STRENGTH AND DESTRUCTION OF BRITTLE MATRICES The destruction of a brittle matrix consists of initiation and propagation of micro cracks caused by local tensions. With increased load or imposed deformation, the dispersed micro cracks are transformed in to a system of macro cracks. Then, one or more major cracks divide the elements in to separate parts and the continuity of the element disappears in conjunction with a rapid decrease in bearing capacity.

In cement-based matrices, the internal micro cracks exist from the very beginning and without any external load being applied. The cracks are concentrated in the interface layers between the aggregate grains and cement paste, but intrinsic cracks also appear in the bulk cement mortar. If a load is applied, then its distribution is uneven because of the high heterogeneity of the material. Certain regions are subjected to much higher local stress and strain than others. Stress concentration are the reason for the cracks development even under relatively low average stress and low values of total load.

The initial slow cracking growth explains two phenomena, which are observed almost from the beginning of the loading: limited acoustic emission (AE) and small deviation from linearity of the stress strain load displacement diagrams. The tests, in which various phenomena of facture progress were mentioned during the initial loading stages, were published several years ago by LHermite (1955).

Under the load corresponding to about 30% of the maximum value the AE event are already numerous and the deviation from the linearity starts to be significant.

The influence of the rate of loading and weak or stiff test machine is also decisive. The determination of the maximum tensile strain is subjected to different factors, such as sensitivity of measuring devices and the magnification power of the optical instrument used for the cracks detection. Therefore, the definition of matrix failure in not precise enough to exclude all ambiguity. In general, it is assumed that Portland cement paste offers a maximum tensile strain of about 100-200.10-6 but smaller values of the order of 60.10-6 are also observed.

As the load is increased to about 70-80% of the maximum, there is an abrupt acceleration in cracking and all other phenomena related to progressing facture. That level of stress is sometimes called the discontinuity point because it corresponds to qualitative modification of several processes:1. Rapid progress of the AE counts due to multiple cracks, which open and propagate1. Inflexion of the transverse strain curve, caused by the cracks, which are measured as an apparent transversal deformation.1. Decrease of the ultrasonic pulse velocity, because ultrasonic waves slow down when they cross the cracks;1. Increase of the relative volume of the material, in which voids start to be an important part of the apparent volume.

The form of stress strain or load displacement curves not only depends upon the material properties, but also on the way the load is applied. There are two criteria for the classification of testing machines: load or displacement control and stiffness corresponding to the capability of energy accumulation. In soft testing machines (load controlled) the energy accumulated is the loading system causes rapid facture after cracking begins because load adjustment is impossible. The corresponding curve is modified by that effect and does not in fact reflect the materials properties. The behavior of the machine may be compared to the actions of the suspended weight directly loading a specimen in a tension test, and in fact such test may furnish only information on the ascending part of the load deformation (or stress- strain) curve. This is the case when testing concrete specimen in in situ laboratories where the concrete strength is determined.

Machines used in the research laboratories are mostly displacement controlled. They are of sufficient stiffness and the actual post-cracking behavior of brittle specimens may be represented thanks to an appropriate decrease of load, which follow gradual reduction of stiffness of the specimen and the deformation or deflection.

It may be concluded that the main factor in the destruction of brittle materials is the propagation and development of cracks. That process is related to a materials structures and particular to the distribution of weak and strong region. It is therefore appropriate that facture mechanics is proposed as the main approach for the explanation and modeling of the facture processes in the brittle matrix composites.

CRACKS IN THE CEMENT MATRIX The cracking processes develop when a system of stresses of any origin is applied to a solid and its tensile strength is exceeded or, in other words, when the ultimate tensile strain is exceeded.

It is usually assumed that for ordinary Portland cement matrix the maximum tensile strain varies between 100 and 150.10-6 and may rarely after attain 200.10-6.

