Why Hairline Cracks in Concrete ?
Hairline Cracks in concrete are extremely common but often misunderstood. Once an owner sees a crack in his slab or wall, especially if the concrete is relatively new, he automatically assumes there is something wrong.
This isn’t always this case. Some types of cracks are inevitable. Best that a contractor can do is try to control the hairline cracking.
This is done by properly preparing this subase, assuring that the concrete isn’t too wet, utilizing reinforcement where needed, and by properly spacing and placing crack control joints and expansion joints.
But, sometimes cracks happen in spite of any precautions taken.
In this ACI (American Concrete Institute) addresses this issue in ACI 302.1-04.
“Even with the ideal floor designs and proper construction, it’s unrealistic to expect crack-free and curl-free floors.
Consequently, every owner Ought to Be advised by both the designer and contractor that It’s normal to expect some amount of curling ans cracking on every project, which such occurrence doesn’t necessarily reflect adversely on either the adequacy of the floor design or the quality of its construction.”
Type of Cracks in Concrete
• Shrinkage Cracks in Concrete
• Hairline Cracks in Concrete
• Settlement Cracks in Concrete
• Temperature and Shrinkage Cracks in Concrete
• Vertical Cracks in Concrete
• Diagonal Cracks in Concrete
• Horizontal Cracks in Concrete
• Structural Cracks in Concrete
• Floor Cracks in Concrete
Shrinkage Cracks in Concrete
There are basically Nine types of everyday cracking. A first, and most common, is shrinkage cracking. When concrete is placed, it’s a liquid. You must keep it a liquid to get it to form the shape you want.
With the right water-cement ratio, you may get a liquid that flows. This makes it easier to push out of a pump or truck to an intended spot.
The amount of water added is tightly controlled. There’s a constant battle between the supplier that has promised the concrete will get into a certain hardness or strength, and the placement contractor whose job it is to get the wet gray material out to the middle of the slab and get it flat.
The supplier wants it drier because that helps achieve strength also placement contractor wants that mud wetter because it makes it much easier to get it into place.
The key point to understand in relation to cracking is that water is a certain percentage of the concrete mix. Any material which contains water will shrink as it dries and the water evaporates.
Concrete is no different. A typical 4″ (10 cm) slab will shrink at least 1/4 inch for every 100 square feet of surface space.
This shrinkage causes several issues. The two main ones are curling and breaking. In this article, we will only focus on cracking. As the shrinkage begins, the concrete will crack at which it is the weakest.
Cracking typically starts within 12 hours of this finishing process, but it may be slowed or accelerated by weather conditions. Shrinkage cracking is typically planned for and handled with control joints.
Control joints are designed cuts that go at least half-way through the thickness of the concrete slab.
All these are intended to cause weakness so that the concrete cracks along the bottom of this control joint, which releases the stress from the evaporating moisture. These joints are typically spaced evenly through a project.
I typically see control joint placement across a slab with the cuts forming squares, which are 10 x 10 or 15 x 15-feet wide.
If the spacing between these control joints is larger, even when the concrete follows the joint control pattern, you end up with extremely wide joints, which become harder to fill and maintain.
Over time with traffic, the edges of wide joints break easier, creating safety issues. Larger sections also have a greater chance to crack in unintended locations.
Hairline Cracks in Concrete
Hairline cracks can develop in concrete foundations as the concrete cures. Hairline cracks don’t cause problems with the stability of this foundation but do cause leakage problems.
If the cracks appear shortly after pouring a concrete foundation, concrete might have been mixed poorly or poured too quickly.
In poured concrete foundations, hairline crack frequently appears in the center of the walls because the wall corners have greater stability.
Settlement Cracks in Concrete
Settlement cracks might appear if the underlying ground hasn’t been compacted or appropriately prepared or when the subsoil wasn’t of the proper consistency.
A settlement crack could also appear as a random crack above areas where the soil of the subgrade was uneven after this concrete was poured.
Settlement cracks are generally more extensive in the top of the crack than the bottom as the foundation “bends” over a single point, allowing differential settlement.
This type of crack is usually continuous and might occur multiple times at a wall.
Temperature and Shrinkage Cracks in Concrete
Horizontal cracks found in the center of this wall are most likely due to an applied load like backfill around foundation compacted improperly or too soon, earth compacting as it settles, hydrostatic pressure against foundation due to the high water table and poor drainage against the foundation wall, or heavy equipment operated too close or too soon to the foundation wall.
Horizontal cracks found high up on the wall is most likely due to frost damage. Sometimes these fine cracks aren’t noticed for years. In a case such as this, a structural engineer should be consulted.
Vertical Cracks in Concrete
Vertical cracks often appear in multiples (multiple cracks in one or more areas). Vertical foundation cracks in poured concrete foundations that tend to appear nearly straight or to wander, normally even in width, intermittent, or more often straight is caused by shrinkage/thermal and are usually low risk.
When there’s significant vertical dislocation or signs of ongoing movement, you should consult a structural engineer. If the cause is shrinkage, it’s probably less of a concern than if because of settlement.
A vertical foundation cracks because of earth loading, or frost could be unusual. A more massive vertical crack may occur when the construction contractors incorrectly prepare a concrete footing, and when the wall had poor steel reinforcement as the workers poured the concrete to the wall. Cracks may also occur during the footing settlement.
Diagonal Cracks in Concrete
Settlement generally causes diagonal cracks, which are almost the full height of the foundation wall. Whenever there’s a settlement problem with footing on one side of the wall, this settlement may also cause a diagonal crack. In a case such as this, a structural engineer ought to be consulted.
If the diagonal crack is wider at a top than at a bottom, then it can be caused by expansion clay soil or frost damage. If the crack is wider in the bottom than the top, there’s likely a problem with the settlement beneath.
Some uniform diagonal cracks are merely caused by shrinkage and will only cause water leakage problems. Diagonal cracks that emanate out of a corner of a window and other openings are known as reentrant cracks and are normally the result of stress built-up in the corner.
Horizontal Cracks in Concrete
Horizontal cracks found in the center of this wall are most likely due to an applied load like backfill around foundation compacted improperly or too soon, earth compacting as it settles, hydrostatic pressure against foundation because of the high water table and poor drainage against the foundation wall, or heavy equipment operated too close or too soon to the foundation wall.
Horizontal cracks found high up on this wall is most likely due to frost damage. Sometimes these fine cracks aren’t noticed for years. In a case such as this, a structural engineer should be consulted.
Structural Cracks in Concrete
Structural cracks in residential foundations normally result from horizontal loading or settlement.
Many (but not all) structural cracks caused by an applied load ( heavy equipment or hydrostatic pressure around foundation wall) are nearly horizontal and appear 16″ to 48″ in the top of the wall. They’re more common in block foundation walls.
Floor Cracks in Concrete
Floor cracks in concrete slabs are relatively usually and common, not worrisome on a structural level. They ought to be repaired to prevent common issues like moisture, insects, and damage from seeping through.
When floor cracks leak, it’s usually caused by hydrostatic pressure or a high water table. Thus, sealing the crack will redirect those issues elsewhere, so fixing the source of those problems beforehand is recommended.
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