Effect of mineral admixtures on formwork pressure of self-consolidating concrete

Owing to enhanced filling ability, self-consolidating concrete offers accelerated casting and superior quality control during construction. However, its high fluidity and high placement rate increase the lateral pressure on the formwork, necessitating an extensive supporting system to retain fresh mixtures in a desired shape. Current recommendations of formwork design for self-consolidating concrete adopt the concept of hydrostatic pressure, even though the measured pressure could be less than the recommended level. This study shows that mineral admixtures such as processed clays can appreciably lessen the formwork lateral pressure. In addition, the correlation between the formwork pressure response and the loss of slump flow is derived, providing an approximate method to estimate the reduction in formwork pressure.     

Effect of steel fibres on mechanical properties of high-strength concrete

Steel fibre reinforced concrete (SFRC) became in the recent decades a very popular and attractive material in structural engineering because of its good mechanical performance. The most important advantages are hindrance of macrocracks’ development, delay in microcracks’ propagation to macroscopic level and the improved ductility after microcracks’ formation. SFRC is also tough and demonstrates high residual strengths after appearing of the first crack. This paper deals with a role of steel fibres having different configuration in combination with steel bar reinforcement. It reports on results of an experimental research program that was focused on the influence of steel fibre types and amounts on flexural tensile strength, fracture behaviour and workability of steel bar reinforced high-strength concrete beams. In the frame of the research different bar reinforcements (2∅6 mm and 2∅12 mm) and three types of fibres’ configurations (two straight with end hooks with different ultimate tensile strength and one corrugated) were used. Three different fibre contents were applied. Experiments show that for all selected fibre contents a more ductile behaviour and higher load levels in the post-cracking range were obtained. The study forms a basis for selection of suitable fibre types and contents for their most efficient combination with regular steel bar reinforcement.     

Evaluating and forecasting the initial and final setting times of self-compacting concretes containing mineral admixtures by neural network

An experimental study was conducted to investigate the effects of using binary, ternary, and quaternary cementitious blends of portland cement (PC), fly ash (FA), ground granulated blast furnace slag (GBS), silica fume (SF), and metakaolin (MK) on initial and final setting times of self-compacting concretes (SCCs). For this purpose, a total of 65 SCC mixtures were prepared at two different water binder ratios. Furthermore, based on the experimental results, neural network (NN) model-based explicit formulations were developed to predict the initial and final setting times of SCCs in terms of the amount of concrete constituents, namely mixing water, PC, FA, GBS, SF, MK, fine (fa) and coarse (ca) aggregates, and superplasticizer (SP). The test results have revealed that the mineral admixtures were very effective on the initial and final setting times of SCCs. Besides, it was found that the model developed by using NN seemed to have a high prediction capability of initial and final setting times of SCCs.     

Microstructure of high-strength foam concrete

Foam concretes are divided into two groups: on the one hand the physically foamed concrete is mixed in fast rotating pug mill mixers by using foaming agents. This concrete cures under atmospheric conditions. On the other hand the autoclaved aerated concrete is chemically foamed by adding aluminium powder. Afterwards it is cured in a saturated steam atmosphere.New alternatives for the application of foam concretes arise from the combination of chemical foaming and air curing in manufacturing processes. These foam concretes are new and innovative building materials with interesting properties: low mass density and high strength. Responsible for these properties are the macro-, meso- and microporosity. Macropores are created by adding aluminium powder in different volumes and with different particle size distributions. However, the microstructure of the cement matrix is affected by meso- and micropores. In addition, the matrix of the hardened cement paste can be optimized by the specific use of chemical additives for concrete.The influence of aluminium powder and chemical additives on the properties of the microstructure of the hardened cement matrices were investigated by using petrographic microscopy as well as scanning electron microscopy.     

The effect of mineral admixtures on the properties of high-performance concrete

The paper presents a laboratory study on the influence of two mineral admixtures, silica fume (SF) and fly ash (FA), on the properties of superplasticised high-performance concrete. Assessment of the concrete mixes was based on short- and long-term testing techniques used for the purpose of designing and controlling the quality of high-performance concrete. These include compressive strength, porosity, oxygen permeability, oxygen diffusion and chloride migration. Measurements were carried out after curing at 20% and 65% relative humidity up to the age of 1 yr. The results, in general, showed that mineral admixtures improved the properties of high-performance concretes, but at different rates depending on the binder type. While SF contributed to both short- and long-term properties of concrete, FA required a relatively longer time to get its beneficial effect. In the long term, both mineral admixtures slightly increased compressive strength by about 10%, but contributed more to the improvement of transport properties of concretes.     

