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Structural Concrete, Vol. 8, no. 2, June 2007

Shear and torsion in prestressed hollow core units: finite element analyses of full-scale tests

H. Broo, Chalmers University of Technology, Göteborg, Sweden
K. Lundgren, Chalmers University of Technology, Göteborg, Sweden
B. Engström, Chalmers University of Technology, Göteborg, Sweden

The present calculation methods for shear and torsion in prestressed hollow core slabs add stresses from various influences without taking into account deformations and compatibility, the softening of cracking concrete, or restraint at the boundaries; therefore, they are most likely conservative. The main purpose of this work is to establish three-dimensional finite element models, which can be used both to analyse the effect of parameters that influence the shear and torsion response and to be included in global models of complete floors. An important aspect was therefore to simplify the models to avoid time-consuming analyses. Coarse meshes with solid elements were combined with beam elements. The established models were validated by simulating a series of full-scale tests conducted on both 200 mm and 400 mm thick hollow core units subjected to various combinations of shear and torsion. In general, although very coarse meshes were used, the finite element analyses of the tests succeeded in describing the overall behaviour, crack pattern, failure mode, and maximum load, with a reasonably good agreement. 

Structural Concrete, Vol. 8, no. 1, March 2007

Integral abutment bridge concept applied to the rehabilitation of a simply supported concrete structure

B. Briseghella, University of Architecture in Venice, Italy
T. Zordan, University of Architecture in Venice, Italy

A case study aiming to transform a simply supported pre-stressed concrete flyover into a frame structure throughout its full length, through refurbishment, is presented. Hogging moment resistance is achieved at the piers by the construction of a rigid transverse cast-in-place concrete diaphragm connected to the piers and to the beams of adjacent bays, in an attempt to fulfil the requirements of the integral abutment bridge (IAB) concept specified. All former bearings and expansion joints are eliminated resulting in an improved durability of the structure and greater comfort for the users. According to the IAB concept, hogging moment resistance is obtained by the flow of tensile forces through the concrete slab thanks to an increased ratio of reinforcement and the transmission of compressive forces directly through the concrete. Shear transmission is ensured by the installation of a convenient number of concrete-to-concrete shear studs on the webs of each of the beams forming the deck. Experimental tests were used to investigate the behaviour of shear studs. The possibility of making a simply supported deck continuous for a certain length depends on the soil-structure interaction and, consequently, on a wide number of parameters, varying from case to case, that will not be investigated in the present paper. 

Structural Concrete, Vol. 8, no. 1, March 2007

Metallic fabric jackets: an innovative method for seismic retrofitting of substandard RC prismatic members

G. E. Thermou, Demokritus University of Thrace, Greece 
S. J. Pantazopoulou, Demokritus University of Thrace, Greece 

This paper presents the results of a recent experimental research study where metallic (high-strength steel cord) fabric jackets (MF jackets) were utilised for the seismic upgrading of substandard reinforced concrete members. The proposed intervention method and its practical application are described in detail. Specimens were cantilevers with a square cross-section, representing a typical building column at half scale. The length of the test region corresponded to half the span of a typical storey building column under lateral sway. Due to lack of adequate seismic detailing the specimens were susceptible to various modes of failure such as web shear failure, buckling of compression reinforcement or failure in the lap splice region. The as-built specimens were first damaged up to failure after being subjected to combined axial loading and cyclic lateral displacement reversals simulating seismic loading. In the next phase, specimens were retrofitted with both composite and metallic fabric jackets and then tested again under the same load history. The results of this preliminary experimental research programme show that the metallic fabric jackets performed in an excellent way compared to glass- and carbon-fibre reinforced polymer jackets, increasing substantially both the strength and the deformation capacity of the repaired members. The excellent mechanical performance of the metallic fabrics combined with many of the advantages of the synthetic wraps (easy handling, no change in member dimensions) and the intrinsic favourable properties of steel (fire resistance), underline the potential of this novel material in repair/strengthening of reinforced concrete as an alternative option for jacketing applications. 

