Mechanical failure:
Type of cracks in concrete:
Overloading
The concrete member is designed with certain loads, determined by strength of the concrete, sizing, and placement of reinforcing bars, and shape of the concrete cross section. Design includes such factors as the strength of the concrete, the number, sizing, and placement of reinforcing bars, and size and shape of the concrete cross section. When a structure is overloaded to the extent not covered in safety factors, concrete may be damaged or fail. Overloading occurs in several ways, such as shear, flexure, tension. Also, the overloads can occur in fatigue or cyclic loading way.
Corrosion of steel
Corrosion of the reinforcing steel in concrete is one of the major issues of concrete failure. The pH level of concrete is high (usually above 12.5) due to the silicate in cement. The reinforcing steel in concrete is able to form a passive layer as a result of the high pH of concrete, thus preventing rust.
There are two major causes of corrosion in the reinforcing steel: chloride penetration and carbonation. When oxygen, chlorides, and moisture all penetrate the concrete, the pH of the concrete will be reduced. Chlorides can be found in water used in mixing concrete, or other environmental factor such as salt water in marine structure or de-icing salts used in winter for de-icing process.
Carbonation occurs when carbon dioxide and moisture infiltrate the concrete. The calcium hydroxide will react with the carbon dioxide and produce Calcium carbonate. The consequence of losing calcium hydroxide is the reducing the pH level of the concrete and increasing corrosion speed of steel.
Freeze-Thaw
Moisture is able to penetrate into the concrete and saturated inside the concrete. When the concrete exposed to freezing environment such as winter, the moisture in the concrete will freeze and expand to 9% more volume than that of water. The stress of the moisture freezing inside the concrete builds up by the freezing, causes defects and cracks. As the frozen water expanded and creates bigger holes, it will lead to more moisture penetration, and thus the freeze-thaw cycle will gradually damage the concrete.
Poor Workmanship
Concrete itself is so variable that properly constructing a concrete structure can be difficult. Some issues related to workmanship are as follows: over/under consolidated aggregates, improper location of rebar, over watering for workability, finishing surface before bleeding occurs. Each of these may end up not mattering overall, or may contribute to a structural failure.
Shrinkage
Concrete shrinkage may occur throughout a structure’s life cycle for different reasons with the majority occurring within the first few months or years after casting. There are two primary categories of shrinkage: plastic (before hardening), and drying (after hardening). Immediately after concrete is poured, there can be settlement shrinkage, construction movement (e.g. formwork movement or removal), and drying shrinkage. After the concrete has fully hardened, a structure will undergo temperature, volume and chemical changes throughout the years. Each of these may also cause concrete shrinkage.
Shrinkage is an expected phenomenon in a concrete structure, and can often be controlled with stress-relieving joints and properly placed reinforcing steel.
Alkali-Aggregate Reaction (AAR)
Aggregates inside the concrete, such as stones and sand, contain certain amount of silica or carbonate may react with alkalis, causing localized expansion in concrete. The alkalis may be also be from within the concrete mix, or may be from outside sources like sea or ground water, or deicing salts. Depending on the type of aggregate, AAR also goes by other names. In siliceous aggregates, the reactions are called "alkali silica reactivity" (ASR). In dolomitic carbonate rocks, the reactions are called "alkali-carbonate reactivity" (ACR).
When these types of reactions occur, it will create an alkali-silica gel, and the gel will swell when moisture react with it. The swelling will create internal stress, which may cause cracks within the concrete.
Sulphates
Sulfates (SO4) in the soil or in groundwater, in sufficient concentration, can react with the Portland cement in concrete causing the formation of expansive products, e.g., ettringite or thaumasite, which can lead to early failure of the structure. The most typical attack of this type is on concrete slabs and foundation walls at grades where the sulfate ion, via alternate wetting and drying, can increase in concentration. As the concentration increases, the attack on the Portland cement can begin. For buried structures such as pipe, this type of attack is much rarer, especially in the eastern United States. The sulfate ion concentration increases much slower in the soil mass and is especially dependent upon the initial amount of sulfates in the native soil. A chemical analysis of soil borings to check for the presence of sulfates should be undertaken during the design phase of any project involving concrete in contact with the native soil. If the concentrations are found to be aggressive, various protective coatings can be applied. Also, in the US ASTM C150 Type 5 Portland cement can be used in the mix. This type of cement is designed to be particularly resistant to a sulfate attack.
Delamation
Instrument:
Petrographic microscope
A petrographic microscope is a type of optical microscope used in petrology and optical mineralogy to identify rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology which focuses on detailed descriptions of rocks.
Concrete moisture meter
A concrete moisture meter is a type of moisture meter used by installers of flooring to measure the moisture levels of concrete. These meters have been used for decades to measure the moisture content in different materials and substances.
Concrete Air meter
A sample of fresh concrete is fully consolidated in the air meter and struck off level-full. A known volume of air at a known pressure is applied to the sample in an airtight container. The air content of the concrete is read from the gauge on the pressure meter apparatus.
