Adhesive bonding for high performance materials

Adhesion as a joining technology assumes greater significance as attempts are made to join newer materials in increasingly exotic combinations and where conventional joining techniques are either less suitable or quite unsatisfactory. This paper describes work directed at the use of adhesives in the fabrication or repair of structures involving modern structural materials.This subject is discussed by reference to research involving fibre reinforced polyether ether ketone (PEEK) and particulate silicon carbide reinforced aluminium alloy as representatives of organic and metal matrix composites respectively.Adhesion is essentially a superficial phenomenon depending as it does upon interactions between the adhesive in its liquid state and the surface of the substrate. The surface preparation of the subject to be bonded is therefore of the greatest importance and this aspect is addressed at length. The success of an adhesive bonding operation is reflected in the strength of the joint generated and in its ability to retain useful joint strength for long periods in the operational environment. These aspects are also considered and in particular the effect of water on the integrity of joints is discussed.Both plasma and corona discharge treatments were found to be effective in the adhesive bonding of the organic matrix material (PEEK) whilst anodising and the use of coupling agents were both found to be useful in the bonding of the metal matrix composite (MMC).The versatility of the adhesion process as a joining technology is highlighted by reference to how the individual problems presented by these very different materials are dealt with.     

Long-term moisture transport in high performance concrete

Moisture is decisive for a large number of binding and transport processes in high performance concrete affecting the durability, shrinkage and performance in various environments. An experimental study on the moisture transport properties of 20 concrete mixes was made during seven years, with type of binder, additives and waterbinder ratio as parameters. An upside-down glass cup method was used to obtain the steady-state flow through concrete discs. The moisture diffusion coefficient decreases with a lower water-binder ratio, increasing amount of silica fume, especially when combined with fly ash and it continues to decrease also after four years for w/B lower than 0.40. The moisture diffusion coefficient is much less moisture dependent for HPC than for normal concrete, which means that steady-state moisture profiles through HPC structures will be almost linear. Long-term moisture profiles in submerged HPC will be affected by self-desiccation for a very long time because of the extremely small moisture flow.

---

Résumé L'humidité a une influence prépondérante sur un grand nombre de processus d'interaction et de transport qui affectent de manière directe la durabilité, le retrait et le comportement des bétons à haute performance (BHP) soumis à différents types de conditions d'exposition. Une étude expérimentale sur les propriétés de transport d'humidité de 20 mélanges de béton a été effectuée durant une période de sept ans. Les variables étudiées concernaient le type de ciment et d'ajouts ainsi que le rapport eau/liant. Une procédure spéciale a été utilisée de manière à être en mesure de réaliser les essais de transfert d'humidité en régime permanent. Les résultats des essais indiquent que le coefficient de diffusion d'humidité diminue avec une réduction du rapport eau/liant, une augmentation de la teneur en fumée de silice, tout particulièrement lorsque celle-ci est utilisée avec des cendres volantes. Pour des bétons de rapports eau/liant inférieurs à 0,40, la réduction du coefficient d'humidité se poursuit toujours même après 4 ans d'hydration. Le coefficient de diffusion d'humidité des BHP semble être beaucoup moins sensible à la teneur en eau que celui des bétons ordinaires ce qui signifie que les profils d'humidité en régime permanent sont presque linéaires. Les profils d'humidité à long terme dans BHP submergés seront fortement affectés par l'autodessiccation.

---

Editorial Note Prof. Lars-Olof Nilsson is a RILEM Senior Member. He participates in RILEM TC 178-TMC ‘Testing and modelling chloride penetration in concrete’.     

Mix proportioning of high performance concrete

High performance concrete (HPC) is that concrete which meets special performance and uniformity requirements that cannot always be achieved by conventional materials, normal mixing, placing and curing practices. Special performance requirements using conventional materials can be achieved only by adopting low w/c, which necessitates use of high cement content. But judicious choice of chemical and mineral admixtures can reduce the cement content and this results in economical HPC. However, the effect of a mineral admixture on the strength of concrete varies significantly with its properties and replacement levels. Mix proportioning methods of normal concrete cannot adequately account for the large variations in the properties of ingredients. This paper presents a modified mix design procedure, which utilises optimum water content and the efficiency factor of mineral admixture. Results of the experimental investigations on mixes using the modified mix design are presented.     