Structural Concrete, Vol. 8, no. 1, March 2007

Effect of admixtures on fresh grout and two-stage (pre-placed aggregate) concrete

A. Nowek, Gdansk University of Technology, Gdansk, Poland
P. Kaszubski, Gdansk University of Technology, Gdansk, Poland
H.S. Abdelgader, Al-Fateh University, Tripoli, Libya
J. Górski, Gdansk University of Technology, Gdansk, Poland

According to two-stage (pre-placed aggregate) concrete technology, special grout is injected into forms or foundation trenches with aggregate, as backing, placed earlier. The fresh grout differs from the ordinary concrete mixture. Good quality two-stage grout can be prepared in high-speed mixers, for example an Ultramixer (3000 rpm). It is characterised by high fluidity, low sedimentation, good viscosity, intensive hydration and a notable increase in the cement particles' surface. However, because of the unique and complex equipment involved, two-stage concrete technology is still underutilised. The aim of the investigation presented in this work is to design a mixture composition similar in features to Ultramixer grout. For this purpose, the new grout was prepared using a normal mixer (140 rpm) and some admixtures. The experimental tests were performed with different mix proportions. On the basis of the obtained results, an optimal composition has been proposed. A comparison of the new grout features with the properties of the grout produced by the Ultramixer has also been elaborated. The new grout design method can improve two-stage concrete technology. Some preliminary two-stage concrete tests are also presented. 

Structural Concrete, Vol. 8, no. 1, March 2007

The effects of the cooling rate on the residual properties of heated-up concrete

R. Kowalski, Warsaw University of Technology, Warsaw, Poland

This paper describes some tests performed on concrete specimens heated up to high temperature and then cooled down in water. The tests were carried out on cylindrical specimens, 103 mm in diameter and 200 mm in height, made of ordinary concrete with siliceous aggregate. The specimens were heated up to 330, 430 and 550 degrees C and then cooled down to room temperature in five various ways. A first group of specimens was cooled in air, while a second was immersed in water for 5, 10, 15 or 20 minutes, respectively; after the rest period in water the specimens were cooled in air. The next day, after the heating and cooling process, the residual concrete compressive strength was measured. Rapid cooling of heated-up concrete had the highest influence on the strength degradation when the maximum temperature peaked at 330 degrees C while, in the case of thermal cycles peaking at 550 degrees C, the strength degradation was less dependent on the method of cooling. It was concluded that rapid cooling of heated-up concrete can cause a significant strength degradation, especially in the range of low temperatures. 

Structural Concrete, Vol. 8, no. 1, March 2007

Design model for socket base connections adjusted from experimental results

R.M. Fernandes Canha, University of São Paulo at São Carlos, Brazil
A.L. H. de Cresce El Debs, University of São Paulo at São Carlos, Brazil
M.K. El Debs, University of São Paulo at São Carlos, Brazil 

This paper presents a theoretical and experimental analysis of socket base connections of precast concrete structures, with the emphasis on pedestal walls. The experimental programme included five specimens subjected to loads with large eccentricities, changing the type of the interface in contact with cast-in-place concrete and the load eccentricities. Three specimens had smooth interfaces and two specimens had rough interfaces. The experimental results indicated the need to revalue the principal design models for this connection. In the case of smooth walls, the friction portion that contributes to the socket connection strength was verified and a design model was proposed and adjusted to the experimental results. Based on the present experimental results, the following conclusions can be drawn: (a) the Leonhardt and Mönnig behaviour model is suitable to represent connections with smooth interfaces; (b) the proposed design model for smooth interfaces provides the closest predictions of the experimental results; (c) the Leonhardt and Mönnig behaviour model is not suitable for rough interfaces; and (d) for rough interfaces, the vertical reinforcement can be designed by bending theory. 

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