It's natural to be concerned when you notice cracks in freshly poured concrete. The truth is that some cracks are unavoidable due to the structure of the surface. Let's explore why your fresh concrete can crack and what you can do about it. Cracks in concrete are common and develop when the stresses in the material exceed its strength.
These cracks can range from being non-structural and unsightly to being detrimental to the structural integrity and safety of a building. Fine cracks, which are usually around 0.08 mm (0.003 inch) wide, are often observed in newly laid concrete and are caused by plastic shrinkage. If someone tells you that your concrete floor shouldn't have any cracks, be careful - they don't know what they're talking about! In hot climates, a concrete slab will expand as it heats up and push against any object in its path, such as a brick wall or an adjacent concrete slab. Certain cracks in the concrete can be repaired by targeted injection of the appropriate material adapted to the diagnosis of the individual crack, followed by a suitable concrete protective coating.
An excessive temperature difference within a concrete structure or its immediate environment causes the coldest part of the concrete to shrink more than the hottest part. During the initial setting of concrete, plastic settling cracks form while the concrete remains plastic. These cracks can be caused by variations in air temperature, concrete temperature, relative humidity, and wind speed on the surface. The primary cause behind the generation of fine cracks in concrete is plastic shrinkage, which is the rapid depletion of moisture from fresh concrete within its plastic state.
The volume reduction in concrete that occurs mainly due to moisture loss after it has hardened is known as drying shrinkage. An expansion joint is used in the concrete to allow it to absorb motion as it expands or contracts with daily temperature variations. In addition to traditional curing methods, concrete additives and curing compounds can help it cure faster and resist cold. If the concrete cover protecting the reinforcing steel is damaged and the joint between the concrete and steel reinforcing bar breaks, then passive layer of steel will break and active corrosion of steel will begin. 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, then reaction forms an alkali silicate gel that swells as it absorbs moisture from cement pore surrounding solution in concrete or environment.
The alkali-aggregate reaction refers to a destructive expansion reaction within concrete that occurs over a long period of time (more than 5 years) in the concrete. Instead of using water to cure your freshly poured concrete, cover it with an insulating plastic sheet or even straw to trap moisture and help it cure at an even rate. If you notice larger or active cracks, or if one side of the crack is higher than the other, you may need a structural engineer to review the work.