An experimental approach for the analysis of early-age behaviour of high-performance concrete structures under restrained shrinkage
Introduction
The 1990's saw a significant increase in the use of high-performance concrete (HPC) in structures. Although HPC can offer superior strength and low permeability, it may develop considerable autogenous shrinkage due to self-desiccation at early age. The main factors responsible for the development of tensile stresses in HPC structures are thermal and autogenous deformations under restrained conditions, and external loads. If these stresses reach the tensile strength of the concrete, cracking will occur and possibly result in premature corrosion of the steel reinforcement, and spalling of the concrete. Considering the practical importance of this problem in HPC structures, there is still a scarcity of systematic experimental investigations on autogenous and thermal deformations of HPC specimens under realistic thermal regimes and restrained conditions [1]. A review of existing approaches for testing linear shrinkage under restrained conditions [2], [3] indicates that most studies have been conducted on mortar specimens of relatively small sizes (usually not larger than 100 mm in width) and under isothermal conditions. These mortar specimens may behave differently from larger size concrete specimens tested under semi-adiabatic conditions.
This paper presents a systematic approach for the experimental testing and thermo-mechanical behaviour analysis of large prismatic HPC specimens at early age under restrained autogenous shrinkage and realistic temperature conditions. Results from the testing of 200 × 200 × 1000 mm specimens made with a 0.34 water–cement ratio (w/c) concrete are presented, analysed and discussed in the paper to demonstrate the application of the proposed approach.
Section snippets
Proposed testing system
The method proposed for the study of the early-age thermo-mechanical behaviour of HPC specimens under restrained shrinkage is based on an approach originally developed by Kovler [4]. The proposed method includes modifications and innovations made to the original approach in order to enable the testing of relatively large-size reinforced HPC specimens under realistic thermal conditions and varying degrees of mechanical restraint. As shown in Fig. 1, a specimen size of 200 × 200 × 1000 mm was
Definition of Time Zero
In restrained shrinkage tests, the precise determination of the time at which stresses start to develop in concrete is of major interest. Deformations occurring before this “Time Zero” can be ignored for stress calculation purposes, since they do not result in stresses. There is no general consensus in the literature on the definition of Time Zero [5], whether it corresponds to the initial setting, final setting, or some other time. The Japanese Concrete Institute [6], however, recommends
Concrete materials
The concrete selected for this study was made of ASTM Type I cement, with a cement–sand–coarse aggregate ratio of 1:2:2 by mass, and a water–cement ratio of 0.34. Table 1 provides the concrete mix design, along with the resulting slump, air content, and 7-day compressive strength (50 MPa). The quantities shown in the table represent the amount of concrete required to make two large-size specimens for the restrained and free shrinkage testing, and a number of small concrete samples for testing
Hygro-thermal conditions
The free and restrained specimens were tested under realistic temperature conditions (i.e. semi-adiabatic conditions). Fig. 5 presents the average temperature and the average RH measured in the concrete specimens over time after casting. The concrete reached an average peak temperature of 41 °C at about 18 h after casting and cooled down to the ambient lab temperature after 2 days. The temperature difference between the core and the surface of the specimens during the heating/cooling period
Summary and conclusions
A complete testing and analysis approach is proposed to study the thermo-mechanical behaviour of large-size high-performance concrete specimens under restrained autogenous shrinkage and realistic temperature conditions. The loading system can apply a partial degree of restraint to enable the characterization of high-performance concrete (HPC) specimens without premature termination of the test. The instrumentation system can measure the strains and other parameters from setting time with high
Acknowledgment
The authors would like to thank Mr. Glendon Pye of NRC for his valuable technical assistance.
References (17)
Applicability of degree of hydration concept and maturity method for thermo-visco-elastic behaviour of early age concrete
Cement and Concrete Composites
(2004)- et al.
Autogenous and thermal deformations
Early age cracking tests
Early age creep and stress relaxation tests
Testing system for determining the mechanical behaviour of early-age concrete under restrained and free uniaxial shrinkage
Materials and Structures
(1994)Experimental determination of the time-zero (maturity-zero)
Committee report on autogenous shrinkage of concrete
- et al.
Concrete
(1981)