Interfaces in cement composites

The interface is a layer between two different phases of a composite material. The structure and composition of that particular layer, known as the interfacial transition zone (ITZ), depend on the properties of both neighbouring phases and also on conditions of mixing, hydration, curing and ageing of the materials. 

In the concrete-like composites, the stresses are transferred from one phase to another through the interface and it should be accepted that the ITZ may be weaker or stronger, and that its structure is always different from that of the bulk matrix. The flow along and across an interface is more intensive than in other phases, because the interface layers have a lower density and may be more penetrable by fluids and gases. It is believed that the ITZ participates in the determination of the overall permeability of the material. Therefore, the influence of the ITZ properties on the mechanical behaviour of the material and on the transport of fluids and gases was studied by many researchers, even though its importance was estimated differently. The problem is, to what extent is the ITZ different from the bulk matrix and how large it is. 

The quantitative determination of the influence of the ITZ is essential for mechanical properties (strength and Young s modulus) and durability (porosity) of cement-based composites, but it is difficult to determine the values of bond between two adjacent materials in concrete. Moreover, all measures aimed at modification of the ITZ, like the application of some kinds of microfillers, have an impact on the properties of the composite material itself, and these effects make analysis of test results more complex. The measurements of the ITZ properties in artificial specimens do not allow all conditions to reproduce correctly and tests performed on specimens built from concrete and stone parts have not snpplied particularly useful findings. Tests performed by several researchers, for example, Roy and Jiang (1995) or Mindess and Rieder (1998) on specimens in which the ITZ was artificially created between concrete and stone parts did not supply results that may be directly used for analysis of the interface between matrix and aggregate grains in concrete. 

The interface in concrete is formed partly by two neighbouring phases or, more often, predominantly by one of them, but in this region strong 

Several important test results and conclusions concerning the interfaces, their structure, properties and influence on overall material properties have been published in many conference proceedings, namely by Mindess and Shah (1988), and later by Maso (1993) and by Katz et al. (1998). 

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Interfaces in cement composites 203 Table 7.1 Tensile strength (MPa) from splitting test (Brandt et al. 1992)... 

A typical thermal expansion curve for PC is shown in Fig. 9. The thermal expansion is important when PC is used in conjunction with other materials such as steel or portland cement concrete since the coefficient of thermal expansions of PC is at least twice as high as those corresponding to steel or cement concrete. Hence, changes in temperature in the composite structure will create shear stresses at the interface between the two materials that may eventually cause deterioration in the structure. 

M. H. Lewis, Interfaces in Ceramic Matrix Composites, in Carbon/Carbon, Cement and Ceramic Matrix Composites, R. Warren, Ed., Elsevier, Oxford, (2000). p. 289 22. 

Effect of water and salt on the adherend. Deterioration of materials such as metals and concrete is often more rapid with salt solution than with water, for example by the action of electro-chemical corrosion. Water itself may be responsible for a number of changes in the adherends concrete is likely to get stronger with further hydration of the cement, and plastic may become weaker by plasticisation. The resin/fibre interface in composite materials is also susceptible to degradation by water. With metals, water may attack... 

Burr, D.B., Schaffler, M.B. and Frederickson, R.G. (1988) Composition of the cement line and its possible mechanical role as a local interface in human compact bone. J. Biomech., 21, 939-945. 

Piezoresistivity [66] was observed in cement matrix composites with 2.6-7.4 vol% unidirectional continuous carbon fibers. The dc electrical resistance in the fiber direction increased upon tensile loading in the same direction, such that the effect was mostly reversible when the stress was below that required for the tensile modulus to decrease. The gage factor was up to 60. The resistance increase was due to the degradation of the interface of the fiber and matrix, which was mostly reversible. Above the stress at which the modulus started to decrease, the resistance abruptly increased with stress/strain, due to fiber breakage. The tensile strength and modulus of the composites were 88% and 84%, respectively, of the calculated values based on the rule of mixtures. 

Yuan CZ, Hua CQ, Feng G, Influence of interface modification in fiber reinforced cement composites on their properties. Mat Res Soc Symp Proc, 211, 209-214, 1991. 

The fibre surface has a considerable influence on the composition of the transition zone. For highly corrosion resistant glass fibres with specially coated surface, like CemFILl, this zone is very porous. This is in contrast to the strong interface formed around steel and asbestos fibres. In various composites with different kinds of fibres and matrices, the transition zone is formed as a result of chemical affinity, quality of the fibre surface and the penetration of the cement paste into the bundles of fibres. Furthermore, the ITZ may be different above and below a fibre due to bleeding and water lenses below fibres. Higher porosity of the ITZ around steel galvanized reinforcements than around ordinary steel rebars was observed by Belaid et al. (2001). 

Bentui A. (1988) Interface in fibre reinforced cements , in Proc. Symp. Bonding in Cementitious Composites, Boston, 2—4 December 1987, Materials Research Society, Pittsburgh, Pennsylvania, pp. 133—44. 

Igarashi, S., Bentur, A., Mindess, S. (1996) The effect of processing on the bond and interfaces in Steel Fiber Reinforced Cement Composites , Cement and Concrete Composites, 18(5) 313-22. 

All adhesives are polymers, and they are used in many ways, for example in composites, automotive tire cords, ply yood, tapes and labels. A particularly demanding application is the cementing of metal joints in military aircraft with polymers such as epoxy resins. The interfaces in such materials must be characterized to determine the strength of the adhesive bond and the relation of such properties as peel strength with... 

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