ALBERT

Description / Table of Contents: Summary Our laboratory is engaged in research on adhesion between concrete and steel, especially as regards surfacereinforced concrete. This expression covers a new method of bonding concrete and steel by means of synthetic resin gluing. Plain concrete has low shear and tensile strength. Reinforcing enables it to acquires strength for such loads and improves its mechanical properties. Bars are embedded in the concrete which, once hardened, will establish satisfactory bonds. However, adhesion of a layer of concrete poured on to a steel plate is very poor. Bond will be effective only if a thin layer of resin is placed between the two materials. Another method consists of gluing metal sheets on to hardened concrete. In this way it is possible to reinforce structures to bear loads for which they were not initially designed. The present paper deals with this latter type of gluing, particularly examining the adhesive strength of the resin which prevents sliding between the steel and concrete surfaces. Adhesiveness depends on: -the composition and qualities of the resin used, -the surface state of materials to be glued, above all their roughness, -the properties of these surfaces. Adhesion is a complex phenomenon. Indeed, we know little of the crystalline structure of resins and the various authors writing on the subject have not given a comparable definition of roughness. Moreover, the properties of surfaces develop with time. To contribute to this research, we have tried to set up stable experimental models, capable of offering reproducible results under set conditions. In so doing, we hope to have improved knowledge of the principal problems of adhesion. The article continues with details of experimental devices to study the shear strength of samples. We define two types of test for shear under tension and compression. In the second and third parts of this paper, we describe the development of all the techniques used during the various tests and controls. This work is the basis for tests being carried out. The techniques have proved that the composites material (steel-resin-concrete) can be studied systematically only if a number of factors are changed: -volumic fraction of components; -structure and mechanical properties of material and their bonding surfaces, -methods for designing and setting up experimental models -loading procedure The scanning electron micrographs show that it is possible to obtain perfect adhesion between bonding surfaces. Measuring the state of bonding surfaces improves accuracy in judging roughness and will help to draw up basic requirements for the preparation of such surfaces on materials to be assembled. By pursuing this work we hope to be able to define the principal characteristics of adhesion.

Description / Table of Contents: Summary A test piece characterized by double symmetrical overlapping and designed by the authors is used under tensile loading to carry out research on the adhesiveness bond strength between adhesive and and steel, depending on the state of the substrates. Several methods are used to determine the surface state, particularly the RNUR-ENSAM method. This paper demonstrates that: - identical sand-blasting carried out on surfaces of similar nature ensure good reproducibility in the measurement of shear strength; - the quality of an adhesive bond depends on the micro-geometrical properties of the substrates; - with the experimental method described, it is possible to determine with accuracy the mechanical properties of an adhesive using a limited number of tests. The fact that there is only slight scattering in results obtained with sand-blasted substrates should facilitate successful research into the time-dependent mechanical properties of the material, that is the danger of ageing.

Description / Table of Contents: Summary The article deals with the impact fatigue test method and results on assembled metal test pieces glued with epoxy resin. The first step defines the value of a critical intensity threshold, below which the test pieces no longer risk being embrittled after one blow. Secondly, the effect of repeated impact of well defined intensity is studied on the service life of models. The test piece is bridge-shaped. A metal plate is bonded to the upper face of two parallelepipeds (steel type XC 90 or aluminium alloy AU 4G) using resin epoxy. Before bonding the substrates underwent fine or coarse surface truing, followed by sanding. The adhesive film is subjected to shear by the hammer of original design modelled on a Charpy apparatus, which ensures that the plate slides parallel to the substrate without warping. Experimental results reveal: - a remarkable intensity threshold which corresponds to an impactor available potential energyE PD of 130 J and beyond which failure no longer occurs in the case of one blow (XC 90 type steel assemblies): - during application of repeated blows of constant energy on assemblies made ofXC 90 type steel andAU 4G aluminium alloy, a fatigue threshold analogous to the one which would be determined from a Wöhler curve (E PD=55 J). Hypotheses on the failure of the adhesive bond are formulated according to the value of the potential energyE PD: microcracks would occur when EPD is below 55 J, onset and propagation of cracks when the values of EPD fall between 55 and 130 J.

Description / Table of Contents: Summary In the course of study of the influence of fillers on the mechanical behaviour of a glued joint, the author was first led to characterize them by spectroscopic analysis and scanning microscopy. These fillers of essentially mineral origin can be classified as common fillers (chalk, calcite, kaolin ...) and reinforcing fillers (aluminia, titanium oxide, ferrous oxide). Roughly spherical in shape, they may reach a diameter of 15 μm, whereas the maximum pore size is close to 20 μm. In a second phase of research, the author determined the radial and tangential stresses in the matrix and in the inclusion at places in the joint under the greatest mechanical stress by a uniform plane field of compressive stresses. The calculation is based on the Mauskhelisvili formulae and shows that the radial and tangential stresses (σ r andσ θ respectively) in the matrix are fairly equal and maximum at points M2 and M 2 ′ -extremities of the diameter of the circular load, in the direction of monoaxial loading p. The values ofσ r andσ θ are 1.45 and 0.45 respectively whatever the type of reinforcing filler. At M1 and M 1 ′ these stresses are nil (M1 and M 1 ′ are the extremities of the diameter perpendicular to M2M 2 ′ ). In the circular inclusion for the same type of filler, the stress tensor is reduced to a compressive stress of 1,45 p at M1, M 1 ′ , M2 and M 2 ′ . Using these results as a basis and applying the Ishaï fracture criterion, the authors show that the onset of microcraking at points M2 and M 2 ′ in the joints occurs for a value of p equal to two thirds of the tensile elastic limit of the resin. By approaching this value of limit load of a circular hole, it is possible to determine a 7μm limit diameter for an inclusion beyond which the filler plays no role in the microcracking of the glued joint. Finally, the authors show that a common fillter fractures well before the onset of microcracking in the matrix. The singularity then behaves like a circular hole under increasing uniform compressive stress p.

Description / Table of Contents: Summary Starting from the theory of Wittmann and Zaitsev describing the propagation of cracks in hardened concrete cement paste, we have analysed microcracking at the ends of a glued joint in a double lap test piece. The type of glue tested has almost total similitude with the hardened cement paste, since it consists of a homogeneous matrix embedding pores of a mean size of 6 μm. The adhesive failure occurs suddenly and is brittle. End A turned towards the inside of the test piece is subjected to a uniform field of compressive stresses in the half-thickness near the glue/central area and the theory of Wittmann and Zaitsev can be applicable to this part of the joint. Taking into consideration the influence of the bubble diameter, we can determine a pore limit diameter of about 15 μm within which the sigularities no longer contribute in the microcracking mechanism. The first microcracks which appear near end A grow from bubbles of larger diameter for tensile forces close to the microcracking thresholdF A determined experimentally. The microcracks at end A propagate more rapidly in the case where two neighbouring pores are present than in the case of a single pore. The singularities are considered as being adjoining when the c/r ratio varies from 1.6 to 4 (c represents the distance separating the two bubbles and r is equal to the pore radius). When two circular holes are adjoining, we demonstrate that the microcracks which run to join together, grow more rapidly than those which propagate towards the exterior of the pores.