Use of recycled concrete aggregate in high-strength concrete

The results of a test programme to study the use of recycled concrete aggregate (RCA) in high-strength, 50 N/mm² or greater, concrete are described. The effects of coarse RCA content on the ceiling strength, bulk engineering and durability properties of such concretes have been established. The results showed that up to 30% coarse RCA had no effect on concrete strength, but therafter there was a gradual reduction as the RCA content increased. A method of accommodating the effects of high RCA content, involving simple adjustment to water/cement ratio of the mix is given. It is shown that high-strength RCA concrete will have equivalent engineering and durability performance to concrete made with natural aggregates, for corresponding 28-day design strengths. The practical implications of the study for concrete construction are discussed.

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Résumé Sont décrits ici les résultats d’une série d’essais destinés à étudier l’utilisation de granulats provenant du recyclage d’éléments en béton (RCA) dans des bétons de haute résistance (50 MPa et plus). Les effets de la teneur en gros granulats recyclés sur la résistance des plafonds et des batiments, ainsi que les propriétés de tels bétons ont été établis. Les résultats ont montré qu’une teneur allant jusqu’à 30% en gros granulats recyclés n’a pas d’effet sur la résistance du béton, mais qu’au dessus de 30%, la résistance diminue progressivement à mesure que la teneur en gros granulats recyclés augmente. Une méthode visant à accommoder les effets dus à une forte proportion de RCA, nécessitant un simple ajustement du rapport eau/ciment dans le mélange, est proposée. Il est prouvé que le béton RCA de haute résistance aura des qualités de résistance et de durabilité équivalentes à celles de bétons constitués de granulats naturels, pour les résistances mécaniques à 28 jours prévues. Les implications pratiques de l’étude sur la réalisation d’ouvrages en béton sont présentées.

     

Effect of sea water from tropical areas on setting times of cements

Summary This paper reports the first results of a programme of research into the influences of sea water and sea atmosphere on cement and concrete in the tropics. The results discussed here show that up to 75% reduction in the normal initial setting time takes place when cement is mixed with sea water, although no influence on the final setting time is perceptible. It is pointed out that prior knowledge of the properties of cement in the local markets in some areas of Africa, notably Sierra Leone, is necessary, in view of the vast difference in properties of available cements of the same brand and production code number. Where sea water is to be used, only cements with initial setting times not less than 30 minutes in excess of the minimum recommended by B.S. 12, 1958, may be permitted without special percautions being taken.

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Résumé Ce rapport décrit les premiers résultats d'un programme de recherche des influences de l'eau de mer et de l'atmosphère marine sur le ciment et le béton dans la région des Tropiques. Ces résultats montrent une réduction de 75% du temps initial de prise normal pour le ciment gaché avec de l'eau de mer; par contre, on ne décéle pas d'influence sur le temps de prise local. On souligne qu'il est indispensable d'avoir la connaissance préalable des propriétés du ciment qu'on se procure sur les marchés locaux de certaines régions d'Afrique, surtout en Sierra Leone, du fait des grandes différences de qualité que présentent des ciments de même catégorie et de même classe. Si l'on doit employer de l'eau de mer, cela ne peut s'admettre sans précaution préalable qu'avec des ciments dont le temps de prise initial n'excède pas de 30 mn le minimum recommandé par B.S. 12, 1958.

     

Experience with high-strength concrete in combination with high-strength steel in precast reinforced and prestressed concrete

This paper deals with the experience gained before the war with centrifugally moulded (spun) concrete masts, tubular and half-tubular beams, reinforced with high strength steel and after the war with prestressed, vibrated high strength precast concrete, including lightweight concrete, with the advantage of gap-grading, particularly when employing phased frequencies at vibration.     

Effect of mineral admixtures and steel fiber volume contents on the behavior of high performance fiber reinforced concrete

High Performance Fiber Reinforced Concrete (HPFRC) is a structural material with advanced mechanical properties. The structural design of HPFRC members is based on the post-cracking residual strength provided by the addition into the mix of the fibers. Moreover, the addition of different types of mineral admixtures influences the overall behavior of this material. In order to optimize the performance of HPFRC in structural members, it is necessary to evaluate the mechanical properties and the post-cracking behavior in a reliable way. As a result, an experimental study on six different sets of HPFRC specimens was carried out. The main parameters that varied were the fiber volume content and the types of mineral addition. The behavior in compression, in flexural tension and the shrinkage properties were evaluated and critically analyzed in order to give a guide for structural use.The results showed that by adding high fiber volume content and the Algerian blast furnace slag into the mix, the HPFRC material obtained has a very good performance and it is suitable for use in practice.     

