Proprietes des betons realises avec des granulats de differentes densites

The recent development of concrete technology is more and more frequently leading to the use of special concretes having different densities compared with the normal concrete. This result is obtained by the use of aggregates with different densities, whose characteristics are so important that they often affect the properties of concrete decisively. whose characteristics are so important that they often affect the properties of concrete decisively. Four different concretes were prepared employing different light-weight aggregates (one composed of expanded clay and another one of sintered pulverised fuel-ash), normal gravel and heavy weight aggregates. An almost monodimensional size of the grains and a constant volumetric ratio between the aggregates and a reference mortar have been fixed up so that the concrete properties obtained can be immediately correlated with the aggregate characteristics. In this paper the results of multiaxial compressive strength tests on cubic specimens are described, with reference to strain measurements too. Furthermore, both the bending and splitting tests and the shrinkage and creep measurements give more information on concrete-aggregate relation and show how the density of aggregates affects the properties of concrete.     

Les betons de demolition source de granulats

In spite of the circumstances, the consumption of aggregates in France is still important. The opening of new gravel pits encounters a more severe regulation and the populations' reticence. Otherwise, in the big agglomerations, a lot of constructions are no longer in accordance with present requirements. So, two solutions are available to meet the requirements: the restoration or the demolition. When the latter solution is chosen, the tipping of the waste concrete is very expensive. From this point of view, the recycling of demolished concrete could become very interesting. The demolished concrete elements already used for filling can be improved in using them as aggregates for new concrete, which will minimize the nomber of extractions of good aggregates, and will partly eliminate the tipping problems. We have shown that the use of all sizes of aggregates coming from the concrete crushing was not suitable. If the concrete exclusively contains aggregates above 4 mm, its mechanical properties approaches those of the reference normal concrete but the characteristics of concrete including recycled sands prove to be very poor. Thus, to specify their influence, we realized some tests on mortar. In that case, the main criterion isn't the size, but the total volume of the recycled particles. Two parameters influence the weakening of mechanical properties: the shape and the nature of particles. For concretes containing only recycled aggregates above 4 mm, concretes present creepings and shrinkages which are 30 % higher than those of the reference concrete, but which have similar strengths. The density and the elasticity modulus, are slightly lower (2 to 4 %).     

Influence de la nature du granulat sur l'etuvage des betons

The effect of the type of aggregate used in concrete on the steam curing temperature is investigated. Using two types of aggregates, basalt and limestone, compressive, flexural and bond strengths are measured on mortar and concrete. Three curing cycles at 40, 60 and 80°C are utilized. One-day results show that while the strengths of basalt concrete increase continuously with the curing temperature up to 80°C, those of the limestone concete increase until 40°C and higher temperatures crete negligable effet. Therefore optimal curing temperatures are 70–80°C for basalt concretes and 40–50°C for limestone concretes.     

Influence des sulfures de fer des granulats du maresme (Barcelone) sur les betons

Several problems which took place in some concrete works performed with aggregates coming from the Mont-Palau quarry are presented. Most of the concrete made with these aggregates in the period 1970–1972 suffered a fast degradation and a state of ruin was eventually reached in some cases. The studies realized at that time, partly by the authors, are reviewed. New analysis of the aggregates used are added. Both paleozoic limestones and slates with variable sulfide content were used. Geological studies show that the operations in the quarry affected external zones of the limestone mass. These proved to be the sources of most unstable aggregates. It is found that limestone aggregates with pyrite contents are less dangerous than slates and other interbedded siliceous rocks which also contain sulfides.     

Granulats elabores par concassage de betons de demolition

Because of the limitation of the extraction of gravels in some alluvial districts and because of the increase of the transportation costs, the French Administration decided, to encourage and to promote the utilization of materials which would be able to replace the traditional materials, the basic principle being to use at the best, all the regional materials. These materials may be from natural or artificial origin. It's in that context, that in 1982 a road contractor, proposed on his own initiative to use the materials produced from the crushing of reinforced concrete issued from the pulling down of buildings so as to supply aggregates for road-building around Paris. This experience proved to be roughly positive, in spite of the problems concerning particularly:

-the crushing and screening plant for the production of aggregates. Though the quality and homogeneity of the reinforced concrete were rather fair (principal origin: pulling down of the “La Villette” slaughter-houses), the crushing and screening plant needed particular engines which were not yet correctly tested to eliminate the undesirable components (iron, plastics, wood...);

-the rate of production;

-the required characteristics concerning aggregates, taking into account the important traffic on this road and of the techniques which were used; the addition of hard aggregates was considered as necessary.

