Strength and Durability Studies on Silica Fume Concrete

Abstract

Concrete is the most widely used construction material utilized since time immortal. However, the environmental concerns necessitate a reduction in clinker usage in the construction industry that is possible only by the use of supplementary cementitious materials, called mineral admixtures in the making of concrete. These mineral admixtures, when introduced in the cement system, react with calcium hydroxide to form calcium-silicate-hydrate gel. Some of the examples of mineral admixtures are silica fume, fly ash, slag etc. Each of these mineral admixtures posses different properties. The objective of the present study is to investigate the synergistic effect of using a combination of silica fume and Indian fly ash. This study is necessary because silica fume is expansive in nature, and fly ash is cheap and abundantly available in India. Therefore, use of small proportion of silica fume along with large proportion of Indian fly ash, is expected to provide overall economy without compensating in the desirable properties of concrete. The investigation is divided into two parts. In the first part, the concentration is on the strength aspects of the resultant concrete. The major objective of the first phase of the work is to choose the most suitable combination of silica fume and fly ash to improve strength characteristics of concrete. The investigation is carried out for a wide range of water-to-binder ratios and under variable curing regimes. Three different water-to-binder ratios ranging from high (0.45) to medium (0.35) and then to low (0.25) are used to ensure wide variation in strength. Under each water-to-binder ratio, eight mixes are prepared using various combinations of silica fume and fly ash. Out of the eight mixes, one mix is a control mix with no mineral admixture, three mixes are binary mixes having only one mineral admixture, that is, either silica fume or fly ash; and four mixes are the ternary mixes with different combinations of silica fume and fly ash. Further, in order to study the minimum number of curing days required for each mix to reach a definite strength level, five different curing regimes are adopted in the study and its influence is studied in terms of strength development and water impermeability of concrete. The curing regimes vary from continuous water curing to continuous air curing. In between these two curing regimes, curing regime with water curing for 7 days followed by air drying is investigated as it is the curing practice normally followed for OPC concrete. Fourth curing regime consists of water curing for 28 days followed by air drying, which is normally suggested in literature for mineral admixture concretes. Along with this, the effect of wet-dry cycles is also investigated by adopting the curing regime of 14 days of initial water curing followed by wet-dry cycles. The effect of water-to-binder ratio, mineral admixtures and curing regimes is studied in terms of compressive strength development, tensile strength development and water impermeability of concrete. Compressive strength development studies are performed at the age of 1, 3, 7, 14, 28, 56 and 90 days for all five curing regimes. Tensile strength is studied through both split tensile test and flexure strength test, conducted at the age of 7, 28, 56 and 90 days. Water impermeability test is performed to check transport rate of water in concrete because it is known that most deterioration processes need aggressive fluids to penetrate through the capillary pore structure of the concrete and react, thereby causing destruction. This test is performed as per DIN 1048 guidelines, at the end of 90 days to see the ultimate transportation rate in various mixes under different curing regimes. Along with these tests, non-destructive tests, like, rebound hammer and ultrasonic pulse velocity tests are also performed on the specimens meant for compressive strength studies, prior to destructive compressive strength test. The objective of performing these tests is to develop a combined relationship between non-destructive parameters and compressive strength, for a wide range of compressive strength values. At the end, economics of the best ternary mix is compared with the binary mixes. From the first phase, it is observed that the ternary mix with 5% silica fume, 15% fly ash and 80% cement is the most suitable mix in terms of strength development and overall economy of the system. For a given strength level, the system makes the concrete cheaper by nearly 15% than the corresponding concrete made by using only silica fume as a mineral admixture. From the combined compressive strength and water impermeability studies, it is obtained that for silica fume concrete and the best ternary combination, seven days of initial water curing is both necessary and sufficient to explore the pozzolanic activity and for reaching a desirable strength and durability level. Results of the experimental investigation are further used to develop a mathematical model for the relationship between compressive strength (20 MPa to 112 MPa) and tensile strengths (split tensile strength and flexure strength) of concrete. Two general models relating tensile strengths of concrete with compressive strength, water-to-binder ratio, concrete age and initial water curing regime are proposed. These models are validated with the data available in literature (having the compressive strengths ranging from 1 MPa to 118 MPa) using appropriate statistical tools to demonstrate the reliability and applicability of the proposed models. Predictability of the proposed models are found to be much superior vis-à-vis other existing models. In the second phase of the study, the chemical resistance of concrete is investigated. It is because the deterioration of concrete due to aggressive environments is an issue of major concern throughout the world and the chemical resistance of concrete decides the service life and hence overall economy of the system. The chemical resistance of concrete is studied in terms of acid attack, sulfate attack and chloride ingress. For studying the chemical resistance, the mixes that are found to be performing best in the binary and ternary combination are chosen and are subjected to aggressive chemicals for a duration of nearly one year. To study the acid attack, three mixes under each water-to-binder ratio are subjected to aggressive chemical environment simulated by 1% sulfuric acid, 1% hydrochloric acid and 1% nitric acid, taken separately. The deterioration process is analyzed periodically up to 48 weeks by studying compressive strength loss and weight loss of the specimens. It is observed that, generally, the use of ternary mixes with 5% silica fume and 15% fly ash is best in resisting any type of acid attack. Also, reduction in water-to-binder ratio helps in making the concrete impermeable and thus reducing the attack. Comparing the usefulness of mass loss study and strength loss study, it is found that the mass loss is a surface phenomenon and thus, does not represent the overall characteristic of concrete. Therefore, studying mass loss alone cannot be taken as a reliable index in judging the efficiency against acidic media. It is also observed that the courses of action of sulfuric acid attack and hydrochloric acid attack are quite different due to the solubility of reaction products formed. Sulfate attack on concrete is studied on four mixes for three water-to-binder ratios. These include the control mix, two binary mixes with silica fume and the best ternary mix. The test solutions for studying sulfate attack include 5% sodium sulfate and 5% magnesium sulfate, taken separately. The maximum exposure duration is kept at 12 monthly cycles of wetting and drying. The deterioration process is analyzed by progressive strength loss and weight loss of the specimens, along with visual observations and measurement of ultrasonic pulse velocity. It is observed that lowering the water-to-binder ratio or the use of combination of silica fume and fly ash tend to improve the performance of concrete in sodium sulfate exposure by resisting the compressive strength loss to the barest minimum value. However, in magnesium sulfate exposure, the use of mineral admixtures tends to aggravate the sulfate attack. Different mechanisms of sodium sulfate and magnesium sulfate attack are proposed based on the observations in the present study and from the literature available. For studying the chloride penetration mechanism, five mixes are chosen for each water-to-binder ratio, which include one control mix, three binary mixes with different percentages of silica fume or fly ash and the best ternary mix. The major objective of studying the chloride ingress is to obtain chloride penetration profiles for mixes with various binary and ternary combinations; and to develop a relationship between free and total chlorides in order to judge the performance of the mix under chloride exposure. The study is carried out with 5% sodium chloride and 5% calcium chloride solutions. The desired solution is ponded on the concrete specimens for a period of 1 year and then a core is drilled out from the centre of the specimen. The core is further cut into thin slices along the depth and then, the slices are further powdered and titrated in order to obtain total chloride and free chloride content at different depths. It is found that lowering the water-to-binder ratio reduces both total and free chloride content at a given depth. The use of mineral admixtures increases the chloride binding capacity of concrete thus reducing the amount of chlorides available for corrosion of steel reinforcement. For both sodium chloride and calcium chloride exposures, ternary mixes are found to perform better than the binary mixes containing either silica fume or fly ash, thus leading to the conclusion that the ternary mixes are best in resisting chloride ingress in concrete.