Mohsen A. Issa

Behavior of Masonry-Infilled Nonductile Reinforced Concrete Frames

This paper presents research on the behavior of a type of building popular in high seismic zones with a lateral-load-resisting system consisting of masonry-infilled reinforced concrete (RC) frames. Older buildings of this type typically were designed for gravity loads in combination with insufficient or no lateral loads, therefore they do not meet current seismic code requirements. Also, the participation of infill panels in the lateral load resistance of RC frames was not recognized in the original design, often resulting in an overly conservative design. In an attempt to determine the seismic vulnerability of this type of structure, an experimental program was carried out to evaluate the behavior of five half-scale, single-story laboratory models with different numbers of bays. The results indicated that infilled RC frames exhibit significantly higher ultimate strength, residual strength, and initial stiffness than bare frames without compromising any ductility in the load-deflection response. Furthermore, the number of bays appears to be influential with respect to the peak and residual capacity, the failure mode, and the shear stress distribution.

Fractal dimension––a measure of fracture roughness and toughness of concrete

Abstract The quantitative description of rough surfaces and interfaces has been an important challenge for many years. This paper addresses the potential application of fractal geometry to characterize the fracture surface and to determine whether there is any correlation between fracture properties and the roughness of the fracture surface. Fractured surfaces of three different size wedge-splitting specimens, dimensions varying from ( width × total depth × thickness ) 420×420×50 mm to 1680×1680×200 mm with four different maximum aggregate sizes of 9.5, 19, 38, and 76 mm, were analyzed using a modified slit-island technique. It was found that fractal dimension, i.e., roughness, increases with an increase in both specimen and maximum aggregate size. A clear correlation exists between roughness (fractal dimension) and fracture toughness: the tougher the material, the higher the fractal dimension.

Assessment and evaluation of fractal dimension of concrete fracture surface digitized images

Abstract Recently, a new non-destructive technique was introduced and tested by the authors [1,2] for the evaluation of fractal dimension of fractured concrete specimens. The objectives of this paper are to expand and eleborate on the advantages of the technique, to provide a basic understanding of fractal geometry and to discuss the potential application of fractal geometry to fracture in cementitious materials. This may eventually lead to a better understanding of the fracture mechanics properties of such materials. Through this technique and the utilization of an Image Analyzer System, the fracture surface was photographed and stored as an image. Different methods of fractal analysis, e.g. modified slit island technique (SIT) and two-dimensional Fourier spectral analyses (2D FFT) were employed. Fractured concrete specimens with a maximum aggregate size of 37.5 mm and a projected fracture area of 367.5 mm long and 125 mm wide were used in the study. Results of the analysis suggest that concrete fracture surface exhibit fractal characteristics over the scales considered. A scaling relation between roughness at different length scales holds only over a limited range of wave lengths. The mean fractal dimension for the modified slit island technique is D=2.21 and the mean stochastic fractal dimension for two-dimensional Fourier spectral analyses is D=2.59. Also, the digitized fracture surface images were found to mimic the actual fracture surfaces.

Size effects in concrete fracture – Part II: Analysis of test results

This paper presents an analysis of the extensive experimental program aimed at assessing the influence of maximum aggregate size and specimen size on the fracture properties of concrete. Concrete specimens used were prepared with varying aggregate sizes of 4.75, 9.5, 19, 38, and 76 mm. Approximately 250 specimens varying in dimension and maximum aggregate size were tested to accomplish the objectives of the study. Every specimen was subjected to the quasi-static cyclic loading at a rate of 0.125 mm/min (0.005 in./min) leading to a controlled crack growth. The test results were presented in the form of load-crack mouth opening displacement curves, compliance data, surface measured crack length and crack trajectories as well as calculated crack length, critical energy release rate, and fracture toughness (G1). There is a well pronounced general trend observed: G1 increases with crack length (R-curve behavior). For geometrically similar specimens, where the shape and all dimensionless parameters are the same, the R-curve for the larger specimens is noticeably higher than that for the smaller ones. For a fixed specimen size, G1 increases with an increase in the aggregate size (fracture surface roughness). For the same maximum aggregate size specimens, the apparent toughness increases with specimen size. It was clear that the rate of increase in G1, with respect to an increase of the dimensionless crack length (the crack length normalized by the specimen width), increases with both specimen size and maximum aggregate size increase. The crack trajectory deviates from the rectilinear path more in the specimens with larger aggregate sizes. Fracture surfaces in concrete with larger aggregate size exhibit higher roughness than that for smaller aggregate sizes. For completely similar specimens, the crack tortuosity is greater for the larger size specimens. The crack path is random, i.e., there are no two identical specimens that exhibit the same fracture path, however, there are distinct and well reproducible statistical features of crack trajectories in similar specimens. Bridging and other forms of crack face interactions that are the most probable causes of high toughness, were more pronounced in the specimens with larger maximum size aggregates.

Size effects in concrete fracture: Part I, experimental setup and observations

AbstractThis paper presents an experimental investigation on the influence of microstructural parameters, such as aggregate size, and macroscopic parameters, such as specimen dimensions, on brittle fracture. Maximum aggregate size was used as a representative parameter of aggregate distribution in agreement with ASTM C 136 standards. Six groups of geometrically similar concrete specimens with various dimensions and aggregate sizes were prepared. Similarity of the specimens was strictly maintained by scaling the specimen dimensions from one group to another by a factor of two starting from a specimen size of (width × total depth × thickness) 105×105×12.5 mm to 1680×1680×200 mm. Two separate sets of removable pre-cast notches were designed to determine the effect of initial notch size. A considerable effort was devoted to the design of the loading fixture to have a reproducible crack initiation and controlled crack growth. Several loading fixtures were evaluated prior to selection of the one used in the experimental program. Quasi-static splitting cyclic loading in edge cleavage configuration was applied. A servo-hydraulic Instron machine was used for testing. The fracture process was monitored by optical and acoustic imaging techniques. Three forms of comparisons of the test results with respect to the specimen and aggregate sizes were adopted. The first corresponded to the various specimen sizes cast with the same maximum aggregate size. The second comparison was based on the geometrically identical specimens cast with various maximum aggregate sizes. The third form of comparison dealt with complete geometrical similarity, i.e., all dimensionless geometrical characteristics including specimen thickness to maximum aggregate size ratio were identical. Results from this study indicated that as the specimen size decreases, the envelope becomes larger within the first and third forms of comparison. In the second form of comparison, i.e., geometrically identical specimens cast with various maximum aggregate sizes, the area under the envelope was greater as the maximum aggregate size increased. The existence of a trend in dimensionless critical load-CMOD envelopes despite the apparent geometrical and physical similarity of the test conditions is the direct indication of a scale effect, i.e., the modified fracture energy,

SPECIMEN AND AGGREGATE SIZE EFFECT ON CONCRETE COMPRESSIVE STRENGTH

\overline {G_F }

Fractal characterization of fracture surfaces in mortar

indicates the existence of a strong scale effect: