Katrien Audenaert

Axial load behavior of large-scale columns confined with fiber-reinforced polymer composites

Confinement of concrete is an efficient technique used to increase the load-carrying capacity and ductility of concrete columns and is of interest for upgrading columns, piers, and chimneys. This article reports on an experimental and analytical study of axially-loaded large-scale columns confined with fiber-reinforced polymer (FRP) wrapping reinforcement; this work updates previous studies on smaller-scale columns. The effective circumferential FRP failure strain and the effect of increasing confining action were investigated. The authors compared the different existing compressive strength models to their study results. The authors then present a revision of an existing model developed previously by the second author. This revised model addresses the effective FRP failure strain that is attributed to localized stress concentrations near failure due to nonhomogenous deformations of the damaged concrete. The authors note that the wrapping configuration of the confinement has a considerable influence on the effectiveness of the FRP wrapping. The authors conclude that although the available models were developed based on small-size cylinders, four models seem to predict the ultimate strength of large-scale columns fairly accurately (Miyauchi et al, Saafi et al, Samaan et al, and Toutanji Revised).

TESTS ON AXIALLY LOADED CONCRETE COLUMNS CONFINED BY FIBER REINFORCED POLYMER SHEET WRAPPING

Wrapping of columns by means of FRP (Fiber Reinforced Polymer) reinforcement enhances the structural behavior of concrete columns considerably. At the Magnel Laboratory for Concrete Research a test program is set-up to evaluate some specific problems in the modeling of FRP confined concrete, i.e., effective circumferential FRP failure strain and effect of increasing confining action. Parameters studies are FRP type, bonded or unbonded wrapping application, column shape and strengthening lay-out. Both wrapped cylinders and wrapped columns are investigated. An analytical verification of the test results is performed according to different models. From the test results obtained so far, the efficiency of this strengthening technique has been demonstrated, both in terms of structural performance and ease-of-application. Quality of the application (voids, protrusion, etc.) and the wrapping concept (bonded or unbonded, partial wrapping, etc.) influence the strengthening effect.

Influence of cracks and crack width on penetration depth of chlorides in concrete

ABSTRACT Chloride induced reinforcement corrosion is the main durability problem for concrete structures in a marine environment. If the chlorides reach the reinforcement steel, the latter will depassivate and start to corrode in presence of air and water. Since the corrosion products have a larger volume than the initial products, concrete stresses are induced, leading to spalling and degradation of the concrete structures. If cracks are present in concrete, the penetration of chlorides is much faster than in uncracked concrete. In this way, the corrosion process is initiated earlier and the service life is decreasing drastically. In order to investigate the effect of cracks on the chloride penetration, a testing program was carried out. Firstly, a method was developed to create cracks in concrete. Afterwards, chloride penetration tests with the non-steady state migration test described in NT BUILD 492 were carried out. From the penetration profiles, the influence of the crack width on the maximum penetration depth and the extent of the crack influencing zone were investigated. This leads to the conclusion that for increasing crack width, the maximum penetration depth is increasing and that the extent of the crack influencing zone is depending on the crack width.

Influence of Cracks on the Service Life of Concrete Structures in a Marine Environment

Chloride initiated reinforcement corrosion is the main durability problem for concrete structures in a marine environment. If the chlorides reach the reinforcement steel, it will depassivate and start to corrode in presence of air and water. Since the corrosion products have a larger volume than the initial products, concrete stresses are induced, leading to spalling and degradation of the concrete structures. If cracks, caused by early drying, thermal effects, shrinkage movements or overstress, are present in the concrete, the penetration of chlorides is much faster compared to uncracked concrete. In this way, the corrosion process is initiated earlier and the service life is decreasing drastically. In order to study the influence of existing cracks in concrete structures on the penetration of chlorides a test program was set up at the Magnel Laboratory for Concrete Research of Ghent University, Belgium in cooperation with the “Politehnica” University of Timisoara, Romania. The first part of the test program consists of concrete specimens with artificial cracks. The chloride penetration into the concrete was realised with a non-steady state migration test and modelled with the finite element method COSMOS/FFE Thermal software. Based on the experimental and numerical results, a crack influencing factor was determined. With this factor, the resulting service life of the cracked concrete construction is determined and compared with the original service life.

Chloride penetration in self compacting concrete by cyclic immersion

Durability and more specifically chloride penetration, is of major importance for reinforced concrete structures. As the conception of self compacting concrete (SCC) is totally different from traditional concrete (TC), some changes in durability behaviour might occur. In order to locate and prevent possible problems, the chloride penetration in SCC has to be investigated. An extended experimental programme was set up: the chloride penetration of 16 self compacting concrete mixtures and 4 traditional concrete mixtures was determined. The test is performed on cylindrical specimens with a diameter of 230mm and a height of 70mm. These specimens are alternately immersed in a solution containing chlorides and exposed to air. One cycle takes approximately I hour. After 6, 12, 18, 24, 30 and 36 weeks the specimens are broken and the penetration depth is determined. For the self compacting concrete, four types of cement and three types of filler (fly ash and two types of limestone filler with a different grading curve) are used and the influence of the amount of powder, water, the water/cement ratio and the cement/powder ratio is studied.

Chloride penetration and carbonation in self-compacting concrete

In this research program, both the steady-state and the non-steady-state migration test are used to determine the chloride diffusion coefficient D (m2/s) of 8 different self-compacting concrete mixes and 1 reference, traditionally vibrated, concrete mix. Simultaneously, the carbonation behaviour of those mixes was investigated. Here fore, the carbonation depth was measured at regular points in time according to NT BUILD 357 (1989) and a carbonation constant A (mm/√year) was deduced. Concerning the chloride diffusion coefficient, test results revealed that the determination of the steady-state migration coefficient according to NT BUILD 355 (1997) is far from easy and question marks could be placed beside the corresponding diffusion coefficient but an explanation for the observed anomalies could not be found yet. The non-steady-state diffusion coefficient according to NT BUILD 492 (1999) was used in order to rank the different concrete mixtures. The carbonation constant could best be measured using an inflated CO2-concentration, resulting in a more linear behaviour of the carbonation depth in function of time. Besides, using this carbonation constant, results reveal that the concrete mixtures could be ranked in the same way as they were by the non-steady-state chloride diffusion coefficient.

Influence of Passive Reinforcement on Creep and Shrinkage of Concrete: Long-Term Observations

Due to time-dependent concrete deformations, a gradual stress transfer occurs between concrete and steel in most concrete structures. In partially prestressed concrete members where restrained concrete deformation may cause significant tensile stresses to develop, this is of particular interest. In the 1970s, a long term research project was started to study this effect. Sixteen concrete prisms were manufactured in 1979 with dimensions of 140 x 150 x 4,000 mm. They differed in applied compressive stress level (σ = 0; 5; 10 or 15 N/mm 2) and passive reinforcement amount (ρ respectively 0; 1.5; 3 and 6%). Unbonded strands were used to apply, at 28 days, compressive force. The prisms were placed at 20 ± 1°C and 60 ± 3% R.H. in an air conditioned room. To determine stress redistribution due to passive reinforcement restraining effect on concrete shrinkage and creep deformation, measurement was carried out during more than 25 years. The creep and shrinkage model given in the CEB-FIP Mode Code 1990 forms the basis of an analysis, which will be compared with deformation measurement results given in this article.