Betons a hautes performances,a granulats artificiels actifs

High strength concretes, in particular at early ages (more than 40 MPa at 3 days and up to 80 MPa at 7 days), are obtained by replacement of certain granular size ranges of sand or gravel by cement-active artificial aggregates in a given concrete composition elaborated with a local supply of aggregates and cement. The strengthening of the paste-aggregate bond by chemical reactions between the artificial aggregates and the cement also involves an improvement of other concrete performances and not only of mechanical strengths. The cracking ability is decreased, the wear strength is higher, and water permeability is low or even nil for certain compositions. The laboratory results are confirmed by tests on the site and show the possibility of achieving high strength concretes without any particular selection of cement and natural aggregates.     

Effect of silica fume on cement hydration and temperature rise of concrete in tropical environment

Investigated herein is the effect of temperature on heat development in cement pastes and concretes with and without silica fume cured at relatively high temperatures often encountered in tropical environment. With an initial temperature of 30°C, adiabatic temperature rise of the concrete with 8% silica fume as cement replacement was similar to that of the control Portland cement concrete up to about 18 h. After 24 h, however, the temperature of the silica fume concrete was lower than that of the control concrete. Since the concrete with 8% silica fume had a higher 28-day compressive strength (72.5 MPa) than the control concrete without silica fume (59.2 MPa), the concrete with silica fume is likely to have a lower temperature rise as compared with the control concrete of equivalent 28-day strength by reducing cementitious materials content with the same water content. The extent of heat evolution in the silica fume pastes was generally greater at lower temperatures of 20-50°C, but less at 65°C than in the control paste. At the relatively high curing temperatures, the degree of cement hydration in the paste with silica fume was lower than that in the control cement paste at early ages. However, the pozzolanic reaction started even before 24 h after water was added.     

Effect of silica fume on cement hydration and temperature rise of concrete in tropical environment

Investigated herein is the effect of temperature on heat development in cement pastes and concretes with and without silica fume cured at relatively high temperatures often encountered in tropical environment. With an initial temperature of 30°C, adiabatic temperature rise of the concrete with 8% silica fume as cement replacement was similar to that of the control Portland cement concrete up to about 18 h. After 24 h, however, the temperature of the silica fume concrete was lower than that of the control concrete. Since the concrete with 8% silica fume had a higher 28-day compressive strength (72.5 MPa) than the control concrete without silica fume (59.2 MPa), the concrete with silica fume is likely to have a lower temperature rise as compared with the control concrete of equivalent 28-day strength by reducing cementitious materials content with the same water content. The extent of heat evolution in the silica fume pastes was generally greater at lower temperatures of 20–50°C, but less at 65°C than in the control paste. At the relatively high curing temperatures, the degree of cement hydration in the paste with silica fume was lower than that in the control cement paste at early ages. However, the pozzolanic reaction started even before 24 h after water was added.     

Failure mechanism of recycled aggregate concrete

The use of recycled aggregate concrete (RAC) acquires particular interest in civil construction regarding sustainable development. Recycled aggregates usually present greater porosity and absorption, and lower density and strength than natural aggregates. Microstructural studies on RAC indicate differences in the characteristics of the interfacial transition zones between the cement paste and the aggregates. At the same time most experiences verify that reduction in concrete stiffness is higher than in strength. The failure mechanisms in RAC can be affected by the above stated factors. In this paper, three Series of concretes with different compressive strength levels are presented. Each Series includes a reference concrete prepared with natural crushed stone and two RAC prepared with two coarse aggregates obtained by crushing a normal strength and a high strength concrete. Flexural tests on notched beams and uniaxial compression tests on standard cylinders were performed. In addition, the characteristics of the fracture surfaces were analysed in order to determine the amount of broken aggregates. RAC present lightly lower strengths (1–15%), lower modulus of elasticity (13–18%) and significant reductions in the energy of fracture (27–45%) and, consequently on the fracture zone size, when it is compared with a concrete prepared with natural coarse aggregates.     

阿利特-硫铝酸钡钙水泥混凝土的试配及力学性能的研究

根据混凝土的配合比设计及试验室现场操作,配制出工作性良好的阿利特-硫铝酸钡钙水泥混凝土。采用对比试验研究的方法,研究了不同水灰比对该水泥混凝土抗压强度的影响,并将其与相同配合比的普通混凝土的力学性能进行比较。试验结果表明:两种混凝土的抗压强度均随水灰比的增大而减小;相同配比下,该水泥混凝土的抗压强度比同龄期的普通混凝土有了明显的改善,尤其早期抗压强度,1 d强度提高了50%~65%。微观结构分析发现:阿利特-硫铝酸钡钙水泥混凝土中水化产物的粒径分布均匀,界面粘结状况较好,结构较为致密。     