In spite of these difficulties, this experiment deserves certainly to be renewed; Important quantities of such materials are available in all important urban districts.     

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.     

粉煤灰加气混凝土吸水性能研究

试验表明，当粉煤灰加气混凝土在整体吸水时的吸水速度和吸水量明显大于粘土砖；粉煤灰加气混凝土一端吸水时的吸水性能与其成型时的膨胀方向密切相关，当吸水方向平行于加气混凝土膨胀方向时，粉煤灰加气混凝土的吸水速度和吸水量明显大于粘土砖；吸水方向垂直于加气混凝土膨胀方向时，粉煤灰加气混凝土的吸水速度和吸水量小于粘土砖。     

粉煤灰加气混凝土吸水性能研究

试验表明,当粉煤灰加气混凝土在整体吸水时的吸水速度和吸水量明显大于粘土砖;粉煤灰加气混凝土一端吸水时的吸水性能与其成型时的膨胀方向密切相关,当吸水方向平行于加气混凝土膨胀方向时,粉煤灰加气混凝土的吸水速度和吸水量明显大于粘土砖;吸水方向垂直于加气混凝土膨胀方向时,粉煤灰加气混凝土的吸水速度和吸水量小于粘土砖。     

Possibilites d'emploid des calcaires de durete moyenne a faible dans les betons hydrauliques

The crushed limestone aggregates with medium to weak hardness present two difficulties concerning first the influence of the variations of the thin elements (0/500 μm) content, (these thin elements are produced between the production of the aggregates 5/20 mm and their utilization) and second the hardness of these materials which is lower than for traditionnaly employed silico-calcareous aggregates. The results of the research described in the paper do no confirm this fear; on the contrary, it is obvious that the limestones allow to elaborate concretes presenting quite satisfactory characteristics; the technology of use does not introduce any particular problem except the necessity of employing wet aggregates. In return, the durability of these concretes, tested through the freezing test, shows that it is better, especially for concretes with a medium cement content, to use an air entraining-agent.     

Granulats et betons hydrauliques. Besoins des entreprises: Regularite dans la diversite et nouvelle norme Française

The design of concrete structures at ultimate limit state, the progress made concerning concrete execution, as well in precast as cast in situ concrete, with very precise striking times, need more and more reliability of the materials. Since approximately ten years, cement plants made large efforts to improve steadiness of their products, and new very useful admixtures are used. In parallel the choice of aggregates has to be more and more selective. These circumstances lead France to review the corresponding standard NF P 18-301 to adapt the needs of construction to the possibilities of deposits and to the cost of production of the suitable aggregates. In such a frame the new french standard proposes basic values for the different characteristics and alternative values with the possiblity, when ordering aggregates, to take either the basic or the alternative value for each characteristic. The paper presents some cases of application which allow to combine the possibility of economy with the best quality of product adapted to the concrete designed.     

Effectiveness of technogenic waste usage in products of building ceramics and expanded clay concrete

In this paper the influence of the burned and the unburned mullite wool waste on the properties of ceramics and expanded clay concrete has been analysed. It has been determined, that this waste could be successfully used in the manufacture of expanded clay concrete and wall ceramic products. Increasing the quantity of mullite wool in the mix of expanded clay concrete, the forecasted frost resistance decreased and the water absorption increased. Products without waste and those having 10% of the waste have similar compressive strength. The performed research has shown that the unburned mullite wool waste is the active micro filler, however it does not decrease the quantity of portlandite formed, but performs a framing function. Modifying the composition of ceramics with the burned mullite wool waste, the density of samples decreased, but the parameters of water absorption and compressive strength increased. To receive stronger ceramic products, 15% of the burned mullite wool waste should be added, then the compressive strength is 23 MPa, however the forecasted frost resistance of such ceramic body would be only 65 cycles. The highest frost resistance (about 200 cycles) of ceramic samples was received applying 5% of the waste additive, then compressive strength is about 16 MPa. The unburned mullite wool waste decreases the frost resistance and density of expanded clay lightweight concrete samples, and increases the water absorption. Replacing 10% of cement with the unburned mullite wool waste almost does not change the compressive strength and density, but decrease frost resistance 36% and increase water absorption 4%.     

