Retardation effect of superplasticizer on different cement fractions

Cement hydration is a complex chemical process strongly influenced by the proportions of the various minerals present in the cement, admixtures, and the size of the cement particles. In this note, it is shows that naphthalene based superplasticizer, used at high dosage, does not affect the total heat of hydration but retards the hydration process of portland cement. This retardation, however, is most effective on the medium size fraction of the cement (4 – 30 μm in our case). The superplasticizer does not affect the hydration of the fine fraction (< 4 μm) to that extent due to its richer concentration of SO₃ and alkalies. The coarse fraction of the cement (30 – 72 μm) does not participate (with or without superplasticizer) in the heat evolution process.     

Characterization of the granular packing and percolation threshold of reactive powder concrete

Reactive powder concrete (RPC) is a new cement-based material developed through microstructural engineering. RPC is composed of very fine powders: sand, crushed quartz and silica fume, all with particle sizes comprised between 300 and 0.02 μm, and a low water content, W/C<0.20. A very dense matrix is achieved by optimizing the granular packing of these powders. This compactness confers to RPC ultrahigh strength and durability.

The hydration kinetic of the cementitious matrix using electrical conductivity and isothermal calorimetry is presented. A linear relationship between the logarithm of conductivity ln(σ) and the degree of hydration , has been found:

ln(σ)=A_v−B_v

where A_v depends on the nature and the dosage of the different mineral and organic components, and B_v is an invariant related to the granular packing. From 19 different RPC compositions, an average value of 12.1 with a standard deviation of 1.7 were found for B_v. The capillary porosity percolation threshold was also deduced from this relationship. It has been found that when the degree of hydration is equal to 26%, the capillary pore space is discontinuous. This last result is in agreement with the prediction of the NIST microstructural model by Bentz and Garboczi [D., Bentz, E. Garboczi, Percolation of phases in a three-dimensional cement paste microstructural model, Cem Concr Res 21 (2) (1991) 325–344]. A continuous determination of the degree of hydration based on electrical conductivity is proposed.     

High performance concrete—An overview

It is suggested that high performance concrete is not fundamentally different from the concrete used in the past, although it usually contains fly ash, ground granulated blastfurnace slag and silica fume, as well as superplasticizer. The cost aspects of the use of silica fume are considered. The content of cementitious material is high and the water/cement ratio is low; the maximum size of aggregate is small. Although ordinary Portland cement is used, it must be compatible with a given superplasticizer; the causes of incompatibility are discussed. The distinct shrinkage behaviour of high performance concrete is considered and the reasons for an absolute necessity of wet curing are given. Some uses of high performance concrete are mentioned. A ‘prediction’ of the future of high performance concrete and of concrete in general is offered.     

Durabilité au gel des bétons de hautes performances mécaniques

Ce texte présente les principaux résultats d'une étude entreprise dans le but de mieux comprendre comment se comporte le béton H. P. face aux cycles de gel-dégel. Le béton H.P. est un matériau nouveau et il n'est pas encore possible d'évaluer en nature sa capacité à résister aux attaques du gel et des sels de déglaçage. A l'Université Laval, la durabilité de bétons H.P. (80–100 MPa) a été analysée en les soumettant en laboratoire à des cycles rapides de gel-dégel dans l'eau (ASTM C666) et à des cycles de gel-dégel journaliers en présence de sels de déglaçage en solution (ASTM C672). Aucune des éprouvettes fabriquées n'a été détruite par les cycles de gel-dégel et toutes ont bien résisté à l'attaque des sels déglaçants, si bien que les différentes variables que nous avons étudiées (rapport eau-liant, \(\bar L\) , cure) n'ont pas influencé la performance au gel de ces bétons. C'est l'emploi d'un ciment H.P. à grains fins avec 6% de fumée de silice et l'utilisation de rapports eau-liant relativement faibles (<0,30) qui sont à la base de la durabilité de ces bétons.     

Role des caracteristiques physico-mecaniques des granulats sur la resistance en compression de betons a tres haute resistance

The construction of very high buildings in concrete instead of steel is now quite common and it requires very high strength concretes (100 MPa and over). This new field of utilization of concrete requires a deeper evaluation of the physico-mechanical properties of concrete. The report presents an experimental study about the production of 100 MPa concretes with different kinds of aggregates quarried in Saint Laurent Lowlands in Québec (Canada) and identified as devonian sedimentary rocks and cretaceous basic intrusive rocks. It appears that the higher strengths are obtained when aggregates and cement paste have similar mechanical properties and if coarse and fine aggregates are produced with the same type of rock, preferably a microgranular limestone; there is no noticeable advantage to utilize aggregates with a strength much more higher than that one of the cement paste.     

A Sequence-Based Object-Oriented Model for Video Databases

Structuration, annotation and composition are amidst the most crucial modeling issues that video editing and querying in the context of a database entail. In this paper, we propose a sequence-based, object-oriented data model that addresses them in an unified, yet orthogonal way. Thanks to this orthogonality, the interactions between these three aspects are properly captured, i.e., annotations may be attached to any level of video structuration, and all the composition operators preserve the structurations and annotations of the argument videos. We also propose to query both the structuration and the annotations of videos using an extension of ODMG's OQL which integrates a set of algebraic operators on sequences. The overall proposal is formalized and implemented on top of an object-oriented DBMS.