WHY IS MY CONCRETE SLAB CRACKING?: INFORMATION FOR HOME OWNERS AND ENGINEERS

You’ve been planning your new home build for months, you’ve signed all the documents, lined up your finance and the construction of your homes foundation slab is underway. As an excited new homeowner you decide to pay your block a visit and look at the freshly placed concrete, but it doesn’t look quite right…there looks to be visible cracks across the surface. “She’ll be right” you can hear the site foreman approaching from the morning smoko break, he continues, “concrete cracks… it’s what it does” . But why is your concrete slab cracking?…

Water is Wet, The Sky is Blue, and Concrete Cracks

It’s one the unfortunate facts of life. Concrete is an inherently brittle material and so it is prone to cracking. In fact, structural engineers expect concrete to crack and factor this into our designs. The issue is when the cracking becomes excessive and can either cause the structure to not perform in a satisfactory manner in the long term or be a sign that the structure is in destress and things are going worn.

Should I be Worried about my Concrete Cracks?

The short answer is, it depends. The long answer is; depending on how large the cracks are in your concrete slab, cracking may not be a major concern, this is referred to as its crack width. It also depends on the environment that your structure is in. The aim of the game here is ensuring that the reinforcing bars within the concrete slab do not corrode and that the structure lasts for its intended design life (which if designed correctly to the concrete design code for buildings, should be 50 years).

Therefore, an acceptable crack width for a concrete slab in a garage may not be an acceptable crack width for a swimming pool.

Concrete cracks can be unsightly and reduce the lifespan of your structure.

Significant structural cracks were found in the FIU bridge in Florida prior to its collapse. At the time the cracks were incorrectly assumed to be non-serious in nature by the site engineer. To read the full story and discover why the FIU collapsed from a Structural Engineering perspective, take a look at THIS article.

If a crack is evident it is a good strategy to monitor the crack over time. If a crack is of an acceptable crack width and it is a “static crack” (does not appear to become worse over time) then you can be pretty sure that the crack is not serious. Below is a guidance table which shows acceptable crack widths dependant on where the concert element is located. Repairs will need to be undertaken if crack widths are wider than specified here.

Location of Concrete ElementAcceptable Maximum Crack Width
Internal within a building sheltered from the weather0.3mm (0.012 inch)
Externally to a building more than 1km from coastline0.3mm (0.012 inch)
Externally to a building within 1km from coastline0.25mm (0.0098 inch)
External to building in a salt-rich arid zone0.20mm (0.0079 inch)
Permanently submerged in water (freshwater)0.20mm (0.0079 inch)
Submerged in water (chlorinated swimming pool)0.00mm1. (0 inch)
Table of acceptable crack widths for various locations of concrete element
  1. Swimming pools are generally treated with a waterproofing membrane. If cracking is evident in your swimming pool, it doesn’t just mean you concrete structure has cracked, it also means that you membrane has failed also. While it is normal for the concrete pool structure to have very thin crack widths, the waterproofing membrane should be flexible enough and not open up along with the underlain concrete. The structure supporting your chlorinated swimming pool should be totally protected by your waterproofing membrane at all times.

These are pretty small measurements, you may be asking “how do I measure a fraction of a millimetre on an inch?” that’s where a crack measuring gauge comes in handy for this exact application…

A crack width measuring gauge helps to determine if your crack widths are acceptable or not.

Concrete isn’t all that bad, it does have great self-healing characteristic. In some cases, very small cracks can self-repair overtime in the concrete structure…

What is Autogenous Healing?

Autogenous healing in concrete allows it to self-heal thin cracks

Concrete is a combination of course stone and sand particles (called aggregate) mixed with cement; a powdered blend of clay and limestone. When water is added to the mix the cement begins to harden and bond with the aggregates, the reaction of the cement and water mix is called Hydration.

Then a crack forms in the concrete and water enters this crack, calcium hydroxide is formed. When the calcium hydroxide is then exposed to the atmosphere, it reacts with carbon dioxide to form calcium carbonate crystals. These crystals expand to fill the crack and heal the concrete, this is called Autogenous Healing.

Because this form of healing requires the presence of water and air, Autogenous Healing usually occurs in structures exposed to the weather, such as bridges, balcony slabs or roof slabs. This phenomenon is also very common in concrete water pipes. You can tell when Autogenous Healing has occurred when the cracking in a concrete element takes on a white appearance (this is the calcium you can see as part of the chemical reactions mentioned above).

Autogenous healing can be spotted where cracks have turned white, this is the visible calcium which forms as part of Autogenous Healing in concrete.

Reason 1: Poor Water Drainage Around your Concrete Slab

You need to keep the soil moisture level as consistent as possible for the life of the house.

