Abdulaziz I. Al-Mana

Influence of atmospheric corrosion on the mechanical properties of reinforcing steel

Abstract This paper reports the results of an investigation carried out to evaluate the mechanisms of atmospheric corrosion of reinforcing steel in arid regions, and their influence on the weight loss, strength, elongation and bendability. The results indicated that atmospheric corrosion begins as a localized attack at discrete points on the metal surface. Upon extended exposure to the atmosphere, this localized attack gradually becomes a uniform form of attack covering the entire metal surface. Further, the atmospheric corrosion did not influence the strength and ductility of the steel.

Effect of Rusting of Reinforcing Steel on its Mechanical Properties and Bond with Concrete

Reinforcing steel samples of 6 different sizes, each of 3 varying compostions, were exposed to the atmosphere for periods up to 16 months. The effect of rusting on the yield sstrength, ultimate tensile strength, and bond strength was investigated. Results indicate that rusting of reinforcement due to atmospheric exposure for periods up to 16 months does not affect the strength poperties. The bond stress values for the rusted reinforcing steels after 16 months of atmospheric exposure were greater than the allowable ACI 318-63 and BS CP 110 values. The maximum weight loss after 16 months of atmospheric exposure was 16 mg/cm squared, which corresponded to an average reduction in diameter of 0.53 %.

29SI MAS-NMR HYDRATION AND COMPRESSIVE STRENGTH STUDY IN CEMENT PASTE

Abstract 29 Si MAS-NMR measurements of cement have been used to follow the hydration process in cement pastes. Samples prepared using a w/c ratio of 0.45 and type I cement have been cured at temperatures over the range 20 to 55°C with curing times of 3 to 28 days. Compressive strength values for samples subjected to the same time/temperature curing regime were also obtained. The compressive strength is found to show a linear dependence on hydration as characterized in terms of the NMR Q 0 , Q 1 and Q 2 silicate polymerization states. Solid state 29 Si NMR measurements appear promising as a means of monitoring cement/concrete strength.

Corrosion of reinforcing steel in concrete containing slag or pozzolans

Use of fly ash, silica fume, natural pozzolans, and blast-furnace slag in concrete is becoming increasingly common. Incorporation of these materials improves the durability of concrete. But as yet there are few quantitative data available on the effect of these materials on long-term corrosion of steel reinforcement. Such data will help the concrete technologists to select the proper type of materials to produce dense and impermeable concrete to withstand aggressive service environments. In this investigation, the long-term corrosion-resisting characteristics of concretes made with two fly ashes, a natural pozzolan, a blast-furnace slag cement, and a preblended portland-fly ash cement were evaluated. Specimens were immersed in 5% sodium chloride solution for a period of more than five years, and the corrosion activity was evaluated by monitoring the half-cell potentials and measuring the corrosion rate of embedded steel using linear polarization resistance technique. Results indicate that addition of pozzolans or slag increases the corrosion-resisting characteristics of concrete. The corrosion rate of steel in concrete made with blast-furnace slag cement is lower than in other concretes.

EFFECT OF FLY-ASH ADDITION ON THE CORROSION RESISTING CHARACTRISTICS OF CONCRETE

Reports results of experiments evaluating the corrosion resistance of plain and fly-ash concrete mixes. Variables were fly-ash additions of 0 to 20 percent as cement replacement and four cement contents. Data were developed both for constant water-cement ratio and constant-workability concrete mixes. The corrosion resistance of concrete samples in which fly-ash replaced an equal quantity of sand was also investigated. Samples were immersed in a 5 percent sodium chloride solution for more than 1000 days and corrosion resistance was evaluated by monitoring the half-cell potentials and measuring the corrosion rate of embedded steel using electrochemical techniques. Results show that additions of fly-ash is effective in inhibiting corrosion of reinforcing bars. The corrosion resistance of concrete samples in which fly-ash was used as an admixture seems to be better than those in which it was used to replace cement. The superior performance of fly-ash concrete samples in inhibiting corrosion of reinforcing steel is attributable to the densification of the cement-paste matrix due to pozzolanic action in the fly-ash concrete mixes.

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.

Evaluation of degree of hydration in concrete using 29Si magic angle spinning NMR in solids

The degree of hydration of cement and concrete can be followed using 29Si MAS-NMR. Preliminary results of the effects of hydration on the distribution of silicate polymers in concretes of known mix designs, as well as the constituent cement, are presented. The spectra of the sand and aggregate used are also given. The technique appears promising as a means of monitoring concrete silicate anion structure and its possible correlation with mechanical properties.

Electrochemical behaviour of steel in plain and blended cement concretes in sulphate and/or chloride environments

Abstract The electrochemical behaviour of reinforcing steel in plain and blended cement concrete specimens placed in sulphate, chloride and sulphate-chloride environments was investigated. The effect of cement composition and the exposure condition on the corrosion behaviour of reinforcing steel was evaluated by measuring corrosion potentials at periodic intervals and conducting potentio-dynamic scans after two and half years of exposure. The corrosion potential data indicated passive corrosion in plain and blended cement concrete specimens placed in the pure sulphate environment. The time to initiation of reinforcement corrosion, however, was higher in blended cements than plain cements in all chloride and chloride-sulphate environments. The concomitant presence of suphate and chloride salts did not significantly influence the time to corrosion initiation. The corrosion rate, however, was significantly influenced by the quantum of sulphate ions associated with the chloride salts. The superior performance of silica fume blended cement, in terms of longer time to corrosion initiation and lower corrosion rate, indicates its potential in enhancing the useful service life of reinforced concrete structures placed in the chloride-sulphate environments.

Corrosion behavior of pre-rusted rebars after placement in concrete

This paper presents the results of a preliminary study in which the effect of the initial rusting on the corrosion behavior of rebars embedded in concrete has been investigated. Concrete specimens were made with pre-rusted and rust free rebars of different compositions. Two concrete mixes, one with a sodium chloride content of 2 kg/m3 of concrete and the other without any sodium chloride were used. The reinforced concrete specimens, immersed in potable water and then transferred to 5% NaCl solution, were subjected to corrosion monitoring for a period of 10 months. The test results indicate that the initial rusting does not have an adverse effect on the corrosion resistance of rebars embedded in concrete.

Effect of field and laboratory curing on the durability characteristics of plain and pozzolan concretes

Abstract The environmental conditions in the Arabian Gulf countries are classified as aggressive and severely corrosive. Field and laboratory studies have shown that concrete in this region should be designed not only for strength but also for durability. Utmost emphasis should be given to produce dense and impermeable concrete, to extend the service life of structures. In this study, concrete samples were prepared using various types of cements and fly ashes at various mix designs. The samples were cured in the field and in laboratory conditions. The permeability was determined by volume of permeable voids and absorption tests. The results showed that continuous water curing is necessary to obtain the least permeable concrete for both plain and pozzolan concretes. Irrespective of curing procedure followed, the fly ash concrete exhibited lower permeability than plain concrete for an initial test period of 7 days during curing. The fly ash concrete samples cured in the laboratory exhibited lower initial surface absorption than control concretes after 90 days of curing for all fly ash additions (10–40%) and cement factors (275–450 kg/m3) used.