Double Feedback Control Method for Determining Early-Age Restrained Creep of Concrete Using a Temperature Stress Testing Machine

Qingbin Li,He Zhu,Yu Hu,Rui Ma

doi:10.3390/ma11071079

Abstract

Early-age restrained creep influences the cracking properties of concrete. However, conventional creep measurements require a large number of tests to predict the restrained creep as it is influenced by the combined effects of variable temperature, creep recovery, and varying compression and tension stresses. In this work, a double feedback control method for temperature stress testing was developed to measure the early-age restrained creep of concrete. The results demonstrate that the conventional single feedback control method neglects the effect of restrained elastic deformation, thus providing a larger-than-actual creep measurement. The tests found that the double feedback control method eliminates the influence of restrained elastic deformation. The creep results from the double feedback method match well with results from the single feedback method after compensation for the effects of restrained elastic deformation is accounted for. The difference in restrained creep between the single and double feedback methods is significant for concrete with a low modulus of elasticity but can be neglected in concrete with a high modulus of elasticity. The ratio between creep and free deformation was found to be 40–60% for low, moderate, and high strength concretes alike. The double feedback control method is therefore recommended for determining the restrained creep using a temperature stress testing machine.

Highlights

The cracking of massive concrete structures due to thermal stresses is a problem that has long been studied by engineers [1]
The measured stress difference between low-heat cement concrete (LHC)-C and LHC-D is insignificant. Both of these testing machine (TSTM) were controlled by the double feedback method, which indicates that the stresses measured by different
Specimens moderate-heat cement concrete (MHC)-B and MHC-C manifested the same pattern as the LHC specimen

Summary

Introduction

The cracking of massive concrete structures due to thermal stresses is a problem that has long been studied by engineers [1]. A typical type of mass concrete, suffers varying temperature and strong restraint effects during the first days following casting. The early-age creep of concrete can relax more than 50% of the restraint stress [2,3,4]. Creep is one of the most important properties that influence temperature stress in concrete due to relaxation effects [5]. Early-age creep has its own unique characteristics: The temperature in the concrete increases due to the hydration heat and must be controlled below a design value to avoid a large temperature gradient. The early-age creep of concrete is difficult to obtain because both the physical and chemical properties of concrete change simultaneously [4,8]

Methods

Results

Conclusion