Ayasha Tamara 1506716611 Chapter 4.docx

Ayasha Tamara 1506716610 Chapter 5 1 Introduction Concrete is a man-made composite material produced in its simplest form from Portland cement, coarse and fine aggregates, and water. In modern times, concrete may contain supplementary cementing materials (such as fly ash, blast furnace slag, or silica fume), various fillers (e.g. limestone), chemical admixtures (i.e. set retarders, set accelerators, water reducers, corrosion inhibitors), and a variety of fibers. 1.1

Physics and chemistry of concrete formation

Portland cement and other Portland cement clinker-based cements are the most common cementing materials used in concrete production. The total world produc- tion of cement is estimated to be above 1.7 billion metric tons per year, enabling the production of more than 10 billion metric tons of concrete annually. Curing of concrete is the most important, though often misunderstood or neglected, part of the concrete-making process. It is during curing that the cement components hydrate, form hydration products of certain chemical and microstruc- tural qualities within a prescribed space. It is the hydration process that leads to development of such important qualities as water-tightness, strength, modulus of elasticity, durability in given environments, etc 1.2

Mechanisms of concrete deterioration All concrete structures are exposed to the environment. Thus, concrete mixture

proportioning, structural design, processing, and maintenance have to take into consideration the environmental conditions under which the concrete is to function during its expected service life.

Because the environments in which concrete may be used vary widely (cold to warm, dry to wet, repeated temperature and humidity changes, seawater, aggressive soils, etc.), it is a mistake to assume that any concrete will be stable under all conditions. This basic concept is often neglected, thus resulting in premature deterioration of structural concrete components and products. Maximum expected service life of a concrete product or structure varies, based on the chemical and atmospheric environment the concrete is exposed to, and clearly depends on the original design, the actual concrete quality, and the level of maintenance.

2 Chemical deterioration of cement paste components

Like the processes leading to the development of concrete properties, the pro- cesses causing premature deterioration of the concrete matrix are usually chemical in nature as well. This fact is often unrecognized or ignored. The chemical pro- cesses of deterioration may have physical (volume change, porosity and perme- ability alteration) or mechanical (cracking, strength loss) consequences, or both. Typical examples are acid attack, alkali–silica reaction, carbonation, and sulfate attack. Porosity is one of the most important properties of hardened concrete, as its magnitude and form controls the permeability of concrete with respect to water vapor and water-soluble ionic species. Most applications of concrete assume that it has certain ‘barrier’ properties protecting it from the environment, therefore control of porosity and permeability is crucial. The most important methods of permeability control are proper mix proportioning and effective curing. 2.1

Aggressive soils and ground water

The most widely spread concrete damage by chemicals present in soil and ground- water is attack by sulfates. Therefore, most of this chapter will be dedicated to various forms of sulfate attack. As in the case of sulfate attack, proper mixture proportioning (low concrete porosity and permeabil- ity), effective curing, and separation of the structure from the source of the soluble chemicals involved are the primary remedies for avoiding deterioration problems. 2.1.1

Sulfate attack

Sulfate attack is a generic name for a series of interrelated, sometimes sequen- tial, chemical and physical processes leading to recrystallization or decomposition of hydration products and deterioration of the concrete matrix microstructure, subsequently inducing degradation of the expected concrete properties, includ- ing strength, volume stability, and durability. 2.1.1.1

Internal sulfate attack

This attack may be caused by (a) out-of-standard amounts of sulfates in the cement or aggregate (composition-induced internal sul- fate attack) or (b) improper processing of the concrete, primarily by excessive curing temperature (heat-induced internal sulfate attack). Heat-induced sulfate attack is often referred to in the literature as delayed ettringite formation or DEF 2.1.2

Prevention of sulfate attack

Because sulfate attack on concrete is not characterized by a single chemical process, when considering the best preventive measures one has to take into consideration the processes involved in concrete production and the environmental conditions to which the concrete will be exposed throughout its service life. However, some basic rules apply in most cases. The most important among them are:

Minimizing access of water-soluble sulfates to the structure (e.g. proper struc- tural design,

1.

drainage, protective barriers/coatings), Production of low-permeability concrete (e.g. use of low w/c mixture propor- tions,

2.

adequate cement content, proper consolidation, adequate curing), and Intelligent selection of cementing materials (e.g. use of low-alumina, low- calcium

3.

cements, including sulfate-resisting cements and cements containing pozzolans or slag).

