International Concrete Abstracts Portal

The characteristics of the steel and the concrete have been changing over the years and this has led to the necessity of new studies on how rotation capacity and the degree of moment redistribution in continuous beams can be affected. Among the factors that have influence on the behaviour of plastic hinges, one could mention the material dependent parameter: steel strength and ductility, concrete strength and bond quality. This work is part of a study on moment redistribution in continuous beams that aimed to verify the effect of bond on the comparisons, normal strength concrete (NCS) beams were also tested. The results of this work have indicated an effect of bond on the plastic rotation capacity of both HSC and NSC beams. As for the plastic hinge length, no effect has been detected, but this should be further investigated.

The effect of varying dosages of a polynaphthalene sulfonate (PNS) superplasticizer on the microstructure of a portland cement paste, and on the microstructural and transition zone characteristics of a portland cement mortar, was investigated using AC impedance spectroscopy. Interpretation of the impedance and resistivity data was carried out in conjunction with data from mercury intrusion porosimetry, scanning electron microscopy, and other techniques. The addition of the PNS superplasticizer influences both the cement paste/sand interfacial region and the bulk paste component in the mortar. The transition zone was more porous at early hydration (around 4 hours) for mortars with high dosages of superplasticzer. The dosage levels also influences the morphology of the hydrates. Larger high frequency arcs and higher electrical resistance values were obtained for all the superplasticized mortars one day and beyond. This suggests a modification in the pore structure and porosity of both the transition zone and the bulk past due to presence of the superplasticizer. Mercury porosimetry, thermogravimetry and conduction calorimetry measurements support the interpretation of the AC impedance spectra.

Due to the advances in the scientific and technological knowledge of concrete several special concretes have been developed. High Performance Concrete (HPC) is one of the most important. It is used for a wide range of field applications as it shows high early strength, low porosity and good durability. HPC has been applied in the construction of many important structures and has been specified where only service life is required. This paper presents test results obtained from drilled cores obtained from 2 years old slabs of 2.00 by 4.00 by .20 m with the purpose of analyzing the physical mechanical properties in field conditions. A conventional concrete and three High-Performance Concretes were evaluated. Compressive and tensile strength, permeability, resistance to abrasion and absorption were measured. The effects of improper consolidation and the introduction of mineral admixtures (a natural pozzolan was incorporated) are discussed.

The objective of this paper is to report test results of gas-permeability of high-performance concretes, measured on laboratory specimens and directly on site. The test refer to three concretes used in two projects: a tunnel (50MPa concrete) and a cable-stayed bridge deck (50 Mpa concrete) and pylon (75 MPa concrete). In particular, the investigation was focused on comparing the permeability of the "covercrete", measured on laboratory specimens, with that measured directly on the site concretes, I.e. subjected to strongly different placing, compaction and curing conditions. The air-permeability of the cover of the three concretes was measured with a non-destructive technique, which takes into account the effect of moisture. Cores were drilled from the same elements and the oxygen permeability measured on them. When core-drilling was not allowed, parallel tests were conducted on large cubes, site cured, cast with the same concrete mixture used for the actual construction. For the three structures investigated, the air-permeability of the site concrete was higher that that measured on the companion laboratory specimens. The largest difference was found for the 75 Mpa-strength concrete; this difference is attributed to thermal cracking in the pylon, the center of which exhibited 55 degrees C temperature rise. This indicates the risk of impairing the potential durability of HPC through inappropriate practices. The results presented show the importance of checking the quality of the concrete, not only on laboratory-prepared specimens, but also directly on site.

Results of an experimental investigation on the performance of cracked fiber reinforced concrete in a simulated marine environment are presented. A total of 111 prismatic specimens (150 by 150 by 510 mm) comprising both lightweight and normal weight concretes were used in this investigation. Cracked specimens with crack sizes of "hairline", .25 mm, 1.0mm, and uncracked specimens were exposed in either simulated seawater for up to a period of 7 years or 5300 alternate wetting and drying cycles. It was found, for both lightweight and normal weight concrete, that the strength development of uncracked specimens is not hampered by alternate wetting and development of uncracked specimens is not hampered by alternate wetting and drying. At the end of 7 years exposure, compressive strength gain of 90% was observed over the seven day moist cured strength for both types of concrete. Corresponding uncracked prismatic specimens showed approximately 25% flexural strength gain; however their post-cracking strength decreased under a prolonged period of alternate wetting and drying. Precracked specimens with cracks of up to .25 mm showed improvement in load carrying capacity up to 1440 wetting and drying cycles. However specimens with cracks of 1.0 mm showed a reduction in load carrying capacity.