Hairline cracks in basements, foundations, and concrete are caused from settline, shrinking, or movement. All of which are very normal. Hairline cracks are not very wide, but some hairline cracks can be very deep and go completely through the basement, foundation, or concrete area. While hairline cracks may appear harmless, they could eventually, if not already, be letting in radon, moisture, insects, and water.
The best repair method for hairline cracks is a high pressure polyurethane resin that uses aluminum injection ports. The aluminum injection ports are injected into the concrete and are meant to intercept the crack. The high pressure from the gun pushes the polyurethane resin deep into the crack. The polyurethane resin comes into contact with water (if you have a dry crack, simply add water) and expands 30-40X to fill the crack from the back of the crack to the front of the crack. This is the only type of repair method that will actually repair a hairline crack. When the basement, foundation, or concrete shrinks, settles, or moves again, the material will flex enough to prevent further cracking but it will maintain its bond to the concrete to keep the already repaired crack repaired. Learn more about the Foundation Armor concrete crack repair kit.
Other cracks use low pressure and plastic ports that sit on the surface. There is absolutely no way for the material to travel far enough into the crack to actually repair kit. With these types of kits, you are left with a partially repaired crack. Eventually, the material will be pushed out of the crack or a new crack will form around the old crack.
Once the hairline cracks have been repaired, seal your basement, foundation, concrete with a deep penetrating sealer. A sealer like the Foundation Armor concrete sealer will penetrate up to 4-6″ to form a crystalline structure beneath the surface. The concrete will become up to 45% stronger to help prevent the concrete from cracking.