فهرست مطالب علیرضا کبودان

Aggregates make up between 60% and 80% of the concrete volume, being surrounded by the cement paste and their properties all have significant impact on the behavior of concrete, during the service life for which it been designed. For this reason, a series of laboratory tests were conducted during this investigation to evaluate the effects of mechanical, durability and geometry properties of aggregates on the compressive strength and permeability of the 28-day concrete specimens. Concrete specimens were made with different types of aggregates, including granite, andesite, silica, limestone, marble and tuff. Studied mechanical properties were compressive, tensile and surface strengths, Los Angeles abrasion index, Schmidt hardness and saturated surface dry density, while permeability, water absorption and porosity were investigated as durability properties. Geometry properties of aggregates, including circularity, solidity and aspect ratio were also measured to investigate the effect of aggregates shape on the concrete compressive strength and permeability. To measure permeability and surface strength, the methods of “Cylindrical chamber” and “Twist-off” were applied, respectively, which were proposed and developed by Naderi. The results obtained revealed that a direct linear correlation, with coefficient of determination higher than 0.89 exists between compressive and surface strengths of the studied concrete specimens and parent rocks. On the other hand, parent rock compressive strength is approximately 140%-270% higher than that of the concrete containing aggregates of the same parent rock, while its permeability is nearly 59%-71% lower than the permeability of the corresponding concrete. It was also observed that concrete compressive strength decreases with increasing its permeability and vice versa. Analysis of the results demonstrated that strong correlations, with coefficients of determination higher than 0.73 exist between concrete compressive strength, concrete permeability and the mentioned mechanical and durability properties of parent rocks and the corresponding aggregates. On the contrary, coefficients of determination lower than 0.1 were obtained for the relationship between geometry properties of aggregates, concrete compressive strength and permeability. This behavior revealed that the impact of aggregates shape on concrete compressive strength and permeability is not significant, compared with the mentioned mechanical and durability properties of the parent rocks and the corresponding aggregates.

Evaluation of concrete durability is commonly performed by measuring its permeability, porosity and electrical resistivity. Moreover, using admixtures in concrete mix design is one of the methods of enhancing durability. For this reason, in this paper, the effect of pozzolanic admixtures (silica fume, fly ash and zeolite) and limestone powder on permeability, porosity and electrical resistivity of 28-day and 120-day cubic concrete specimens with 15cm dimension is investigated. For concrete preparation, 5, 10, 15 and 20 percent of cement mass were replaced by these admixtures. In this paper, the innovative method of “Cylindrical chamber” was used for measuring concrete permeability. The results obtained revealed that except the 28-day concrete specimen that 20 percent of its cement mass was replaced with limestone powder, other concrete specimens had lower permeability and porosity and higher electrical resistivity than the concrete specimen without any admixtures. In addition, volumetric porosity is a more accurate index for assessing concrete microstructure than surface porosity calculated from the images taken using scanning electron microscope. XRD test results also showed that pozzolanic material produce more calcium silicate hydrates by consuming calcium hydroxide which results in the reduction of permeability and porosity and increase of electrical resistivity, compared with the concrete specimen without any admixtures. Also, a slight reduction in calcium hydroxide and calcium silicate hydrate peaks was observed for the concrete containing limestone powder. This behavior showed that limestone powder decreases permeability and porosity and increase electrical resistivity of concrete, mainly due to the filler effect of its particles.

Concrete is one of the widely used materials in hydraulic structures. The permeability of these structures is considered to be one of the most important factors. Therefore, in this paper, the effects of aggregates, cement paste, transition zones, and concrete surface strength, on the penetration of surface water into the concrete are presented. For this study, 150 mm concrete cubes containing granite, andesite, siliceous, limestone, marble, and tuff aggregates were prepared. These specimens also contained type 2 Portland cement, silica fume, fly-ash, zeolite, and limestone powder. These admixtures replaced 10% of the cement content. While the “cylindrical chamber” was used for the permeability measurement, the “Twist-off” method was used to estimate the surface strength of the concrete specimens. Regression analysis of the permeability readings of the parent rocks, cement paste, interfacial transition zone length, and concrete surface strength revealed that the penetrated water volume into the concrete specimens could be predicted, using the proposed regression equation. It was also observed that, compared with other considered parameters, the cement paste, and concrete surface strength had the highest and lowest impact on the concrete permeability, respectively.

