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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. 

Structural Concrete, Vol. 7, no. 4, December 2006

Determining the coefficient of concrete strength variation during non-destructive testing

V. Klevtsov, State Research Institute for Concrete, Moscow, Russia 
M. Korevitskaya, State Research Institute for Concrete, Moscow, Russia

All non-destructive methods for concrete strength control are indirect methods, which is why concrete strength on each specific area is determined with some error. In this paper, probably for the first time, a method to account for this error is presented. Theoretical grounds for the method and its experimental confirmation are also presented.

Structural Concrete, Vol. 7, no. 4, December 2006

Calculation of the coefficients of oxygen permeability of mortar samples using PORECOR analysis

N. Shafiq, University of Technology Petronas, Perak Darul Ridzuan, Malaysia
J. G. Cabrera, University of Leeds, UK

It has been established that the durability of cement-based composites is generally controlled by the transport characteristics of fluid in their pore network. Experience has shown that the monitoring of transport properties, such as the coefficient of permeability, in the laboratory is a very exhaustive task. The pore network in a cementitious composite, which provides passage for fluid transportation, is a major controlling factor for transport characteristics. In the last few years, many efforts have been made to correlate the microstructure and the coefficient of fluid permeability, and/or diffusion, for a cementitious composite. In this study, a set of ordinary Portland cement (OPC) and OPC/fly ash mortar samples equilibrated in different relative humidity were analysed using PORECOR analysis and the coefficient of oxygen permeability was calculated and compared with the coefficient of permeability of another set of the same samples tested in the laboratory, and valid statistical correlation was obtained.

Structural Concrete, Vol. 7, no. 3, September 2006

Limit states of cracking in beam-and-block floor systems using pretensioned ribs

R. S. Camposinhos, Instituto Politecnico do Porto, Portugal
A. Serra Neves, University of Porto, Portugal

Beam-and-block floor systems using pretensioned ribs enable very efficient industrialised production. By ensuring that the ribs are produced under a strict quality assurance programme and that they are safely transported, assembled and properly detailed, an equally monolithic structural performance is achieved. However, calculation methods to control cracking in these structural elements must ensure the durability according to the surrounding environment's severity and predefined exposure classes. The design of beam-and-block floor systems is particularly affected by serviceability limit states. This fact is closely linked to the verification conditions for crack control. Therefore, new design models have been developed to take into account the peculiarities and characteristics of these floor systems. The aim is to find practical and effective crack-control methods for composite beam-and-block floor systems with pretensioned ribs or beams. The verification of the limit state of crack widths has so far been disregarded in the design of this type of lightweight slab. In fact, the current procedure is simply to compare the tensile stress in the lower fibre of the beams with the characteristic strength of the rib's concrete. This paper presents a method to assess crack control. The procedure implies the quantification of a limit bending moment depending on the physical and geometric characteristics of the sections. Simplified calculation methods and verification rules are presented, which allow the establishment of tables and design charts for the indirect verification of this limit state through bar size and spacing limits. 

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