A.J. Al-Tayyib

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.

LABORATORY STUDY ON THE EFFECT OF SULFATE IONS ON REBAR CORROSION

This paper presents the findings of a laboratory investigation in which the role of sulfate ions on reinforcing steel corrosion has been investigated. 1020 carbon steel specimens were immersed in saturated Ca(OH)2 solutions containing different concentrations of sulfate ions and the corrosion rate was determined by linear polarization resistance and A.C. impedance techniques. The tests were carried out at 22°C and 50°C. The test results indicate that in the presence of sulfate ions and at 22°C, active corrosion results from modification of the protective passive film to a sulfate film which is less protective than that of the original iron oxide film. Moreover, the combined effect of sulfate ions and elevated temperature results in a seven fold increase in corrosion rate compared to that of the well known chloride ions at normal temperature.

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.

Effect of polypropylene fiber reinforcement on the properties of fresh and hardened concrete in the Arabian Gulf environment

The effect of polypropylene fiber reinforcement (0.2% by volume of concrete) on the workability, plastic shrinkage, drying shrinkage and strength of concrete has been studied in a detailed test program. Concrete specimens of different mix designs with and without polypropylene fibers have been prepared, cured in laboratory conditions and hot weather environment of the Arabian gulf, and then tested at different ages for the determination of various properties. The results indicate that the inclusion of polypropylene fibers eliminates the plastic shrinkage cracking of concrete, and reduces the drying shrinkage to some extent. It also slightly improves the tensile and flexural strength of concrete.(a)

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.

The effect of thermal cycling on the durability of concrete made from local materials in the Arabian Gulf countries

Abstract This paper presents results of a study on the effect of heating/cooling cycles on the durability of concrete made from available limestone aggregates in the Arabian Gulf countries. The coefficients of thermal expansion of three different limestone rocks, mortar and concrete were determined. Specimens of limestone rocks, mortar and concrete were heated in a temperature controlled oven to 80°C for 24 hours and then cooled in the oven to room temperature for another 24 hours. After 30, 60 and 90 cycles, specimens were tested for compressive and flexural strengths, pulse velocity and permeability. The test results indicate that the experimentally determined values of the coefficient of thermal expansion vary from 5.07 to 9.99 × 10 −6 /°C and that the limestone rocks tested are thermally anisotropic. Also, it is indicated that heating/cooling of concrete affects its durability performance as it loses up to 27 and 32% of its compressive and flexural strength, respectively. For all tested specimens, the greatest loss of strength is observed after 30 cycles.

USE OF POLYPROPYLENE FIBERS TO ENHANCE DETERIORATION RESISTANCE OF CONCRETE SURFACE SKIN SUBJECTED TO CYCLIC WET/DRY SEA WATER EXPOSURE

This paper presents results of a testing program in which the effects of addition of polypropylene fiber to concrete mix and adequate concrete curing in enhancing the deterioration resistance of concrete surface skin subjected to cyclic wet/dry seawater exposure have been evaluated. The results indicate that the addition of polypropylene fibers effectively retards the deterioration process of the surface skin of the concrete specimens cured in hot weather environment. Tests were carried out on 30 concrete slab specimens of dimensions 75 x 375 x 750 mm made with and without polypropylene fibers. Some specimens were cured under laboratory-controlled conditions and were subjected to the wet/dry cycles for 85 weeks, while others were cured under field conditions and were subjected to the same cycles for 50 weeks.

Effect of sulfate ions on the corrosion of rebars embedded in concrete

Abstract The corrosive action of sulfate ions on reinforcing steel has been studied and compared with that of chloride ions. Reinforced concrete specimens were made with equal dosages of sulfate and chloride ions (0, 0·6, 1·8 and 1·8 kg/m3 of concrete) and were immersed in potable water for a period of 808 days and then in 3% SO2−4 and 3% Cl− solutions for a period of 60 days. Half-cell potential and corrosion rate measurements were made during the entire exposure period of 868 days. The test results indicate that the sulfate ions are corrosive to reinforcing steel but their corrosivity is less than that of chloride ions.

EFFECT OF SULFATE AND CARBONATE IONS ON REINFORCING STEEL CORROSION AS EVALUATED USING AC IMPEDANCE SPECTROSCOPY

The electrochemical behavior of carbon steel in saturated calcium hydroxide solutions has been studied using AC impendance spectroscopy and linear polarization resistance techniques. Sulfate and carbonate ions help to break down the protective passive film, resulting in an increaesd rate of corrosion. The behavior of the sulfate is different from that of CO2 since the sulfate forms a less protective film than the original iron oxide film. The CO2, however, has two effects. It modifies the Ca(OH)2 solution with the formation of CaCO3 and also modifies the original passive film, leading to increased corrosion rates. With an increase in temperature from 22 to 50 C, the corrosion rate may be increased by as much as a factor of about 20. Two-time constants intially characterize the electrochemical reactions caused by both sulfate and carbonate ions, but these reactions are eventually controlled by charge transfer.

EFFECT OF HOT WEATHER ON MODULUS OF RUPTURE AND SPLITTING TENSILE STRENGTH OF CONCRETE

Several costly precautions for reducing temperature of concrete are specified in construction specifications for concreting in hot weather so that workability and compressive strength of concrete are not adversely affected. This paper presents the results of tests conducted for modulus of rupture and splitting tensile strength of concrete prepared at different temperatures and moist cured in hot weather conditions. It has been found that, even if the required compressive strength of concrete in hot weather conditions is obtained, the modulus of rupture and splitting tensile strength of concrete could be reduced by 22 and 11 percent respectively. (Author/TRRL)