Shrinkage of concrete stored in natural environments

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Abstract

The paper reports on the influence of the natural environment on the drying shrinkage of a range of concretes, with and without steel fibre reinforcement. A combination of increasing cement content, the addition of silica fume (SF) and reduced water/binder ratio was used to obtain a wide range (C30–C70) of concrete strengths. Both prism and cylinder test specimens were used in the study and a fibre concentration of 2% (by weight) was used in the fibre reinforced concrete (FRC) mixes. The experimental results have been compared with predicted shrinkage strains obtained from the ACI 209 model. The results show that the effect of varying relative humidity and temperature in the natural environment had only a limited effect on the drying/autogenous shrinkage. The addition of 2% fibre to the various mixes had a negligible effect on shrinkage for the lowest strength (C30) but the restraint on the development of shrinkage was enhanced as the strength of the concrete was increased. Comparison between experimental results and predicted shrinkage strains was not good for the high-strength concretes, although good correlation was observed for the lower strengths (C30–C45). Further research is required to improve the prediction models for use with high-strength concretes with particular emphasis being given to the rapid development of drying/autogenous shrinkage during the first month after casting the concrete.

Introduction

The time-dependent properties of concrete have been researched since the early decades of the last century. This area of research continues to be active for a number of reasons including the significant structural effects caused by shrinkage and creep in modern concrete structures and, in particular, the rapid development of high-performance concretes in recent years. Most shrinkage and creep studies have been carried out in well-founded research laboratories with excellent temperature and relative humidity control facilities. Great care has been taken in such research to maintain constant environmental conditions throughout the storage period.

In practice, the design engineer has little knowledge of the probable time of year of manufacture of concrete elements which are being designed. Hence, concrete elements can be subjected to quite different environmental conditions during their early storage period resulting in a range of early shrinkage strains being developed. Long-term investigations into the drying shrinkage of concrete subject to natural environmental conditions are not extensively reported in the literature. An exception is an extensive study by two of the authors [1] on the seasonal variation in shrinkage data acquired during monitoring of two major glued segmental viaducts. The segments for one bridge were cast in the Spring, whereas the segments for the other were cast in late Summer, resulting in significantly different seasonal effects during storage. More recently, Vandevalle [2] has reported on a study of creep and shrinkage in which the ambient conditions were varied cyclically to simulate the yearly variations in temperature and relative humidity in Belgium.

Shrinkage is a complex phenomenon which is influenced by many factors including the constituents, the temperature and relative humidity of the environment, the age when the concrete is subjected to the drying environment and the size of the structure or member. Shrinkage results in a reduction in volume and may be due to a number of sources. The shrinkage of hardened concrete due to drying is referred to as `drying shrinkage', while `plastic shrinkage' is used to describe the shrinkage of fresh concrete. `Autogenous shrinkage', which occurs when a concrete can self-desiccate during hydration, and which becomes more significant as the strength of concrete is increased, is analogous to drying shrinkage [3]. A range of concrete strengths were used in this study but a distinction between the autogenous and drying component is not possible since all the test specimens were unwrapped. Hence the term `drying shrinkage' in this paper includes any contribution which arises due to autogenous shrinkage.

The paper reports on the influence of the natural environment on the drying shrinkage of a range of concretes, including high-strength concrete, with and without fibre reinforcement. The experimental work was carried out at the Technical Faculty of the University of Mazandaran located in the North of Iran. The test specimens were stored in three environments with different relative humidities and temperatures. Some of the specimens were stored as control specimens and were kept in the control room where the environmental conditions were kept constant at 60±5% RH and 23±2°C. Similar specimens were kept in two natural environments of 77±8% RH and 23±7°C (green area) and 71±8% RH and 25±7°C (roof of building).

Section snippets

Experimental details

Details of the environmental conditions in the green area and on the roof of the building are presented in Fig. 1. Average daily readings of temperature and relative humidity were recorded over a 12 h period (8.00 a.m. to 8.00 p.m.) for a period of approximately one year. These results have been smoothed by taking 10-day average values to give the results presented in Fig. 1. Fig. 1(a) presents details of the relative humidity whereas Fig. 1(b) presents the corresponding information for the

Results and discussion

The results for the fresh concrete properties for the plain and FRC mixes are presented in Table 2, Table 3, respectively. As expected, the Vebe test results are more meaningful for the FRC mixes. The results show that satisfactory workability was achieved in all cases by the use of increasing amounts of SP.

The increase in compressive strength was achieved partly by the use of SF and partly by reducing the water content. The 28-day compressive and tensile strength results for the plain and FRC

Prediction of shrinkage strain

A number of shrinkage prediction models have been developed from experimental data gathered from test specimens subjected to constant relative humidity and temperature conditions for various periods of time [6]. However, the predicted shrinkage deformations often show significant errors. In the full study [7], the experimental values for drying shrinkage in the three test environments were compared with the predicted shrinkage strains given by two of the current common shrinkage models, i.e.,

Conclusions

The paper presents experimental shrinkage results obtained from prisms and cylinders stored in two natural environments and under controlled conditions (60±5%RH, 23±2°C). The experimental results have been compared with predicted shrinkage values obtained from the ACI 209 model. The main conclusions from the study are as follows:

1. One of the strengths of the study is that detailed information was obtained regarding the environmental conditions in the two natural storage areas used in the

Acknowledgements

The second author wishes to express his thanks to the Technical Faculty, University of Mazandaran for the financial support to carry out the work reported in this paper and for supporting his three period of study at Cardiff University, UK.

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