The carbonation of concrete depends both on the carbon dioxide penetration through the pore network and on the accessibility of the calcium-containing compounds such as calcium hydroxide or C-S-H gel. The aim of this work was to determine the influence of the W/C on the microstructural characteristics of cement pastes. Investigations were performed during the hydration process at early age and on 28-day and 2-year old hydrated cement pastes made with a Type I normal portland cement. The W/C were .25, .35, .45, and .60. The main results are as follows: Porosity and autogenous strain measurements, completed with SEM observations, show that the microstructrual characteristics are very different between pastes having WC above and below .35-.40. Above this value: The porosity strongly increases; The autogenous deformations exhibits a swelling phase before self-desiccation shrinkage; The calcium hydroxide appears in large crystals. SEM examinations and EDX analysis show that, for an equivalent degree of hydration, the calcium hydroxide amount increases and that the CaO/SiO2 ratio of the C-S-H decreases as the W/C increases. Explanations of the results are proposed, taking into account the ability of the water to diffuse thought the structure and to react with unhydrated cement. The implication of these results in carbonation process are also discussed.

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.

Hysteresis effects are very frequent in experimental studies of porous building material. In the case of frost deicing salt resistance, hysteresis effects between freezing and thawing were reported in calorimetric and expansion experiments. They often were explained by a difference in the active pore diameter. However, our calorimetric and ultrasonic pulse velocity measurement show much smaller hysteresis effects which support another explanation: supercooling. A model based on non-connected pore water which may supercooling. A model based on non-connected pore water which may supercool was developed to explain the experimental differences. This model allows a reinterpretation of experimental expansion data. The moment of damage formation in frost deicing salt resistance tests can be studied. The analysis of the causes of hysteresis effects therefore leads to improved models of frost deicing salt damage mechanisms.

The behavior of ordinary concrete and high strength concrete under chemical attack was studied over a period of more than 5 years. The concrete specimens were immersed in a variety of chemical solutions including 5%, 2.5%, .5% and .1% ammonium sulfate, and 10%, 5%, 1% and .1% ammonium nitrate. The strength and weight changes, and the penetration depths of the attacking ammonium nitrate ions into the specimens were measured. The influence of the initial concrete strength and the concentration level of the aggressive solution on the behavior of the immersed concretes were evaluated.

Heat curing is very commonly used in the production of precast concrete elements to allow for a rapid demoulding and frequent reuse of the casting beds. Curing temperatures up to 70 degrees C are often used to obtain high early strengths from 30 to 55 MPa. at ages ranging from 6 to 16 hours. Heat curing may influence negatively the durability of the precast concrete elements in several ways such as strength loss, formation of micro cracks which are preferential paths for ingress of aggressive agents and delayed ettringite formation and subsequent cracking. This paper examines the use of novel polycarboxylic either base superplasticizers which allow for a high water reduction (up to 40%) in the concrete mix, in order to obtain high early strengths at curing temperatures considerably lower than those utilized at present and in some cases, eliminating the heat curing . Several case histories of applications in Europe are presented where the benefits of the low water-cement ratio of the concrete mixes, combined with a very high workability, contribute to enhance the durability of precast concrete structures.