nan Rasheeduzzafar

Factors affecting threshold chloride for reinforcement corrosion in concrete

Three cements with variable C3A contents were mixed with different levels of chloride, alkali and sulfate contents to study the effect of these parameters on pore solution composition. Effect of exposure temperature was also studied by curing the chloride-treated specimens at 20 ° and 70 °C. Pore solution was extracted using a high pressure pore solution extrusion device and analysed for chloride and hydroxyl ion concentrations. Threshold chloride for onset of reinforcement corrosion was computed using threshold [Cl−OH−] ratio of 0.3. The results showed that C3A content and exposure temperature have very strong influence on threshold chloride content. Alkali content of cement has marginal effect whereas presence of sulfates along with chlorides has moderate effect on the threshold chloride content.

EFFECT OF CEMENT COMPOSITION ON CHLORIDE BINDING AND CORROSION OF REINFORCING STEEL IN CONCRETE

Abstract Pore solution study has been carried out on 2.43 and 14% C3A hardened cement pastes. Data have been analyzed in conjunction with the data developed in two pore solution studies made by Page and Vennesland and Diamond using 7.37 and 9.1% C3A mature cement pastes. The results show that C3A and alkali contents of a cement have significant effect on its chloride-binding capacity. For similar alkali content, the levels of free chlorides in the pore solutions of 2.43 and 9.1% C3A cement pastes are respectively 4.7 and 2.8 times more than in a 14% C3A cement. The alkali content of a cement appears to have an inhibiting effect on its chloride-binding capacity. However, this effect is overshadowed by a conjoint strong elevation of the OH− ion concentration in the pore solution due to cement alkalies, causing a net lowering of the Cl−/OH− ratio which roughly ascertains corrosion risk. Threshold chloride values have been evaluated for different C3A cements. The threshold chloride content for a typical Type I portland cement with C3A upto 8% and Na2O equivalent upto 0.60%, may be taken as 0.4% chlorides by weight of cement. However, for a similar alkali cement with a high C3A content of about 14%, the chloride threshold value is 2.5 times higher and may be taken as 1.0% by weight of cement.

Effect of tricalcium aluminate content of cement on corrosion of reinforcing steel in concrete

Abstract Results of accelerated laboratory studies reported in this paper show that a high tricalcium aluminate content of cement has a significant beneficial effect on reinforcement corrosion resistance performance of concrete structures. On an average, a 9.5% Type I cement performs 1.62 times better than a 2.8% C3A Type V cement in terms of corrosion initiation time for embedded reinforcement. This appears to be due to the complexing ability of C3A with free chlorides in cement.

CHLORIDE THRESHOLD FOR CORROSION OF REINFORCEMENT IN CONCRETE

Cement mortar specimens made with three different C(subscript)3A cements with a steel bar embedded centrally were partially immersed in a 5 percent NaCl solution, and half-cell potentials were monitored. When the potential value reached -270 mV versus saturated calomel electrode (SCE), taken as the threshold potential for the onset of corrosion of the embedded bar, the specimens were taken out and pore solution extracted from the mortar surrounding the bar. The pore solutions were analyzed for Cl~ and OH~ concentrations and threshold Cl~/OH~ ratios calculated. The threshold Cl~/OH~ ratio seemed to depend on the pore solution pH and was found to range from 1.28 to 2.0 for a pore solution pH of 13.26 to 13.36. The free chloride concentration in the pore solution was converted into threshold free chloride and total chloride contents. It was found that the threshold free chloride content was 0.22 to 0.29 percent by weight of cement and was independent of the C(subscript)3A content of the cement. However, the threshold total chloride content was found to depend on the C(subscript)3A content of the cement and varied from 0.48 to 0.59, 0.73 to 0.85, and 1.01 to 1.20 percent for 2.43, 7.59, and 14 percent C(subscript)3A cements, respectively.

Corrosion Resistance Performance of Fly Ash Blended Cement Concrete

Accelerated corrosion tests were carried out on reinforced concrete specimens made with plain and fly ash blended cements. The fly ash blended cements were formulated by replacing 30 percent by fly ash on weight basis. Corrosion initiation time and corrosion rate of steel reinforcement in the post-corrosion initiation period were measured for plain and fly ash blended cements. To explain the corrosion resistance performance of steel in fly ash blended cement concrete, the effect of fly ash blending on pore solution composition and physical characteristics of hardened concrete have been evaluated. Results show that partial cement replacement by fly ash caused significant pore refinement, reduced permeability to water and chloride ions, and increased electrical resistivity. The observed superior corrosion resistance performance of fly ash blended cement concrete compared to plain cement concrete in terms of corrosion initiation time and corrosion rate is attributable to the improved physical structure of the cement matrix due to fly ash blending.

