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Rust Crack Status REPACK


When a hole for the service pipe line is drilled through the concrete, the coring process can cause damage to the concrete structure by creating cracks. The core hole can often provide a direct path for water to pass through the concrete structure.




Rust Crack Status



Control joints are pre-planned and installed to prevent concrete cracking due to shrinkage during curing. A control joint is saw-cut into the curing concrete when the concrete is just hard enough, usually within 6-12 hours after the concrete has been poured. The timing depends on the concrete mix and the surrounding environment. The cuts should be made at a predetermined spacing, depth and pattern to meet structural engineering specifications and only after the concrete has obtained sufficient strength, but before internal cracking begins.


An expansion joint is used in concrete to allow the concrete to absorb predicted movement by expanding or contracting with daily temperature variations. Lack of expansion joints may lead to uncontrolled cracking.


Cracks in concrete are of common occurrence and they develop when stresses in the concrete exceed its strength. Cracks are often caused by normal shrinkage of the concrete when hardening and drying. Concrete cracks can range from being a non-structural and unsightly, to being detrimental to the structural integrity and safety of a building.


Structural cracks may endanger the safety and durability of a building. They can form due to incorrect design, faulty construction and/or overloading. Non-structural cracks are mostly formed due to internally induced stress in building materials and do not result in weakening of the structure. However, if left untreated, a non-structural crack may facilitate ingress of moisture and other destructive environmental substances which may lead to corrosion of reinforcement, making the concrete structure unsafe.


Cracks that are identified as small and fine (less than 0.3 mm in width), are generally deemed acceptable as part of minor settlement depending on the purpose and intent of the concrete structure, the environment it is placed in, the design-life and the durability design. Nevertheless, a qualified and licenced professional should be consulted to classify the severity of the crack and appropriately investigate the cause of the crack and determine the most effective repair method if required.


Plastic settlement cracks form while the concrete is still plastic, during the initial setting of concrete. At this time, bleed water is still rising and it covers the surface while the aggregate and cement settle under the force of gravity. This separation forms a weaker layer of concrete near the surface. These types of cracks occur on the surface before the concrete has set. Plastic settlement cracks are typically mirroring the pattern of the restraining elements such as reinforcements.


Plastic settlement cracking that is identified as small and fine (less than 1mm in width), is generally deemed acceptable as part of minor settlement. However, a qualified and licenced professional should be consulted to classify the severity of the crack and appropriately investigate the cause of the crack and determine the most effective repair method if required.


Occasionally, some plastic settlement cracks can be wide at the surface and may extend to the reinforcement steel or other types of restraining elements. This may expose the reinforcement steel to the elements and increase the risk of corrosion and thereby pose a threat to the durability of the concrete slab. Further drying shrinkage can potentially lead to full depth cracking of the concrete slab.


In hardened concrete: Cracks may be v-cut with a concrete crack chaser and filled with suitable material. This is the best method to help improve the durability of the surface and prevent a corrosive environment developing around the reinforcement.


Plastic shrinkage cracks form due to rapid early drying and a low rate of bleeding while the concrete is still plastic (not set). Plastic shrinkage occurs mainly in concrete elements with a high surface to volume rate, such as slabs and pavements. Air and concrete temperature, wind speed, the warming and drying action and relative humidity all play an important role in the rate of moisture evaporation from setting concrete. Plastic shrinkage cracking occurs when the rate of evaporation from the surface exceeds the rate at which moisture is being supplied to it (via bleeding from the concrete).


Rapid drying of the surface of the concrete causes it to shrink and crack (similar to the cracking that occurs in clay soil as it dries). The cracks are not always evident and may not be discovered until the next day.


Plastic shrinkage cracks may form in a random pattern or they can appear in an almost parallel pattern. The cracks are often almost straight, ranging in length from 25 mm to 2 m but are usually 300 to 600 mm long. They can range from 1 to 3 mm wide at the surface and are superficial.