Excessive wetting and drying of the soil may cause shrinking and swelling, especially in highly reactive soil types. This movement may be excessive enough to overstress your slab and house foundation resulting in cracking. While seasonal changes in weather and rainfall are unavoidable, these effects are magnified if the surrounds of your house are poorly drained.

A great place to start is to make sure that the the ground level external to your house slopes away and downwards to allow rainfall runoff to accumulate a distance away from the house and its foundations.

cross section view of sloping soil towards the edge of a house slab foundation, this causes poor performance of your foundation
Poor drainage next to your property such as the ground level sloping towards your house could cause water ponding and subsequent excessive movement to your foundations.
cross section view of sloping soil away from the edge of a house slab foundation, this improves the long-term performance of your house foundation
Draining the ground away from your house ensures that the moisture content of the soil remains more consistent throughout the seasons ensuring that your house slab and foundation perform better.

For even better protection, it is very good practice to border your house with a paving slab to further prevent moisture from accumulating adjacent to the foundations.

Plan view of a house footprint with a continuous paving slab around the perimeter falling away from the house.
Plan view of example house footprint with perimeter pavement indicated, this is very good practice especially when located in reactive clayey soils.
cross section view of sloping a paving slab away from the edge of a house slab foundation, this improves the long-term performance of your house foundation
Cross section view at edge of house, for added moisture control next to you foundations, include a perimeter pavement slab draining away from you property.

Reason 2: You have a Large Tree Planted Next to your Concrete Slab

Large trees with thick and vast root systems can also effect the moisture level for the foundations of you house slab. Large trees should be planted well clear of you property. You should consult with an arborist prior to planting any substantial trees on your block. Generally speaking, the root system can stretch at least the same distance from the tree trunk as the above tree canopy. Advice from an arborist on the likely size a tree will grow and its canopy size will help you in planning your landscaping to ensure that it doesn’t adversely affect your house foundations.

Cross section view of the damaging effects a large tree planted close to your property can cause
Cross section view of the damaging effects a large tree planted close to your property can cause

So what about our future homeowner at the beginning of this article? The house slab and foundations were freshly poured and the concrete had not long hardened, why is that slab cracking? This leads us to the wonderful world of how concrete behaves while it is being placed, while it is being cured and while it begins its service life supporting your house or building…

Plastic Shrinkage Cracks

Plastic shrinkage cracks are caused when the exposed surface of the concrete element dries too rapidly while the concrete is setting, this may be caused by:

  • Strong windy environment
  • High ambient temperatures
  • Low humidity environment.

The above issues increase the rate of evaporation of the water within the concrete mass. The cracking occurs when the hydration process (which is the reaction when water mixes with cement and it begins to set) does not occur evenly across the thickness of the concrete member, this causes internal stresses which results in this form of cracking. If plastic shrinkage cracks have occurred this means that the builder has not properly cured the concrete element while it was setting.

Concrete curing is the application of surface measures to an exposed concrete face while it is setting to ensure that rapid water loss through evaporation does not occur. Methods of concrete curing may include one of the following:

  • Continuously misting/fogging the concrete surface for a specified period (spraying with water)
  • Applying a damp cloth to ensure moisture is retained (hessian bags can be used, however they need to be applied to the concrete surface once the surface has set enough to ensure that the bags do not damage the finish of the concrete).
  • Laying plastic sheets on the exposed concrete surface to reduce evaporation caused by wind.
  • Application of chemical curing compounds. The chemical curing compound forms an impermeable chemical barrier to prevent moisture loss through evaporation.

Plastic Settlement Cracks

Plastic settlement cracks occur when the concrete mass starts to consolidate when the concrete is partially set. When the concrete consolidates (settles) and the reinforcing bars don’t, this causes cracks to form above the reinforcing bars and voids to form immediately bellow the reinforcing bar.

Plastic settlement cracks are identifiable by a cracking pattern which resembles the arrangement of the reinforcing bar within the concrete mass.

Plastic settlement cracks are identifiable by a cracking pattern which resembles the arrangement of the reinforcement within he slab
Plastic settlement cracks are identifiable by a cracking pattern which resembles the arrangement of the reinforcement within he slab

Plastic settlement cracks are caused because the builder has not adequately compacted the concrete while he/she was placing it. This may have left tiny air pockets within the concrete mass, which over time while the concrete is curing, begin to close up which causes this settlement to occur.

Shrinkage Cracking

During the hydration process, the concrete begins to dry as the water is both taken up by the cement paste and also drawn out of the concrete by the curing process.

The drying of the concrete causes shrinkage to occur, if the slab is restrained at multiple locations (preventing it from shortening) the shrinkage can result in cracking across your slab.