Identification et caractérisation géotechnique des sédiments de retenues des barrages de Tunisie

Reservoirs of Sidi Boubaker, Nebeur, Sidi Saad and Sidi Salem dams are silting. This study includes the grain size (sand, silt and clay), mineralogy, and geotechnical characteristics (specific area, water content, plasticity and undrained shear strength) of deposited sediments. Results of correlation between different parameters are exposed. From the laboratory experiments of consolidation, we simulated the sediments behaviour in the reservoir during the consolidation phase, and we identified five stages of consolidation. The results allow to formulate recommendations for desilting works.     

Recherche de developpement d'emploi des sables issus de roches massives siliceuses et silicatees dans les betons hydrauliques

Because of the depletion in some parts of France of the alluvial sands normally used for hydraulic concretes, consideration has been given to using sands produced by crushing massive rocks as a substitute. Rocks of twelve different mineralogical types, representing most of the types of rock found in France, have been investigated. Attempts were made:

- to determine the maximum acceptable filler content of the sands, in terms of the properties of the mortars and concretes in both the fresh and hardened conditions;

- to assess the risks of alkali-aggregate reactions;

- and to work out the best techniques for the industrial production of these sands, so that the filler content and particle size distribution are compatible with the results of the research on mortars and concretes.

     

Utilisation des fillers ultrafins dan les betons

The authors consider the possibilities of achieving high strength hydraulic concretes by reducing the porosity of the interstitial cement paste with ultrafine particles filling the cement intergranular spaces. The possibility of decreasing the water-cement ratio of a water/cement + ultrafines mix is studied, using a very simplified model of viscosity. The experimental results are given for two types of filler incorporated in mortar compositions of constant workabilities: a silica fume (d_m?2 μm) and a calcareous rock filler (d_m?2 μm).     

La porosite des granulats de laves basaltiques de la Reunion

Basaltic lava from the Reunion are characterised by a high closed porosity. Water can only penetrate open voids on the surface. When the density increases, water content also increases. In fresh concrete, aggregates attract and gradually retain water during the setting process of the cement; this greatly modifies the workability and the strength of concrete. Results of the measurement of water content of sands and stones versus time are exposed.     

Stabilisation des sols gonflants: cas d'une argile en provenance de Mila (Algérie)

The construction of highways using swelling materials causes problems in many countries of the world. A number of authors have studied this subject, but the treatment of swelling clay remains a problem. This paper describes the problems associated with the black cotton soil in Algeria. It is hoped that it will help designers and contractors when making decisions in countries with similar soils and climate. A new apparatus capable of measuring swelling to a volume change of less than 10^–4 was used. Due to the difficulty of dealing with black cotton soils, these materials are frequently discarded as unusable for engineering works because of their sensitivity to water and their swelling potential. Mixing the black cotton soil with slake lime or another admixture has been found to modify the physical and chemical characteristics of the soil such that a significant decrease in swelling is observed and the material can be used in certain situations. The cationic exchange capacity (CEC) is modified by the addition of lime and the time of storage, while the swelling potential is significantly affected by the addition of 6% lime, even within a 7-day period. The influence of time is clearly shown. In both cases, 3% additive was mixed with the Mila clay. Using the first method the swelling potential dropped from 6% to less than 1% over a period of between 7 and 90?days, while using the second mode of storage over the same time span, the swelling potential dropped from 0.35% to less than 0.1%. The study demonstrates the method of storage, the percentage of mixed material added to the clay and time all have an important effect on the physical characteristics of the material, including the swelling potential.