Since a portion of concrete is made of the cement paste, its impact on the concrete permeability can be considerable. For this reason, in this paper, permeability of the concretes containing different types of cements (type 1-425, type 2, type 5, pozzolanic and white cements) and mineral admixtures (microsilica, fly ash, zeolite and limestone powder) was studied, using the results obtained from “Cylindrical chamber” method. Cement replacement levels with mineral admixtures were 5%, 10%, 15 and 20%. Considering the durability problems of the concrete structures located in Iran’s seashores, in addition to the tap water, seawater was used for the permeability assessment of the concrete specimens containing different types of cements. The results tend to show that, except the concrete specimen containing limestone powder with a replacement level of 20%, other concrete specimens have lower permeability than the concrete specimen without any mineral admixtures. Microstructural properties of the concretes containing the mentioned mineral admixtures with replacement levels of 0% and 5%, obtained from analyzing scanning electron microscope images were also consistent with the results obtained. Furthermore, it was observed that water type has great impact on concrete permeability. In this regard, concrete specimens containing type 1-425 and pozzolanic cements had the lowest permeability when exposed to potable water and seawater, respectively. On the contrary, concrete specimens containing pozzolanic and white cements showed the highest permeability against potable water and sea water, respectively. Weak correlation was also seen to exist between permeability and compressive strength of the studied concrete specimens.

Permeability is one of the most effective parameters on concrete durability. Therefore, in this paper penetration of water into concrete is studied. Although most of the researchers have considered the coefficient of permeability obtained from one dimensional Darcy’s equation, in the present paper due to movement of water in all directions, two-dimensional diffusion equation which defines penetration of fluid into a porous material has been used for first time. For this purpose, cubic concrete specimens with different W/C ratios were prepared and their permeability was measured using “Cylindrical chamber” method. In this method, applied pressures and durations of water penetration were varied. The two-dimensional equation considered, was solved using Laplace and Henkel transformations and the obtained results were compared with the “Cylindrical chamber” results. Comparison of the theoretical and experimental results showed that the average respective percentage errors calculated for the estimation of wet curve, maximum penetration depth, average penetration depth and wet surface were 23.07, 13.64, 21.41 and 1.66. The coefficients of determination between pressure magnititude and duration of water penetration, considering the variables of maximum penetration depth, average penetration depth, wet surface, penetrated volume and optimum diffusion coefficients were seen to be higher than 0.95. Furthermore, no reliable correlation was observed between the optimum diffusion coefficients and the mentioned variables.

Reinforced concrete is the most commonly used construction material. The durability of reinforced concrete mostly depends on the surrounding environmental and exposure conditions such as carbonation, corrosion, and freezing and thawing. Corrosion of steel in concrete is one of the major causes of premature deterioration of reinforced concrete structures, leading to structural failure and is largely controlled by the permeability of the concrete cover to protect the steel reinforcement from corrosion. Therefore, the effect of silica fume, fly ash and zeolite pozzolans on the compressive strength, permeability and reinforcement corrosion is evaluated in this investigation. The compressive strength of 7 and 28-day concrete samples results show that silica fume results in compressive strength increase with respect to the control sample (without any pozzolans). Except for the samples containing zeolite with replacement weight percentages equal to 5, 10 and 15 percentage and fly ash with replacement weight percentage equal to 5 percentage, in other cases the compressive strength of 28-day samples containing fly ash and zeolite are lower than that of the control sample. The results of cylindrical chamber test also represent the permeability reduction of the 7-day samples containing these pozzolans with respect to the control sample. Based on the half-cell method test results, it is also observed that the resistance to corrosion of reinforcement concrete containing silica fume and zeolite is improved by using these pozzolans.

