Prediction Models of Shrinkage and Creep in Industrial Floors and Overlays

University essay from KTH/Byggvetenskap

Abstract:

All form of activity in a building is in need for a sound platform to be able to operate. The performance of concrete floor in industrial spaces is of importance because there are greater demands on industrial concrete floor.  By making sure that the necessary recommendation regarding casting an industrial concrete floor is followed, the risk for failure is reduced. The Swedish Concrete Association (2008) recommends a w/c ratio of approximately 0.55 for industrial floors. This would result in a concrete strength class of C30/37 with an abrasive resistance adequate for most industrial floors.

From an economical perspective an approach for concrete rehabilitation is the bonded overlay, which has been used for many years and has the intension to extend the life of structural concrete slabs. For concrete overlay having a matching material to the substrate is the main recommendation, in addition in a fully bonded overlay to the substrate there is less risk for cracking and edge lifting.

From a mechanical perspective a crack develops when the stresses in concrete exceeds the tensile strength. Shrinkage and creep of concrete in both overlays and industrial floors are important factors that contribute in development of cracks which have considerable effect on failure of the structure; therefore it is useful to find an accurate prediction model to predict shrinkage and creep.

The literature study that has been carried out in this thesis is mainly about shrinkage, creep, industrial floor, overlays and tensile stress prediction. In addition the following calculation models for prediction of creep and shrinkage of concrete have been reviewed in this paper: Eurocode 2, fib Model Code 2010, ACI 209R-92, Swedish code BBK04 and Swedish concrete manual Material. In order to study the differences between the mentioned models two example cases for calculation of shrinkage and creep in industrial floor and  bonded overlay with the mentioned methods have been carried out, also the tensile stress development in bonded overlay has been calculated according to a method proposed by Prof. Silfwerbrand (1997). Since it was not possible to consider all types of concrete and conditions which will affect the outcome, only one type of concrete C30/37 with w/c ratio of 0.55 for industrial floor and w/c ratio of 0.40 for overlay is taken as an example in this thesis.

Some of these methods take only a few factors into consideration; however the others are more detailed and treat numerous factors. The simple methods such as BBK04 or the graph in Eurocode 2 for calculating the creep coefficient can be used in the lack of existence of sufficient input parameters to be able to roughly estimate the final value. The predicted shrinkage did differ by using different models, however the final value of the creep coefficient was quiet similar regardless of the used model.

There are some important parameters such as ambient relative humidity, age at loading, duration of drying and duration of loading which should be included in predicting models since these parameters affect shrinkage and creep considerably. Also aggregate type has an important role in both creep and shrinkage; however aggregate type (modulus of elasticity of aggregate) was not included in any of the studied models. Choosing a factor according to the type of aggregate might be helpful for better prediction.

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