PERFORMANCE OF MINERAL BASED CONCRETE IN MARINE ENVIRONMENT

Abstract

Ordinary Portland Cement (OPC) concrete continues to be the pre-eminent construction materials for use in any type of CIVILENGINEERING structures both in normal as well as in severe marine environments because of numerous advantages over other construction materials like steel, timber etc. At the same time some problems are still associated with OPC concrete i.e., lower durability of OPC concrete in severe marine environment and environmental pollution in production of OPC. Concrete with OPC, which performs very well over a period of about 100 years in the normal environment shows substantial damage within a few years of construction in the marine environment. This happens due to the external and internal causes, associated with the concrete. The external causes are severe physical and chemical actions of marine environment, whereas internal causes are non-homogeneity and poor microstructure of concrete. Large energy requirement and environmental pollution in production process of OPC are other constraints in the use of OPC concrete. Production of one ton OPC requires energy of about 4 G Joule and releases about one ton Carbon dioxide (CO2) in the environment. At present the cement industry produces approximately 7% of total CO2 produced in the world, which is very alarming to the protective ozone layer. In such situation, use of mineral admixtures may be a better solution. Blast Furnace Slag, Fly Ash and Silica Fume are most common mineral admixtures used in concrete. Out of these, Blast Furnace Slag has been found to be most beneficial, due to its latent hydraulic property. Blast Furnace Slag is a by-product of Pig Iron, which is formed in the Blast Furnace during reduction process of iron ore. Physical state of the slag during its cooling is fundamental to its cementitious properties. A rapid cooling, either by granulation or pelletization of melt slag converts it into a glassy material with latent hydraulic property. Chemical composition of Blast Furnace Slag is most close to OPC in comparison to other mineral admixtures and variation in composition from source to source is also relatively small. Hydration product of Blast Furnace Slag is poorly crystalline Calcium-Silicate-Hydrate, which is broadly similar to the hydrates of OPC. In the presence of OPC, its hydration depends upon the break down and dissolution of the glassy slag structures by hydroxyl ions, which is released during the hydration of OPC, and also on the alkali content of OPC. The Blast Furnace Slag is being used as Portland Slag Cement since more than 120 years, but the use of Ground Granulated Blast Furnace Slag (GGBFS) as mineral admixture in concrete is still uncommon. IV Numerous studies in literature have confirmed the supremacy of GGBFS blended concrete in durability aspects like Sulphate attack, Chloride attack, Aggregate-Silica reaction, Freeze-thaw attack and Carbonation, but very few studies have reported on the strength aspects, especially on tensile, bond or shear strength of concrete. The concrete made with GGBFS is expected to have enhanced resistance to ingress of aggressive chemicals from sea water, which is attributed to dense morphology of C-S-H and pore refinement in concrete. Present study examines, whether such attributes could also improve their mechanical properties like compressive strength, split and flexural tensile strength, modulus of elasticity, bond strength and shear strength of concrete in both normal and marine conditions of exposure. Broad aims of the present study are: 1. To study the effect of GGBFS blending on compressive strength, split tensile strength, flexural tensile strength and modulus of elasticity of plain concrete in both plain water and in marine conditions. 2. To study the effect of GGBFS blending on bond and shear strength of concrete of RC beam elements in both plain water and in marine conditions. 3. To study the effect of GGBFS blending on compressive strength and tensile strength of mortar in both plain water and in marine conditions. An extensive experimental program with GGBFS Grade 100 and OPC Grade 43 is carried out in the laboratory. Experiments are planned for all forms of cement products i.e., paste, mortar, plain concrete and reinforced concrete. Three mix proportions belonging to low, medium and high strength are prepared on the basis of statistical analysis of trial mixes. In each mix proportion OPC is replaced by GGBFS from 0 to 70%on equal weight basis. Ratio of water to binder remains constant at all replacement level in each mix. Thermo mechanically treated steel bars of 8, 10 and 12 mm diameter are used as tensile reinforcement in RC beams. An artificial marine environment is created in laboratory. Standard concentration of ions in prepared sea water is obtained by dissolving commercial grade NaCl (Salt), MgCb, MgS04, CaS04, K2SO4, CaC03 and MgBr2 in potable water. Two physical effects of marine environment, namely submersed condition (SUB) and alternate wetting drying condition (AWD) are considered in the present work. Chemical attack on the concrete has