Excess paste method to formulate a self-compacting concrete

On the basis of the principle that a concrete is composed of a liquid phase (paste) and a solid phase (aggregates with fixed gravel/sand ratio), the concrete self-compacting properties come necessarily from those of the paste. The present work is the continuity of a first phase of the testing already conducted, which resulted in obtaining an optimal self-compacting cement paste composition. This paste will be used to prepare a self-compacting concrete (SCC), while passing from the scale of the cement paste to that of the concrete, by injecting wet aggregate to the self-compacting paste. The excess paste theory was used to determine the thickness of the paste coating each aggregate with a given diameter of constituting granular skeleton, then generalized for the determination of the quantity of total paste allowing the flow of the concrete by decreasing frictions between the grains of its granular skeleton. This approach was also experimentally validated. The influence of the granular distribution was minimized by the use of the approach based on the determination of the average diameter of the aggregates. This required the determination of a homothetic factor “k” similar for all concretes with different aggregate grading. Formulation of a self-compacting concrete passes initially by the determination of a sufficient quantity of paste allowing its flow without frictions between its aggregates and to balance the mixture by the quantity of water retained by the aggregates. The self-compacting concrete characteristics would come from those of the cement paste which composes it

Study of concretes properties based on pre-saturated recycled aggregates

The aim of this study was concretes waste recovery by replacing natural aggregates (NA) by recycled aggregates (RA) at rates of 0%, 50% and 100%. Recycled  aggregates  coming from construction and demolition waste (CDW) present a low  quality  compared  to  natural aggregates,  the  water  absorption being their  main  drawback.  Quantities of water absorbed and their variations in times were measured according to the European standard (EN 1097-6). Adding absorbed water during mixing have caused concretes segregations and an excess volume of water was observed at the surface of the concrete. The added water was not totally absorbed by aggregates. The pre-saturation of aggregates 24 hours before mixing and introducing them as a component in concrete was the best method. The initially saturated aggregates did not consume concretes mixture water. Workability by slump test and mechanical strengths developed with pre-saturated aggregates (100 % RA) and (50% RA - 50% NA) were measured on concretes, and then were compared to those of the reference concretes made with 100 % of natural aggregates

Valorization of Dredged Sediments as a Component of Vibrated Concrete: Durability of These Concretes Against Sulfuric Acid Attack

Abstract Hydraulic facilities are subjected to significant siltation which, in a very short period of time, can render them unusable. In Algeria, the silting-up of a great number of dams, built for drinking water needs and for irrigation, implies the necessity and urgency to take action. Therefore, the maintenance work, which leads to dredging the deposited silt, constitutes an unbearable obligation for the preservation of the environment. Chorfa dam (western Algeria) may be mentioned as a concrete example. This study is part of a long research whose objective is to contribute to the valorization and the optimization of the formulations economically that are easy to implement and which enable to use the dredged materials in the formulation of ordinary concretes by partial substitution to cement (10, 20 and 30%) of dredged sediments, after calcination at 750 °C to make them active. Tests were carried out on concrete that was vibrated in the fresh state (setting time) and hardened state (compressive strengths and durability of concrete exposed to sulfuric acid attack) in order to determine their characteristics. The results obtained confirmed the possibility to develop concretes containing calcined silt, with proportions up to 30%, and which can meet the economic, ecological and technological objectives