AN EXPERIMENTAL STUDY ON THE BEHAVIOUR OF REINFORCED CONCRETE IN MARINE ENVIRONMENT

Abstract

Concrete, both conventionally reinforced and prestressed, are being used extensively in the construction of various types of marine structures for a variety of reasons. Such structures are required to withstand the destructive physical as well as chemical action of sea water throughout their life. The physical actions consisting of various loadings caused due to cyclic sea waves, high and low tides, ocean currents, hydrostatic pressure, freeze-thaw cycles, temperature gradient etc. have their own independent damaging effects on the exposed concrete structures. While, the chemical actions consist of slow decomposition of cement mortar matrix and corrosion of the embedded reinforcement due to the reaction of various salt ions present abundantly in sea water. Thus, the concrete structures in a marine environment are subjected to various environmental loads apart from the normal loads considered for their structural design. Good quality concrete provides excellent protection to the embedded steel reinforcement against corrosion. Chemical protection is provided by the high alkalinity of the concrete and the physical protection is afforded by the cover concrete acting as a barrier to the access of aggressive species. However; despite of these inherent protective qualities, failure and deterioration of R.C. structures have occasionally been noted in the tidal/splash zone where the exposed concrete experiences the concentrated effect of salt ions due to various actions of sea water. Under unavoidable circumstances, the use of sea water and sea dredged aggregates for making concrete may become the potential source of salt ions in concrete. Moreover, the cracks in concrete structural elements offer a direct path for the corrosive ions to the interior of the concrete. Thus, the penetrated salt ions easily cause the threshold values of corrosion at the rebar level to be exceeded. Specially, under tropical/semitropical climates, the high ambient temperature and humidity conditions lead to a greater reaction between the salt ions and the hydrated cement products; easily initiating the electrochemical process of rebar corrosion. The (iii) formation of expansive compounds and the corrosion products, both being larger in volume than the volumes of the original materials, result in cracking, spalling and ultimately structural failure. However, the part of the R.C. structures permanently submerged under sea water, are reported to show little or no corrosion. The primary objective of this study was to carry out an indepth study regarding the performance of reinforced concrete exposed to splash/tidal zone conditions in a laboratory simulated marine environment. The physical as well as chemical aspects such as the effect of concrete grade, quality of mixing water, stress level, type of rebar, cover depths, crack widths etc. have been studied both in normal and accelerated environment to provide useful data relevant to the design, construction and maintenance of marine structures. In the present study, the environmental effects have been brought out by simulating the following marine environments in the laboratory in which normal as well as accelerated test technique have been used over periods of 3,6,9,12,18 and 24 months, (a) Controlled humidity and temperature environment (b) Controlled temperature environment (c) Ambient environment with varying temperature (d) Freezing and thawing environment. In the controlled humidity and temperature environment, the concrete specimens were exposed to sea water of concentrations IN, 5N and ION to investigate their performance under the submerged (SUB) and the alternate wetting-and-drying (AWD) state at .27°C and a relative humidity of 90 to 92%. The test specimens were also subjected to plain water action under identical environmental conditions for comparison. In the controlled temperature environment, the specimens were exposed to AWD state in IN, 5N and ION sea water at a constant environmental temperature of 50°C. Some of the specimens were also kept under sea water to simulate the submerged state of exposure. (iv) The test specimens in ambient environment have been subjected to AWD state of exposure in the sea water of above mentioned concentrations at temperatures varying from 25 - 40 C over the period of investigation. The reinforced concrete specimens were also subjected to alternate freeze-thaw action of both plain water and sea water of concentrations IN and 3N under SUB state of exposure with the temperature varying between -27°C to +27°C in one cycle of 32 hours. Normally, high strength concrete is used for marine construction works. For the present study, a relatively high, medium and a low grade concrete namely A, B and C with mix proportions of 1: 1. 37: 1. 86; 1: 1. 84: 2.22;1:2.44:2.68 were used. The water cement ratios adopted for these mixes were 0.40, 0.48, 0.58 respectively by weight. The 28 days compressive strength of concretes A, B and C achieved were 47.5, 37.5 and 28.0 MPa respectively. Three different types and sizes of test specimens have been used. (i) RC cubical specimens of 150 mm and 100 mm sizes for concrete strength and rebar corrosion studies; (ii) Plain concrete cylindrical specimens of 150 mm diameter x 150 mm height for permeability studies; (iii) RC beam specimens of 100 x 100 x 500 mm size for rebar corrosion study in stressed concrete. 0PC has been used as a binding material throughout the investigation period. Both plain and sea water were used for making concrete specimens. In addition to bare medium strength steel bars, galvanised rebars were used at different cover depths of 15, 25, 40 and 50 mm to study its comparative corrosion resistance performance. A total of 2200 specimens of different types and sizes were cast and precured in plain water for 28 days at 27 C before exposure to different sea environments. To study the deteriorative characteristics in stressed concrete, the precured beam specimens have been stressed back to back in pairs to a desired level with a suitable nut-bolt arrangement. The prestressed as well as precracked beam couples, thus obtained, have been exposed to different sea environments for 3-18 months exposure periods. (v) The effect of simulated marine environment on the test specimens has been studied visually and by measuring the changes in weight, volume, permeability, compressive strength, carbonation depth, chloride and PH levels at various depths of concrete and the nature and extent of rebar corrosion. The test results have been presented both in graphical and tabular Rational discussions have been made and suitable conclusions drawn therefrom of