Design of high performance concrete corbels

Currently, concretes of higher compressive strengths have been produced and increasingly used by designers and contractors for both reinforced and prestressed concrete construction. The design of concrete corbels using the strut-and-tie model considers two failure modes: failure by crushing concrete in the strut and by yielding of the main steel reinforcement. To consider the failure by diagonal tension, a third equation is included in the truss plastic model proposed by Rogowsky and MacGregor. To use the truss plastic model, an efficiency factor is applied to reduce the effective concrete strength. An efficiency factor is proposed in this paper for high performance concrete. The proposed model using the new efficiency factor gives an improved correlation with experimental data. Based on the test results reported from literature review, the design method proposed by AFREM for high stregth concrete corbels is modified in this paper to make design of high strength concrete corbels more reasonable. The comparison with experimental data shows that the modified AFREM model predicts fairly well the shear capacity of the corbels.

---

Résumé à ce jour, les bétons de hautes performances connaissent de multiples utilisations par des concepteurs et des constructeurs pour des éléments en béton armé et précontraints. Le dimensionnent de consoles courtes selon le modèle de bielle-et-tirant prend en considération deux types de rupture: celle par palstifiction des aciers de flexion et celle par écrasement de la bielle du béton. Pour tenir compte de la rupture diagonale des consoles, une troisième équation a été introduite dans le modèle de treillis plastique par Rogowsky et MacGregor. Pour ce modèle, nous proposons un nouveau coefficient d'efficacité pour le béton de hautes performances qui permet une meilleure corrélation avec les données expérimentales. Sur la base de résultats des essais trouvés dans la littérature, le modèle de dimensionnement des consoles en béton de hautes performances proposé par l'AFREM a été modifié afin de prédire la capacité en cisaillement des consoles.

---

---

Editorial Note M. Saafi is a RILEM Student Member.     

Study on high performance roller compacted concrete

Results are presented of a study about mixture design, production and characterisation of a concrete conjugating the typical features of roller compacted concrete with the mixture features of high performance concrete. The compressive strength, the tensile strength and the abrasion resistance of the material produced were determined together with the modulus of elasticity. It has been verified that the material had a very high compressive strength, an abrasion resistance higher than the very coarse aggregate, which formed its composition, a modulus of elasticity comparatively proportional to the compressive strength and a tensile strength not very developed. It has also been assumed that, due mainly to the energy spent in the vibration of that concrete, the binding material used was three times more effective than when used in normal concrete.

---

Résumé On présente les résultats d'une étude sur le mélange, la production et la caractérisation d'un béton associant les caractéristiques typiques d'un béton compacté au rouleau avec les caractéristiques de dosage d'un béton à haute performance. La résistance à la compression, la résistance à la traction et la résistance à l'abrasion du matériau produit ont été déterminées simultanément avec le module d'élasticité. On a vérifié que le matériau avait une résistance à la compression très haute, une résistance à l'abrasion plus grande que celle de l'agrégat lui-même, entrant dans sa composition, et, finalement, avait un module d'élasticité relativement proportionnel à la résistance à la compression, pour une résistance à la traction peu élevée. On a aussi constaté qu'à cause de l'énergie utilisée pour la vibration de ce béton, le matériau agglomérant utilisé a été trois fois plus efficace que s'il avait été utilisé dans un béton courant.

---

---

Editorial Note LNEC (National Laboratory of Civil Engineering) is a RILEM Titular Member     

Discrete fracture in high performance fibre reinforced concrete materials

In this paper a simple, but effective methodology to simulate opening mode fracture in high performance fibre reinforced concrete is presented. The main contribution of the paper is a technique to extrapolate the load displacement curves of three point bending experiments on fibre reinforced concrete. The extrapolation allows the full work of fracture to be determined, from which the fracture energy may be obtained. The fracture energy is used in the definition of a cohesive softening function with crack tip singularity. The softening relation is implemented in an embedded discontinuity method, which is employed for the numerical simulation of three point bending experiments. The experimental work includes a size effect study on three point bending specimens. The numerical simulation provides a satisfactory prediction of the flexural behaviour and the size effect observed in the experiments.     

