A number of different types of interfacial bonding occur in concrete, including:

i) bonding between various phases (including anhydrous cement) in the hydrated cement paste (hcp) system

ii) bonding between cement and aggregates

iii) bonding between cement or mortar and the fibres in fibre-reinforced concrete; and

iv) bonding between concrete and steel reinforcing bars or prestressing cables.

The importance of these types of bonds with respect to the mechanical behaviour of concrete is discussed. It is concluded that in some systems, the mechanical properties are governed primarily by the interfacial bond; in other systems, however, this bond has only a secondary effect.

The mechanical behavior of concrete is thought to be affected by the microstructure of the paste-aggregate interface, which differs in several respects from the microstructure of bulk paste. General aspects of interfacial microstructure, which appear to be fairly well understood, are reviewed. Recent microstructural studies using backscattered electron imaging show that there is a zone along the interface (approximately 50 μm wide) within which there are generally few large unhydrated clinker grains, many large voids (greater than 5 μm), a generally porous microstructure, many hollow-shell hydration grains, and in some cases little calcium hydroxide. The tendency for cracks to develop or grow along the paste-aggregate interface is discussed in relation to specific features of the interfacial microstructure.

On a series of test specimens made with five kinds of rock and six kinds of cements hydrated between 1 and 8 weeks, the structure of the existing interfacial zone, the cleavage strength and the mechanism of failure were studied. The structure of the interfacial zone of different rockcement combinations was similar but not identical. Significant differences between different rock-cement combinations existed with respect to the measured cleavage strength and mechanism of failure.

In this paper, the influence of the matrix-aggregate bond on the strength and brittleness of concrete is studied. Six different matrixaggregate interfaces are used to evaluate the interfacial bond capability. The results obtained on the strength and brittleness index of concrete show that strengthening and toughening of concrete can be obtained simultaneously, if the interfacial bonds are changes so that they conform to a rational distribution according to aggregate size. These results are discussed in terms of the energy dissipation and crack propagation during failure of concrete.

In order to improve the properties of concrete, the composition and structure of the interfacial zone must be modified. This may be done by pretreating the inert aggregate with special reagents to form a reactive surface, which will then react with the cement paste, resulting in an enhancement of the bond strength between cement paste and aggregate through the combination of the physical, chemical and mechanical interlocking effects. A cylindrical hole was drilled in a limestone cake and the surface of the hole coated with special solutions; cement slurry was then poured into the hole and the specimens were cured in a fog room. The shearing strength was determined to evaluate the effect of the aggregate pretreatment on the bond strength. The mechanical strength, porosity, electric conductivity of the concrete, orientation degree of the Ca(OH)2 and morphology in the interfacial zone were also examined. It was found that the compressive and flexural strengths increased, and the bond strength between aggregate and cement paste also increased; the porosity, average pore size, number of large harmful pores, electric conductivity and the orientation index of the Ca(OH)2 decreased.

Concrete is a composite material, consisting of hardened cement paste (HCP), aggregate and pores. The interfacial bond between HCP and aggregate is very important in determining some of the properties of concrete, such as the mechanical behaviour and durability. Recently, the microstructure of the transition zones between different aggregates and HCP has been studied and a mechanism for the formation of the transitional zone has been proposed by Maso. The objective of the present work has been to study the relationship between interfacial bond and mechanical behaviour of concrete in the light of both theoretical and experimental research on the microstructure of the interfacial zone.

The ordinary fine aggregate in concrete has been replaced by ground and sieved steel slag fine aggregate, treated and exposed to air for three months. Compared with concrete made from natural sand, properties such as compressive strength, flexural strength, elastic modulus, permeability and abrasion resistance are considerably improved. The improvement increases with a decrease in w/c ratio, an increase in curing time and an increase in the replacement weight of sand. These results are due to the fact that the steel slag contains some active minerals such as C3S, C2S, C4AF, etc., and shows favorable surface physical characteristics that improve the bond between steel slag particles and cement paste. The results of XRD, SEM and EPM microhardness showed that there are heavier concentration of ions, with finer crystals and a lower degree of CH orientation at the interfacial zone between steel slag particles and cement paste. The study also found small cementitious and fibrous C-S-H crystals growing from the fine aggregate, which are linked with hydrated products from cement paste making the bond and structural characteristic more favorable with cement. The steel slag fine aggregate is an active mineral similar to cement. The bond between the aggregate and cement paste is strengthened both physically and chemically.