Resistance of high-strength concrete to projectile impact

This paper presents the results of an experimental study on the impact resistance of concrete with compressive strengths of 45–235 MPa when subjected to impact by 12.6 mm ogive-nosed projectile at velocities ranging from ∼620 to 700 m/s. The results indicate that the penetration depth and crater diameter in target specimens exhibit an overall reduction with an increase in the compressive strength of the concrete. However, the trend is not linear. Further increase in the compressive strength requires a reduction in the water-to-cementitious material ratio and the elimination of coarse aggregates. However, doing these does not result in reduction of the penetration depth and crater diameter. The presence of coarse granite aggregates appears to be beneficial in terms of reducing penetration depth, crater diameter, and crack propagation, thus contributing to impact resistance. Incorporation of steel fibers in the concrete reduced the crater diameter and crack propagation, but did not have a significant effect on penetration depth. An increase in the curing temperature from 30°C to 250°C did not alter the impact resistance of the concrete significantly. Based on the present findings and cost consideration, high-strength fiber-reinforced concrete with a compressive strength of ∼100 MPa appears to be most efficient in protection against projectile impact.     

Stress-strain curve for the design of high-strength concrete elements

Although the great advances in concrete technology have led to the possibility of obtaining ready-mix concrete with compressive strength around 100 MPa, some national and international codes for concrete structures do not cover concrete strengths above 50 MPa. Many codes are under revision, but some of them (including the Brazilian Code) will still not include high strength concretes. Due to the different characteristics of higher strength concrete some design procedures traditionally used in normal strength concrete structures have to be changed. Different types of stress-train relationships for concrete have been proposed for the non-linear analysis of member behavior and for the ultimate state analysis of high strength concrete elements under combined flexure and axial load. In this work comparisons are made between proposed stress-strain curves and between the axial load-moment interaction diagrams based on these curves. Comparisons of test results with these diagrams, for columns subjected to eccentric compression, give an idea of the different degrees of safety obtained using those curves.

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Résumé Malgré les grands progrès de la technologie en matière de béton, qui ont rendu possible l'obtention de bétons résistant à une compression d'environ 100MPa, quelques codes nationaux et internationaux relatifs aux constructions en béton ne couvrent pas des résistance du béton supérieures à 50 MPa. Plusieurs codes sont en cours de révision, mais beaucoup (dont le code brésilien) ne s'intéressent toujours pas aux bétons à haute résistance. En raison des différentes caractéristiques des bétons à haute résistance, certains procédés de définition traditionnellement utilisés pour les constructions en béton normal doivent être modifiés. Différents types de relations contraintes—déformations ont été proposés pour le béton dans le cadre de l'analyse non-linéaire de son comportement et dans celui de l'analyse finale des éléments en béton armé en flexion combinée avec une charge axiale. Cet article compare les courbes contraintes—déformations proposées et les diagrammes interaction charge axiale—moment qui résultent de ces courbes. Les comparaisons effectuées entre les résultats expérimentaux et ces diagrammes, pour des colonnes soumises à des compressions excentriques viennent illuster les différents degrés de sécurité obtenus grace à ces courbes.

     

Plastic rotation capacity of high-strength concrete beams

This work describes an experimental study on the plastic rotation capacity of high strength beams. Nine simply supported isotatic beams were tested, by applying comprising two symmetrical concentrated loads at approximately one-third and two-third's span. A method of analysis is defined that leads to the development of a parameter that characterizes the plastic rotation capacity at the failure section by means of a plastic analysis of the tested beams. The influence of concrete strength and the longitudinal tensile reinforcement ratio on the capacity for plastic rotation is examined and discussed. The results are discussed and compared with previous studies.

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Résumé Ce travail décrit une étude expérimentale sur la capacité de rotation plastique de poutres en béton à haute résistance. Neuf poutres isostatiques ont été testées avec une charge constituée par deux forces concentrées et symétriques situées environ au tiers et aux deux tiers de la portée. Une méthode d'analyse est définie, conduisant au développement d'un paramètre qui caractérise la capacité de rotation plastique de la section de rupture, utilisant une analyse plastique des poutres testées. L'influence de la résistance du béton et du taux d'armatures longitudinales de traction sur la capacité de rotation plastique est examinée et discutée. Les résultats sont analysés, discutés et comparés avec des études antérieures.

     

Critical parameters for the compressive strength of high-strength concrete

This study investigates the influence of several material properties underlying the failure mechanism of high-strength concrete (HSC) under uniaxial compression. An experimental-numerical characterization of a single inclusion block (SIB) – an idealized composite comprising of a granite cylindrical core embedded within a high-strength mortar (HSM) matrix – is first carried out. Parametric studies are next conducted with the calibrated SIB model, to identify the critical parameters governing the failure of the idealized composite. The qualitative understanding obtained from the SIB is then utilized to design a series of experiments, exploring the extent of influence of the identified critical parameters on the compressive strength of HSC. Complementary experimental data in literature are also examined. For the range of specimens considered, it is found that the lateral strain capacity of mortar matrix has the most influence on the compressive strength of HSC.     

