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

The effectiveness of concrete surface treatment materials such as silanes, siloxanes, etc., in preventing concrete deterioration due to sulfate attack, carbonation, and chloride-induced reinforcement corrosion was investigated. The effectiveness of these materials in decreasing the sulfate attack was evaluated by measuring the reduction in compressive strength. The chloride diffusion in the coated and uncoated concrete specimens was also evaluated. The performance of these materials in reducing carbonation was studied by exposing the coated and uncoated concrete specimens to a saturated carbon dioxide environment. The reduction in compressive strength due to sulfate attack in the concrete specimens coated with silane/siloxane in conjunction with an acrylic topcoat was very low compared with that in the uncoated concrete specimens and those coated with other surface treatment materials, the chloride diffusion was also least in the concrete specimens coated with silane/siloxane followed by a topcoat. Among all the surface treatment materials tested, silane/siloxane with an acrylic topcoat and two-component acrylic coating were most effective in preventing carbonation of concrete. Furthermore, silane and silane/siloxane with a topcoat were effective in reducing chloride-induced reinforcement corrosion in in-service structures.

Cracking of concrete due to plastic shrinkage is frequently noted in hot weather conditions. These cracks form focal points for other forms of concrete deterioration by allowing moisture and oxygen to diffuse to the reinforcing steel. They are formed when the rate of evaporation of moisture from the concrete surface is more than the rate of bleeding. Since the mix constituents control the rate of bleeding, it is apparent that plastic shrinkage cracking can be minimized by choosing appropriate mix design and adopting other necessary precautions during placement and consolidation of concrete. This paper provides a brief review of literature pertaining to plastic shrinkage cracking of concrete, particularly in hot and arid environments. This literature review is supplemented by the results of studies conducted by the authors on this important, and yet scantly studied, concrete deterioration phenomenon. It was noted that both environmental conditions and concrete mix variables influence plastic shrinkage cracking of concrete. One of the important findings of this study was that plastic shrinkage cracking was the least in a lean-stiff concrete mix, such as that made with a cement content of 300 kg/m 3 and water-cement ratio of 0.40. Further, plastic shrinkage cracking in blended cement concretes was more than that in plain cement concretes. Therefore, appropriate precautions should be taken when using these cements to utilize the technological benefits accruing out of their use.

The effectiveness of concrete surface treatment materials, such as silane, siloxane, acrylic coating, etc., in reducing chloride-induced reinforcement corrosion was investigated. Two sets of reinforced concrete specimens were cast. In the first set, reinforcement corrosion was accelerated by impressing an anodic potential of 2 V and the time to cracking was monitored. The second set of concrete specimens were immersed in the chloride solution and reinforcement corrosion was monitored by measuring corrosion potentials and corrosion current density. Among the surface treatment materials investigated, silane, silane/siloxane with top coat and acrylic coating were effective in reducing the rate of reinforcement corrosion. Furthermore, the data developed in this investigation indicated that the performance of coatings can be quickly evaluated using impressed current technique.

The effectiveness of epoxy resins and cementitious repair materials, including silica fume cement, in improving the functional performance of beams and slabs with corroded reinforcement was evaluated. Reinforcement corrosion in the beams and slabs was accelerated by application of a direct current for various periods of time. The deteriorated specimens were repaired and tested for flexural strength. The results indicate that not all the repair materials are able to restore the original strength of the components. The improvement in the load-carrying capacity was related to the increase in the bond between the parent concrete and the repair material, inter alia, the steel reinforcement for an effective load transfer. One of the epoxy resin mortars investigated, as well as silica fume cement concrete, to some extent, were able to restore the original strength of the component. Furthermore, the improvement in the functional performance of the repair materials, vis-à-vis, epoxy resin mortar, was observed to be dependent on the degree of reinforcement corrosion. The repair using this material was only effective when the degree of reinforcement corrosion was less than 10%.

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.

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.

An effective formulation for optimum design of two-span continuous partially prestressed concrete beams is described in this paper. Variable prestressing forces along the tendon profile, which may be jacked from one end or both ends with flexibility in the overlapping range and location, and the induced secondary effects are considered. The imposed constraints are on flexural stresses, ultimate flexural strength, cracking moment, ultimate shear strength, reinforcement limits cross-section dimensions, and cable profile geometries. These constraints are formulated in accordance with ACI (American Concrete Institute) code provisions. The capabilities of the program to solve several engineering problems are presented.