that culminated in tests to failure of in excess of 95 two-way action large scale models of concrete plates accompanied by detailed analysis and development of a hypothesis on how two-way action cracks have been generated. The fracture grid controlled by the spacing of the nodal intersections of the reinforcing bars or wires proved to be the controlling factor in the generation, spacing and width of flexural cracks in two-way action. This work was extended to two-way slab tests and analysis of 12 specimens reinforced with glass fiber reinforced plastic bars. They verified the extension of this work to structural two-way floors reinforced with non-metallic reinforcing bars. The existing literature shows only two or three additional sources of limited work by other investigators and only on small scale reinforced concrete two-way slabs, perhaps due to the high expense and elaborate infrastructure needed for the necessary experimental tests to verify any proposed hypothesis or expressions. The resulting expression presents the criteria applicable to almost all normal boundary condition for concentrated and uniformly distributed loads in supported two-way slabs. It enables the choice of the size and spacing of the reinforcement that can control the propagation of the crack width, as reported in the first edition in 1972 and the several subsequent editions of ACI 224 Report “Control of Cracking in Concrete Structures.” A modified expression for crack control in walls of liquid retaining tanks is presented as well as a table of tolerable crack widths in concrete structures.

This paper reports design recommendations for flexural cracking and short-term deflection control in reinforced concrete (RC) beam-type elements. The proposed crack control procedures, which have evolved from existing design provisions in ACI 318-08 and AASHTO LRFD 2007, aim at controlling flexural cracking either directly -through the calculation of crack widths-, or indirectly -by prescribing a maximum spacing for the tension reinforcing bars-. The main feature of the proposed crack control procedures is that they are explicitly set up as a function of a limiting crack width value. This approach gives designers the ability to control specific serviceability requirements stipulated by the Owner/User of the structure and is very convenient in those instances where the limiting crack width is less than that tacitly linked to the ACI 318-08 crack control procedure. The proposed deflection control procedures are indirect in nature, aiming at controlling deflections by specifying maximum span/depth ratios based on the concept of limiting curvature and also as a function of both the deflection/span ratio and the maximum allowable crack width. Even though the serviceability design formulations are general in form, emphasis is given to RC beam-type elements in marine infrastructure.

Cracking is a major durability problem for bridge decks. Studies in Kansas dating back 20 years that correlate the density of cracking with the concrete properties, construction procedures, and environmental conditions during construction provide specific guidance on the steps needed to limit cracking in bridge decks. Since 2002, the lessons learned in those studies have been successfully implemented through training and material and construction specifications as part of an 11-year pooled-fund study to develop aggregate, concrete, and construction specifications for low-cracking high-performance concrete (LC-HPC) bridge decks. Twenty-three bridge decks have been constructed using a combination of best practices, including a reduction in the cement paste content of the concrete while maintaining workability and finishability through the use of optimized aggregate gradations, maintaining adequate air content and moderate slump, deemphasizing the importance of high compressive strength and low concrete permeability, controlling the temperature of the concrete at the time of placement, providing improved curing, and reducing the rate of drying after curing is complete. The background and specifications for the study are presented, along with a description of field experience and crack survey results, which demonstrate that the procedures used for LC-HPC decks result in crack densities that are significantly below those observed in conventional bridge decks.

This paper discusses the importance of deflection control in reinforced concrete structures and highlights a number of problems which can complicate accurate deflection calculations. The tension stiffening phenomenon is then described in some detail followed by descriptions of the approaches used to accommodate it in major design codes. Recent experimental work which has resulted in new recommendations for the treatment of long term tension stiffening effects is summarised and areas of current research are indicated. Finally, the paper comments that the tension stiffening phenomenon, in view of its important influence on deflection control, is likely to remain a topic of considerable interest for the foreseeable future.