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.
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.
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.
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.
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.
Crack opening near reinforcement bars in concrete structures
K. Tammo, Lund Institute of Technology, Sweden S. Thelandersson, Lund Institute of Technology, Sweden
Current concrete codes impose limitations of crack widths at the concrete surface. To investigate how the crack width at the surface affects risk for reinforcement corrosion, the related crack width close to the bar should be determined. An experimental study to investigate how concrete cover affects the crack width at the reinforcement level is presented. Axially loaded concrete prisms with a central 16 mm reinforcement bar were tested. Three different concrete covers, 30, 50 and 70 mm, were used. By continuous monitoring of strains and load, it was also possible to see how the crack width near the bar is affected by crack spacing. The test results showed that the crack width at the concrete surface is more than twice the crack width at the level of reinforcement. The influence of concrete cover seems to be rather small for the crack width at reinforcement level. It was also found that the crack width close to the bar is not affected by short term cyclic variations of five cycles or less with steel stresses in the range 200 - 400 MPa.
