The Canada Centre for Mineral and Energy Technology (CANMET) of Natural Resources Canada, Ottawa, has played a significant role in Canada for over 40 years in the broad area of concrete technology. In recent years, CANMET has become increasingly involved in research and development dealing with supplementary cementing materials, high-performance normal-weight and lightweight concretes, and alkali-aggregate reactions. In May 2004, CANMET, in association with the American Concrete Institute and several other organizations in the United States, sponsored the Seventh CANMET/ACI International Conference on Recent Advances in Concrete Technology in Las Vegas, U.S.A. Seventeen refereed papers from more than 10 countries were presented and distributed at the conference. The proceedings consisting of refereed papers were published as ACI SP-222. Note: The individual papers are also available as .pdf downloads.. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP222

Lignosulphonate is a widely used plasticizing admixture in concrete. It is well documented that different qualities of this material give different performance in concrete. Depending on what kind of concrete that is needed, workability can be controlled by adding different amounts or qualities of the lignosulphonate. This investigation compares the adsorption of lignosulphonate on three different portland cements, to the rheological properties of cement pastes made from the same cements. The adsorption isotherms were calculated from depletion experiments. A rheometer with bob-cup geometry was used to measure the rheological properties of the cement pastes. The plasticizing effect of lignosulphonates in cement paste slurries was confirmed. Recent advances have given a novel lignosulphonate with superplasticizer performance. This investigation demonstrates these improved properties achieved by this novel lignosulphonate by determining the differences in adsorption of the different lignosulphonates, on cements with different chemical characteristics.

Magnetic Resonance Imaging (MRI) is a nondestructive technique that can be used to spatially resolve distributions of certain nuclei. Lithium is a relatively sensitive nucleus for MRI. Therefore, it is possible to directly measure the distribution of lithium in cement based materials. Lithium salts are used in concrete to suppress alkali-silica reaction. The MRI relaxation parameters associated with lithium in cement-based materials are relatively short by traditional MRI standards. Due to the short relaxation parameters, special MRI measurement techniques and hardware considerations had to be developed in order to quantify lithium distributions in cement based materials. MRI has the potential to play an important role in concrete technology. While this method has been developed for laboratory studies, measurements could be made on cores extracted from existing concrete structures.

A comprehensive research program was undertaken to determine the influence of coarse aggregate concentration, binder type and content, and the use of set-modifying admixtures on lateral pressure exerted by self-consolidating concrete (SCC). Experimental columns measuring 200 mm in diameter and either 2100 or 2800 mm in height were used to determine the distribution of lateral pressure during the plastic stage of cement hydration. The effect of thixotropy of the concrete on pressure variations was investigated. Test results show that lateral pressure exerted by SCC is significantly affected by the development of shear strength properties of the plastic concrete, namely internal friction and cohesion. Mixtures incorporating greater coarse aggregate volumes and/or lower binder contents were found to exhibit higher degree of internal friction. This can reduce the mobility of the concrete and result in lower initial pressure. However, given that internal friction is an inherent property of the material which remains constant with time, the rate of drop in pressure was shown to depend mainly on the increase in cohesion. Therefore, mixtures containing higher binder contents and/or a set-accelerating admixture can exhibit sharper rate of pressure drop with time. Concrete with higher degree of thixotropy was found to develop lower initial lateral pressure and higher rate of pressure drop with time. This is attributed to the stiffening effect which enables the material to re-gain its shear strength when left at rest with-out any shearing action.

Ettringite observed in macroscopic paste cracks during DEF is often held responsible for concrete deterioration. However, some authors have raised the hypothesis of an homogeneous paste expansion resulting from ettringite crystallisation in the C-S-H porosity, which does not attribute any mechanical effects to the subsequent formation of ettringite crystals in the voids. Thus the role of ettringite is still controversial. In this paper, we are tempting to link the two previous hypotheses in a more global mechanism: During heat treatment, thermal decomposition of ettringite can occur, whereas higher amount of sulphate and aluminate are trapped into C-S-H porosity. Then, at ambient temperature, ettringite forms in the porosity corresponding to the network of C-S-H layers. If the volume of ettringite reaches the limit of C-S-H porosity, the following ettringite crystallisation induces an homogeneous expansion of the paste. This expansion can generate peripheral cracks around aggregates. Then, the local shrinkage around voids resulting from the dissolution of the small ettringite crystals incorporated in C-S-H layers and the following precipitation of ettringite massive crystals in the cracks due to Ostwald rippening, outbreaks radial cracks. When the material is weaken by a multidirectional crack network, the pressure exerted by massive ettringite crystallisation can propagate existing cracks by strain localisation at the crack's tip, even if the crystallisation pressure is small in these conditions.