Quantification of Crack Formation Using Image Analysis and its Relationship with Permeability

In this study a relationship between permeability of concrete and fractal dimension of crack is established. For this purpose four series of specimens of fiber reinforced cementitious composites are prepared. Specimens are subjected to uniaxial tension in order to create targeted damage (cracking) prior to permeability test. Image analysis is done on the cracked surface and fractal dimension of cracks are calculated using box counting method. Maximum crack width is found to have correlation with the coefficient of permeability. However, such correlation was observed neither between coefficient of permeability and crack area nor between coefficient of permeability and crack density. Relationships of fractal dimension of cracks is established with the maximum crack width, crack area and crack density. Trilateral relationship among coefficient of permeability, the maximum crack width and fractal dimension are established

Strain hardening behavior of lightweight hybrid polyvinyl alcohol (PVA) fiber reinforced cement composites

Experimental results on the strain hardening and multiple cracking behaviors of polyvinyl alcohol (PVA) fiber reinforced cementitious composites under bending are reported in this paper. Different hybrid combinations of PVA fibers with different lengths and volume fractions are considered to reinforce the mortar matrix. Among different hybrid combinations, the composite containing 2% thicker PVA fibers of 12 mm length and 1% thinner PVA fibers of 6 mm length and the composite containing 2% thicker PVA fibers of 24 mm length and 1% thinner PVA fibers of 6 mm length showed the best performance in terms of highest ultimate load, largest CMOD (crack mouth opening displacement) at peak load and multiple cracking behavior. The effects of four types of light weight sands on the strain hardening and multiple cracking behavior of hybrid fiber composites are also evaluated in this study. It has been observed that the ultimate load and CMOD at peak load for all light weight hybrid fiber composites are almost the same irrespective of volume fractions of light weight sand. The composites containing finer light weight sands exhibited higher ultimate load than those containing coarser light weight sands. It is also observed that the hybrid fiber composite containing normal silica sand exhibited higher ultimate load than the composites with light weight sands

Experimental study on restrained shrinkage-induced cracking of mortars with different toughness

In this paper, experimental results on restrained shrinkage test of cement mortars and light weight high performance fiber reinforced cementitious composites (HPFRCC) are presented. Two types of light weight hybrid HPFRCC and two types of premix mortars are included in the experiment. Results show the multiple cracks, as many as 49, in light weight hybrid HPFRCC specimens compared to a localized single crack in premix mortar-II and about six cracks in premix mortar-I specimen. The maximum crack width in the premix mortar-II and mortar-I specimens is about 1.4 mm and 0.40 mm, respectively. However, the scenario is quite different in light weight hybrid HPFRCC specimens, where the width of almost all cracks are less than 0.10 mm except only one crack the width of which is about 0.40 mm

Tailoring high-strength SHCC

Multiple cracking and strain hardening can be achieved in cement-based specimens subjected to uniaxial tension by increasing the volume fraction of steel fibers with hooked ends, or by using plastic fibers with and without steel fibers, or by means of high bond steel fibers (e.g., twisted fibers or cords). To better understand why relevant mechanical performances are obtained in such situations, an analytical micro-mechanical model was proposed. The model, capable of predicting the average distance between cracks as measured in some experimental campaigns, is here used to tailor a high performance fiber-reinforced concrete. Specifically, a two High-Strength and Strain Hardening Cementitious Concrete (HS-SHCC), reinforced with different types of steel fibers, are introduced. By combining direct uniaxial tensile tests, performed on the so-called dumbbell-shaped specimens, and the results of the micro-mechanical model, the critical value of the fiber volume fraction can be evaluated. It should be considered as the minimum amount of long steel fibers which can lead to the formation of multiple cracking and strain hardening under tensile actions. The aim of the present paper is to reduce such volume as much as possible, in order to improve the workability and reduce the final cost of HS-SHCC

Synergy assessment in hybrid Ultra-High Performance Fiber-Reinforced Concrete (UHP-FRC)

Ultra-High Performance Fiber-Reinforced Concretes (UHP-FRC) subjected to uniaxial tensile loads are investigated in the present paper. The study comprises a new procedure to assess the effectiveness of the hybridization, herein obtained by reinforcing UHP-FRC with micro and macro steel fibers. A comprehensive experimental campaign is also performed on monofiber and hybrid UHP-FRC. In all the concretes, the distance between the cracks and the minimum fiber volume fraction, which produces strain hardening response and multiple cracking, are theoretically and experimentally evaluated. If the bond parameter of the macro-fibers is properly calculated, the results of the analytical model, in terms of crack-spacing vs. fiber volume fraction, are in good agreement with the test data. Moreover, to increase the number of the cracks, and to reduce crack spacing, the hybridization is suitable only when the amount of macro-fibers is within a well-defined range