Also an initial system of defects exists in all cement based materials. These are cracks and micro cracks, voids and pores, hard un-hydrated cement grains, weaker and stronger regions etc. which exist before the application of any load or imposed deformation and may be recognized with different methods. These defects caused concentration of high stresses even under small external actions. The cracks propagations of high stresses even under small tensile strength. Such systems of initial defects exist on a micro-, meso- and macro- level in all brittle materials.

The main causes of cracking in cement based composites are:1. Volume changes during the hydration of cement and hardening of matrices in presence of restraints e.g. shrinkage, temperature variation, chemical influence on particular material components, swelling of reinforcing bars during their corrosion (these are intrinsic cracks)1. Tensile stresses due to loads and deformations imposed during exploitation.

Different types of intrinsic cracks, which appear during processes of hydration at the early age of a cement based matrix may be distinguished

Plastic shrinkage cracks are considered to be caused by the loss of water in a concrete mix because of evaporation to the air or suction by old, neighboring concrete. This phenomenon is the origin of the formation of micro cracks in the cement paste.

The crystals are surrounded by the gel with low tensile strength, cracks appear, which may be detected both by microscopical observation and by increased permeability of the paste. It is estimated that crack width varies between 0.125m and 1.0m. Plastic shrinkage can be controlled by keeping the concrete surface wet since the very beginning.

The direct cause is the evaporation and the crack may be considerably reduced if the loss of water can be limited during at least first eight hours after casting. Cracks also develop in the cement based composites due to other kinds of shrinkage. Thermal variations may cause cracks, because in these highly heterogeneous materials the thermal expansion of various components are slightly different and produce a system of tensile and compressive stresses due to local restraints.

The overall deformations of the elements also produce stresses and possible cracks if the displacements are restrained by reinforcing bars, neighboring members and layers of old concrete or other rigid material. The cement paste is restrained at a local scale by grains of sand and most of all by coarse aggregate grains. As a result, in any kind of construction, the material has no possibility of free deformation.

Restraints may cause compressive and tensile stresses and because of low tensile strength of concrete-like materials, cracks appear particularly at early age. It has to be admitted that micro cracks in concrete are unavoidable, but the excessive cracks, due to shrinkage and thermal effects, are usually not acceptable and may be attributed to inadequate curing.

The intrinsic cracks and flaws are dependent on material composition and ambient conditions humidity, temperature and restraints and are subjected to modifications in time together with these conditions. The structure of concrete composites is also macroscopically anisotropic in the direction of casting. The voids beneath aggregate grains are formed and partly filled with water. These voids create weak matrix-aggregate bonds in horizontal planes and are the origin of cracks when favorable stress fields are imposed due to external loading or other action.

The cracks in the matrix open and propagate under excessive local stress. This means that because of a very non-uniform distribution of stresses in the material structure, the local stresses produce local cracks even if in the macro-scale the load is relatively small and is considered to be causing an average stress below limits of elastic behavior. There are two characteristic points in the cracking process: crack initiation; that is, opening of the first crack in the matrix, and the beginning of unstable cracking. After the first crack opening, also called the discontinuity point, the stable crack propagation starts, provided that additional energy is supplied, for instance by an increase in load. When the unstable cracking stage is reached, the cracks progress rapidly without any input of new energy and lead to the failure.

In ordinary- and high-strength materials, the matrix has lower strength than the aggregate grains. Under load the cracks are crossing the matrix and contouring the grains.

In lightweight composites the situation is opposite: the cracks propagate across the grains

It should be remembered that what is macroscopically called a matrix also contains grains and inclusions of lower level; that is, the mortar which, with the aggregate grains, forms the ordinary concrete structure is itself composed of cement paste and grains of sand Non-hydrated grains of cement and particles of micro-fillers like fly ash and SF are surrounded by interfacial layers. The increase in load corresponding to the first crack opening in elements subjected to bending or tension is an important objective in material design. It may be achieved by several methods: increase of the tensile strength of the matrix itself, improvement of bond strength to aggregate grains and reinforcement, transformation of major cracks to micro cracks by adequate reinforcement, etc.6