Durability of Concrete Structures
A long service life is considered synonymous with durability. Since durability under one set of conditions does not necessarily mean durability under another, it is customary to include a general reference to the environment when defining durability. According to ACI Committee 201, durability of Portland cement concrete is defined as its ability to resist weathering action, chemical attack, abrasion, or any other process of deterioration; that is, durable concrete will retain its original form, quality, and serviceability when exposed to its environment. No material is inherently durable; as a result of environmental interactions the microstructure and, consequently, the properties of materials change with time. A material is assumed to reach the end of service life when its properties under given conditions of use have deteriorated to an extent that the continuing use of the material is ruled either unsafe or uneconomical.
Concrete Deterioration can be caused by:
– The use of inappropriate materials.
– Poor construction practices.
Environmental Related Causes of Concrete Durability Problems
The inferior durability characteristics of concrete may be caused by the environment that the concrete is exposed to. The following environmental condition can affect the concrete durability:
– Physical factors.
– Chemical factors.
– Biological factors.
These factors may be due to weathering conditions (temperature, and moisture changes), or to abrasion, attack by natural or industrial liquids and gases, or biological agents.
Durability problems related to environmental causes include the following: steel corrosion, delamination, cracking, carbonation, sulfate attack, chemical attack, scaling, spalling, abrasion and cavitation.
The influence of shrinkage and creep on concrete cracking: under restraining conditions in concrete, the interplay between the elastic tensile stresses induced by shrinkage strains and the stress relief due to the viscoelastic behavior is at the heart of the deformations and cracking in most structures.
To understand the reason why a concrete element may not crack at all or may crack but not soon after exposure to the environment, we have to consider how concrete would respond to sustained stress or to sustained strain. The phenomenon of a gradual increase in strain with time under a given level of sustained stress is called creep. The phenomenon of gradual decrease in stress with time under a given level of sustained strain is called stress relaxation.
Both manifestations are typical of viscoelastic materials. When a concrete element is restrained, the viscoelasticity of concrete will manifest into a progressive decrease of stress with time (Fig. 4-1 curve b from Mehta textbook). Thus under the restraining conditions present is concrete, the interplay between elastic tensile stresses induced by shrinkage strains and stress relief due to viscoelastic behavior is at the heart of deformations and cracking in most structure.
In general, solids expand on heating and contract on cooling. The strain associated with change in temperature will depend on the coefficient of thermal expansion of the material and the magnitude of temperature drop or rise. Except under extreme climatic conditions, ordinary concrete structures suffer little or no distress from changes in ambient temperature. However, in massive structures, the combination of heat produced by cement hydration and relatively poor heat dissipation conditions results in a large rise in concrete temperature within a few days after placement. Subsequently, cooling to the ambient temperature often causes the concrete to crack. Since the primary concern in the design and construction of mass concrete structures is that the completed structure remains a monolith, free of cracks, every effort to control the temperature rise is made through selection of proper materials, mix proportions, curing conditions, and construction practices.
With low tensile strength materials, such as concrete, it is the shrinkage strain from cooling that is more important than the expansion from heat generated by cement hydration. This is because, depending on the elastic modulus, the degree of restraint, and stress relaxation due to creep, the resulting tensile stresses can be large enough to cause cracking.
For instance, assuming that the coefficient of thermal expansion of concrete is 10 × 10?6 per °C, and the temperature rise above the ambient from heat of hydration is 15 °C, then the thermal shrinkage caused by the 15 °C temperature drop will be 150×10?6. The elastic modulus (E) of ordinary concrete may be assumed as 3 × 106 psi. If the concrete member is fully restrained (Dr = 1), the cooling would produce a tensile stress of 450 psi. Since the elastic tensile strength of ordinary concrete is usually less than 450 psi, it is likely to crack if there is no relief due to stress relaxation.
Factors Affecting Thermal Stresses
Degree of restraint (Kr ) . A concrete element, if free to move, would have no stress development associated with thermal deformation on cooling. However, in practice, the concrete mass will be restrained either externally by the rock foundation or internally by differential deformations within different areas of concrete due to the presence of temperature gradients.