Properties of concrete prepared with crushed fine stone,furnace bottom ash and fine recycled aggregate as fine aggregates

This paper presents the results of a study to compare the properties of concretes prepared with the use river sand, crushed fine stone (CFS), furnace bottom ash (FBA), and fine recycled aggregate (FRA) as fine aggregates. Two methods were used to design the concrete mixes: (i) fixed water–cement ratio (W/C) and (ii) fixed slump ranges. The investigation included testing of compressive strength, drying shrinkage and resistance to chloride-ion penetration of the concretes. The test results showed that, at fixed water–cement ratios, the compressive strength and the drying shrinkage decreased with the increase in the FBA content. FRA decreased the compressive strength and increased the drying shrinkage of the concrete. However, when designing the concrete mixes with a fixed slump value, at all the test ages, when FBA was used as the fine aggregates to replace natural aggregates, the concrete had higher compressive strength, lower drying shrinkage and higher resistance to the chloride-ion penetration. But the use of FRA led to a reduction in compressive strength but increase in shrinkage values. The results suggest that both FBA and FRA can be used as fine aggregates for concrete production.     

Mechanische Hochtemperatureigenschaften von flugaschebasierten Geopolymerbetonen

Mechanical properties of fly ash‐based geopolymer concretes at high temperature At present, concretes based on alkali‐activated binders, so‐called geopolymer concretes, are investigated intensively in the building materials industry and by the research community as environmentally friendly alternative to Portland cement‐based concretes. These inorganic binders, which are based on industrial by‐products such as fly ash and ground granulated blast furnace slag, exhibit high resistance against corrosive acids and salts, if properly designed. The mechanical properties of fly ash‐based geopolymer concretes at high temperatures are subject of systematic investigations at the Bundesanstalt für Materialforschung und ‐prüfung (BAM) to create a basis for the structural design of fire exposed concrete members based on alkali‐activated binders. The concrete specimens, produced with quartz aggregates or lightweight aggregates and heated to a maximum temperature of 750 °C, exhibited a decrease of compressive strength up to temperatures of ca. 300 °C, attributed to formation of microcracks caused by dehydration. At higher temperatures the compressive strength of the investigated geopolymer concretes recovered partly, due to sintering processes starting from ca. 500 °C. Because of this beneficial property when compared to conventional concretes, geopolymer concretes can potentially be applied in infrastructure facilities where fire resistance is critical. From the results of the thermomechanical tests stress‐strain relationships are derived that can be used for the structural design of members made from geopolymer concretes.     

Splitting tensile strength and modulus of elasticity of self-compacting concrete

In this experimental work, the splitting tensile strength and the modulus of elasticity are studied for self-compacting concretes of different ages. At the same time, its porous structure is analysed, since this aspect is directly related with mechanical properties of the material. Eight different concretes were used, four self-compacting and four normally-vibrated, with different water/cement ratios and different types of cement. The results obtained show that in self-compacting concretes made with limestone filler the splitting tensile strength is on average 15% less than that of normally-vibrated concretes. It is therefore suggested that the standard expressions that are normally used for normally-vibrated concretes should be modified for these concretes. In addition, it is seen that the cementitious paste’s modulus of elasticity is greater in the self-compacting concretes because the addition of limestone fines creates lower porosity and a finer microstructure. Despite this, the concrete’s modulus of elasticity tends to be around 2% less in the self-compacting concretes as a result of the greater amount of cement paste in the self-compacting concretes.     

阿利特-硫铝酸钡钙水泥混凝土的试配及力学性能的研究

根据混凝土的配合比设计及试验室现场操作,配制出工作性良好的阿利特-硫铝酸钡钙水泥混凝土.采用对比试验研究的方法,研究了不同水灰比对该水泥混凝土抗压强度的影响,并将其与相同配合比的普通混凝土的力学性能进行比较.试验结果表明:两种混凝土的抗压强度均随水灰比的增大而减小;相同配合比下,该水泥混凝土的抗压强度比同龄期的普通混凝土有了明显的改善,尤其早期抗压强度,1d强度提高了50%～65%.微观结构分析发现:阿利特-硫铝酸钡钙水泥混凝土中水化产物的粒径分布均匀,界面粘结状况较好,结构较为致密.     

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.     

Recycled concrete made with different natural coarse aggregates exposed to high temperature

The recycled aggregates obtained from crushed waste concretes have different characteristics from those of natural aggregates. For that reason, the mixture proportions and the fresh and hardened properties of recycled concretes are different. The performance of recycled concrete exposed to high temperatures is not a very well-known subject since most studies have been conducted on conventional concretes. Recycled concretes with water/cement (w/c) ratios of 0.40 and 0.70, and made with three different types of natural coarse aggregate were exposed to 500 °C for 1 h. These concretes were evaluated by the ultrasonic method, resonance frequency, static modulus of elasticity and compressive strength, before and after heating, and compared with those obtained on similar conventional concretes containing the same type of natural coarse aggregate. The conventional and recycled concretes made with quartzitic coarse aggregate performed better after the heat treatment.     

