Structural Cracks

This can be caused by incorrect design, faulty construction, overloading, poor soil bearing, swollen soil, and poor construction sites. It can be found in the column, beam, joist, slab or wall and may be dangerous based on its severity. Structural engineers must examine the damaged part in order to assess the integrity of the structure.

Scaling and Exfoliation

Scaling and exfoliation happens on the surface and can be observed as the peeling off or flaking of a certain part of the exterior concrete or mortar. This can be caused by the penetration of expansive agents. Freeze/thaw attacks, sulphate solutions, salt recrystallization and near surface corrosion of steel reinforcements.

Popouts

Popouts can leave a small crater in the concrete and can reach a diameter of 100 mm and depth of 40 mm. It starts of as a semicircular crack progressing to an inverted cone. The portion of the concrete that breaks may include a remnant of the aggregate. It happens due to the presence of expansive materials such as alkali silica gel, iron sulphide minerals and freeze/thaw attack in aggregates.

Mapcracking

Also known as pattern cracking, this type of crack forms a network of fine cracks extending through the upper concrete surface. Expansion caused by alkali-silica reaction (ASR), drying shrinkage and restrained thermal contraction are some of its causes.

Shrinkage cracks

Moisture change in concrete is the primary cause of shrinkage cracks. Presence of shrinkable aggregates, plastic & drying shrinkage and bleeding in concrete are some causes.

Expansion cracks

Cracks are formed once the expansion in concrete can’t be accommodated anymore. It can be due to the expansion brought by reformation of ettringite, alkali-aggregate reactions, reinforcement corrosion, thermal gradient from the environment, freeze/thaw attacks and salt recrystallization.

METHODS FOR CRACK REPAIR:

• Resin injection

• Dry packing

• Stitching and Bandaging

• Muting and sealing

• Drilling and plugging

• Vacuum impregnation and Polymer impregnation

• Autogenously healing

• Flexible sealing

References:

Poole, A. B., & Sims, I. (2016). Concrete Petrography: A Handbook of Investigative Techniques (2nd ed.). CRC PRESS.

https://www.chamberlinltd.com/consultants-corner-repairing-cracked-concrete/

https://ohiobasementauthority.com/difference-between-structural-cracks-and-non-structural-foundation-cracks/

https://www.tarmac.com/media/960524/non-structural-cracking-leaflet-a4-sbn-0419.pdf

https://theconstructor.org/concrete/prevent-cracks-in-concrete-structures/13457/

There are three main components for the reaction to proceed:

1. Presence of reactive aggregates

These are aggregates that contain reactive silica or amorphous silica which may react with the highly alkaline pore solution. Examples of reactive silica are cryptocrystalline silica, unstable varieties of quartz and volcanic glass.

2. Alkali concentration in the pore solution

The sodium and potassium ions present in the alkaline concrete pore fluid will interact and attack the varieties of silica present in the concrete. This will produce the alkali-silica gel.

3. Moisture availability

Exposure to moisture will trigger the expansion and swelling of the alkali silica gel. It will absorb the water into their structure and will expand causing strain to the concrete which will eventually lead to cracking if left unchecked.

Prevention of Alkali-Silica Reaction

1. Proper selection of aggregates

2. Usage of low-alkali cement

3. Fixing the water access and preventing exposure to moisture

4. The use of admixtures such as lithium and barium salts

5. Partial replacement of cement

References:

Alkali Silica Reaction - Proactive Avoidance. (n.d.). Retrieved from https://www.engr.psu.edu/ce/courses/ce584/concrete/library/chemical/asrproact.html

Alkali-Aggregate Reaction. (n.d.). Retrieved from https://www.cement.org/learn/concrete-technology/durability/alkali-aggregate-reaction

Poole, A. B., & Sims, I. (2016). Concrete Petrography: A Handbook of Investigative Techniques (2nd ed.). CRC PRESS.

DEF can occur in cracks, along aggregate boundaries and in air voids. This could result to expansion which can damage concrete structures.  It appears as needle-like crystals under the microscope.

References:

Poole, A. B., & Sims, I. (2016). Concrete Petrography: A Handbook of Investigative Techniques (2nd ed.). CRC PRESS.

These are the chemical reactions that take place:

Ca(OH)2 + CO2 → CaCO3 +H2O

Calcium silicate hydrate + CO2 → various intermediates → CaCO3 + SiO2•nH2O + H2O

Aluminate hydrates + CO2 → CaCo3 + hydrated alumina

Ferrite hydrates + CO2 → CaCO3 + hydrated alumina and iron oxides

Carbonation and Corrosion

The alkalinity of concrete protects and preserves the thin passive layer on steel reinforcements. The steel is protected from corrosion due to the concrete having a pH of around 12.5. However, carbonation of the cement paste lowers the pH of the pore solution. This will be a problem to steel reinforcements due to the acidic conditions. Carbonated concrete has a pH around 9.5 which could cause the thin film protecting the steel to be damaged thereby making it vulnerable to corrosion.

Prevention and Mitigation

• Make a denser well compacted concrete having low water/cement ratio to reduce the porosity and permeability of the cement paste

• Coat the surface of the concrete

• Regularly monitor the surface cracks

• Apply penetrating sealers in areas where carbonation are observed to prevent further deterioration of the concrete.

References:

Poole, A. B., & Sims, I. (2016). Concrete Petrography: A Handbook of Investigative Techniques (2nd ed.). CRC PRESS.

https://theconstructor.org/concrete/carbonation-of-concrete-structures/7791/

https://www.understanding-cement.com/carbonation.html