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strength; therefore a majority of cement mortar elements are designed to take advantage of the greater compressive strength of the material. The variables that have an effect on the strength of cement mortar are shown in Figure 2.13.

Figure 2.14: Factors influencing concrete strength [adopted from Crane and Charles, 1997].

Compressive strength is usually defined as the measured maximum resistance of a concrete or mortar specimen to axial loading. It is normally expressed in Mega-Pascals (MPa) at an age of 28 days and is usually specified by the symbol fc. Compressive strength can be used as an index to judge flexural strength, once the relationship has been established for the particular mix design and size of the unit (Crane & Charles, 1997).

2.13.2 Flexural Test

Direct testing of cement mortar under tension is not possible because of its brittleness since it is difficult to grip and align. Eccentric loading and failure at or in the grips is very hard to avoid. Therefore, the flexural test is preferred. This provides a way of measuring the

material‟s behaviour subjected to beam loading. Flexural strength is the ability of a beam or slab to resist failure in bending. The flexural strength is usually expressed as Modulus of Rupture (MR) in MPa. Hence, laboratory mix design based on flexure may be necessary, or cement content may be chosen from past experience to yield the required design of MR. MR can be employed for field control and in the acceptance of pavements. Flexural testing is used on a very few occasions for structural concrete (Crane & Charles, 1997).

2.13.3 Fracture of Cement Mortar

Even though cement mortar is a primitive and most commonly used material, a lot of properties and characteristics of cement mortar are neither easily nor accurately understood, and research is still being continued using various techniques to acquire better knowledge of the characteristics of cement mortar. Due to its main uses in building and construction works, as well as a selection of other heavily used applications (e.g. roadwork), understanding of its mechanical behaviour is receiving attention from engineers and scientists using a variety of testing methods; but owing to its complex nature, there are issues of concern to both the manufacturers and users. Cement mortar behaves like a structure of two dissimilar materials (aggregate and cement paste) which becomes more complex by the various quantities of materials that either make up the cement paste or function as the aggregate. The interface between the aggregate and cement is usually considered as the weakest link (Jan & Mier, 1997) in cement mortar which consequently has a major effect on the mechanical behaviour of cement mortar.

The interfacial bonding between the cement and aggregate performs a crucial function in the strength of concrete. With the hardening of fresh cement mortar, loss of moisture starts occurring in the cement paste causing shrinkage. Shrinkage does not occur in the aggregate material; the boundary conditions of the structure or the object during casting resist the

shrinkage of the cement paste. The boundary conditions, non-uniform distribution of shrinkage strain and the restraints from the aggregates cause increase to tensile stresses.

These tensile stresses give rise to internal flaws and cracks within the concrete before the application of any external load (Crane & Charles, 1997). The mechanical behaviour of concrete is controlled by the existence and propagation of these internal cracks during loading.

It is difficult to observe the fracture nature of concrete because of its complexity. This makes the crack propagation in concrete complex and it chooses a path based on the structure and constituents of the material, hence its behaviour is not predictable. The design of concrete structures, according to Adachi et al., (2002), is becoming increasingly necessary to look at crack growth and propagation to avoid catastrophic failure. Failure in concrete is usually due to crack propagation. Understanding the reasons and the circumstances under which concrete fails are important for design of concrete structures, as well as developing new cement-based materials.

When it is difficult for cracks to grow, a material is said to be tough and when crack propagation is easy, it is known as a brittle material. It would therefore clearly be useful to have a technique that is able to detect a crack and its propagation (Crane & Charles, 1997). A number of techniques exist that are categorised as non-destructive testing techniques, which are designed to detect, and usually size, stationary cracks without damaging the serviceability of the component. However one technique exists and is readily available, which can detect a growing crack but not an inactive crack; this passive technique is called acoustic emission (AE). It is the application of this AE technique which is considered in this work.

