A.S. Al-Gahtani’s research while affiliated with King Fahd University of Petroleum and Minerals and other places

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.

This study was designed to evaluate the relative corrosion and sulfate resistance of concrete made with portland cements containing 2%-14% C(3)A without and with 50%, 60%, 70%, and 80% cement replacement by blast-furnace slag (BFS). The results show that BFS blended-cement concretes had a significantly superior corrosion-resistance performance. The best corrosion protection was obtained with 50% BFS cement, which, depending on the C(3)A content of the parent cement, showed a 3.82-3.16 times better performance in terms of corrosion-initiation time compared to the parent plain-cement concrete. BFS blending was specially beneficial in improving the corrosion-resistance performance of Type V low C(3)A cements. Performance on exposure to sodium-sulfate (NS) solution, replacement level only at 70% and above, showed sulfate resistance to be better than that of the Type V sulfate-resistant cement. BFS blending, even with high C(3)A cement (9%, 11%, and 14%) at 70% and above-replacement level, imparted a high degree of sulfate resistance. The cement with high C3S/C2S ratio has a perceptible adverse-interactive effect and-causes sulfate deterioration even with low-C(3)A sulfate-resistant cements. In MS-NS environment, due to the magnesium-gypsum type of attack, the 60% BFS cement deteriorated even more severely than the plain Type V and Type I cements.

Using a technique for accelerated corrosion monitoring, corrosion-resisting characteristics of reinforcement of four plain and 36 fly ash blended cement concretes have been evaluated. Three fly ashes of bituminous, sub-bituminous and lignite origins have been used, in conjunction with four plain cements having C3A contents of 2%, 9%, 11%, and 14%. The 36 blended cements were formulated such that each of the four C3A cements had 10%, 20%, and 30% cement replacements by each of the three fly ashes. Results of corrosion monitoring tests show that fly ash blending of plain cements by 30% partial replacement improved the corrosion-resistance performance twofold to threefold over plain type I and type V cement concretes, respectively, in terms of corrosion-initiation time. Fly ash of lignite origin exhibited better impermeability and corrosion-resisting characteristics than bituminous and sub-bituminous fly ashes. Level of replacement is a significant performance parameter, with the best performance observed for 30% replacement. The time to initiation of corrosion and weight loss were significantly influenced by the C3A content of the cement; 9%, 11%, and 14% C3A cements performed 1.75, 1.93, and 2.45 times better than the 2% C3A cement in terms of corrosion-initiation time. The beneficial C3A effect was also operative in fly ash blended cement concretes, although on a reduced level.

Concrete cover, concrete quality, and bar size have a significant effect on corrosion initiation and corrosion cracking. This paper attempts to quantify the effect of these three parameters in providing corrosion protection to reinforcing steel. It is found that the cover-to-bar-diameter (c/d) ratio is a more definitive protection parameter against corrosion cracking than either cover or bar diameter separately. In view of the importance of c/d ratio, clear cover specifications without consideration of the bar size leads to inadequate and misleading design for corrosion protection, especially in concrete where internal chlorides are present in concrete from the time of manufacturing, making the corrosion propagation time prior to cracking an important phase in the service life of structures. A concept of corrosion cracking resistance factor, cf'c/d or c/dw incorporating cover, bar diameter, and concrete quality either in terms of strength (f'c) or water-cement ratio (w) has been developed to quantify the relative corrosion protection provided by a particular set of detailing and strength parameters.