However, it is important to keep a close eye on these cracks as they can potentially form a weakness in the concrete. With subsequent drying shrinkage, thermal movement and/or loading, the cracks can be deepened, widened and/or extended, which may eventually cause a crack to penetrate right through the concrete element. This crack may lead to water ingress problems since it can start acting as a passageway for moisture and dissolved salts. This issue can lead to rusting reinforcement steel, concrete deterioration and spalling if left untreated.


In pre-hardened concrete: While the concrete is still plastic, the surface can be worked with surface vibrators to close the cracks whilst ensuring that the concrete re-liquifies so that the cracks close fully.


In hardened concrete: Where they are of concern for watertightness and/or protection of the reinforcement steel, or the cracks are 2 mm or wider, the cracks should be filled with a suitable proprietary filler. The cracks may be v-cut with a concrete crack chaser and filled with suitable material.


Early thermal contraction cracks are common in cantilever walls often used in reservoirs, dams, concrete tanks, retaining walls, bridge abutments and basements. They are also common in other large pours that exceed 2 meters, such as pavements.


Excessive temperature difference within a concrete structure or its immediate environment causes the cooler portion of the concrete to contract more than the warmer portion. This leads to stresses greater than the tensile strength of the concrete and early thermal cracks appear.


Changes in ambient temperature conditions from moderate day-time temperature to low night-time temperature may cause rapid cooling of the exposed surface of the concrete and the contraction is likely to cause thermal cracking.


If concrete is free to contract without restraint, thermal cracking will not occur. However, there is always some internal and external restraint. One example of an external restraint is where concrete is cast onto a previously hardened base, or adjacent to similar elements, without the provision of an expansion joint. The internal restraint is the warm interior of the concrete, which will contract later as it cools. Depending on the temperature difference and the strain capacity of the concrete, differential thermal strains may cause the concrete to form early thermal contraction cracks.


Unless the structural tolerance level is exceeded, early-age cracking formed by early thermal contraction are not considered to affect the general safety of the concrete structures. That being said, the aesthetic appearance may be greatly affected if the cracks are not repaired. Furthermore, if the cracks are left untreated, they may lead to ingress of water, oxygen and minerals, which may bring on the corrosion of reinforcement steel, costly maintenance and potential durability issues.


Whilst early-age cracking may initially be considered as a minor shortcoming, it is advised that building owners and building maintenance managers appoint qualified remedial repair contractors to correctly repair these cracks sooner rather than later in order to help prolong the service life of the concrete structures.


Structures such as concrete dams, reservoirs, tanks and roofs are especially at risk. If early-age cracks are left to further develop, it may affect the bearing capacity of the structure and speed up fatigue failure.


In many situations, early thermal contraction cracking cannot be avoided, but it can be reduced by avoiding excessive heat of hydration, reducing restraint where possible and using an adequate quantity and distribution of reinforcement.


Crazing is the development of fine random cracks on the surface of the concrete caused by shrinkage of the surface layer. Crazing is caused by drying out of the concrete surface, particularly when the surface has been exposed to low humidity, high air or concrete temperature or hot sun during placement for the concrete mix.


The cracks are typically shaped like irregular hexagon patterns not more than 50 to 100 mm across. They are rarely more than 3 millimetres deep and are more noticeable on overfloated or steel-trowelled surfaces.


Many craze cracks are not visible until the surface gets wet and starts to dry. Apart from appearance, crazing cracks do not affect the strength or durability of the concrete to a great degree as long as water intrusion is not occurring, which may on lead to subsequent deterioration of the concrete.


The build-up of corrosion causes tensile stresses as it grows in thickness. The pressure causes the concrete to form cracks near the steel that will with time lead to more extensive cracking as the rust builds up until the concrete starts to break away from the reinforcing steel bars (spalling of concrete) and expose the corroded reinforcing steel rods. 041b061a72


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