International Concrete Abstracts Portal

This work describes a modification to the two-parameter fracture method’s experimental procedure aimed at removing this operator/equipment dependence. With this method, three compliances are used to determine the focal point at which these compliances intersect. This focal point is then used to determine the slope of the unloading compliance that corresponds to the peak of the load vs. CMOD curve. The unloading compliance that corresponds to unloading at the peak load and initial compliance are then used to determine Ktc and CTODc as normally done with the Two Parameter Fracture Model. Use of this method makes it possible to remove operator and machine dependence, especially if the materials are extremely brittle, such as in pastes or high strength concrete, thereby permitting the loading and unloading to be programmed using testing software removing the need for manual operator loading changes. Tests on 15 mortar beams with 4 different notch lengths and initial unloading points ranging from 97% to 75% of maximum load are used to validate this approach. The experimental results are typically more consistent and better correlate to results from the peak load test method. These results indicate that utilizing the focal point correction typically reduces Ktc and CTODc by 12% and 38% respectively for the mortar tested thereby causing the TPFM and peak load method results to coincide even more closely.

Early-age cracking can occur in concrete if free shrinkage is prevented by the surrounding structure. This paper highlights recent findings to illustrate that shrinkage cracking is influenced by the geometry of the structure. A series of experimental results are presented from three different ring specimen geometries to illustrate that although these specimens had the same residual strain level (and similar residual stress), the age of cracking varied with specimen geometry. A second series of experiments was performed to illustrate that a geometry dependence also exists in specimens with moisture gradients. This paper describes how fracture mechanics concepts can explain this geometry dependent behavior under a uniform moisture distribution. Residual stress levels are computed, non-linear fracture mechanics failure criterion is applied to develop the time and geometry dependent tensile stress resistance curves, and the age of cracking is predicted. The theoretical simulations were found to compare reasonably with the experimental observations. A discussion is provided to illustrate how these considerations may be extended to the specimens with moisture gradients.

Fracture surface provides valuable information on internal structure and mechanical behavior of composite materials. Loading rate affects the roughness of the fracture surface of composites. A higher loading rate, in general, results in a smoother fracture surface. Similarly, aggregate size influences the roughness of the fracture surface. Larger aggregates cause a rougher fracture surface under the same loading rate. The roughness of the fracture surface of concrete is experimentally studied using concrete specimens made of the different aggregate sizes under different loading rates. Fractal dimension is used to evaluate the surface roughness of concrete specimens. A new fractal fracture model is developed which correlates the fractal dimension with concrete mix design parameters, such as volume fraction and size of aggregate, as well as loading rate. The model prediction agrees with test data very well.

Crack propagation in cement-based matrices reinforced with micro-fibers of steel and carbon was studied using contoured double cantilever beam specimens. Influence of fibers, sand and silica fume was quantified using crack growth resistance curves. It was demonstrated that these fibers enhance the resistance to both nucleation and growth of cracks, and that such fundamental fracture tests are very useful in developing micro-fiber composites with a high performance. The influence of number of variables which would otherwise have remained obscured in normal tests for engineering properties become apparent in the fracture tests. The paper points out the desired durability characteristics of these composites and discusses their current and future applications.

The testing proccdurc for the uniasial tension of concrctc in or&r to monitor tension softening bchwior is not cstablishcd )zt, bccausc thcrc arc scvcral difficulties for performing the tat. Various test conditions v,,hich wcrc somctimcs on the contrary have been proposed in the pzst 30 years. This paper suggests a rccommcndablc testing proccdurc for the uniwxial tension with fundamcnt:rl csplanations. In the prcscnt proccdurc, prismatic spccimcns having primary and guide notches on the laterals v,‘crc cmploycd. In addition, an original gear system w;is adopted for complctc elimination of secondary flcxurc or for maint;rining an artificial flcsurc at a conskrnt Icvcl. It was shown that the proposccl proccdurcs cnzblc to monitor tension softening curves :rnd to provide rcliablc test results.