Shrinkage cracking may occur due to one of two reasons:

  1. The Structural Engineer has not detailed adequate temporary movement joints to allow the concrete to shrink while its being cured which causes the slab to be restrained at multiple locations.
  2. Inadequate crack control reinforcement may have been provided within the slab, particularly around areas such as re-entrant corners.

Lets take a look at each example in a little more detail…

For the first example, lets consider an existing house slab shaped like a reverse “C”. The home owners want to make an extension to their house by filling in the open area inside the “C”. The new slab (shown in green) and the old slab (shown in black) need to be structurally tied together to prevent differential settlement and cracking in the floor tiles within the house. The Structural Engineer has specified bars to be drill and epoxied into the side of the existing house ground beams, then the new slab poured against it.

As the new slab begins to cure and shrink through drying, the slab dries towards the centre of its volume (a focal point where the direction of slab tends to move towards due to shrinkage). The connection to the existing slab however prevents this movement from recurring and restrains the slab. Something has to give in this situation and unfortunately it is cracking of the new slab while it sets.

The cracking can be prevented if a temporary movement joint is detailed where the old and new slab meet (allowing the new slab to freely shrink as it sets) then once the majority of the shrinkage has occurred, the joint is locked so either slab can not move independently (protecting the floor finish within the house as intended).

Example of how shrinkage cracking can occur in a slab if it is restrained on two or more sides.
Example of how shrinkage cracking can occur in a slab if it is restrained on two or more sides.

The second example is cracking at a re-entrant corner of a ground slab. A re-entrant corner is an inside corner that forms an angle of 180o or less at a slab perimeter. For this example we will consider a reverse “L” shape slab for a brand new house. The geometry is a little more complex than a simple rectangle, the shrinkage behaviour will tend towards two separate shrinkage focal points. The boundary where these two focal points are pulling the slab in two different directions is where the re-entrant crack will occur.

Example of how a re-entrant corner crack can occur if inadequate reinforcement is provided at this location.
Example of how a re-entrant corner crack can occur if inadequate reinforcement is provided at this location.

This form of cracking can be prevented by providing specific re-entrant corner reinforcement to stop this edge from opening up like a zipper…

Re-entrant corner reinforcement prevents this edge from cracking due to shrinkage.
Re-entrant corner reinforcement prevents this edge from cracking due to shrinkage.

For more on construction joints and temporary movement joints in slabs, take a look at THIS article which details how to structural check critical construction joints.

Thermal Cracking due to Heat of Hydration

Cracking caused by heat of hydration is a form of cracking that is quite rare in the residential space however can occur more frequently on larger scale high-rise buildings.

When hydration occurs (the reaction between cement and water), heat is released. In very thick concrete members the outer layer of the member closest to the atmosphere cools quicker and acts as an insulator for the hotter internal core of the member. Temperatures within the member can reach heights up to 45-60o C (113 – 140o F). It is the differential in heat between the outer layer and the inner core that causes internal stresses and potential cracking to occur. This is because the different temperatures of concrete are moving at different rates due to thermal expansion and contraction.

Thermal cracking caused by heat of hydration starts to become a concern for concrete members 1.5m (3.3 foot) thick or more and with concrete mixes with high cement ratios (generally high strength-concrete). Areas where 1.5m+ thick concrete members may exist in a high-rise building include transfer beams or core box raft foundations.

pre-pour photo of a very deep lift box raft foundation
Cracking caused by heat of hydration is more common in very thick concrete members such as this raft foundation beneath a high-rise lift/stair box.

There are techniques in reducing the chance of thermal cracking induced by heat of hydration which all aim to lower the overall heat of the concrete and/or the heat differential across the thickness of the member:

  • Use low heat cement if possible
  • Use “cool” concrete mix, this may include using chilled water in the mix, cooling the aggregate prior to mixing etc.
  • If possible, construct the member in two stages
  • In extreme cases, embed water pipes into the poured concrete and continuously pump cooled water through the system during curing. These pipes are then left imbedded in the concrete member once its fully set.

Corrosion of Steel Reinforcement within Concrete Slabs

Cracking caused by rusted reinforcement is usually a result of an initial smaller crack which existed and allowed the reinforcing bar to rust in the first place. This can also be caused through in-sufficient cover to the reinforcement (the distance from the bar to the surface of the concrete).

When steel corrodes, it can expand up to 7 times its original volume through the chemical processes which causes it to rust. This expansion, when confined by concrete, can cause Oxide Jacking (sometimes called rust burst), as the outward pressure from the steel expanding pushes out the surrounding concrete which can cause significant cracking and large sections of concrete to spall away.

