International Concrete Abstracts Portal

SP192 In 2000, CANMET, in association with ACI, the Japan Concrete Institute, and several other organizations in Spain and Canada, sponsored a fifth international conference held on June 4-9, 2000, in Barcelona, Spain. More than 120 papers from 35 countries were received and peer reviewed in accordance with the policies of the American Concrete Institute; 73 were accepted for publication. The accepted papers deal with all aspects of concrete durability. In addition, several sessions dealing with sulfate attack, superplasticizers and supplementary cementing materials, and near surface testing for the durability of concrete were organized. In addition to the papers that have been published in the refereed proceedings, more than 30 papers were presented at the conference.

Leaching degration of cement composites is a phenomenon involving the removal of various solid constituents and may result in loss of strength. The leaching of calcium is not a major problem for most concrete structures, as the degradation rate is very slow. However, it is important for structures such as marine structures, dams, and radioactive-waste repositories that are in an environment that direct contact with water for a long term. In order to evaluate the leaching behavior, investigation of the fundamental degradation mechanism must be undertaken. Described in this paper is the degradation mechanism of cement composite derived from accelerated testing. Investigations were conducted using three different accelerated test methods, I.e. the immersion test method, the dissolution test method and the permeation cell test method. The main results of the study were as follows: 1) 30-50% of calcium in cement composites was leached out in several moths in the accelerated tests (the immersing test method and the permeation test method); 2) in all tests, calcium leaching from Ca(OH)2 and C-S-H was observed, and the change in the porosity and physical properties corresponding to the leaching degree was found; and 3) a possibility to predict the long-term degradation behavior of structures by using numerical analysis was demonstrated.

The state-of-the-art on thaumasite formation is discussed, stressing aspects of its formation in mortars an in portland-cement concretes, and also the effect its formation has on their durability. Methods for material synthesis, mentioned in the bibliography including a detailed report of the characterization of thaumasite through XRD, IR, DTA/TG, NMR, electron microscopy and microanalysis, are described.

Mortars or concretes that have been cured at high temperature. This phenomenon is usually referred to Delayed Ettringite Formation (DEF) because ettringite deposits have been observed in expansive structures. However no correlation has been established between the amount ettringite detected in expansive heated mortars and the degree of expansion. Ettringite is prevalent in old structures regardless of the occurrence of deterioration. This study is aimed at investigating the expansion mechanism and in particular the role of ettringite relating to expansion. The results indicate that there is no direct cause between the ettringite detected by XRD and expansion. Instead, evidence is presented for the possible implication of the C-S-H of expansive and non-expansive heated mortars that is the presence of ettringite formed in the outer C-S-H from calcium monosulfoaluminate, present within the outer C-S-H, and released sulfate from the C-S-H. Such ettringite can cause expansion because its formation takes place in site in the outer C-S-H. The paste expands and detaches from the non-expansive components such as aggregates forming gaps at the paste/aggregates interfaces.

A number of unusual microscopic features have been documented in studies of the alteration of permeable concretes undergoing sulfate attack in semi-arid climates. The high S/C of the concretes examined here has permitted complete penetration of sulfate-bearing ground water through the concrete, and deposition of crystallized salts on evaporative surfaces, including the upper surfaces of slabs. The microstrucrual alterations observed in response to this penetration of sulfate-bearing ground water include the deposition of secondary ettringite and the accompanying local expansion and cracking and the deposition of gypsum, but are much more extensive and complicat4d than that. Calcium hydroxide is often entirely removed throughout the concrete, leaving open areas, particularly in the interfacial zones around aggregates. Various new compounds are deposited in these spaces, and elsewhere within the paste. In addition to ettringite and gypsum, the substances deposited often include monosulfate; brucite and magnesium silicate hydrate may be formed where the ground water is rich in magnesium; and thaumasite may be developed where carbonation accompanies sulfate attack. Partial decalcification of the C-S-H gel is common, and magnesium silicate hydrate may locally replace it. Even the large residual unhydrated cement grains, usually stable indefinitely in most concretes, are profoundly altered. The C2S and C3S in such grains may be completely dissolved and the spaces thus provided within the outlines of the cement grains may also be filled by secondary deposits. It is evident that sulfate attack produces profound internal changes in the paste microstructure, leading to the softening, exfoliation, and other external symptoms of sulfate attack reported in the literature.