Hairline Cracks in Concrete: Causes, Types, and Management

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 in walls are inevitable. Best that a contractor can do is try to control the hairline cracking.

Understanding the crack hair meaning can also help in assessing the situation. 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 are hairline cracks in new concrete normal and it’s normal to expect some amount of curling and cracking, including hairline crack in new concrete, on every project.

Such occurrences don’t necessarily reflect adversely on either the adequacy of the floor design or the quality of its construction.

Types of Cracks in Concrete 

1. Shrinkage Cracks in Concrete.
2. Hairline Cracks in Concrete.
3. Settlement Cracks in Concrete.
4. Temperature and Shrinkage Cracks in Concrete.
5. Vertical Cracks in Concrete.
6. Diagonal Cracks in Concrete.
7. Horizontal Cracks in Concrete.
8. Structural Cracks in Concrete.
9. Floor Cracks in Concrete.

1. 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.

2. Hairline Cracks in Concrete

Hair cracks in concrete can develop in concrete foundations as the concrete cures. Hairline cracks in new concrete slab don’t cause problems with the stability of this foundation but do cause leakage problems.

It’s essential to know the hair crack meaning to assess the severity. 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.

3. 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 types of crack is usually continuous and might occur multiple times at a wall.

4. 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.

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.

5. 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.

6. 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.

7. 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.

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.

8. 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.

9. 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.

FAQ: Hairline Cracks in Concrete

Are hairline cracks in concrete normal?

Yes, hairline cracks in concrete are normal and common. They typically occur due to shrinkage as the concrete cures and do not usually indicate a structural problem.

What causes hairline cracks in new concrete?

Hairline cracks in new concrete can be caused by shrinkage during the curing process, improper concrete mix, rapid drying, or external stresses like temperature changes or settling of the underlying soil.

Can hairline cracks in concrete be prevented?

While it is not possible to completely prevent hairline cracks, they can be controlled by proper preparation of the subbase, ensuring the concrete mix isn’t too wet, using reinforcement where necessary, and properly spacing and placing control and expansion joints.

Should I be concerned about hairline cracks in my concrete slab?

Hairline cracks in a concrete slab are rarely a cause for concern. They are usually superficial and do not affect the structural integrity of the slab. However, if the cracks are unusually wide or appear to be growing, it may be worth consulting a professional.

What are control joints and how do they help?

Control joints are pre-planned cuts in the concrete that go at least halfway through the thickness of the slab. They are designed to control where the concrete will crack due to shrinkage, thus preventing random cracking and directing cracks to less noticeable locations.

What types of cracks are common in concrete?

Common types of cracks in concrete include:

1. Shrinkage cracks
2. Hairline cracks
3. Settlement cracks
4. Temperature and shrinkage cracks
5. Vertical cracks
6. Diagonal cracks
7. Horizontal cracks
8. Structural cracks
9. Floor cracks

What are settlement cracks and why do they occur?

Settlement cracks occur if the underlying ground was not properly compacted or prepared, or if the subsoil was uneven. These cracks often appear as random, continuous cracks above areas with uneven subgrade.

How can temperature affect concrete cracking?

Temperature changes can cause concrete to expand and contract, leading to temperature and shrinkage cracks. These are usually horizontal cracks found in the center of the wall and can be caused by loads such as backfill, poor drainage, or frost damage.

What should I do if I see structural cracks in my concrete?

If you notice structural cracks, especially if they are horizontal and caused by applied loads like heavy equipment or hydrostatic pressure, it is important to consult a structural engineer. These cracks may indicate more serious issues that require professional attention.

Can floor cracks in concrete slabs cause problems?

Floor cracks in concrete slabs are common and generally not structurally concerning. However, they should be repaired to prevent issues such as moisture infiltration, insect infestation, and further damage. If the cracks are leaking, addressing the source of moisture is crucial before sealing the cracks.