Concrete is a porous material. Water and other fluids can penetrate it and effect its durability. So permeability is one of the most effective parameters of concrete structures durability. Design and construction of concrete, which has a less permeability in addition of providing the desirable compressive strength is of great importance. In this paper, the permeability of 7-day concrete samples containing different pozzolans was investigated using cylindrical chamber method. The mass percentages of silica fume, zeolite and fly ash equal to 5, 10, 15 and 20 percentage that replace type II Portland cement and the existing method of British standard (BS EN 12390-8:2009) were used to compare the obtained results. The percentage of the permeable voids volume was also measured based on ASTM C642-06 standard and used as an index to evaluate permeability. Samples strength was also investigated using the results obtained from the twist-off method. The results of the permeability tests show that all of the pozzolans used in this investigation decrease the percentage of the permeable voids volume and consequently decrease the samples permeability, while silica fume increases the samples compressive strength and zeolite and fly ash decrease samples compressive strength. A very good correlation was also observed between the cylindrical chamber and British standard method results.

Due to penetration of harmful agents into concrete and reduction of its durability, having knowledge about the value of concrete permeability is of importance. Permeability tests are conducted using a specified pressure and duration in concrete codes. While pressure value and its duration are important factors effecting permeability. So, the effects of the test pressure value, its duration and the direction of the casting on the permeability of cubic concrete samples made of dried aggregates and different water to type II Portland cement ratios equal to 0.4, 0.5 and 0.6 at the ages of 7, 28 and 91 days are evaluated using ‘‘cylindrical chamber’’ method. ‘‘Twist-off’’ method is also used for determination of concretes strength. The obtained results show that a linear relationship exists between shear stress of twisting moment and compressive strength and permeability of the samples in casting direction is lower than the permeability perpendicular to casting direction and the exponent obtained to approximate the relationship between penetration volume and depth with pressure is approximately constant in a specific age and test direction, regarding the water to cement ratio. This issue wasn’t observed in approximation of the relationship between penetration volume and depth with the pressurizing time using power function. An inverse relationship between compressive strength and permeability of samples was also observed. The relationship between penetration volume and depth was approximated with high accuracy using linear function. The calculated Reynolds number also shows that flow regime is laminar in concrete samples.

Permeability is an index, describing the easiness of fluid movement into a porous body, such as concrete. Water containing deleterious substances, such as chloride ions, sulfate ions, aggressive chemicals, etc. can penetrate into concrete and, subsequently, affects its short-term and long-term durability. Therefore, it is of great importance to evaluate the water permeability of concrete. For this purpose, the results of the permeability of the 7-day concrete specimens, containing silica fume, fly ash, zeolite, and limestone powder with replacement levels of 5, 10, 15 and 20 percentage of type II Portland cement mass are presented in this paper. ``Cylindrical chamber'' method, which was devised and developed by Naderi, was used during this investigation, for permeability measurements. Furthermore, one-dimensional permeability and diffusion coefficients, calculated based on cylindrical chamber test results, were used to evaluate the permeability of the concrete specimens. Percentage of the permeable pore space volume was also measured based on ASTM C642-06. The results tend to show that silica fume increases the compressive strength of the concrete specimen without any admixture (control specimen), while fly ash, zeolite, and limestone powder decrease the compressive strength of the control specimen. Moreover, the permeability and diffusion coefficients of the specimens containing the mentioned admixtures, except limestone powder with a replacement level of 20 percentage, were revealed to be lower than those of the control specimen. The results of the regression approach performed also show that the relationships between the dimensionless coefficients of permeability and diffusion and dimensionless coefficients of the test results (penetrated water depth, penetrated water volume, average water flow rate, and percentage of permeable pore space volume) can be estimated with high accuracy using the regression functions; however, no equation was found to estimate the relationship between the dimensionless coefficients of permeability and diffusion and compressive strength of the studied concrete specimens.