Reduction of concrete sorptivity with age through carbonation W.P.S. Dias* Department of Civil Engineering, University of Moratuwa, Moratuwa, Sri Lanka Received 1 July 1999; accepted 17 May 2000 Abstract Ordinary Portland cement (OPC) concrete specimens that had been air - dried for 3.5 years were found to have an increase in weight and reduction in sorptivity, with weight and sorptivity changes being greater for specimens with higher original sorptivity. Since this was attributed to carbonation of the surface zone, the porosity differences between end slices and the specimen interior in cylindrical specimens from another test series (of 4 years of age) were measured, together with the depths of carbonation. It was found from sensitivity analyses performed on a neural network model that the porosity difference can be attributed, in increasing order of importance, to (i) depth of carbonation, (ii) original sorptivity (which reflects both the quality of the mix and the efficiency of curing), and (iii) whether the end slice was a top or bottom one (reflecting the direction of casting and compaction). Tests on existing structures also showed that sorptivity decreased with age and also that (carbonated) surface sorptivities were lower than interior sorptivities, further confirming that the reduction of sorptivity with age is due to surface carbonation and that this carbonation occurs in and benefits surfaces with poorer initial quality more. This implies that carbonation could have mutually compensatory effects on some aspects of concrete durability. D 2000 Elsevier Science Ltd. All rights reserved. Keywords: Aging; Carbonation; Curing; Porosity; Sorptivity 1. Introduction Sorptivity, which is an index of moisture transport into unsaturated specimens, has been recognised as an important index of concrete durability, because (i) the test method used for its determination reflects the way that most concretes will be penetrated by water and other injurious agents and (ii) it is an especially good measure of the quality of the near surface concrete, which governs durability related to re- inforcement corrosion [1]. For example, the extent of curing is considered to be a major influence on the quality of cover zone concrete, and sorptivity is very sensitive to the type and duration of curing [2]. This paper reports some results on the influence of age on sorptivity of ordinary Portland cement (OPC) concretes. It goes on to identify carbonation as the cause for the reduction in sorptivity that was observed with increase in age, and to substantiate this through the measurement of porosity and carbonation depths, and also through some tests on existing structures. 2. Materials and methods The test programmes described were designed for other primary objectives, which are described below. Nevertheless, the specimens that were prepared for them were also used to study the influence of age on sorptivity, and to ascribe reasons for it. The cement used in all cases was OPC. 2.1. Test programme 1 A full factorial experimental programme was carried out to study the relative effects of (i) mix, (ii) slump, (iii) compaction, and (iv) curing on sorptivity. Two commonly used volume batched mixes were used for this programme, namely 1:2:4 and 1:1.5:3. The variation in slump corresponded to 0±10 mm for low slump and 100 mm for high slump. The specimens were cast in cylindrical moulds of 100 mm diameter  100 mm height. Half the specimens were * Tel.: +94-1-647-567 or +94-1-647-568; fax: +94-1-647-622. E-mail address: priyan@civil.mrt.ac.lk (W.P.S. Dias). 0008-8846/00/$ ± see front matter D 2000 Elsevier Science Ltd. All rights reserved. PII:S0008-8846(00)00311-2 Cement and Concrete Research 30 (2000) 1255 ± 1261