Influence of Shrinkage-Reducing Admixtures on Development of Plastic Shrinkage Cracks

The term plastic shrinkage cracking is generally used to describe cracks that form between the time when concrete is placed and the time when concrete sets. This paper discusses how the evaporation of water causes concave menisci to form on the surface of fresh concrete. These menisci cause both settlement of the concrete and tensile stress development in the surface of the concrete, which increase the potential for development of plastic shrinkage cracks. Specifically, this paper studies the development of plastic shrinkage cracks in mortars containing a commercially available shrinkage-reducing admixture (SRA). Mortars containing SRA show fewer and narrower plastic shrinkage cracks than plain mortars when exposed to the same environmental conditions. It is proposed that the lower surface tension of the pore fluid in the mortars containing SRA results in less evaporation, reduced settlement, reduced capillary tension, and lower crack-inducing stresses at the topmost layer of the mortar.

Keywords: cracking; durability; plastic shrinkage; settlement; shrinkage,

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

INTRODUCTION

One of the age-old problems in concrete is cracking. In addition to being unsightly, cracks have the potential to act as weak planes for further distress or as conduits for accelerated ingress of aggressive agents that may reduce durability. In particular, fresh concrete may be susceptible to plastic shrinkage cracking, which occurs between the time of placement and the time of initial set.1

In the literature, plastic shrinkage cracks are generally attributed to four driving forces. The first driving force that leads to plastic shrinkage cracking is the rapid evaporation of water, which creates menisci and high tensile stresses in the capillary water near the surface.2-4

According to the Young-Laplace equation,2 the maximum capillary tension that develops during drying is proportional to the surface tension of the pore fluid and inversely proportional to the radius of the pores being emptied. This relationship has been used to illustrate why finer cements and mixtures containing silica fume are more susceptible to plastic shrinkage cracking.3 The second driving force commonly thought to lead to plastic shrinkage cracking is differential settlement, because plastic shrinkage cracks are frequently observed above reinforcing steel or at locations where there is a sudden change in cross-sectional thickness.1,5,6 Concrete with reduced slump or with fiber addition is less sensitive to plastic shrinkage cracking due to a decrease in the settlement that occurs in these mixtures.6 A third potential driving force for plastic shrinkage cracking is differential thermal dilation in which a temperature gradient develops inside fresh concrete due to evaporation of water from the surface. Simpkins et al.7 showed that the temperature of a gel system is reduced during evaporation, while Kovler8 reviewed the effects of evaporative cooling for hardened concrete. The fourth driving force that has been linked to an increased potential for plastic shrinkage cracking is autogenous shrinkage in the plastic phase. Autogenous shrinkage has been defined as "the bulk deformation of a closed, isothermal, cementitious material not subjected to external forces."9 This deformation is caused by the formation of liquid-vapor menisci inside the material due to the consumption of water by the hydration reactions. Experiments10,11 have shown that fresh concrete may undergo severe (autogenous) shrinkage that could lead to cracking even if evaporation is prevented. While these four factors are often addressed independently, they will act simultaneously in practical applications and contribute to the potential for cracking.

Over the last two decades, significant research has examined the use of shrinkage reducing admixtures (SRA) in concrete. The majority of studies have focused on long-term shrinkage12 and on the reduced potential for shrinkage cracking in hardened concrete.13 A recent review article14 reported that concretes containing SRA generally have lower shrinkage, lower or equal chloride penetration indexes, reduced sorptivity, and reduced cracking potential, despite having similar or slightly lower strength, modulus of elasticity, and fracture toughness compared with plain concrete. Concrete containing SRA generally shows less cracking due to a lower rate of shrinkage and a reduction in the overall magnitude of shrinkage.13

However, substantially fewer studies have been conducted on the role of SRA in fresh (that is, plastic) concretes. In a study on drying of gels, Simpkins et al.7 observed that the use of a surfactant, which lowered the surface tension of the pore fluid, like an SRA, greatly reduced the occurrence of cracking. Bentz et al.15 examined the role of SRA in pastes and mortars at early ages using x-ray absorption. To explain the lower evaporation of mixtures with SRA and a change in the drying profile, they hypothesized that initial drying causes the formation of a concentrated solution of SRA at the surface, which hinders further evaporation of water. Mora et al.16 presented an experimental study where a reduction of evaporation and of plastic shrinkage cracking was observed in concrete with three different types of SRA.