Concrete floors in industrial or warehouse environments must meet a range of performance requirements. One of the most common concerns is cracking. Certain cracks are primarily cosmetic, while others can grow into major defects, potentially affecting durability, serviceability, or—though less frequently—structural integrity.
This article, written by Tim Walker, Director of Face Consultants (NZ) and CoGri Limited (NZ), provides an overview of the most common causes of cracking in industrial slabs on grade, based on FACE Consultants’ decades of global expertise.
Plastic Shrinkage Cracking
What it Involves
This type of cracking emerges when the surface of new concrete loses water faster than bleed water can replace it while the concrete is still in a plastic state (i.e. before it sets).
This causes the surface to shrink and creates fine, shallow fractures.
Though typically superficial, these cracks can extend deeper if conditions allow or open more in the future due to long-term shrinkage.
Key Contributors
- Elevated temperatures and direct sunlight
- Low humidity and high wind speeds
- Low bleed rates
Common Appearance
Plastic shrinkage cracking often appears as fine “map” cracks or short random lines across the surface. Although mostly aesthetic, they can cause durability issues in some circumstances.
See figure 1.
Plastic Settlement Cracking
What it Involves
Plastic settlement cracks arise when fresh concrete settles around internal elements like reinforcement or embedded fixtures, causing small but noticeable lines directly above those obstructions.
While more common in suspended slabs or thicker elements like basement raft foundations, they can still occur in slab-on-grade construction under the right conditions.
Key Contributors
- Highly fluid mixes with excessive bleed profiles
- Congested reinforcement or large embedded elements (for instance PT ducts)
- Movement or vibrations during the initial set
Common Appearance
Plastic settlement cracking often presents as thin, straight lines over bars or around objects in the slab, highlighting areas of disrupted settlement.
See figure 2 below.
Thermal Cracking
What it Involves
Thermal cracking may occur during the hydration process as the concrete temperature can rise and the surface cools faster than the interior.
This differential thermal gradient creates tensile stresses within the concrete causing cracks.
Key Contributors
- High cement content or other mix design properties causing high heat of hydration.
- Thicker slab sections – note that you won’t expect to see thermal cracking in a typical warehouse slab on grade, but it might be a concern in a large ASRS raft foundation.
- Rapid drops in ambient temperature – as this will increase the thermal gradient between the hot internal slab temperature and the edges of the slab.
Common Appearance
Thermal cracks may be wide at the surface and can penetrate deep into the slab.
They frequently appear in straight lines, sometimes oriented perpendicularly to free edges or joints.
Early Age Shrinkage Cracking
What it Involves
Early age shrinkage cracking occurs in the first 24 hours from slab casting.
The shrinkage can be induced by water evaporation, autogenous shrinkage from the concrete hydration process, or due to temperature changes – i.e. the slab cools rapidly overnight. Restraint from the slab being tied into other structures, or from sub-base friction, will mean that the shrinkage creates tensile stresses that can lead to cracking. Note that the slab tensile strength will be low at an early age.
Key Contributors
- Mixes with high shrinkage potential (e.g., excessive cement content)
- Rapid cooling of the slab overnight due to cold weather
- Insufficient joint detailing or suboptimal joint spacing
Common Appearance
Look for cracks near edges, corners, and around columns, or in random patterns on the slab surface. These cracks can be slightly wider and more noticeable than typical plastic shrinkage cracks. A key diagnostic sign of early age shrinkage cracking is when cracks open directly adjacent to saw cuts in a slab – indicating that the slab cracked before the saw cut was installed.
Long-Term Drying Shrinkage Cracking
What it Involves
Concrete shrinkage continues beyond the early-hardening phase, often lasting 18-24 months.
This ongoing shrinkage will cause cracking if design and construction practices do not allow enough movement.
See figure 3.
Key Contributors
- Continual moisture loss from the slab’s surface over the first 18-24 months of the slab life
- Restraining the slab by tying it into other elements like foundations or columns
- Subbase friction
- Mix designs that inherently produce higher shrinkage
Common Appearance
Long-term drying shrinkage cracking often develops gradually and can grow in width over time if unaddressed. Common sites include re-entrant corners, mid panel or 1/3rds of slab panels (depending on aspect ratio) and at areas of restraint such as columns or walls that the slab is tied into.
See figure 3.
Settlement or Deflection-Induced Cracking
What it Involves
When the support beneath a slab (often the subgrade or base layer) settles or deflects, the slab may rotate or bend.
This can create tensile forces on the upper surface, resulting in cracks.
Key Contributors
- Inconsistent sub-base/subgrade conditions with differential settlements
- Voids beneath the slab from erosion or washouts
- Having piles or piled foundations in some locations under the floor (these will not settle), while most of the floor is ground supported and subject to settlement.
Common Appearance
Settlement or deflection-inducing cracks will occur over areas where the slab rotates or “hogs” at the surface; for instance over a stiff ground beam that doesn’t settle comparative to the rest of the slab area. The severity of the crack will depend on the level of differential settlement or deflection, and these cracks can combine with long term drying shrinkage to open further.
See figure 4 below.
Load-Induced (Structural) Cracking
What it Involves
Although most warehouse floors rely on thickness, subbase support, and nominal reinforcement to carry loads, excessive loading or weak points in the subbase can cause structural cracking.
Heavy traffic, racks, and machinery all contribute to the demands on the slab.
Key Contributors
Loads exceeding design capacity – in our experience this is most common with heavy vehicle loads or in bulk storage warehouses
- Inadequate slab thickness or reinforcement – most common in steel fibre reinforced slabs as the steel fibre suppliers will often offer free design advice that does not consider all of the floor’s requirements or try to promote thin/low dosage designs with limited safety factors to win contracts.
- Weak subgrades – for example, where the slab should have been piled-supported but wasn’t in an attempt to reduce costs.
Common Appearance
Structural cracks often develop where bending stresses are highest —
commonly in mid-slab spans, at supports, or slab joints and corners.
They can widen with repeated loading cycles and then cause new cracks to form.
See figure 5.
The Role of Restraint and Shrinkage
Concrete slabs shrink as they dry and cool, but if they’re locked into position—whether by internal reinforcement or external elements like columns and walls—tensile stresses will develop.
Such restraint intensifies cracking:
- Internal restraint: Reinforcement, or variations in curing across sections
- External restraint: Fixed edges, dowelled joints, or tied-in walls
- Subgrade friction: Large slabs may resist movement due to friction against the ground
By reducing or eliminating restraint you will go a long way to eliminating cracking. In my next article on cracking, I will discuss mitigation – best practices for avoiding these common types of cracks—from proper mix selection to ideal curing procedures and effective slab design.
Understanding the Importance of Cracking in Industrial Flooring
Industrial concrete floors can develop cracks from diverse factors, including plastic shrinkage, thermal fluctuations, subgrade issues, and direct loading.
In most cases, the type and pattern of cracking point to an underlying root cause
Some cracks may not compromise performance, but others necessitate closer attention or repair. Cracks in slabs are largely avoidable, and in our follow-up article we will discuss mitigation strategies.
Contact us today
If you have cracks in one of your projects that need a diagnosis or repair strategy, or you are sick of having cracking and would like to avoid it on your next slab – reach out to your local FACE Consultants office as we can definitely assist you and your warehouse floor.
Tim Walker
Tim Walker is the Director of Face Consultants (NZ) and CoGri Limited (NZ). He has two decades of experience in the industry and provides the link between the Logistics Industry and Property and Construction; matching the requirements of one with the capabilities of the other.Contact Tim and his team today or follow us on LinkedIn:
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