The use of the sand concrete makes it possible to carry out a Hollow Bricks concrete having physico-mechanical properties answering the structural exigences and having economic and environmental advantages compared to the classical concrete. The present study Hollow Bricks aims to connecting the parameters of a formulation based on an empirical formula of Caquot in order to optimize, on the one hand the couple compressive strength/absorption of water under various degrees of hygrometry, and on the other hand more precisely to use the concrete sand in the public works sector in the prefabrication of prestressed beams and hollow bricks. The results show the importance of the type of formulation used because it takes into account the percentages of fillers of sand which is a co-product (waste) of massive rock crushing. In addition, the use of fillerized sands, which are wastes of crushing basaltic rocks and containing a small percentage of fillers, is efficient in the manufacture of prestressed beams. As for the hollow bricks, a fillerized basalt sand, containing a high percentage of filler, as well as a sand dune, gives satisfactory results.

The environmental issues [1] , socio-economic and sustainable development in the public works sector are increasingly taken into account by public authorities and industrial companies in all developed and developing countries. Concrete is today the most widely used building material in the world. Its success is due to its high mechanical properties, its durability, its geometric adaptability, its high fire resistance and especially its availability [2] . Moreover, the construction of buildings, bridges and road infrastructure requires significant amounts of quality materials. Indeed, the public authorities are looking for local and sustainable alternatives, in order to preserve the environmental quality of the country. Whatever the construction works, the characteristics of the used materials must meet minimum quality requirements. Furthermore, several categories of construction materials can be used in different construction sectors. However, in order to reduce construction costs, engineers are forced to take into account the transport distances and the means of exploitation.

However, at present, more criticisms are being made about the concrete material because of the environmental impacts generated by the production of cement, its main constituent. It should be known that, globally, 5% of total CO₂ emissions are from cement industries, which also consume 2% of total primary energy [3] . The cement is obtained by calcination of limestone and clay rocks at very high temperature of about 1450˚C. This process requires high energy consumption (coal, natural gas, fuel oil, etc.) and generates very significant CO₂ emissions. In addition, the CO₂ emitted by the means of transport and the production of electricity necessary for the operation of the cement plants is very important. According to a study published in June 2009 by [4] , the average global amount of CO₂ emitted per ton of clinker produced was 866 kg CO₂/t in 2006.

Country like Senegal must invest in a research and development program on sand concrete for the following reasons: development of its local resources; cost reduction of constructions in the public works sector; scarcity of aggregates; abundance of raw materials (sands and filler sands) found in almost inexhaustible quantity (Senegal is covered with more than 70% of sand on the one hand and secondly, the filler sands are co-produced by crushing massive rocks available in quarries. Indeed, the research and experiments carried out by the partners of the “SABLOCRETE” National Development Research Project [5] have shown that the technique of sand concrete pavements brings advantages in terms of the economy, the preservation of natural resources and the environment, in areas rich in sand. Then, several research studies synthesized in [6] [7] [8] [9] and [10] have shown that the mechanical performance of lightweight aggregate concrete could be sufficient for use as structural concrete. In this work we are interested in the exploitation of local materials in construction instead of using materials that require a very expensive supply, and as Senegal is very rich in sand dune we thought to exploit the sand for the manufacture of sand concrete [6] [11] [12] . Therefore, sand concretes have the same cement content as traditional concretes (250 to 400 kg/m³); the compactness is reached by a complementary addition of fines, generally limestone. Sometimes, certain uses of concrete require characteristics poorly assured by traditional concrete and sand concrete can better satisfy, among these features we quote: Handiness, cohesion and absence of segregation, small particle size and small dimension grains, surface appearance and the most interesting, its non-cracking character [4] , which encourages us to use it as a repair material. The other interest for this material is the possibility of employing industrial fillers to increase its compactness. To have a multi-purpose construction material, denser with a very fine porosity, more impermeable and therefore more durable; silica smoke was introduced into the sand concrete formulation to see its effect on the concrete.

This article consists of a continuation of many studies already carried out on sand concretes in order to optimize the compression resistance/water absorption at different degrees of hygrometry. This experimental work deals both with the study of the physical and mechanical properties of sand concretes as a function of the water content, the addition of adjuvant and the percentage of fillers. It should also be applied on a large scale in order to confirm the possibility of the use of this concrete in the prefabrication of prestressed beams and hollow bricks.

In this study, the physical properties as well as their particle-size parameters of beach sand, sand dune and basalt 0/3 are presented through a characterization process. Then, different formulations of concrete using these local materials, with or without adjuvants, are detailed in Section 3. Indeed, the specimens of concretes corresponding to real prestressed beams and real hollow bricks have been tested in laboratory to determine their mechanical performance.