Interfaces between hardened portland cement matrix and aggregates are considered relatively weak and a source of microcrack initiation. Based on optical and scanning electron microscopic studies of fractured concrete specimens, researchers have studied the role of interface in the fracture mechanism of cement composites. Acoustic Emission source location technique and laser holography provide noninvasive and nondestructive tools to study microcracking as well as the pertinent displacement field associated with cracking. In this paper, some results obtained using laser holography, speckle methods and acoustic emission source loction measurements for model concrete specimens containing circular aggregate inclusions are described. Effects of diameter of circular inclusions on mode of cracking are reported.

This paper contains a review of the results from the studies at the University of California at Berkeley on various factors influencing the microstructure of the transition zone in concrete. Two types of aggregate, two different cement, and three mineral admixtures were investigated. Using cement paste-polished aggregate composite specimens cured up to three years, X-ray diffraction, scanning electron microscopy, and microhardness testing techniques were used for characterization of the transition zone.

Compared to the transition zone between a quartz aggregate and an ASTM Type I portland cement, transition zones with smaller and less preferentially oriented crystals of calcium hydroxide were obtained when using a Type K expansive cement, or limestone aggregate, or mineral additives, such as condensed silica fume, granulated blast-furnace slag, and fly ash.

The effectiveness of condensed silica fume as a strength enhancing additive in concrete has been attributed to its ability to modify the interfacial zone between paste and aggregate. This paper describes a microstructural investigation of this interface using backscattered electron (bse) imaging combined with quantitative image analysis.

Composite specimens were made in which a single piece of aggregate was embedded in cement paste. Granite, dolomite and garnet aggregates were used. After curing, the specimens were sectioned perpendicular to the surface of the aggregate particles and polished. The variation in porosity, amount of anhydrous material and calcium hydroxide (CH), with distance from the aggregate surface was measured. It was found that the porosity increases in the paste close to the interface, while the content of anhydrous grains decreases. No significant increase in CH content was found near the interface.

The results confirm the applicability of the bse - image analysis technique, but indicate that the interfaces in specimens prepared in this manner may not be representative of aggregate paste interfaces in concrete.

The application of quantitative image analysis to the measurenent of microstructural gradients in the interfacial zone in concrete is described. Some preliminary results are presented and discussed.

Mortar bars of tricalcium silicate or ordinary portland cement with small aggregate particles have been sectioned and examined in the transmission electron microscope (TEM). The presence of calcium silicate hydrate (CSH) gel at the interface is seen by TEM in many cases in intimate contact with the aggregate. In some cases calcium hydroxide (CH) is observed near the interface but is seldom in contact with the aggregate. Microanalysis in the TEM shows that in the case of reactive silica aggregate a gel of relatively low Ca:Si ratio is found at the interface. Aggregates of different degrees of reactivity and mortars of different ages are compared. It is suggested that interface fractures may occur beyond the coating of CSH rather than at the true interface in the case of the type of particles we have studied.

The strength of high strength silica fume concretes is usually attributed to the reduction in w/c ratio and the refinement of the pore structure. A study of concretes and pastes, with and without silica fume, suggests that the contribution of the silica fume to strength is also the result of the densification of the transition zone. It is argued here that this influence is as important as the one due to the reduction in w/c ratio. It is suggested that the densification of the transition zone is the result of the effect of the silica fume on the nature of the fresh concrete.

