International Concrete Abstracts Portal

The harsh environmental conditions in the Arabian Gulf region calls for the use of high performance concrete and utilizing the recent developments in concrete technology in terms of materials and practices to produce concrete with high quality. This paper presents results of an investigation to evaluate the effect of curing and the addition of polypropylene fibers on the properties of high strength concrete. In this study, four high strength mixes with compressive strengths in the range of 65 to 80 MPa were made and tested. The first was a plain mix with a cementitious content of 450 kg/m3, the second was a plain mix with polypropyplene fibers, the third was a silica-fume blended mix (10% cement replacement), and the fourth was a silica-fume mix with polypropylene fibers. These specimens were subjected to three curing conditions: moist curing, dry curing and use of curing compound. The specimens were tested for compressive strength, tensile strength, drying shrinkage and plastic shrinkage cracking. The results clearly show the importance of curing on the quality of concrete. The use of silica fume contributes to the increase of concrete strength and reduction of drying shrinkage. The results also show the contribution of polypropylene fibers in reducing plastic shrinkage cracking and the need for early curing to prevent plastic shrinkage cracking in fresh concrete.

This paper deals with a new generation of corrosion inhibitors, the so called mixed inhibitors. Mixed inhibitors influence both the anodic and the cathodic sites in reinforced concrete by the formation of an adsorbed layer. Nitrogen is usually the active atom in the adsorbed layer. Typical compounds of nitrogen used as inhibitors are amines. The bond strength of the adsorbed layers is due to the nitrogen atom’s high density of electrons and ability to form co-ordinate bonds. Extensive tests have shown that this type of corrosion inhibitors do not negatively influence the fresh and hardened concrete properties. The mode of action of these inhibitors has been investigated by different methods, cracked beam tests, X-ray Photoelectron Microscopy (XPM), X-ray Photoelectron Spectroscopy (XPS), Secondary Ion Mass Spectroscopy (SIMS) and Secondary Neutron Mass Spectroscopy (SNMS).

Highly flowable concrete has been receiving particular attention in recent years. With highly flowable concrete, the use of a vibrator for consolidating is not necessary because the concrete is self-consolidating within the form. To produce self-consolidating highly flowable concrete, the concrete must have high fluidity, resistance to segregation and the ability to pass between steel reinforcing bars. For this paper, one of these properties, the ability to pass between steel reinforcing bars was studied. The mechanism of coarse aggregate accumulation between the bars, considered the main factor influencing this ability, was studied and then confirmed through execution of two experiments. Based on the results of these experiments, the conditions related to volume and shape of coarse aggregate contained in the concrete and the clearance between and direction of the steel reinforcing bars which would permit the coarse aggregate to pass between the bars were clarified.

In this study, the wedge splitting test(WST) was carried out for the fatigue crack growth behavior of concrete. Selected test variables were concrete compressive strength of 28, 60 and 118 MPa, and stress ratio with 2 levels (6, 13 %). In order to make the designed stress ratio, the maximum and the minimum fatigue loading were 75-85 % and 5~10 % of ultimate static load, respectively. Fatigue testing was preceded by crack mouth opening displacement(CMOD) compliance calibration and fracture energy test, and then the fatigue crack growth was computed by crack length vs. CMOD compliance relations acquisited by the CMOD compliance calibration technique. In fatigue test, the frequency of loading cycle was 1 Hz, and the initial notch length(%) was 30 % of specimen height. To evaluate the validity of CMOD compliance calibration technique, the crack length measured by the method suggested in this study was compared with that predicted by linear elastic fracture mechanics(LEFM). On the basis of the experimental results, a LEFM-based empirical model for fatigue crack growth rate(da/dN-AKi relationships) considering the effects of concrete strength was presented. The fatigue crack growth rate increased with the strength of concrete. It appears that the da/dN-AK1 relationships was influenced by stress ratio, however, the effect is diminished with an increase of strength. In addition, the effect of initial notch length on the fracture energy were shown, and the comparisons between CMOD compliance calibration technique and LEFM gave the validity of CMOD compliance calibration technique for the WST.

The expanded use of fiber reinforced polymer (FRP) tendons as prestressing and post-tensioning tendons requires that these tendons be subjected to high axial loads while bent or harped around a curved surface. This paper describes the behavior of one type of CFRP tendon when subjected to combined axial loading and harping. Four test series to study the tendon fiber strain distribution and seven test series to study the tendon ultimate strength were conducted. The tendons were subjected to various combinations of axial loading and harping around a curved surface. Test parameters for the ultimate strength tests included tendon axial load, bend angle, and curvature of harping point. Harping points were set to either 1 in. (25 mm), 5 in. (127 mm), or 20 in. (508 mm) radius. Ultimate strength tests included static strength tests and fatigue tests. Results of the static tests indicate that at failure, the flexural strains at harped point far exceed the average fiber strain recorded in uni-axial tensile strength tests. The flexural strain is defined as the difference between strain readings from a 0.125 inch (3.2 mm) strain gage placed on the tendon bend outside face and the tendon average axial strain. No flexural strains were observed away from the tendon bend beyond about 6 in. (152 mm). Based on the tests, an expression for the tendon flexural strain at harped point as a function of axial load, bend angle, and harping plate curvature was developed. This expression for flexural strain at harped point was used in a maximum strain failure criteria model that accurately predicts the failure of specimens tested in this research. The results of the fatigue testing indicated no degradation of tensile strength due to fatigue loading.