2.2

Industrial and agricultural chemicals

Concrete is seldom attacked by solid industrial or agricultural chemical waste, but may be easily damaged by certain kinds of liquid or water-soluble chemicals. If there is a pressure and/or temperature difference between the exposed and nonex- posed concrete surfaces, the rate of damage may increase. 2.3

2.3.1

Atmospheric deterioration

Carbonation

Because Portland cement-based concrete always contains high proportions of calcium hydroxide and other calcium-containing compounds, exposure to carbon dioxide (CO2) in the presence of moisture will induce formation of calcium car- bonate. This may lead to several possible problems, especially, but not exclusively, in thin-wall products: 1. Shrinkage of 2. Decreased

the cement paste leading to cracking,

alkalinity (pH) of the cement paste enabling increased rate of cor- rosion of the

reinforcing steel

3. Decreased

2.2.1

wear resistance of carbonated surfaces.

Acid rain

The concrete matrix is a highly alkaline material. Thus, its resistance to acids is relatively low. However, for well-produced concrete, where the matrix permeabil- ity is low, concrete products and structures withstand occasional deterioration by atmospheric acids quite well. The source of most atmospheric acid is sulfurous combustion gases that react with moisture to form low-concentration sulfuric acid. Other sources of acids include industrial or municipal sewage, as mentioned above.

2.2.2

Other deterioration mechanisms

The effect of seawater on concrete could have been categorized under sulfate attack or corrosion of reinforcement or otherwise but, because of the complex chemistry of seawater and the multiple potential deterioration mechanisms involved, it will be briefly discussed as a separate issue.

2.3.3.1 Exposure of concrete to seawater Some of the most important and envi- ronmentally vulnerable concrete structures are located in marine environments. This enhanced vulnerability is related to continuous changes in temperature (including wetting/drying and freezing/thawing cycles), pressure changes in the tidal zone, reoccurring capillary suction, surface evaporation, and multiple chemical interactions with seawater components. Affected are the cement paste components, paste– aggregate interfaces, and the reinforcing steel. Deterioration of concrete exposed to seawater is known to

be caused primarily by two independent chemical mecha- nisms: (a) corrosion of reinforcing steel embedded in concrete, and (b) deterioration of the concrete matrix itself. The rate of damage varies widely, depending upon the concrete quality, its maintenance, and the severity of the environment.

. Under different conditions, the Ca(OH)2 can dissolve, leading to increased permeability.

CO2 + Ca(OH)2 → CaCO3 xCO2 + xCaO·SiO2 aq. → xCaCO3 + SiO2 aq. (aragonite) 3 Deterioration of aggregates in concrete

Aggregates make up about 70–80% of the volume of typical concrete. Thus, their quality and sensitivity to the environment of concrete exposure is of utmost impor- tance. Depending upon the chemical and mineralogical composition, and on the geological origin of the rock used, aggregates may be sensitive to the microenvi- ronment within the concrete matrix or to the external environment in which the concrete is used. 3.1

Alkali–aggregate reaction

Alkali–aggregate reaction (AAR) is the generic name for interactions of aggre- gate components with alkali cations and OH− ions present in the pore solution of concrete. The most important mechanisms are alkali–silica reaction (ASR), alkali–carbonate reaction (ACR), and various, less common, oxidation and hydra- tion reactions of aggregate components (e.g. oxidation of pyrite, FeS2; hydration of MgO). 3.1.1

Alkali–silica reaction

Alkali–silica reaction is a potentially severe damage mechanism affecting concrete worldwide. The rate of reaction and the severity of damage vary widely, as they depend on numerous factors, including the type of aggregate (type and concentra- tion of silica, alkali content), alkali content of the cement, concrete quality, struc- tural design (access of water), and local reaction conditions such as temperature, humidity, and concrete permeability. 3.1.1.1 Prevention of alkali–silica damage The primary preventive measures are as follows: 

Use of aggregate containing low concentrations of amorphous/reactive silica,

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Use of low-alkali cements, aggregates, and other cementing materials

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Protection of concrete from moisture, especially from water containing alkalis (e.g. seawater)

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Mixed design to produce cement paste of low porosity/permeability (low w/c, controlled cement content) and low free calcium hydroxide content (blended cements; high-silica supplementary cementing materials).

