Rasheeduzzafar’s research while affiliated with Aston University and other places

The combined effect of chloride and sulphate contamination and temperature on the pore solution chemistry, particularly the OH- , Cl- and SO42- concentration, in plain and blended cements was evaluated. The cement mortar specimens were contaminated with sodium chloride and sodium chloride plus sodium sulphate and exposed to temperatures in the range 25 to 70°C. Results indicate that the chloride binding capacity of cement is affected by both the temperature and the sulphate contamination. In the chloride contaminated specimens, the chloride binding was influenced by the exposure temperature, while in those contaminated with sodium chloride plus sodium sulphate, it was affected by the presence of sulphate ions. The sulphate concentration in the plain and blended cements was also influenced by the exposure temperature and chloride contamination. The Cl- / OH- values increased with temperature in both plain and blended cements, indicating the vulnerability to accelerated steel depassivation in concrete exposed to elevated temperature and contaminated with chloride and sulphate ions.

Cement mortar specimens made with three different C3A 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 computed. The threshold Cl-/OH- ratio seemed to depend on the pore solution pH and was found to vary from 1.28 to 2.0 for a pore solution pH of 13.26 to 13.36. The free (water-soluble) chloride concentration in the pore solution was converted into threshold free chloride and total (acid-soluble) 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 C3A content of the cement. However, the threshold total chloride content was found to depend on the C3A 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 C3A cements, respectively.

The effect of reinforcement corrosion on the flexural strength of a uniformly loaded and simply supported one-way slab was investigated. In addition to the flexural strength, the effect of different degrees of reinforcement corrosion on the deformational behavior ductility, and the mode of failure of the slabs were also evaluated. The critical level of reinforcement corrosion that renders the strength contribution of steel negligible was evaluated by comparing the strengths of slabs with highly corroded reinforcement with the strengths of plain concrete slabs. In order to induce different levels of reinforcement corrosion, a calibration curve establishing a relationship between the duration of the impressed current and reinforcement corrosion was prepared. The magnitude of reinforcement corrosion was measured as gravimetric loss in weight of the steel bars. The results indicate a sharp reduction in the ultimate flexural strength of slabs with an up to a 29% reinforcement corrosion; thereafter, the strength decreased at a somewhat reduced rate with further increase in reinforcement corrosion. The ultimate deflection of the slabs decreases with an increase in the magnitude of reinforcement corrosion, leading to a marked and progressive reduction in the ductility of the slabs. Furthermore, the strength of plain-concrete slabs and those reinforced with Steel bars of less than 60% corrosion was almost similar.

This research was conducted to evaluate the concomitant effect of chloride and sulfate contamination and high temperature and humidity on carbonation in plain and blended cements. The cement mortar specimens were contaminated with chloride and sulfate ions and exposed to 55°C, 75% relative humidity (RH) and 3% CO2 atmosphere. The progress of carbonation was evaluated by measuring the depth of the carbonated layer and the weight gain. Morphological changes due to carbonation were examined using scanning electron microscopy. The results indicated greater carbonation in the contaminated specimens than in the uncontaminated specimens. This was observed in both plain and blended cements. The increased carbonation in the contaminated specimens is attributed to changes in the pore structure of cement due to the inclusion of contaminants.

The effect of reinforcement corrosion on the bond strength between steel and concrete was investigated. The bond behaviour of reinforced concrete elements, including the ultimate bond strength, free-end slip, and the modes of failure in precracking, cracking and postcracking stages was studied. Also, the effect of different crack widths and the rib profile degradation for various degrees of corrosion on the bond strength were evaluated. In order to establish different levels of corrosion, a calibration curve establishing a relationship between the duration of the impressed current and the corresponding degree of corrosion was prepared. The magnitude of corrosion was measured as gravimetric loss in weight of the reinforcing bars. The results indicate that in the precracking stage (0–4% corrosion) the ultimate bond strength increases, whereas the slip at the ultimate bond strength decreases with an increase in the degree of corrosion. The degradation of bond results from the crushing of concrete keys near the bar lugs. When reinforcement corrosion is in the range of 4 to 6%, the bond failure occurs suddenly at a very low free-end slip. At this level of reinforcement corrosion, a large slip was noted as the ultimate failure of the bond occurred due to the splitting of the specimens. Beyond 6% rebar corrosion, the bond failure resulted from a continuous slippage of the rebars. The ultimate bond strength initially increased with an increase in the degree of corrosion, until it attained a maximum value of 4% rebar corrosion after which there was a sharp reduction in the ultimate bond strength up to 6% rebar corrosion. Beyond the 6% rebar corrosion level the ultimate bond strength did not vary much even up to 80% corrosion. In terms of the effect of rib profile, a sharp reduction in the bond strength was initiated when its degradation exceeded 25%. This decrease in bond strength continued up to 45%. Thereafter, there was no significant effect of the rib profile degradation on the bond strength.

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.

The repair performance of resinous and cementitious materials exposed to static and fluctuating temperature conditions has been evaluated in this study, which is of importance for the repair of concrete structures in the Arabian Gulf region. Specimens were exposed to a total of 90 thermal cycles between 25°C and 70°C, simulating the temperature variation of concrete surfaces on typical summer days in the Arabian Gulf region. The slant shear bond strengths as well as failure characteristics have been observed at 0, 60, and 90 cycles. The results show that the shear slant bond strength undergoes significant reduction with thermal fluctuations due to the thermal incompatibility between the concrete and the repair materials. It is found that for resinous materials the reduction varied from 9.3-20.47% for 60 cycles, and from 18.98-36.43% for 90 thermal cycles. For the cementitious materials, the corresponding values were 3.2-17.46% for 60 cycles, and 8.07-34.80% for 90 cycles. It is also seen, in general, that the mode of failure of the test specimens changed from crushing of concrete to combined crushing-joint00 failure at 60 cycles, and then to a distinct joint failure at 90 thermal cycles.

Cement pastes with water to cement ratio of 0.60 were prepared using three cements with C3A 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 SO3 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 Friedel's 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 C3A content of the cement.