For example, assuming a rigid foundation, there will be full restraint at the concrete-rock interface (Kr = 1.0), however, as the distance form the interface increases, the restraint will decrease, as shown in the following Figure (from Mehta textbook).
Temperature change. The hydration of cement compounds involves exothermic reactions which generated heat, and increase the temperature of concrete mass. Heating causes expansion, and expansion under restraint results in compressive stress. However, at early ages, the elastic modulus of concrete is low and the stress relaxation is high, therefore, the compressive stress will be very small, even in areas of full restraint. In design, to be conservative, it is assumed that a condition of no initial compression exists.
Thermal Properties Of Concrete
Coefficient of thermal expansion is defined as the change in unit length per degree of temperature change. Selecting an aggregate with a low coefficient of thermal expansion when it is economically feasible and technologically acceptable, may, under certain conditions, become a critical factor for crack prevention in mass concrete. This is because the thermal shrinkage strain is determined both by the magnitude of temperature drop and the linear coefficient of thermal expansion of concrete; the latter, in turn, is controlled primarily by the linear coefficient of thermal expansion of the aggregate which is the primary constituent of concrete.
The reported values of the linear coefficient of thermal expansion for saturated Portland cement pastes of varying water/cement ratios, for mortars containing 1:6 cement/natural silica sand, and for concrete mixtures of different aggregate types are approximately 18, 12, and 6 to 12 × 10?6 per °C, respectively. The coefficient of thermal expansion of commonly used rocks and minerals varies from about 5 × 10?6 per °C for limestones and gabbros to 11 to 12 × 10?6 per °C for sandstones, natural gravels, and quartzite. Since the coefficient of thermal expansion can be estimated from the weighted average of the components, assuming 70 to 80 percent aggregate in the concrete mixture, the calculated values of the coefficient for various rock types (both coarse and fine aggregate from the same rock) are shown in Fig. 4-24. The data in the figure are fairly close to the experimentally measured values of thermal coefficients reported in the published literature for concrete tested in moist condition, which is representative of the condition of typical mass concrete.
Specific heat is defined as the quantity of heat needed to raise the temperature of a unit mass of a material by one degree. The specific heat of normal weight concrete is not very much affected by the type of aggregate, temperature and other parameters. Typically the values of specific heat are in the range of 0.22 to 0.25 Btu/lb.F.
Thermal conductivity gives the flux transmitted through a unit area of a material under a unit temperature gradient. The thermal conductivity of concrete is influenced by the mineralogical characteristics of aggregate, and by the moisture content, density, and temperature of concrete. Typical values of thermal conductivity for concretes containing different aggregate types range between 23-25 25 Btu in/h.ft2.F.
Extensibility and Cracking
As stated earlier, the primary significance of deformations caused by applied stress and by thermal and moisture-related effects in concrete is whether or not their interaction would lead to cracking. Thus the magnitude of the shrinkage strain is only one of the factors governing the cracking of concrete. From 4-1 it is clear that the other factors are:
• Modulus of elasticity. The lower the modulus of elasticity, the lower will be the amount of the induced elastic tensile stress for a given magnitude of shrinkage.
• Creep. The higher the creep, the higher is the amount of stress relaxation and lower the net tensile stress.
• Tensile strength. The higher the tensile strength, the lower is the risk that the tensile stress will exceed the strength and crack the material.
The combination of factors that are desirable to reduce the advent of cracking in concrete can be described by a single term called extensibility. Concrete is said to have a high degree of extensibility when it can be subjected to large deformations without cracking. Obviously, for a minimum risk of cracking, the concrete should undergo not only less shrinkage but also should have a high degree of extensibility (i.e., low elastic modulus, high creep, and high tensile strength).
In general, high strength concretes are more prone to cracking because of greater shrinkage and lower creep; on the other hand, low strength concretes tend to crack less, probably because of lower shrinkage and higher creep.