Alkali Aggregate Reaction

Alkali aggregate reactions within Australia generally occur through that of Alkali Silica Reactions (ASR) due to the aggregates use locally in this region.

ASR is a chemical reaction which occurs within the concrete. The reaction occurs between alkali hydroxides an silica minerals. This reaction produces a silica gel which expands when produced and further expands when exposed to additional moisture. As the expansive gel grows within the setting concrete, internal pressure builds up which eventually results in significant cracking.

Aggregates used in Australia which may contain the form of silica prone to causing ASR within concrete include:

  • Basalt
  • Rhyolite
  • Dacite
  • Granite
  • Quartzite

Damage caused by ASR cracking is usually identifiable by an expansive network of interconnecting cracks (with an appearance resembling a web).

Cracking caused by Alkali Aggregate Reaction can appear as a vast network of cracks looking like a web.
Cracking caused by Alkali Aggregate Reaction can appear as a vast network of cracks looking like a web.

Methods to reduce the risks of ASR include:

  • Use low alkali cement blends
  • Where possible, use aggregates with lower reactivity and presence of silica.

Structural Cracks

Cracking can also be caused by structural forces within the concrete caused by applying a load to a spanning member, these are called structural cracks.

Structural cracking presents as either flexural cracking or shear cracking. Cracking of spanning concrete members is expected and factored into the Structural Engineering design. However concrete design codes provide guidance on controlling flexural and shear cracking, the intention being that while the concrete section may crack, the crack width shall not be excessively large to ensure durability of the structure and not cause unsightly visual cracks which may alarm occupants of a building.

The design codes do this by placing restrictions on the allowable tension stress which can be placed on a reinforcing bar which therefore reduces its elongation and therefore reduces the resultant crack width which is generated.

Magnified cross section of a concrete spanning member with tension in the bottom layer reinforcement resulting in concrete cracking (flexural cracking).
Magnified cross section of a concrete spanning member with tension in the bottom layer reinforcement resulting in concrete cracking (flexural cracking).

Flexural cracking in reinforced concrete members are vertical cracks which occur in the tension zones of the section when the member undergoes bending. The crack extends from the surface of the member to the internal neutral axis. The crack widths are widest at the highest tension zones (mid-span at the soffit or directly over a support on the top surface of a continuous or cantilevered member) .

Shear cracks occur at areas of high shear within a concrete member (usually near a support) and are distinguishable from flexural cracks as they are usually diagonal in nature.

Visual representation illustrating the appearance of flexural cracking and shear cracking.
Visual representation illustrating the appearance of flexural cracking and shear cracking.

How to Repair Concrete Cracks

There are many different products on the market for repairing cracked concrete, this is a reflection on how prevalent concrete cracking is. Common manufactures of such products in Australia include Ardex, Sika and Parchem.

The approach to repairing the cracks will be dependant upon:

  • Weather the crack is “static” (not getting worse over time) or “active” (opening up more over time)
  • If the crack is causing structural integrity concerns
  • If the crack is causing more aesthetic and durability concerns rather than structural integrity concerns.

The following list is not an exhaustive list on crack repair methods however covers the most common approaches:

  • Applying a Coating Over Cracks: If the cracks are mainly posing a visual concern, an applied finish (painted or otherwise) can be used to cover the cracking. This approach works for thinner crack widths that are likely to not move further over time (static cracks). You will need to consult the manufacturer of the finish to ensure that their product has crack bridging capability for this application.
  • Apply Flexible Sealant: For cracks that are active and likely to continue to move over time, a flexible sealant should be used. The reinforcement within the concrete needs to be protected for long-term durability and the crack repair needs to be able to flex and move with the continuing movement of the crack itself. Flexible sealant products are usually silicone based and are specialised for this type of application.
  • Resin Injection: This form of crack repair requires specialised equipment and epoxy resin to repair the cracks. High pressure injecting equipment is used to ensure that the cracks are adequately filled and the resin penetrates the concrete mass effectively. This method of repair if effective at re-gaining structural integrity to the effected concrete.
  • Stitching of the Cracks: This form of crack repair is used if the tensile strength of the concrete needs to be fully restored. It involves providing “U” shaped staples which straddle the crack. The staple is drill and epoxied either side of the crack and provides an alternate load-path for the tension within the concrete which has been compromised by the crack.

Conclusion

Cracking is a common occurrence in concrete and needs to be minimised as much as possible to ensure that the structure lasts as long as intended.

If you are a home owner and in doubt about the cracking and condition of your structure, you should seek guidance and advice from a qualified Structural Engineer on the best approach to take for your situation.

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Quentin Suckling is a full time practicing Structural Engineer based in Melbourne Australia. He has been practicing in the local market at tier 1 engineering consulting firms over the last 16 years.

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