The strength and strain-rate sensitivity of cement paste and mortar is studied as a function of water-cementitious material ratio (W/C) and silica fume content. W/C's of 0.30 and 0.35 are used for materials without silica fume, while W/C's ranging from 0.336 to 0.436 are used for material containing silica fume. The volume fractions of cement paste matrix and sand are held at 63 and 37 percent, respectively, for all mortars. Strain rates of 30, 3000, and 300,000 microstrain per second are used. The results indicate that materials with silica fume are less strain-rate sensitive of than materials without silica fume. The replacement of cement by silica fume appears to (1) reduce rather than increase the bond strength between cement paste and sand and (2) increase the compressive strength of mortar primarily by increasing the strength of the cement paste matrix.

The aggregate/hydrated paste interface represents the weakest link in very high strength river gravel concrete, due to the surface smoothness of these aggregates.

Microstructural examination of the aggregate/hydrated paste interface in four different (very low W/C ratio) very high strength concretes with and without silica fume shows major differences in the nature of the transition zone at the interface level. In the non-silica fume concretes, hydrated lime and ettringite are found quite exclusively at the interface, while in silica fume concretes, only C-S-H is observed.

The modulus of elasticity can be correlated to the compressive strength by the equation, , with a low correlation index (78%) for non-silica fume concrete, whereas in silica fume concrete it becomes MPa, with excellent correlation in ex of 95%.

These results can be explained by the nature of the aggregate/hydrated paste interface, which is stronger in silica fume concrete.

Portland cement pastes were mixed with predissolved naphthalene sulfonate superplasticizer at normal water:cement ratios. Solutions were separated from the fresh pastes at intervals and the residual concentration of the superplasticizer determined by UV spectrophotometry. At low dosage levels essentially all of the superplasticizer was found to be removed from solution within a few minutes; at high dosage levels a substantial concentration was maintained in solution at least to approximately the time of set. In pastes in which silica fume replaced 10% by weight of the cement, it was found that the incorporation of silica fume significantly increased the uptake of superplasticizer. In separate trials it was found that the silica fume by itself adsorbed little superplasticizer, even from high pH solution simulating that of cement paste.

Factors such as the type of aggregate, the addition of silica fume, and the roughness of the aggregate surface, which influence the orientation index of Ca(OH)2 crystals in the interface layer, are discussed, based on the experimental results of Maso [3] and the author. Then, a hypothesis about the mechanism of this orientation is proposed, involving the Ca(OH)2 crystal structure and the principles of physical chemistry of surfaces.

From this hypothesis, it is suggested that the reduction of the orientation index of Ca(OH)2 crystals when silica fume is added is due to physical aspects other than the large water requirement of silica fume. The same is also true when the aggregate surface is rough.

The microstructure of the matrix in the vicinity of the fibre surface is quite different from the microstructure of the bulk paste matrix. This can have an important effect on the processes that take place at the interface, such as crack-fibre interaction and debonding. The present paper describes the special structure of this zone, in monofilament and bundled fibre reinforced cements, and discusses its effects on some characteristics of the mechanical performance of the composites, which cannot be predicted by analytical models assuming a uniform matrix up to the fibre surface. The modification of the microstructure at the interface as a means for improving properties in some composites is described.

Results of Scanning Electron Microscope (SEM) and Energy Dispersive X-ray (EDX) studies on the fracture of polypropylene fiber-hydrated cement interface are presented.

The fiber reinforced concrete specimens were tested in direct tension. The resulting fracture surfaces were observed, including surface adhesion and mechanical bond between the fibers and the hydrated cement paste.

The nature of the fiber fracture was also studied. Mechanical bond was achieved due to the surface and shape of the fibers. Adhesion between the fibers and the cement paste was observed microscopically and confirmed by (EDX) analysis. The nature of the fiber reinforcement was observed at the tensile fracture.

A series of fiber-cement composite materials was prepared by dispersing different amounts of polyacrylnitril (PAN) fibers in portland cement suspensions of variable water/solid ratios. The samples were used to study the effect of the volume of fibers and the water-cement ratio on the physico-mechanical properties of the material. The distribution of the fibers within the cementitious matrix and the fracture mechanism were studied by SEM and compared with those existing in glass fiber-cement composites.