The modulus of elasticity of high performance concrete

To determine the modulus of elasticity of concrete precisely is very important for structures that require strict control of the deformability. International codes propose a wide variety of formulae which establish a relationship between modulus of elasticity and compressive strength. Most of these codes are valid up to 50 MPa, the usual limit for normal concretes. A research programme has been carried out to establish the modulus of elasticity-compressive strength curve for low and high strength concretes. The effect of the aggregate also has been studied. This is an important factor to take into account, sometimes more important than the concrete strength itself or even age. A model code formula provides good correlation with experiment up to a compressive strength of 120 MPa. Other aggregates not considered for this code have been studied and results given. Also, the influence of age on the modulus of elasticity is discussed.     

Stress curves and mechanical properties of high performance concrete

Abstract

In this research, high performance concrete (HPC) was designed by the minimum void ratio method, and slag and silica fumes partially replaced cement, as well as fly ash replacing about 15% of sand. Stress curves for compressive, splitting and flexure strengths of HPC specimens were measured and indicated the experimental concretes had better pastes to void ratios than control batches ratio N=V_p /V_v =1.3. The result indicates that pozzolanic materials provide not only a chemical strength effect, but also a physical packing effect. The compressive stress curves may keep growing as the concrete ages.     

Toughness testing of ultra high performance fibre reinforced concrete

In this paper an investigation is made of the applicability of the ASTM C 1609 procedure for testing toughness of ultra high performance fibre reinforced concretes containing a large amount of fibre (≥2% by volume) and exhibiting deflection hardening behaviour. All mixtures exhibited deflection hardening behaviour, and the parameters varied included (1) the amount of steel fibres, (2) the type of steel fibres, (3) the size of the longest fibre, (4) the addition of polypropylene fibres, and (5) the size of the maximum aggregate grain in the concrete matrix. Based on comparison of the curves obtained from flexural toughness tests with the evaluation of the test results obtained according to ASTM C 1609 and with the statistical analysis, the authors recommended additional toughness parameters (P_100,3.00, P_100,4.00, P_100,6.00, T_100,3.00, T_100,4.00, and T_100,6.00) for the evaluation of toughness results. Such additional toughness parameters are calculated using a similar procedure as that specified in ASTM C 1609.     

Copper slag as sand replacement for high performance concrete

This paper reports on an experimental program to investigate the effect of using copper slag as a replacement of sand on the properties of high performance concrete (HPC). Eight concrete mixtures were prepared with different proportions of copper slag ranging from 0% (for the control mix) to 100%. Concrete mixes were evaluated for workability, density, compressive strength, tensile strength, flexural strength and durability. The results indicate that there is a slight increase in the HPC density of nearly 5% with the increase of copper slag content, whereas the workability increased rapidly with increases in copper slag percentage. Addition of up to 50% of copper slag as sand replacement yielded comparable strength with that of the control mix. However, further additions of copper slag caused reduction in the strength due to an increase of the free water content in the mix. Mixes with 80% and 100% copper slag replacement gave the lowest compressive strength value of approximately 80 MPa, which is almost 16% lower than the strength of the control mix. The results also demonstrated that the surface water absorption decreased as copper slag quantity increases up to 40% replacement; beyond that level of replacement, the absorption rate increases rapidly. Therefore, it is recommended that 40 wt% of copper slag can used as replacement of sand in order to obtain HPC with good strength and durability properties.     