Type of cracks in concrete:
Overloading
The concrete member is designed with certain loads, determined by strength of the concrete, sizing, and placement of reinforcing bars, and shape of the concrete cross section. Design includes such factors as the strength of the concrete, the number, sizing, and placement of reinforcing bars, and size and shape of the concrete cross section. When a structure is overloaded to the extent not covered in safety factors, concrete may be damaged or fail. Overloading occurs in several ways, such as shear, flexure, tension. Also, the overloads can occur in fatigue or cyclic loading way.
Corrosion of steel
Corrosion of the reinforcing steel in concrete is one of the major issues of concrete failure. The pH level of concrete is high (usually above 12.5) due to the silicate in cement. The reinforcing steel in concrete is able to form a passive layer as a result of the high pH of concrete, thus preventing rust.
There are two major causes of corrosion in the reinforcing steel: chloride penetration and carbonation. When oxygen, chlorides, and moisture all penetrate the concrete, the pH of the concrete will be reduced. Chlorides can be found in water used in mixing concrete, or other environmental factor such as salt water in marine structure or de-icing salts used in winter for de-icing process.
Carbonation occurs when carbon dioxide and moisture infiltrate the concrete. The calcium hydroxide will react with the carbon dioxide and produce Calcium carbonate. The consequence of losing calcium hydroxide is the reducing the pH level of the concrete and increasing corrosion speed of steel.
Freeze-Thaw
Moisture is able to penetrate into the concrete and saturated inside the concrete. When the concrete exposed to freezing environment such as winter, the moisture in the concrete will freeze and expand to 9% more volume than that of water. The stress of the moisture freezing inside the concrete builds up by the freezing, causes defects and cracks. As the frozen water expanded and creates bigger holes, it will lead to more moisture penetration, and thus the freeze-thaw cycle will gradually damage the concrete.
Poor Workmanship
Concrete itself is so variable that properly constructing a concrete structure can be difficult. Some issues related to workmanship are as follows: over/under consolidated aggregates, improper location of rebar, over watering for workability, finishing surface before bleeding occurs. Each of these may end up not mattering overall, or may contribute to a structural failure.
Shrinkage
Concrete shrinkage may occur throughout a structure’s life cycle for different reasons with the majority occurring within the first few months or years after casting. There are two primary categories of shrinkage: plastic (before hardening), and drying (after hardening). Immediately after concrete is poured, there can be settlement shrinkage, construction movement (e.g. formwork movement or removal), and drying shrinkage. After the concrete has fully hardened, a structure will undergo temperature, volume and chemical changes throughout the years. Each of these may also cause concrete shrinkage.
Shrinkage is an expected phenomenon in a concrete structure, and can often be controlled with stress-relieving joints and properly placed reinforcing steel.
Alkali-Aggregate Reaction (AAR)
Aggregates inside the concrete, such as stones and sand, contain certain amount of silica or carbonate may react with alkalis, causing localized expansion in concrete. The alkalis may be also be from within the concrete mix, or may be from outside sources like sea or ground water, or deicing salts. Depending on the type of aggregate, AAR also goes by other names. In siliceous aggregates, the reactions are called "alkali silica reactivity" (ASR). In dolomitic carbonate rocks, the reactions are called "alkali-carbonate reactivity" (ACR).
When these types of reactions occur, it will create an alkali-silica gel, and the gel will swell when moisture react with it. The swelling will create internal stress, which may cause cracks within the concrete.
Sulphates
Sulfates (SO4) in the soil or in groundwater, in sufficient concentration, can react with the Portland cement in concrete causing the formation of expansive products, e.g., ettringite or thaumasite, which can lead to early failure of the structure. The most typical attack of this type is on concrete slabs and foundation walls at grades where the sulfate ion, via alternate wetting and drying, can increase in concentration. As the concentration increases, the attack on the Portland cement can begin. For buried structures such as pipe, this type of attack is much rarer, especially in the eastern United States. The sulfate ion concentration increases much slower in the soil mass and is especially dependent upon the initial amount of sulfates in the native soil. A chemical analysis of soil borings to check for the presence of sulfates should be undertaken during the design phase of any project involving concrete in contact with the native soil. If the concentrations are found to be aggressive, various protective coatings can be applied. Also, in the US ASTM C150 Type 5 Portland cement can be used in the mix. This type of cement is designed to be particularly resistant to a sulfate attack.
Delamation
Instrument:
Petrographic microscope
A petrographic microscope is a type of optical microscope used in petrology and optical mineralogy to identify rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology which focuses on detailed descriptions of rocks.
Concrete moisture meter
A concrete moisture meter is a type of moisture meter used by installers of flooring to measure the moisture levels of concrete. These meters have been used for decades to measure the moisture content in different materials and substances.
Concrete Air meter
A sample of fresh concrete is fully consolidated in the air meter and struck off level-full. A known volume of air at a known pressure is applied to the sample in an airtight container. The air content of the concrete is read from the gauge on the pressure meter apparatus.