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Section 20.2 of ACI 318-08 gives provisions for determining required dimensions and material properties for the strength evaluation of an existing concrete structure. Subsection 20.2.3 states “If required, concrete strength shall be based on results of cylinder tests from the original construction or tests of cores removed from the part of the structure where the strength is in question. For strength evaluation of an existing structure, cylinder or core test data shall be used to estimate an equivalent fc’.” The commentary cites two methods presented in ACI 214.4R for determining an equivalent-to-specified fc’ using cores from an existing structure. There are no criteria provided or references cited, however, suggesting how to compute an equivalent fc’ based on cylinder strength data. The paper first identifies necessary conditions for using original cylinder test data to determine a concrete compressive strength for structural evaluation. To investigate strength compliance during initial construction, methods to compute an equivalent-to-specified fc’ value are presented that are based on inverting the conventional acceptance criteria given in ACI 318-08 Section 5.6.3.3. To evaluate an older structure, a method to determine an equivalent-to-specified strength based on the lower-bound fractile of the concrete strength represented by fc’ is presented.

The study of failures can help teach students how to ensure the satisfactory performance of buildings and bridges. A number of failure case studies have been developed for use in courses teaching reinforced concrete design, as outlined in the book “Beyond Failure.” These include failures in the construction process and in formwork, such as the Willow Island Cooling Tower. Some of the shear design provisions in current codes can be traced back to two collapses of Air Force warehouses in the mid-1950s. Three building collapses in the 1970s and 1980s showed the importance of punching shear. There is also much to be learned from reinforced concrete building performance under extreme conditions, such as the terrorist attacks on the Oklahoma City Murrah Building and the Pentagon (9/11). Another classic case is the collapse of a major section of the Ronan Point apartment towers in the UK in 1968, illustrating the need to properly tie precast building elements together. The collapse of the Laval, Quebec concrete bridge abutments in Canada shows the importance of providing continuity of reinforcement. This case study also offers the opportunity to illustrate the application of strut and tie models to analysis of complex reinforced concrete structures.

The Oklahoma City Bombing is used as a case study to demonstrate how structural damage investigations can lead to code changes and improvements in design. This paper describes the use of seismic detailing to improve blast resistance. Such detailing has been incorporated into the soon to be published ASCE/SEI Standard on Blast Protection of Buildings.

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.