高强再生骨料混凝土的配制及性能研究

采用再生粗骨料配制强度在50MPa或更大的高强再生骨料混凝土,并对其变形能力和耐久性进行测定,为高强再生骨料混凝土在工程上的应用提供理论和实验基础。通过一系列的抗压实验确定再生粗骨料的强度极限,并通过对水灰比的调整,使配制的高强再生骨料混凝土在强度上达到设计值,并以再生粗骨料取代率为0、30%、50%、80%和100%的高强再生骨料混凝土为研究对象进行实验。当再生粗骨料取代率为30%时,对再生混凝土的强度影响不大;之后混凝土强度随再生骨料的增加而降低。高强再生骨料混凝土与天然混凝土在耐久性上具有相似的性能,可以将高强再生混凝土应用于工程中。     

CFRP筋-高强钢筋/高强混凝土柱的抗震性能

为了研究高强钢筋和碳纤维增强树脂复合材料(CFRP)混合配筋/高强混凝土柱的抗震性能,对CFRP筋-高强钢筋混合配筋的高强混凝土柱进行了低周反复荷载试验和有限元分析,研究了CFRP筋的粘结条件、不同轴压比以及高强混凝土种类等参数对其抗震性能的影响。结果表明:所有的高强混合配筋高强混凝土柱均发生延性破坏;在相同条件下,高强混合配筋混凝土中分别添加了钢纤维活性粉末和钢纤维后,表现出更好的耗能能力和延性;有粘结CFRP筋混合配筋高强混凝土柱比无粘结CFRP筋混合配筋柱的变形能力和承载力分别提高了9.6%和17.1%,但是延性系数降低了22.5%;在延性破坏的条件下,随着轴压比的增加,CFRP筋-高强钢筋混合配筋柱的屈服强度和极限强度明显增大,极限位移和耗能能力也逐渐减小;高强钢筋和CFRP筋配筋率越高,高强混合配筋柱的极限承载力和变形能力越大。      

Effect of shrinkage reducing agent on pullout resistance of high-strength steel fibers embedded in ultra-high-performance concrete

The interfacial bond strength of long high-strength steel fibers embedded in ultra-high-performance concrete (UHPC) reinforced with short steel microfibers was investigated by conducting single-fiber pullout tests. In particular, the influence of the addition of a shrinkage-reducing to a UHPC matrix on the pullout resistance of high-strength steel fibers was investigated. The addition of a shrinkage-reducing agent produced a noticeable reduction in the fiber pullout resistance owing to the lower matrix shrinkage, although the reduction of pullout resistance differed according to the type of fiber. Long smooth and twisted steel fibers were highly sensitive to the addition of the shrinkage-reducing agent whereas hooked fibers were not. Among the various high-strength steel fibers tested, twisted steel macrofibers showed the highest interfacial bond resistance, although twisted fibers embedded in UHPC showed slip softening pullout behavior rather than the typical slip hardening behavior observed in mortar.     

Snap-back softening instability in high-strength concrete beams

Three-point bending tests on pre-cracked slabs of high-strength concrete are interpreted on the basis of a virtual crack propagation model. As theoretically shown by the model, a snap-back softening instability appears only for initial crack lengths smaller than 0.3 times the beam depth. As a limit-case, when the material is sufficiently brittle or the specimen size is sufficiently large, such an instability can be predicted by the LEFM condition K₁=K_IC.     

Recent trends in steel fibered high-strength concrete

Steel fibered high-strength concrete (SFHSC) became in the recent decades a very popular material in structural engineering. High strength attracts designers and architects as it allows improving the durability as well as the esthetics of a construction. As a result of increased application of SFHSC, many experimental studies are conducted to investigate its properties and to develop new rules for proper design. One of the trends in SFHSC structures is to provide their ductile behavior that is desired for proper structural response to dynamic loadings. An additional goal is to limit development and propagation of macro-cracks in the body of SFHSC elements. SFHSC is tough and demonstrates high residual strengths after appearance of the first crack. Experimental studies were carried out to select effective fiber contents as well as suitable fiber types, to study most efficient combination of fiber and regular steel bar reinforcement. Proper selection of other materials like silica fume, fly ash and super plasticizer has also high importance because of the influence on the fresh and hardened concrete properties. Combination of normal-strength concrete with SFHSC composite two-layer beams leads to effective and low cost solutions that may be used in new structures as well as well as for retrofitting existing ones. Using modern nondestructive testing techniques like acoustic emission and nonlinear ultrasound allows verification of most design parameters and control of SFHSC properties during casting and after hardening. This paper presents recent experimental results, obtained in the field SFHSC and non-destructive testing. It reviews the experimental data and provisions of existing codes and standards. Possible ways for developing modern design techniques for SFHSC structures are emphasized.