Influence of Freezing-and-Thawing Damage on Behavior of Reinforced Concrete Elements

The purpose of this research is to provide a basis on which physical and chemical actions can be included in the design process of reinforced concrete structures by adopting appropriate material models that include deterioration aspects. As an example of physical and chemical deterioration mechanisms, freezing-and-thawing cycles, together with the absorption of capillary water, have been chosen and analyzed.

The variation of concrete elastic modulus and bond behavior due to the applied freezing-and-thawing cycles is examined. The results of this work are then implemented in a computational model developed at the University of Waterloo, Waterloo, Ontario, Canada. The program generates the moment-curvature relation for reinforced concrete structures subject to bending and axial deformations. The model is based on a layer analysis of a cross section, where the bond behavior is particularly considered. The computed moment-curvature relation is compared with data obtained from tests on freezing-and-thawing damaged and undamaged beams.

Keywords: bond; design; deterioration; freezing-and-thawing.

(ProQuest-CSA LLC: ... denotes formulae omitted.)

INTRODUCTION

The failure of reinforced concrete elements is often due to an interaction of different damage mechanisms. In addition to mechanical loads, reinforced concrete structures are also subject to physical and chemical environmental actions. In design, however, often only the mechanical loads are taken into account.

Physically- and chemically-induced damage to concrete (for example, due to attack by freezing-and-thawing cycles) is usually considered as a problem of durability of the concrete surface only, and the influence of this type of damage on the mechanical behavior is not accounted for.

Within the framework of the presented research project, cycling freezing-and-thawing actions together with water absorption have been chosen for investigation to quantify their influence on the mechanical behavior of reinforced concrete elements. This deterioration mechanism causes internal damage to the concrete, which can be quantified by the ultrasonic measurement technique. The freezing-andthawing- induced microcracks in the cement paste cause a relatively homogenous damage, which can be formulated in terms of deterioration coherences.

RESEARCH SIGNIFICANCE

Reinforced concrete buildings are subject to mechanical loads and environmental actions. These loads and actions induce damage processes, which include loosening of bond in the micro and meso area of the building material, causing a macroscopic change in its mechanical behavior.

Even though there is no question that such damage processes take place, they have been ignored in planning, modeling, and structural design. Increasing safety in the structural engineering design of buildings requires considering the deterioration of mechanical performance due to various damage processes. This can also allow predictions and appropriate rating of the operating life of the designed buildings.

This paper presents a systematic quantification of the freezing-and-thawing damage effect on the mechanical behavior of reinforced concrete structures. This can provide a basis for reinforced concrete constitutive material equations, which include damage considerations (mechanical or physical-chemical).

General aspects of cycling freezing-and-thawing loads

When saturated concrete is subjected to freezing-andthawing cycles, external and internal damage occur. Powers1 found out in 1945 that these two types of damage can occur independently and therefore can be considered separately. External damage indicates weathering of thin outer mortar layers and aggregate (deicer scaling). Mechanical properties of the remaining concrete are not influenced by external damage. Internal damage causes microscopic cracks in the cement paste, leading to change in mechanical parameters of the concrete. External damage is caused by using deicing salts, whereas internal damage dominates when concrete is exposed to freezing-and-thawing-cycles without deicing salts. These mechanisms can often be traced back to the same phenomenon, but due to different combinations or intensities of the different mechanisms, they have to be considered separately. Because only internal damage affects mechanical properties, the next part of this paper deals only with internal damage.

The damage due to a freeze attack is based primarily on the freezing-and-thawing-induced pump effect of the concrete.2,3 The micro-ice-lens model describes the abnormal contraction during freezing after the first ice formation (frost shrinkage) and the equivalent expansion during heating. The chemical and mechanical stability criteria are developed from surface thermodynamics. Microscopically, the model points to a redistribution of water in the pore space from paste to larger pores during cooling and a water uptake from external sources during heating if water is available. With each freezing-andthawing cycle, water from external sources is absorbed by the concrete, and it becomes more and more saturated. The moving boundary condition given by the progressing ice front (cooling) and melting front (heating) enhances the process. After reaching a critical degree of saturation, there is no space in the cement paste left to compensate for the 9% expansion at the change from water to ice. The hydraulic pressure at the critical saturation point, caused by freezing, is then counterbalanced by a microcrack initiation. If this critical degree of saturation is exceeded, the well-known damage models can be applied.4 This information is essential for accurate testing methods and simulation of practical conditions.