3.2 Frost damage to aggregate: D-cracking

As is the case with other solid materials, the resistance of aggregates used in con- creting to frost depends on their porosity and permeability. These are controlled by the overall pore structure (size distribution, shape, connectivity, etc.). Depend- ing upon the size and accessibility of the pores, under humid conditions some of the pores will be water-filled; under saturated conditions, all the pores may be water-filled. Under freezing conditions, water in some pore sizes may freeze and, in near-saturated (critical saturation) conditions, the hydraulic pressures caused by ice formation may not be accommodated, thus leading to expansion. 4 Corrosion of steel reinforcement

Corrosion of the steel reinforcement in concrete structures is a significant problem throughout most of the world. Under ideal conditions, concrete is capable of chem- ically protecting the internal steel through the formation of a passivation film at the surface of the steel. Unfortunately, there are a number of conditions that can cause failure of the protective layer and allow corrosion to initiate, the primary factor being the introduction of chloride ions into the concrete. This can occur anywhere that reinforced concrete is exposed to seawater or deicing salts, making it a very widespread problem. 5 Damage to concrete by repeated freezing and thawing

If not properly designed and protected, concrete saturated with water can be sus- ceptible to significant damage from exposure to freezing. When exposed to tem- peratures sufficiently low enough to induce freezing of the pore solution within the hydrated cement paste, internal

tensile stresses develop that can grow large enough to induce fracture of the hardened paste. This type of deterioration is actually a combination of a number of mechanisms.

6 Mechanical deterioration of concrete

Concrete is susceptible to physical damage due to progressive mass loss induced by contact with moving objects or materials. This type of damage is also an attrition process, and usually takes many cycles of repetitive contact or a continuous exposure to the damage source. 6.1

Abrasion

Abrasion damage is caused by direct physical contact with a moving object or material. Typically, this refers to wear induced by moving equipment, but can also apply to the flow of dry particles.

6.2

Erosion

Erosion refers to wear or deterioration that is induced by contact from solid particles suspended in a liquid. This type of damage is common in canal linings, spillways, sewage lines, and bridge piers. 6.1

Cavitation

Cavitation is another source of damage common to concrete structures designed to carry liquids. The mechanism of damage in this case is the formation, and subse- quent collapse, of vapor

bubbles. Such bubbles form naturally within flowing water and later violently collapse when entering a turbulent or high velocity zone. 6.2

Prevention of mechanical damage

Protecting concrete from abrasion or erosion is typically done through the use of higher strength concrete. The American Concrete Institute Committee 201 rec- ommends minimum values of 28 and 41 MPa, respectively, to provide adequate abrasion and erosion resistance for most exposures. Additionally, it is important to use hard, durable, wear-resistant aggregates and to minimize air contents relative to the exposure conditions. 7 Structural consequences of environmental damage

Depending on the type and extent of the damage, environmental degradation of con- crete may lead to serious consequences in terms of structural failures, cost of repair, and indirect economic losses such as caused by delays in transportation and the inability of using the industrial and military infrastructures. 6 Economic considerations

The importance of concrete industry becomes obvious when one realizes that con- crete is the most widely used man-made product and is second only to water as the most utilized substance by the humanity. On the positive side, concrete and related industries employ hundreds of thou- sands of people worldwide, and the products produced by these industries are basic to industrial progress and societal well-being. Transportation and housing infra- structures, private and governmental

construction, and even energy and water util- ities all strongly depend on the productivity, efficiency, and quality of the concrete industry. For example, in the United States, the construction materials companies employ about 200,000 people and the gross product of the cement and concrete manufacturing segment of the construction industry exceeds $35 billion annually. Yearly worldwide per capita production of concrete exceeds one metric ton.