MATERIAL CHARACTERISTICS OF CONCRETE IN MARINE ENVIRONMENT

Abstract

Concrete has been increasingly finding extensive application in the construction of marine structures for a variety of reasons. Concrete structures in marine environment have to withstand the effects of destructive physical and chemical actions of sea water throughout their life-span. The physical actions consisting of alternate wetting-and-drying, high hydrostatic pressure, freeze-thaw cycles and temperature variations have their own independent damaging effects on concrete. In addition, attack in any one of these forms renders the material more susceptible to the action of remaining agents of destruction. The chemical actions on concrete are due to the presence of sulphates, chlorides, carbonates and other salt-ions abundantly available in sea water. Thus concrete structures in marine environment are required to withstand several environmental loadings apart from the normal loading for which a structure is designed in non-marine environment. In the present study, an attempt has been made to investigate these environmental effects on concrete by simulating the following marine environments in the laboratory. (a) Controlled humidity and temperature environment (b) Controlled temperature environment (c) Ambient environment of varying temperature (d) Controlled pressure-temperature environment, and (e) Freezing and thawing environment. In the controlled humidity and temperature environment, cylindrical concrete specimens have been exposed to sea water of concentrations IN, 3N, 6N and 12N to investigate their performance under the submerged and the alternate wetting-and-drying states at 27 C and a relative humidity of (v) 92 to 95%. For comparison, concrete specimens have also been kept in the submerged state in plain water but in the above mentioned environment of controlled temperature and humidity. Concrete above the splash zone in sea environment is exposed to a higher temperature than the portion in the splash zone and below. In India, marine structures are normally designed for temperatures upto 50°C. Keeping the above in mind and with a view to assess accelerated effects, a controlled temperature environment of 50°C has been created and concrete has been exposed to sea water of IN, 3N, 6N and 12N concentrations under the submerged and the alternate wetting-and-drying states. In the ambient environment of varying temperature, concrete specimens have been exposed to sea water of the various above concentrations to an ambient varying temperature 24 hour duration cycles of 27°C to 40°C to 30 C in a predefined manner in the submerged and the alternate wettingand- drying states. Effects of upto 365 such cycles have been studied. For exposing concrete to the effects of enhanced hydrostatic pressures at different temperatures, special cylindrical pressure vessels have been designed and fabricated provided with heating devices and the necessary monitoring instruments. Herein, concrete specimens have been subjected to 3N concen tration of sea water under different combinations of hydrostatic pressures of 1.5 MPa and 3.0 MPa and temperatures of 27°C, 50°C and 65°C for various time durations. Concrete has also been subjected to freeze-thaw cyclic loading in the sea water submerged state with the temperature varying between -27°C and +27 C in 32 hours for one cycle. For comparison of results, concrete specimens submerged in plain water and in dry state have also been subjected to the same environment. (vi) Normally high strength concrete is used for marine structures. For the present studies, concretes of grade M4 8.0 (i.e. 28-day characteristic strength = 48.0MPa) and M53.5 have been selected for investigations. The mix proportions by weight for M48.0 concrete are 1:1.15:2.20 with water/ cement ratio of 0.38, while those for M53.5 concrete are 1:1:2.10 with water-cement ratio of 0.35. Cylindrical concrete specimens of size 150 x 150 mm have been adopted due to the requirement of the specimen size for carrying out the standard permeability test. A relationship between the compressive strength of these cylinders to that of the standard cylinder of size 150 x 300 mm has been obtained for reference. All the specimens have been first cured in plain water at 27°C for 28 days before subjecting them to the various predecided environment states. It may be seen that the various environment parameters considered in the present study are the temperature, pressure, salt concentration, humidity, alternate wetting-and-drying and freezing-and-thawing and two or more of these parameters have been suitably combined to obtain the different environment states as described. Concrete specimens have been subjected to these environment states for various time periods of 28, 90, 180 and 365 days. However, freeze-thaw environmental loading has been applied in a cyclic fashion as defined in the appropriate chapter of the thesis. Deterioration of concrete in various environment states has been studied in relation to the effects on concrete in the submerged state in plain water and in the non-submerged state while keeping all other environmental parameters identical. For purpose of averaging, three specimens have been used for each environment state. A total of 960 concrete specimens have thus been tested in the present investigation. (vii) The effect of simulated marine environment on the concrete specimens has been studied visually and by measuring changes in the weight, the volume, the permeability, and the compressive strength including stress-strain characte ristics of concrete. X-ray diffractometry studies have also been carried out to identify and estimate various products of chemical reactions. Test results have been presented both in graphical and tabular forms. These results have been discussed and suitable conclusions drawn. Some of the conclusions drawn from the present study arc summarised below. Concrete exposed to sea water acquires a lime grey colour from the original dark grey colour. The fractured surfaces of the tested cylinders show a uniform distribution of coarse aggregates over the entire surface and a vesicular mortar structure. There is a gain in weight to the maximum extent of 1.5 per cent when concrete is kept in the fully submerged state. The gain in weight is to a smaller extent when concrete is subjected the alternate wetting-anddrying environment. The maximum loss of weight (about 1.5 per cent) occurs in the atmospheric state of different sea environments. Studies of volume change indicate that the maximum volume increase of about 0.12 per cent occurs for concrete in the submerged state followed by a smaller volume increase in the alternate wetting-and-drying state, whereas the maximum decrease in volume of 0.11 per cent occurs in the atmospheric state of different sea environments. Study of the permeability reveals that it decreases for concrete exposed to different sea environments for the first about 90 days beyond which it starts increasing with exposure time. The increase in the permeability of concrete exposed to sea water over a period of 365 days is about ^ to 14 times the permeability of normally cured concrete for the same period of time. (viii) It has been observed that all concretes show a significant loss of comp ressive strength in all the environments studied. The overall loss after the 365-day exposure lies in the range of about 6 to 19 per cent as compared to the 28-day compressive strength of normally cured concrete, whereas the loss is about 24 to 33 per cent when compared to the 365-day compressive strength of concrete cured in plain water at 27 C. In the freezing and thawing environment of sea water, these losses are very high amounting to 70 - 75 per cent of the 28~day/365-day compressive strength of plain water cured concrete. Although various environments affect the concrete strength differently, the deteriorated concrete exhibits similar stress-strain characteristics as the undeteriorated concrete of the same ultimate compressive strength. The ultimate compressive strengths a invariably correspond to very nearly a strain of 0.2 per cent in all cases. X-ray diffraction studies reveal that although the chemical compounds formed in the concretes under different sea environments are the same as those due to hydration and hydrolysis of cement in concrete, their percentages differ significantly. The differences in the relative amounts of the expansive and the non-expansive compounds formed help in explaining the behavioural changes of concrete to a large extent in different sea environments. Of all the environments, freezing-and-thawing action of sea water is the most detrimental to concrete. Even high strength concretes, as used in the present study, show substantial erosion and splitting on the surfaces after 365 cycles. The loss in the weight and the decrease in the volume are respectively 16.5 per cent and 1.7 per cent, while there is a significant increase in the permeability of concrete observed to lie in the range of 4 3 to 67 x 10 m/scc.