Relationship between delayed ettringite formation and delayed expansion in massive shrinkage-compensating concrete

Abstract

The possibility of delayed expansion induced by delayed ettringite formation in massive shrinkage-compensating concrete was investigated. Mortars were prepared using ordinary Portland cement and a sulfoaluminate-based expansive agent. They were cured in three different moist conditions at elevated temperature in a temperature match conditioning process to simulate the temperature development in the interior of massive concrete, and then at ambient temperature. Changes in quantity and morphology of ettringite were examined using semi-quantitative X-ray diffraction and scanning electron microscopy. The time-dependent length change of restrained mortars was also measured. The results show that, in addition to the quantity of ettringite, the structure of mortar, morphology of ettringite and curing conditions also influenced expansion of mortars induced by delayed ettringite formation. The expansion mechanism of delayed ettringite formation and the relationship between curing conditions and length change, delayed expansion and damage to concrete structure are discussed.

Introduction

Nowadays, sulfoaluminate-based expansive agents are widely used in China to construct shrinkage-compensating concrete with large section size for the foundations of high-rise buildings and for huge infrastructure. The designed strength class and dimension of these concrete structures increase continuously with the increase of the height and volume of the buildings. For example, the concrete foundation of Shanghai Jinmao Building, which is the tallest building in China, was placed in the strength class of C50 at a thickness of 4 m [1]. The concrete foundation of Beijing Orient Plaza was placed in the strength class of C30–C40 at a thickness of 5.1 m [2]. A large amount of concrete placed in a short period of time results in an obvious rise of temperature in the interior of the massive concrete, due to the release of hydration heat of cement. A peak temperature of 97 °C was recorded in 40 h after placing the concrete foundation of Shanghai Jinmao Building [1].

It is known that the main hydration product of sulfoaluminate-based expansive agents is ettringite. Its formation leads to a volume expansion which compensates the cooling and drying shrinkage of concrete. Ettringite formed in the early hydration period decomposes when it is subjected to temperatures above 70 °C. This phenomenon was first recognised by Ludwig and his co-worker [3] as “delayed ettringite formation (DEF)” and has been confirmed as a destructive phenomenon occurring in many concrete structures [4]. Taylor et al. [5] defined DEF as the formation of ettringite in a cementitious material by a process that begins after hardening is substantially complete, that involves none of the sulfate originates from an external source. We have most recently demonstrated that DEF can also occur in massive shrinkage-compensating concrete [6].

Research work on DEF has largely been focused on heat-cured, precast concrete elements, for example railway sleepers in which cracking appears mainly on the top surfaces where there is little restraint on the concrete [7], [8], [9], [10]. Unconfined mortar or concrete samples were often prepared, cured under saturate atmosphere in the temperature range of 80–100 °C for a few days, then kept humid at ambient temperature. The dimension variation of samples and ettringite's characteristics were investigated to verify whether DEF occurs [11], [12], [13]. Yang et al. [14] emphasized that the late formation or regrowth of ettringite in mortars cured at ambient temperature may not result in expansive damage. The condition in the massive concrete is, however, very different from that in small concrete elements. There is strong restriction and it is difficult for the migration of water in the massive reinforced concrete foundation. So the critical question is whether or not DEF can induce a delayed expansion, or even cracking, under these conditions. In this paper, the relationship of DEF and delayed expansion of restricted mortar is investigated under conditions simulating a massive reinforced concrete foundation.