Reinforcement corrosion-resisting characteristics of four plain and eight silica-fume blended cements have been evaluated using an accelerated corrosion monitoring technique. The four plain cements had C3A contents of 2, 9, 11, and 14 percent to evaluate the effects of C3A factor on corrosion-resistance characteristics of plain cements. Eight blended cements were formulated in a manner that each of the four plain C3A cements had 10 and 20 percent cement replacements by silica fume. Results of accelerated corrosion monitoring tests show that the time to initiation of corrosion of reinforcement is significantly influenced by the C3A content of the cement. The 9, 11, and 14 percent C3A cements performed respectively 1.75, 1.93, and 2.45 times better than the 2 percent C3A cement. Silica-fume blending of plain cements by 10 and 20 percent partial replacement very significantly improves corrosion-resistance performance in terms of corrosion initiation time. On an average, 10 and 20 percent silica-fume blended cements respectively perform 3.45 and 3.75 times better than the parent plain cements. Corrosion resistance of 9, 11, and 14 percent C3A cements blended with 10 percent silica fume was found to be 5.12, 7.35, and 7.39 times better compared to the performance of Type V 2 percent C3A plain cement commonly used in the Middle East and in marine environments. Hardly any tangible advantage was observed in terms of corrosion initiation time by increasing silica fume from 10 to 20 percent as cement replacement. The beneficial C3A-chloride complexing effect was found to be operative in blended cements also, although on a reduced scale compared to plain cements.

Plain and microsilica blended cement pastes with water-cement ratio of 0.6 were prepared using a 14% C3A cement. Two levels of chloride from NaCl corresponding to 0.6% and 1.2% by weight of cement were added through mix water. The pastes were allowed to hydrate in sealed containers for 180 days and then subjected to pore solution expression. The expressed pore fluids were analyzed for chloride and hydroxyl ion concentrations. The results show that the OH− ion concentration in the pore solutions of both chloride-free and chloride-bearing pastes drop steeply with increasing cement replacement by microsilica. For 10% microsilica cement pastes the pH for both 0.6% and 1.2% chloride addition was found to be around 13.30. However, the pH drops to a level below that of saturated Ca(OH)2 solution when cement replacement by microsilica is increased from 10% to 20%. This is ascribable to the consumption of Ca(OH)2 by microsilica as shown by the DTA/TGA results. 10% and 20% microsilica blending more than doubles the free chloride ion concentration in the pore solutions of the chloride-bearing pastes. 10% microsilica replacement raises the Cl−/OH− ratio 4 to 5 fold, whereas for 20% microsilica replacement, the Cl−/OH− ratio is increased to 77 and 39 folds over the corresponding values for the plain cement pastes for 0.6% and 1.2% chloride additions respectively. Accelerated corrosion monitoring tests carried out on steel bars embedded in plain and microsilica blended cement concretes exposed to 5% NaCl solution show a 3 fold superior performance of microsilica blended cement concretes in terms of corrosion initiation time. This corrosion behaviour is contrary to the prediction from the increased aggressivity of pore solution composition in terms of highly elevated Cl−/OH− ratios. This is attributable to the densification of cement matrix by the pozzolanic reaction between microsilica and calcium hydroxide. No discernable advantage in terms of corrosion initiation time is evident by increasing microsilica blending from 10% to 20%.

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.

Data have been developed on the effects of curing period length, type of curing water, aggregate washing, and the degree of consolidation on corrosion resistance characteristics of concrete. The effects of curing period and consolidation on the sulfate resistance of concrete have also been evaluated. Results show that concretes cured for 28 days performed 4.4 times better in terms of corrosion of reinforcement and showed 59 percent strength reduction and 40 percent weight loss improvements to sulfate resistance compared to concretes cured for 7 days. The beneficial effect of aggregate washing is, on average, about 15 to 20 percent for the aggregates tested in this study. Degree of consolidation has a significant effect on concrete durability.

Results of accelerated corrosion monitoring and exposure site tests show that corrosion of reinforcement in concrete is significantly influenced by the C "SUB 3" A content of the cement. A 9.5% C "SUB 3" A Type I cement, on an average showed 1.73 times better corrosion resistance performance in terms of corrosion initiation time compared to a 2.8% C "SUB 3" A Type V cement. The consistent beneficial effect of the C "SUB 3" A content in cement is shown by the time to initiation of corrosion results on concrete specimens made with four C "SUB 3" A cements of 2, 9, 11 and 14%. The 9, 11 and 14% C "SUB 3" A cements performed respectively 1.75, 1.93 and 2.45 times better than the 2% C "SUB 3" A cement. (A)