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).     

高铝水泥耐火混凝土火灾高温后强度及耐久性试验研究

研究了以高铝水泥为胶结材料 ,以陶粒、耐火砖块、陶砂等为骨料配制的 4种耐火混凝土 ,在模拟火灾高温后的强度及氯离子渗透性的变化。试验表明 ,在慢速升温的条件下 ,不同骨料的高铝水泥耐火混凝土强度均有所下降 ,经高温后耐火砖混凝土、陶砂陶粒混凝土和普砂陶粒混凝土较普通混凝土表现出良好的耐火性能。     

Effect of coarse aggregate size and cement paste volume on concrete behavior under high triaxial compression loading

When massive concrete structures (high-rise buildings, tunnels, dams, nuclear power plants, bridges, protection structures, …) are subjected to extreme loadings (aircraft shocks, rock falls, near-field detonations, ballistic impacts, …), the material undergoes triaxial compression loading at a high confinement. In order to reproduce high stress levels with well-controlled loading paths, static triaxial tests are carried out on concrete samples by mean of a very high-capacity triaxial press. It is a well-known fact that the cement paste volume and the coarse aggregate size are two important parameters of concrete formulation. This article focuses on identifying the effect of coarse aggregate size and cement paste volume on concrete behavior under high triaxial compression. This article shows that at low confinement, the concrete strength slightly increases as the coarse aggregate size increases. At high confinement, the coarse aggregate size has a slight influence on concrete deviatoric behavior and a significant influence on concrete strain limit-state. The higher the coarse aggregate size, the lower is the mean stress level corresponding to concrete strain limit-state. Furthermore, this article highlights that at low confinement, the concrete strength significantly increases with an increase in cement paste volume. Increasing confinement tends to reduce cement paste volume effect on concrete strength. At high confinement, contrary to what has been observed in unconfined compression, the cement paste volume has little effects on concrete deviatoric behavior. Otherwise, decreasing cement paste volume increases concrete deformation capacity. At very high confinement levels and at very high deviatoric stress levels, the axial tangent stiffness of concrete increases as the coarse aggregate size or the cement paste volume is reduced.     

Influence of class F fly ash on the abrasion–erosion resistance of high-strength concrete

This study investigates the abrasion–erosion resistance of high-strength concrete (HSC) mixtures in which cement was partially replaced by four kinds of replacements (15%, 20%, 25% and 30%) of class F fly ash. The mixtures containing ordinary Portland cement were designed to have 28 days compressive strength of approximately 40–80 MPa. Specimens were subjected to abrasion–erosion testing in accordance with ASTM C1138. Experimental results show that the abrasion–erosion resistances of fly ash concrete mixtures were improved by increasing compressive strength and decreasing the ratio of water-to-cementitious materials. The abrasion–erosion resistance of concrete with cement replacement up to 15% was comparable to that of control concrete without fly ash. Beyond 15% cement replacement, fly ash concrete showed lower resistance to abrasion–erosion compared to non-fly ash concrete. Equations were established based on effective compressive strengths and effective water-to-cementitious materials ratios, which were modified by cement replacement and developed to predict the 28- and 91-day abrasion–erosion resistance of concretes with compressive strengths ranging from approximately 30–100 MPa. The calculation results are compared favorably with the experimental results.     

Effect of environmental conditions on the properties of concretes with different cement types

The paper reports on the changes in properties of concretes with different cement types associated with environmental conditions. Three strength classes with three different cement types (ordinary portland cement PC 42.5 (CEM I 42.5), portland composite cements PKC-A 42.5 (CEM II/A-M 42.5) and PKC-B 32.5R (CEM II/B-M 32.5R)) were used in the study. Also, a mixture was prepared with PC 42.5 and silica fume (SF). The effects of variable ambient conditions on plastic shrinkage of fresh concrete and cement paste, compressive strength, modulus of elasticity, capillary absorption and drying shrinkage of hardened concrete were investigated. In contrast to PC 42.5 cement paste, plastic shrinkage cracks were observed in PKC-B 32.5 and PKC-A 42.5 pastes. Water absorption coefficients of all concretes stored in natural environment were higher at all ages as compared to coefficients of concretes kept in laboratory. Drying shrinkage values of concrete with SF, except the first week, were significantly lower than those of others. Although different behaviors for different cement types were observed, water–cement ratio was one of the dominating factors determining the behavior of concrete. This ratio should be lowered to improve the durability of concrete.     