Influence of sulfates on chloride binding in cements

Abstract Cement pastes with water to cement ratio of 0.60 were prepared using three cements with C 3 A contents of 2.43, 7.59 and 14 percent. The chloride treatment levels of 0.6 and 1.2 percent by weight of cement, derived from sodium chloride, were used in conjunction with sulfates. Sulfates derived from sodium sulfate, were added in such quantities that for each of the two 0.6 and 1.2 percent chloride-bearing cement pastes the total SO 3 content of the cements were raised to 4 and 8 percent on a weight basis. The pastes were allowed to hydrate in sealed containers for 180 days and then subjected to pore solution expression. The expressed pore solutions were analyzed for chloride and hydroxyl ion concentrations. It was found that the alkalinity of the pore solution is significantly increased by the addition of sodium sulfate in the chloride-bearing hydrated cement pastes. This is attributable to the formation of sodium hydroxide as a result of reaction between sodium sulfate and calcium hydroxide liberated during cement hydration. The addition of sulfates also caused a significant increase in the chloride ion concentration in the pore solution, for both chloride levels in all the three cements tested. DTA results show that the sulfate addition reduces the formation of Friedels salt, which possibly results in an increase in the chloride ion concentration the pore solution. The interactive effect of increase in alkalinity and chloride ion concentration with sulfate addition is not a consistent increase or decrease in the Cl − /OH − ratio of the pore solution. For a given chloride level, whether sulfate addition increases or decreases the Cl − /OH − ratio of the pore solution, and hence the corrosion risk, depends upon the interactive effect of equivalent alkali content and C 3 A content of the cement.

Response of sabkha to laboratory tests: A case study

Abstract Sabkha is a saline, evaporative flat soil that forms under and climates. It is generally associated with saturated watertables that are very close to the ground surface. There are typically two major types of sabkhas; coastal and continental or inland. The presence of brines in the sabkha matrix and the crystallization of diagenetic minerals therein can lead to the highly variable mechanical properties of such a soil. This investigation was carried out to evaluate the engineering properties of this salt-laden and water-sensitive sabkha soil. Several laboratory tests were conducted, including compaction, permeability, unconfined compression, direct shear, triaxial, CBR, specific gravity measurements and grain-size distribution analysis. The investigation also focussed on the effect of distilled water and/or sabkha brine on the properties of this unusual type of soil.

PREDICTION OF LONG-TERM CORROSION RESISTANCE OF PLAIN AND BLENDED CEMENT CONCRETES

This investigation evaluated the relationship between the early age properties, such as compressive strength, pulse velocity, porosity, and permeability, and the long-term corrosion resistance of plain, fly ash, pozzolanic, and blast furnace slag cement concretes. The details of the study are described. The data that was developed were statistically analyzed to establish relationships bestween the long-term corrosion rate and the early age properties of plain and blended cement concretes. The results of regression analyses indicate excellent correlation between permeability and corrosion rate, and porosity and corrosion rate for both plain and blended cement concretes.

DEGRADATION OF BOND BETWEEN REINFORCING STEEL AND CONCRETE DUE TO CATHODIC PROTECTION CURRENT

The details are given of the study in which bond strength and chemical analysis results obtained from current-treated specimens were compared with the corresponding values measured in the control specimens, and it was found that a sustained impressed current on reinforcing steel will cause a deterioration in the bond between steel and concrete. The magnitude of bond deterioration was found to be a function of the current density and chloride content of concrete from NaCl source. Bond reduction is almost proportional to the accumulation of sodium and potassium ions in the vicinity of reinforcing steel. However, the migration of chlorides away from the steel significantly brings down the chloride ion concentration at the steel concrete interface, thereby reducing the corrosion risk. Increase in the NaCl-supplied chloride content of the concrete decreases the bond due to the additional accumulation of Na near the steel surface.

Effect of temperature on pore solution composition in plain cements

Abstract Cement pastes with a water-cement ratio of 0.6 were prepared using three ordinary portland cements with C 3 A contents of 2.43, 7.59 and 14%. Three levels of chlorides 0.3, 0.6 and 1.2% by weight of cement, derived from sodium chloride, were added through mix water. The pastes were allowed to cure in sealed containers at 20 and 70°C for 180 days and then subjected to pore solution extraction. The expressed pore solutions were analyzed for chloride and hydroxyl ion concentrations. Results show that increase in temperature from 20 to 70°C increased unbound chlorides and decreased hydroxyl ion concentration of pore solutions for all the three cements. The simultaneous increase in unbound chlorides and decrease in hydroxyl ion concentration drastically increased Cl − /OH − ratio of the pore solution, thereby indicating an increase in corrosion risk. This adverse effect of increase in the Cl − /OH − ratio of the pore solution with increase in temperature is higher in the high 14% C 3 A cement than in the low C 3 A cements, and is also higher for the low 0.3% chloride treatment level than the higher chloride inductions. Increase in temperature is also expected to cause an increase in ionic diffusion to steel embedded in concrete as well as in the rate of corrosion reaction. All these factors tend to increase corrosion risk of steel reinforcement in concrete with an increase in temperature.