CONCREAT

Properties, microstructure, brittle fracture and applications of CONCRETE

Concrete:
Concrete is a composite material composed of coarse aggregate bonded together with a fluid cement which hardens over time. Most concretes used are lime-based concretes such as Portland cement concrete or concretes made with other hydraulic cements, such as ciment fondu. However, asphalt concrete which is very frequently used for road surfaces is also a type of concrete, where the cement material is bitumen, and polymer concretes are sometimes used where the cementing material is a polymer.
Famous concrete structures include the Hoover Dam, the Panama Canal and the Roman Pantheon. The earliest large-scale users of concrete technology were the ancient Romans, and concrete was widely used in the Roman Empire. The Colosseum in Rome was built largely of concrete, and the concrete dome of the Pantheon is the world's largest unreinforced concrete dome.Today, large concrete structures (for example, dams and multi-storey car parks) are usually made with reinforced concrete.

Crystal structure of  Concrete:

Microstructure of  Concrete:

            

         

a.                                             (b)

b.                    (a) Diagrammatic representation of bleeding in freshly deposited concrete;

c.                    (b) shear-bond failure in a concrete specimen tested in uniaxial compression.

Internal bleed water tends to accumulate in the vicinity of elongated, flat, and large

pieces of aggregate. In these locations, the aggregate-cement paste interfacial transition

zone tends to be weak and easily prone to microcracking. This phenomenon

is responsible for the shear-bond failure at the surface of the aggregate particle

marked in the photograph..    

Microstructure is the subtle structure of a material that is resolved with the help of

a microscope. A low-magnification (200¥) electron micrograph of a hydrated cement

paste shows that the structure is not homogeneous; while some areas are dense, the

others are highly porous. In the porous area, it is possible to resolve the individual

hydrated phases by using higher magnifications. For example, massive crystals of calcium

hydroxide, long and slender needles of ettringite, and aggregation of small

fibrous crystals of calcium silicate hydrate can be seen at 2000 ¥ and 5000 ¥ magnifications.

The unique features of the concrete microstructure can be summarized as

follows:

 First, there is the interfacial transition zone, which represents a small

region next to the particles of coarse aggregate. Existing as a thin shell, typically

10 to 50 μm thick around large aggregate, the interfacial transition zone is generally

weaker than either of the two main components of concrete, namely, the

aggregate and the bulk hydrated cement paste; therefore, it exercises a far

greater influence on the mechanical behavior of concrete than is reflected by its

size. Second, each of the three phases is itself a multiphase in character. For

instance, each aggregate particle may contain several minerals in addition to

microcracks and voids. Similarly, both the bulk hydrated cement paste and the

interfacial transition zone generally contain a heterogeneous distribution of different

types and amounts of solid phases, pores, and microcracks, as will be

described later. Third, unlike other engineering materials, the microstructure of

concrete is not an intrinsic characteristic of the material because the two components

of the microstructure, namely, the hydrated cement paste and the interfacial

transition zone, are subject to change with time, environmental humidity,

and temperature.

The highly heterogeneous and dynamic nature of the microstructure of concrete

are the primary reasons why the theoretical microstructure-property relationship

models, that are generally so helpful for predicting the behavior of

engineering materials, are not of much practical use in the case of concrete.

A broad knowledge of the important features of the microstructure of each of

the three phases of concrete, as provided below, is nevertheless essential for

understanding and control of properties of the composite material.

Fracture effects on concrete:

                         

                                         Microstructure of brittle fractured concrete

Fracture propagation in concrete and mortar has been generally analyzed when the crack advances orthogonally to the maximum principle stress, in pure Mode I(opening mode). Effectively, in concrete structures we

have always experimentally observed fracture propagation in Mode I, even in presence of bi- and triaxial state of stress, as in the collapse of large beams and plates, in the collapse for tear and for punching. Consequently

, the strength and toughness parameters definition for collapse in Mode II and III is often considered useless. It is opportune however distinguish between crack initiation and limit state. This is essential nowadays

as semi-probabilistic approaches for design at limit state split the two aspects and tend to assure structural integrity with respect to catastrophic collapse. In fact, crack initiation of the single micro-crack is always governed by local tension stress(Mode I) generated by singularities due to micro-structural heterogeneities and to pre-existent defects, while meso and macro-phase of propagation impose the interaction of in-plane shear or sliding (Mode II) and antiplane shear or tearing (Mode III).

Fracture propagation in Mode II and III has been observed, for concrete, in all dynamic shear test, during impact resistance and in bullet penetration tests.In all these cases, a generalized fracture toughness has been determined, and it has been obtained that When the microstructural roughness is involved in propagation resistance mechanisms, it is necessary to define correct mechanics parameters for Mode II and II.

Applications of concrete:

Concrete is widely used for making architectural structures, foundations, brick/block walls, pavements, bridges/overpasses, highways, runways, parking structures, dams, pools/reservoirs, pipes, footings for gates, fences and poles and even boats. Concrete is used in large quantities almost everywhere mankind has a need for infrastructure. Concrete is one of the most frequently used building materials in animal houses and for manure and silage storage structures in agriculture.

Properties:

Concrete has relatively high compressive strength, but much lower tensile strength. For this reason it is usually reinforced with materials that are strong in tension (often steel). The elasticity of concrete is relatively constant at low stress levels but starts decreasing at higher stress levels as matrix cracking develops. Concrete has a very low coefficient of thermal expansion and shrinks as it matures. All concrete structures crack to some extent, due to shrinkage and tension. Concrete that is subjected to long-duration forces is prone to creep.