再生骨料高强高性能混凝土配制研究

通过对不同强度等级旧混凝土破碎而成的再生粗骨料与天然粗骨料性能的对比研究 ,表明再生粗骨料与天然粗骨料性能具有相似性 ,能够满足行业标准对碎石的性能要求 ,并尝试将再生粗骨料用于配制高强混凝土和自流平高性能混凝土 ,结果表明 ,采用原强度在 15～ 2 0MPa以上的旧混凝土破碎而成的再生骨料均可用于配制高强混凝土 ,但在配合比相近情况下 ,原混凝土强度等级越高 ,再生混凝土各龄期强度越高 ,其再生利用价值也越高。再生粗骨料亦可用于配制自流平高性能混凝土 ,水胶比为 0 4 0 ,净用水量为 2 0 5kg m3左右时 ,配制得到的新拌混凝土坍落度和坍落扩展度分别在 2 4 5、5 6 0mm以上 ,其 2 8、6 0d强度分别在 5 8 0、6 7 3MPa以上     

Beeinflusst die Kornsteifigkeit der Gesteinskörnung im Beton den Degradationsprozess infolge zyklischer Druckbeanspruchung?

Does the Stiffness of Aggregate in Concrete Influence the Degradation‐Process Due to Cyclic Compression Loads? Structures like bridges, cooling and wind towers etc. are not only exposed to environmental influences and constant static loads but also often to cyclic loads. Due to the latter, very fine microcracks can occur due to load cycles leading to a degradation in the microstructure of concrete and to a reduction of stiffness and strength. Up to now it is still uncertain, if and to what extent different aggregates and therefore different aggregate stiffnesses influence this degradation‐process and which consequences may result for the stiffness of concrete. Within the scope of the Collaborative Research Center 398 (SFB 398), lifetime‐oriented design concepts with the aspects of damage and deterioration were developed. In one of the subprojects (A13), the degradation of material properties due to cyclic loading was experimentally investigated. This paper gives an account of the investigations on concrete with different aggregate characteristics under cyclic compressive loading. Thereby aggregates with significant different characteristics (sandstone, basalt, quartzite) were selectively included with furthermore constant concrete composition. Specimens of these concretes were subjected to single stage cyclic loads with lower and upper stresses being constant during the whole test series. During the cyclic loading, the changes in the strains and the stiffness (Young's modulus) of concrete caused by the proceeding degradation in the microstructure were determined. Microscopic analyses at different numbers of cycles were done as well. The results of the investigations showed that the ratio between the stiffness of the coarse aggregates and the hardened cement paste significantly influences the degradation‐process. At the concretes with sandstone the lower stiffness of aggregates led to a comparatively lower reduction in concrete's stiffness and to a increase of microcracks in the concrete's microstructure after defined load cycles in comparison to concretes with quartzite or basalt.     

国外高性能混凝土的发展及其在复杂工程和超大型工程中的运用

高性能混凝土具有许多卓越的特性,但是其最重要的性能就是强度高。其高强度的产生是由于高性能混凝土内部所具有的特殊紧密矩阵所形成的。高性能混凝土的其它优良特性一般包括良好的延性和强度、收缩性;具有和普通混凝土相同的骨料但其弹性模量明显高于普通混凝土。高性能混凝土的弹性模量几乎可与铝相比较[1];非常好的不可渗透性,其不可渗透性低于自然界中的任意种类的岩石[2];卓越的抗磨损性,其抗磨损性可以与自然界中最坚硬的岩石相媲美[3];杰出的抗冻融循环性[4];非常低的徐变性[5]和较高的弯曲强度。本文将重点关注高性能混凝土运用其本身的卓越性能如高强度、硬度及延性而建造的复杂结构体系及超大型结构工程。同时在此文中还要展示运用高性能混凝土所带来的其它方面的益处。     

Durability of lightweight concretes with lightweight fly ash aggregates

This paper presents the effects of aggregate properties such as strength, porosity, water absorption, bulk density and specific gravity on the strength and durability of lightweight fly ash aggregate concrete (LWAC). The influence of properties of four aggregates (sintered lightweight fly ash aggregates, cold-bonded lightweight fly ash aggregate and normalweight aggregate) on mechanical and durability properties of concrete is discussed. Experimental results revealed that durable high-strength air-entrained lightweight concretes could be produced using sintered or cold-bonded lightweight fly ash aggregates, having comparable performance with the normalweight concretes. The use of lightweight aggregates (LWA) instead of normalweight aggregates in concrete production decreased the strength and stiffness due to the higher porosity and lower strength of the aggregate included in the concrete. However, permeability of sintered fly ash aggregate lightweight concretes was comparable and slightly lower than normalweight concrete whereas permeability of cold-bonded fly ash lightweight concrete was greater than the others. All concretes had a durability factor greater than 85, which met the requirements by showing quite perfect resistance to freeze–thaw.