Small, thin cracks usually won't cause any problems and are better left alone. However, if your new concrete develops cracks more than ⅛” wide, it's better to seal them off from water with a high-quality concrete caulk so they don't get worse.
Before the concrete dries, it's referred to as “plastic,” a particularly temperamental stage. Hairline cracks in new concrete are completely normal because of rapid moisture loss, also known as “plastic shrinkage,” before it dries.
Cracks that are 0.3 mm or greater can pose an issue for durability and watertightness. This can eventuate into concrete deterioration, affecting its strength and durability.
If you had hairline cracks that would be normal for concrete. Since yours have opened that indicates movement. So, unless you can stop the movement (very difficult to impossible) the cracks will reappear and may continue to grow over time.
Evaluating Concrete Crack Severity
Cracks smaller than 0.3 mm in width are generally not serious and of not structural impact. However, they should be monitored for any growth. Cracks of greater width than 0.3 mm may present structural durability issues.
You can repair hairline cracks in concrete with a grout made of Portland cement and water. Add just 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.
Your new layer needs to be at least 2 inches thick. If it's any thinner than that, it'll easily crack under pressure and won't adequately bond to your surface.
Apply a Sealant
Latex or silicone caulking is often a popular choice for crack filling, but the only downside is that the crack will still be visible once it's sealed. Epoxies and concrete sealers are other options, but the sealers will eventually wear off over time.
Luckily, most cracks are completely normal in all sorts of houses, even new builds, and are simply a sign that the house is settling. Other causes of cracks include change in temperature or humidity levels and vibrations from traffic if you live near a busy or fast road.
Cracks are present in every concrete slab, and they are unavoidable due to the rigid nature of concrete. Uneven drying, shrinkage and temperature changes can all cause fractures in your slab. To avoid water seepage and further damage, it's critical to seal any cracks that develop.
Some signs that may indicate that a crack is structural in nature may be that the crack is ⅛” or larger, a crack is horizontal in direction, the crack has shifted forward, one side of a crack projects out further than the other side of the crack, a wall is bowing, and there may be stair step cracks in the mortar ...
Although this definition doesn't give a numerical value for width (or require the viewer to wear bifocals to see the crack), it probably applies to cracks less than about 0.003 inch (0.08 mm) in width. Hairline cracks can be very shallow or quite deep, depending on the cause.
This means the concrete should be free of cracks, buckling, and breaks. If you notice any small cracks or minor issues, it's in your best interest to repair them before you pour the new concrete. Otherwise, your new concrete will have the same issues.
Small, thin cracks usually won't cause any problems and are better left alone. However, if your new concrete develops cracks more than ⅛” wide, it's better to seal them off from water with a high-quality concrete caulk so they don't get worse. You can repair cracks yourself, or hire a professional to caulk the cracks.
The thin concrete layer of overlays, like other thin materials, is more susceptible to cracking than traditional concrete surfaces.
Concrete shrinkage cracks are hairline and barely visible. If you see another kind of crack, it probably wasn't caused by shrinkage.
Hairline cracks form due to concrete settling while it's curing. They are usually very thin cracks, but they can run very deep in the slab, as well. Because hairline cracks run deep, they pose a risk for more serious cracking once the concrete has hardened.
Hairline cracks or cracks under 1/4-inch wide may not indicate serious underlying damage, but if water gets into them and repeatedly freezes and expands, and vehicles continue to drive over them, the cracks can get bigger and the problem can worsen. These cracks can usually be repaired.
Hairline cracks
They can appear during a home's normal settling process or after minor fluctuations in temperature and humidity. Hairline cracks can run in any direction and don't require immediate attention.
Some cracks in the foundation, including hairline cracks, are considered normal foundation cracks. These typically appear due to settling or concrete shrinkage. Other types of foundation cracks might indicate a serious structural problem.
Shrinkage Cracking
As the concrete begins to dry, the water within evaporates, and as it does, the concrete will begging to shrink. A concrete slab will shrink about ¼ inch for every 100 square feet of surface space. When concrete dries and shrinks, cracking can occur hours after being poured.
QUIKRETE® Concrete Bonding Adhesive (No. 9902) permanently bonds new concrete, plaster, and stucco to existing concrete, plaster, and stucco. Eliminates the need for roughing the surface before the application.
Moist curing is a common method of concrete curing. It involves wetting the concrete slab often with water (5-7 times per day) for the first 7 days. This method ensures your concrete slab will be extremely strong and durable, because it allows the moisture to evaporate slowly, preventing cracks and shrinks.