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

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.

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.

Four constant current densities of 32.3, 108, 215 and 538 ma/m2 (03, 10, 20 and 50 ma/ft2) at the steel surface area were used in the test program which also included chloride content of concrete as the second variable. These four current densities were maintained in steel of reinforced concrete specimens for an activation period of 14 months. At the end of the activation period, concrete powder samples were obtained from three locations of a particular specimen; near the steel, near the anode and the midpoint of steel and anode.These samples were then analyzed for Cl− , Na+ , K+ , Ca++ and Mg++ ion concentrations. It is observed that cathodic protection (CP) current drives away Cl− ions from near the steel to a short distance away. Due to the negative polarity of steel in a cathodically protected reinforced concrete structure, Na+ , K+ , and Ca++ ions from the concrete pore solutions get accumulated near the steel surface. K+ ions migrate towards the steel surface at a higher percentage when compared with the migrations of other cations. The effect of CP current on the migration of Mg++ ions towards the steel in reinforced concrete specimen is not clear from the results obtained in the test program.

Cement pastes with water-cement ratio of 0.60 were prepared using four cements with C,A contents of 2.04, 7.59, 8.52, and 14 percent. Four levels of chlorides corresponding to 0.3, 0.6, 1.2, and 2.4 percent by weight of cement were added to the 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. It was found that the free chloride concentration in the pore solution decreases significantly with an increase in the C,A content of the cement. Typically for a 0.6 percent chloride addition, the unbound chlorides decreased from 41 to 12 percent when the C,A content of the cement was increased from 2 to 14 percent. The high C,A content was found to be especially beneficial for binding chlorides in the range of 0.3 to 0.6 percent. With increasing level of chloride addition, although the absolute amount of bound chloride increases, the ratio of bound to total chlorides decreases. For example, in the 14 percent C,A cement, the ratio of bound to unbound chloride is about 14 times higher for the 0.3 percent chloride addition compared to 2.4 percent chloride addition. For a threshold Cl-/OH- ratio of 0.30, the threshold chloride values for the 2.04, 7.59, 8.52, and 14 percent C,A cements were found to be 0.42, 0.62, 0.68, and 1.0 percent by weight of cement. The effect of the C,A content in significantly influencing corrosion is also confirmed by the corrosion initiation times, which were found to be 1.75, 1.93, and 2.45-fold more for the 9, 11, and 14 percent C,A cements compared to 2 percent C,A cement. The pore fluid analysis indicates some chloride binding even in the low 2.04 percent C,A cement when chlorides are added at the time of mixing.

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.

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.

Performance data based on accelerated corrosion-monitoring and exposure site tests indicate that cement type, reflecting particularly the C3A content, significantly affected concrete durability with respect to corrosion of reinforcing steel. On average, Type I cement (C3A = 9.5 percent) performed 1.7 times better than Type V cement (C3A = 2.8 percent) in terms of time of initiation of corrosion. Accelerated sulfate-resistance tests show that a 20 percent microsilica blended with Type I 14 percent C3A cement performed 1.4 times better against sulfate attack than a Type V portland cement with 1.88 percent C3A. Also, sulfate deterioration data indicate that, in addition to the C3A content, the C3S/C2S ratio of the cement has a significant effect on the sulfate resistance of the cement. Additional study results are discussed.