You can repair fine cracks in concrete with a grout made of Portland cement and water. Add enough water to the cement to form a thick paste. Moisten the old concrete along the hairline crack with water for several hours before adding the grout. Fine cracks are commonly observed in newly laid concrete and their appearance is due to the phenomenon of plastic shrinkage. As the name implies, these cracks are very small, about 0.003 inches (0.08 mm) wide and can be very shallow.
In addition to cement grout, you can use any other filler or sealant material to fill the cracks. Keep in mind that an ideal adhesive should have a low surface tension and viscosity. These properties help the adhesive penetrate deep into the space to fill it. Both fine cracks and larger structural cracks are caused by stress relief. A crack forms when tensile stress builds up in concrete and exceeds the material's ability to withstand those stresses.
Most large structural cracks in countertops are formed due to bending, either because a faucet was too tight or the house was installed. Fine cracks often occur due to shrinkage, either from drying or by heat. These types of cracks are more difficult to control. Because they usually occur close to the surface, reinforcement does not help prevent them. The best prevention is to use a good mixture design that has a low tendency to shrink.
Even so, fine cracks can occur, and they are often located near moisture areas (sinks and dishwashers), where dry concrete absorbs moisture repeatedly and then dries. Over time, these wetting and drying cycles will cause concrete to crack, just as a piece of steel will eventually crack if bent back and forth enough times. Repairing fine cracks can be a challenge because making the crack disappear requires skill, patience, practice and a keen eye for color matching. A definition of a fine crack is a crack that has not been opened. That means there is very little room to fill it with anything other than a very low viscosity liquid. Since most fine cracks do not affect the structural integrity of a countertop, sealing and concealing the crack are the primary goals of a repair.
Common topical sealants can be used for concrete countertops, but getting them to penetrate and fill a thin crack can be difficult. Most concrete countertop sealants have a low solids content. But using a material with a high solids content will ensure that as the sealant cures, the material left in the crack will not shrink and separate from the concrete or, just as bad, leave a vacuum. A highly fluid epoxy (such as a fluid grade granite epoxy) will fill and seal a crack very well if it can get it into the crack. Keep in mind that materials such as epoxy are effective because they wet concrete.
Functionally, this helps to get the epoxy into the crack. Aesthetically, this can obscure the appearance of the crack, especially if the sealant used on the countertop does not darken concrete as much as epoxy. In Australia market, elastomeric masonry crack filler is widely applied with a caulking gun to fix narrow cracks of concrete. Instead, there are many different products available in bottles with some generic applicator tip. There is no doubt that small cracks (fine cracks) are often repaired using a vinyl concrete patch material that is worn over the crack and leveled with a flexible knife or trowel afterwards. As for the Australian market, for very fine cracks, putty or crack filler application is also very good.
Beware of cracks in your home; they can be narrow cracks and they can also be deep, often spreading over the entire width of the concrete slab. Therefore, it is wise to start repair by filling foam backing rod into narrow crevices to make a base for holding restoration material. The back-up bar comes in solid form and of different sizes; using rod width that is somewhat larger than width of crack. Defects in concrete structures are commonly caused by penetrations through concrete made for service lines or created by Z-bars that are used to create formwork braces or temporary ties in concrete. The important thing is that these cracks should not be visible nor should they affect overall performance of structure. Depending on temperature difference and deformation capacity of concrete, differential thermal deformations can cause concrete to form cracks due to early thermal shrinkage.
Floor cracks in concrete slabs are relatively common and common; not of concern at structural level. Cracks generally do not extend to perimeter of slab and rarely impair strength of concrete element. However, a crack should be inspected and diagnosed to classify severity of crack, cause of crack whether it is latent or living crack to determine most effective method of repair. Changes in ambient temperature conditions from moderate daytime temperature to low night temperature can cause rapid cooling of exposed concrete surface and shrinkage is likely to cause thermal cracking. Plastic settlement cracks are often identified in pours of deep sections such as on top of beams and columns. Idle Crack Repair: Inactive cracks are stable and no future movements are anticipated or in other words they are unlikely to open, close or spread further. Cracks in concrete can range from being non-structural and unsightly, to being detrimental to structural integrity and safety of building.
Before you can begin repair of fine cracks in concrete evaluate crack to determine best solution. If cracks appear soon after pouring concrete base it is possible that concrete has mixed poorly or poured too quickly. Low viscosity epoxy resin is mainly used for repair of structural cracks when future movements (latent cracks) are not anticipated.