Interaction between thermal dilation and autogenous deformation in high performance concrete

The paper concerns the driving forces to stress generation and cracking in high performance concrete,i.e. thermal dilation and autogenous deformation and presents experimental results from a recently finalized Dr. program. The paper focuses on the effect of temperature, and discusses how to separate thermal dilation and autogenous deformation with the view to describe each with a model for use in stress calculations. The results show clearly that especially autogenous deformation is very complicated since autogenous deformation from isothermal tests appears to be fundamentally different from autogenous deformation in tests using realistic temperature development. The implication is that autogenous deformation under realistic temperature development cannot be predicted from isothermal test results and that the simple maturity concept cannot be used. The paper gives a new experimental approach to separate thermal dilation and autogenous deformation. The results from such tests show, that the thermal dilation coefficient varies more systematically with temperature. Consequently, the maturity concept appears to be more usable for the thermal dilation coefficient.

---

Résumé Cet article concerne les mécanismes physiques qui sont à la base de la génération de contrainte et de la fissuration des bétons hautes performances: la dilatation thermique et le retrait endogène. Il présente les résultats expérimentaux acquis dans le cadre d'une thèse de doctorat récement soutenue. Il traite des effets de la température et des méthodes de séparation des composantes thermiques et endogènes du retrait. L'objectif final est de modéliser chacune des ces contributions, pour les utiliser dans des calculs de développement de contraintes. Les résultats présentés montrent clairement que la caractérisation du retrait endogène est difficile puisque celui qui est mesuré dans des conditions isothermes est fondamentalement différent de celui mesuré dans des conditions d'évolution, réaliste de la température. L'implication de ces travaux est que le retrait endogène se produisant dans des conditions non isothermes ne peut pas être prédit au travers d'essais isothermes et que le concept de maturité ne peut pas être utilisé. Cet article décrit enfin une nouvelle approche expérimentale pour séparer la dilatation thermique et le retrait endogène. Son application montre que le coefficient de dilatation thermique varie assez systématiquement avec la température. En conséquence, le concept de maturité semble plus adéquat pour la détermination de coefficient de dilatation thermique.

---

---

Editorial Note This paper was presented at the International RILEM Workshop Shrinkage of Concrete (‘Shirnkage 2000’) held on 16–17 October 2000 in Paris, France. Prof. E. J. Sellevold is a RILEM Series Member     

Is the cube test suitable for high performance concrete?

Tests have been performed in order to evaluate the possible variability of the cylinder/cube conversion coefficient, for high performance concrete (HPC). The influence of the various relevant parameters was maximized, while the tests complied with usual standard procedures. In this context, the ratio has been found to range from 0.71 to 1.02. The practical impact of this experimental fact is analysed, from the viewpoints of both safety and economics. It is concluded that grinding the test faces is mandatory for obtaining reliable HPC cube strength values. Also, the cylinder/cube coefficient must be measured for each particular HPC used in any actual project.     

Hygro-thermo-chemical modeling of high performance concrete. I: Theory

This study deals with the formulation, calibration, and validation of a new hygro-thermo-chemical model for high-performance concrete (HPC) suitable for the analysis of moisture transport and heat transfer at the early age and beyond. In Part I of this study the theoretical formulation is presented and discussed in detail. Classical macroscopic mass and energy conservation laws are written by using humidity and temperature as primary variables and by taking into account explicitly various chemical reactions, such as cement hydration, silica fume reaction, and silicate polymerization. The effect of cement hydration is modeled through the classical concept of hydration degree. Silica fume reaction and silicate polymerization are modeled by introducing the degree of silica fume reaction and the concentration of silicate polymers, along with their evolution laws. The present model can simulate early age phenomena, such as self-heating and self-desiccation, with great accuracy. Numerical implementation, calibration and validation of the model by comparison with experimental test data are postponed to Part II of this study [Di Luzio G, Cusatis G. Hygro-thermo-chemical modeling of high performance concrete. II: Numerical implementation, calibration, and validation. Cem Concr Compos, in press].     

The AFREM method for the mix-design of high performance concrete

The article presents the results of work by the AFREM/HPC group on the formulation of high-performance concretes. This is an updating of LCPC's grout method. The goal is a methodology making it possible, for a given binder (or mixture of binders), to determine the saturation doses of a certain number of superplasticizers and compare them in terms of their effectiveness and the duration of the rheological behaviour they impart. The innovations with respect to the old method are the following: i) the starting concrete can vary over a broader range, ii) part of the sand is added in the grout, iii) mixing and measurement are better codified, and iv) the method of determining the saturation proportion is objective. The method has successfully passed tests of repeatability and reproducibility (between the six member laboratories of the group).