been increased by use of higher concentration of ions in sea water (3N and 6N), although standard concentration of ions (IN) is also used. Specimens are exposed to different conditions of marine environment from 3 to 18 months time. Sea water of tanks is replaced after every four month. Compressive, split tensile, flexural tensile strength and modulus of elasticity of concrete are observed on conventional cube (150 mm), cylinder (150 x 300 mm) and prism (100 x 100 x 500 mm) specimens. Effects of marine conditions on compressive and tensile strength are evaluated by using small cylinders (100 x 200 mm), while prisms (100 x 100 x 500 mm) are used for flexural tensile strength. Workability of concrete is evaluated by slump and compaction factor test. Bond strength of concrete is observed by tests on RC beam (1.2 m long) with central splice (200 mm) in tensile reinforcement. Beams are tested under four point loading arrangement and failed under splitting mode. Two parts of bond strength i.e., before appearance of first crack and from first crack to ultimate failure, are calculated from the loads at first crack and at failure. Effect of GGBFS blending and marine environment on ultimate bond strength and bond strength after first crack is evaluated. Shear strength of concrete is observed by tests on RC beam (1.2 m long) without shear reinforcement. Beams are tested under four point loading arrangement with shear span to depth ratio of 3.2. All beams failed in shear characterized by sudden failure with some sound. Two parameters related to shear behaviour, ultimate shear capacity and contribution of interlocking in shear capacity are calculated. Ultimate shear capacity is taken as failure load of RC beam. Interlocking part of shear capacity is determined by assuming uniform stress distribution on major diagonal crack at failure. Effect of GGBFS blending and marine environment on shear capacity and contribution of interlocking in total shear capacity of RC beam is evaluated. Mortar specimens of cubes (70.6 mm) and briquettes (76.20 mm x 44.45 mm with cross section of 25.4 x 25.4 mm) are used for compressive and tensile strength respectively. Consistency and setting time of binder paste is observed on Vicat's apparatus while flow ability is measured on flow table. All tests are carried out as per the relevant Indian Standard or ASTM Standard. Experimental results are presented in both graphical and tabular forms. The performance of OPC and GGBFS blended mixes in marine conditions is compared by absolute value of strength in an exposure condition and by relative value of strength in marine and PW SUB conditions. Relative strength is the percentage ratio of strength of identical specimens in marine and normal condition of exposure. Equations proposed in standards like ACI, New Zealand, British, Indian, Canadian and EURO are used to compare compressive and tensile strength of OPC and GGBFS concrete. vi Somefindings of the present study on performance of concrete in normal environment are: • Strength of GGBFS blended concrete is found to be less than that of OPC concrete up to 28 days, but in later ages it surpasses the strength ofOPC concrete. • Gain in strength form 28 to 180 days is more in GGBFS blended concrete and its value increases with increase in GGBFS content. • Optimum GGBFS content for maximum strength characteristics of blended concrete in later ages is found to be equal to 40 %. • Bond strength and shear capacity of blended RCbeams are comparable to OPC beams at 28 days, while in laterages strength of blended RCbeams is clearly higher. • Tensile strength of the GGBFS blended concrete is higher than that of OPC concrete having equal compressive strength. • Blending of GGBFS in concrete increases the contribution of interlocking in total shear capacity of RC beam. • Bond strength and shear capacity of RC beam increase with concrete grade, curing age and diameter of bars. Some findings of the presentstudyon performance of concrete in marine environment are: • GGBFS blending reduces the deteriorating effect of marine environment on various strength properties of concrete. Blending of GGBFS is not only effective in delaying the deterioration but it also reduces quantitative loss in strength under marine conditions. • Marine environment has larger deteriorating effect on bond strength than on shear strength of concrete. • Marine environment has more deteriorating effect on tensile strength than on compressive strength of concrete and mortar. • Slight corrosion of steel reinforcement helps in bond and shear strength of RC beam. • For same exposure conditions, loss in ultimate bond strength of concrete is higher than that on bond strength after first crack. • Effect of marine environment on interlocking part is larger than that on shear capacity of beam. Contribution of interlocking in shear capacity reduces with marine exposure. • Deteriorating effect of marine environment on mortar is lower than that on concrete specimens. • Alternate wetting-drying condition of sea water is more deteriorating than submersed condition on strength characteristics of concrete and mortar. vn