2.13.4 Microstructure of Concrete

Many characteristics of concrete do not abide by the laws of mixture even though it is a composite material. For example, if both the aggregate and the hydrated cement paste under compressive loading are separately tested, it would fail elastically, whereas concrete itself exhibits inelastic behaviour prior to fracture. In addition, the strength of concrete is normally much lower than the individual strength of the two components. These irregularities in the behaviour of concrete can be justified on the basis of its microstructure, particularly the vital function of the interfacial transition zone between coarse aggregate and cement paste (Akpabio et al., 2012).

It is essential to understand the microstructure of concrete to understand the crack propagation in concrete. It is very difficult to establish a clear pattern of the microstructure of concrete from which an opinion of the material‟s behaviour can be formed with confidence since concrete has a highly heterogeneous and complex microstructure. The developments in the area of materials have resulted mainly from recognition of the principle that the properties originate from the internal microstructure, i.e. modification can be made to the properties of materials by making appropriate alteration in the microstructure of a material.

The distinctive features of the concrete microstructure are (Crane & Charles, 1997):

i) Interfacial Transition Zone, representing a small region next to the particles of coarse aggregate. It exercises a far greater influence on the mechanical behaviour of concrete than is reflected by its size.

ii) Each of the three phases is itself a multiphase in character. For example, each aggregate particle may contain several minerals in addition to micro-cracks and voids;

iii) The microstructure of concrete is not an intrinsic characteristic of the material, because the two components of the microstructure (interfacial transition zone and hydrated cement paste) are subject to change with time, environmental humidity and temperature. The

theoretical microstructure-property relation models are not much helpful for predicting the behaviour of concrete mainly because of the highly heterogeneous and dynamic nature of the microstructure of concrete. To understand and control the composite material such as concrete, a broad knowledge of the important features of the microstructure of each of the three phases of concrete is nevertheless important (Crane & Charles, 1997).

2.13.5 Toughening Mechanism in Concrete

The toughness is a measure of energy while the strength is a measure of the stress required to fracture the material. Thus two materials may have very similar value of strength but different toughness values. With the propagation of cracks in concrete, many toughening mechanisms start taking place. The inelastic zone around a crack tip is expressed as the fracture process zone and is the location of these toughening mechanisms. Crack shielding, crack deflection, aggregate bridging, crack tip blunting and crack branching are some of the most common toughening mechanisms known until the present moment (Crane & Charles, 1997).

Crack shielding takes place when the major crack propagates into a zone that consists of a high density of flaws, such as water-filled pores, air voids attained during casting process and shrinkage cracks. Part of the energy being introduced by the applied load is consumed by the high-density flawed region. Compared to the main crack, the flawed region has a random orientation and therefore does not make contribution to the propagation of the main crack.

When the main crack must alter its direction of propagation due to a strong particle, such as an aggregate lying in its path, crack deflection occurs. If the main crack path is altered more, then the greater amount of energy must be introduced into the material to cause fracture.

When a crack has advanced beyond and through a particle, such as an aggregate, which is capable of distributing stresses from one side to the other (of the main crack), bridging

occurs. This transfer of stress is continued until the particle ruptures or is pulled out. Bridging is at times purposely introduced (glass slide in this investigation) into concrete by adding small fibres to serve as bridges across the surface of the cracks. Some of the commonly used fibres are steel, polypropylene, aramide and glass fibres (Crane & Charles, 1997). The propagation of the main crack is occasionally terminated by a large internal void; this toughening mechanism is termed crack tip blunting. When a crack tip propagates into a void, the tip of the crack becomes blunt and an extra amount of energy is needed to propagate the crack with a blunt tip. When the main crack splits into two cracks, the toughening mechanism of crack branching is introduced into the specimen. More energy is needed to propagate two cracks through concrete than it does to propagate one crack.

These toughening mechanisms take place amongst one another and absorb a part of the energy being introduced into a concrete specimen by an external force or movement. The fracture mode of a cementitious material relates very closely to the nature of fracture process that takes place in that material, based on the understanding of the conditions under which a number of toughening mechanisms can occur in a given material, it may be possible to control the fracture mode by tailoring the material microstructure (Crane & Charles, 1997).