---

Résumé L'article présente le résultat du travail du groupe AFREM/BHP sur la formulation des bétons à hautes performances. Il s'agit d'une remise à jour de la méthode des coulis du LCPC. Le but en est de fournir une méthodologie permettant, pour un liant (ou mélange de liants) donné, de déterminer les doses de saturation d'un certain nombre de superplastifiants, de comparer leurs efficacités respectives et leur propension à conférer un comportement rhéologique stable dans le temps. Les nouveautés introduites, par rapport à l'ancienne méthode, sont les suivantes: i) béton initial pouvant varier dans une plus large gamme, ii) introduction d'une partie du sable dans le coulis, iii) malaxage et mesure mieux codifiés, iv) méthode objective de détermination de la dose de saturation. La méthode a fait l'objet d'essais de répétabilité et de reproductibilité (entre les six laboratoires membres du groupe) satisfaisants.

     

The durability characteristics of high performance concrete: a review

Durability problems of ordinary concrete can be associated with the severity of the environment and the use of inappropriate high water/binder ratios. High-performance concrete that have a water/binder ratio between 0.30 and 0.40 are usually more durable than ordinary concrete not only because they are less porous, but also because their capillary and pore networks are somewhat disconnected due to the development of self-desiccation. In high-performance concrete (HPC), the penetration of aggressive agents is quite difficult and only superficial. However, self-desiccation can be very harmful if it is not controlled during the early phase of the development of hydration reaction, therefore, HPC must be cured quite differently from ordinary concrete. Field experience in the North Sea and in Canada has shown that HPCs, when they are properly designed and cured, perform satisfactorily in very harsh environments. However, the fire resistance of HPC is not as good as that of ordinary concrete but not as bad as is sometimes written in a few pessimistic reports. Concrete, whatever its type, remains a safe material, from a fire resistance point of view, when compared to other building materials.     

Application of high performance polypropylene fibers in concrete lining of water tunnels

In this study, the application of high performance polypropylene fibers (HPP fibers) in concrete lining of water tunnels, was investigated experimentally. A comparison between the behavior of steel fiber reinforced concrete and HPP fiber reinforced concrete with ordinary concrete is drawn. Advantages and shortcomings of HPP fibers used for concrete lining of water tunnels are also presented.The obtained results showed that the HPP fibers were not effective in compressive strength when compared to steel fibers, but the effects of HPP fibers on tensile strength, flexural strength, toughness and energy absorption of concrete were significant. Based on the results, the effects of HPP fibers on concrete characteristics such as the flexural toughness, concrete permeability and resistance to chloride penetration were higher than those of steel fibers. The results also showed that with application of HPP fibers, durability and serviceability of the concrete linings can be improved.     

变形钢筋/超高性能混凝土搭接黏结性能

超高性能混凝土(UHPC)是一种高强度、高韧性和高耐久性的水泥基复合材料。为了研究钢筋/UHPC的搭接黏结性能,进行了21组考虑搭接长度、纤维掺量和配箍率影响的钢筋搭接对拉拔出试验,3组考虑锚固长度影响的钢筋直接拔出锚固试验;试验出现了劈裂拔出破坏和钢筋拉断破坏2种破坏模式;钢筋/UHPC平均黏结强度随钢筋埋置长度的增大而减小,随配箍率的增大而增大;钢纤维掺量的增大,有利于增大对UHPC的约束作用,增加配箍率和适当增大纤维掺量均能减小钢筋/UHPC的临界搭接长度;结合前人的试验结果,拟合得到平均锚固和搭接黏结强度计算公式及临界锚固和搭接长度计算公式,根据混凝土结构设计规范,建立了钢筋/UHPC锚固和搭接长度简化算法,计算结果较为准确。