Most cracks in concrete occur within 2-3 days after it is laid. It is important to check that the formwork is firm and secure before the concrete is poured. After a month of settling, the cracks that were initially visible may become almost impossible to see. Any crack that is an eighth of an inch or less is considered standard.
Plastic shrinkage cracks are one of the most common causes of early cracking in concrete. When the concrete is still in its plastic state, it is filled with water which takes up space and makes the slab a certain size. As the tile loses moisture during curing, it becomes smaller and this shrinkage creates stress on the slab. The tension created can literally separate the slab as the concrete shrinks and drags through its granular subbase.
When the stress becomes too great for the now-hardened concrete, it cracks to relieve stress. This can happen as soon as a few hours after pouring in hot climates. Even if concrete cures slowly, large slabs such as patios or sidewalks may still crack due to shrinkage as temperatures change and water is consumed in the hydration process. Control joints are essential to anticipate and direct any future cracks. These intentional weaknesses are cut into the slab to about a quarter of its depth.
As the concrete shrinks, it will most likely crack in these weaker parts. Caps and bolts should be removed and Z-bar holes should be inserted with cement-based plugs or patched with sand and cement mix. Pipe penetrations should also be sealed with a cement-based plug or patch from the inside of the building during construction. The plug does not seal gaps in the entire wall or slab and can fail over time, allowing water to enter voids and seep through the concrete around the pipe penetration. An expansion joint should be used in the concrete to allow for natural movement as it expands or contracts with daily temperature variations. To determine maximum joint spacing, multiply the planned thickness of the concrete (in inches) by 2.5.Cement cures gradually and needs to retain moisture to achieve maximum strength.
Mass repair of concrete structures will depend on width of crack, depth, whether it is inactive or alive, and service conditions of structure. If you would like an expert opinion, contact a nearby concrete contractor for an in-person consultation evaluation. For smaller projects such as decorative items, you can add wire mesh during pouring to improve strength and reduce cracking. When non-crystalline silicon dioxide (mainly originating from Portland cement) reacts with alkali hydroxide in concrete or alkalis present in environment such as sea spray or groundwater, an alkali silicate gel forms which swells as it absorbs moisture from cement pore solution in concrete or environment. When you hear someone talk about 4,000 psi concrete, it refers to the fact that it would take 4,000 pounds per square inch of pressure to crush it. A product that reacts quickly with water can be used to fill cracks and create a strong bond with the concrete and a flexible waterproof seal that prevents future water leaks. Construction joints are sometimes necessary when there are multiple concrete placements but they can be a weak link that causes service problems.
Cracks that are identified as small and fine (less than 0.3 mm wide) are generally considered acceptable as part of minor settlement based on purpose and intention of structure, environment it is placed in, service life, and durability design. This problem can cause oxidation of reinforcing steel, deterioration of concrete, and chipping if left untreated.