which some are summarised below. Concrete exposed to sea water acquired a reddish gray colour from the original off white colour. The fractured surfaces of the tested specimens showed a uniform distribution of coarse aggregates over the entire surface and a vesicular mortar structure. The concretes exhibited again in weight to a maximum extent of 1.1 percent when exposed to sea water under SUB state. A comparatively lower weight gain was observed under AWD state of exposure. The maximum loss in weight of about 11% occurred when the concrete was subjected to 270 cycles of freezing and thawing under SUB state in 3N sea water. The studies related to the volume changes indicated that the maximum increase in volume of about 0.13 percent occurred for concrete under SUB state of exposure. The changes in volume under AWD state were however, comparatively smaller under similar environmental conditions However, the concrete suffered asubstantial loss in volume of about 12 percent under the action of 270 cycles of freezing and thawing in sea water due to extensive erosion and splitting on the concrete surfaces. The permeability data for concretes exposed to different environmental conditions revealed that it decreased initially upto a period of 6 to 9 months followed by a gradual increase at the later stages. Concrete exposed to plain water showed a continuous reduction in permeabilities with time. After exposure periods of 18 to 24 months the permeabilities of the concretes exposed to plain and sea water were found to lie in the range of 0.5 to lexio'13 m/sec and 8 to 27 x 10~13 m/sec respectively. However, the increase in concrete permeability (vi) under freeze-thaw action of sea water was found to be very high, ranging -13 from 27-to 59 x 10 m/sec. It was observed that the concretes lost their compressive strengths in all the environments studied. The overall losses were observed to lie in the range of 5 to 30% as compared to the normally cured concrete of similar age. The sea water concrete is found to have 5 to 12% lower compressive strengths than that of plain water concrete under identical environmental conditions. No significant effect on strength deterioration was observed due to a nearly two fold increase in temperature. On the other hand, the concrete suffered a strength loss of 45 to 70% when subjected to 270 cycles of freezing and thawing in sea water. The rate of chloride diffusion in concrete was seen to be significantly affected by the grade, stress condition and the state of exposure of the concrete specimens. The lower grade concretes B and C showed 1.5 to 3.0 times higher chloride content as compared to the higher grade concrete A. The higher grade concrete also considerably limited the deeper penetration of chloride ions and tended to concentrate them near the concrete surfaces. Concrete made with sea water exhibited 15 to 40% higher chloride concentration than that of plain water concrete specimens. In stressed concrete, the chloride diffusion in tension zone was found to be 30 to 80% higher than that in compression zone. Moreover, concrete exposed to similar environment exhibited 10 to 30% higher chloride content under SUB state as compared to AWD state of exposure. The observed chloride data reveals that its penetration increases significantly at the extreme temperatures viz at 50 C and at reversible temperature ranging from -27 C to +27 C. The chloride content under freeze-thaw action was observed to be about 1.3 to 2 times as that of concrete at 27°C. It was also roughly 30 to 70% higher than that at an elevated temperature of 50 C. (vii) Regardless of grade, stressing condition and the quality of mixing water, the concrete exposed to different sea water environments exhibited only a marginal change in concrete alkalinity. Over the entire exposure periods, the PH of all types/grades of concretes corresponding to the depth level of 15 to 50 mm lie in the range of 11.56 to 11.70 which are above the critical values. Thus, the overall reduction in concrete alkalinity due to increased environmental temperature and salt ion penetration was not significant enough to initiate rebar corrosion by carbonation process. Carbonation study indicated a little or no measurable depths of carbonation in the deteriorated concretes. In most of the cases, it was seen to be limited on the concrete surface only. However, under AWD state and at elevated temperature, carbonation depths ranging from 0.5 to 1.5 mm were observed at some spots of afew specimens. Whereas, the concrete under SUB condition exhibited no carbonation at all. The overall corrosion of rebars embedded at various depths in concretes exposed to different sea environments were observed to be small, ranging from 0 to 0.28%. The maximum cover depths of 40 and 50 mm were considered to be insufficient in resisting corrosion. In stressed concretes, the rebar corrosion was observed to be 40 to 80% higher than that in unstressed concrete and the crack widths in excess of 0.15 mm were found to be critical from corrosion initiation point of view. The higher grade concrete Ashowed roughly 2 to 7 times greater endurance in corrosion resistance over the lower grade concretes B and C. Rebar corrosion in sea water concrete was found to be nearly 1.1 to 2.5 times of that in plain water concrete. Whereas, the specimens under SUB state showed negligible corrosion. An increase in the environmental temperature from 27°C to 50°C leads to roughly a two fold increase in the rate of corrosion. On the other hand, the rebar corrosion under freeze-thaw environment was observed to be the only marginal although the cover concrete showed the maximum chloride content at rebar level. Galvanized rebar exhibited around 30 to 50% lower corrosion than that of bare steel under identical exposure conditions. But galvanizi ng (viii coating was observed not to give full protection to rebar in severe chloride bearing environment simulating the tidal/splash zone of marine environment although it considerably delayed the onset of corrosion. Of all the environments, the freeze-thaw action of sea water was found to be the most detrimental to concrete. In contrast, reinforced concrete exposed to AWD state in sea water at high ambient temperature was seen to be deteriorated mainly due to the hazards of rebar corrosion. Sea water was found unsuitable in making structural concrete for tidal/splash zone of marine environment due to its higher contribution of free chlorides and dampness characteristics. However, a higher grade concrete (> M40; W/C < 0.40) made from salt free ingredient materials with cover depths in excess of 50 mm together with proper quality control can ensure adequate durability in critical marine locat ions.