Why is there an ugly crack in my concrete???

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It’s the bane of every engineer and architect – and often a source of annoyance or concern for the owner of the property:  Cracks in concrete.  The questions start flying: “Why did my slab crack?”  “Could this have been prevented?”  And, more commonly – “Who is to blame?

With off-form-concrete suddenly flavour of the month again and being featured on many houses and commercial buildings, we now find concrete facades, walls, and polished floor slabs are turning up everywhere and are the focus of attention.   But what if you focus a little too closely and you see a crack?  What if the crack is so obvious, you don’t even need to focus?

The old adage about there being two certainties in life missed the mark.  There are actually three:  Death, taxes, and concrete will shrink.  It’s how we deal with or attempt to combat this shrinkage that will govern whether or not your concrete cracks in unsightly ways.   If you’re an architect or designer who specifies concrete pavements or floors, or if you’re a builder who deals with concrete on your projects, this article is specifically intended to assist you.

Let’s set some ground rules first.  Concrete is essentially made up of just three ingredients: Cement, aggregate and water.  (The aggregate is both fine and coarse, i.e. sand and gravel respectively).  As soon as concrete is poured, the hardening or curing process starts.  An exothermic chemical reaction is taking place in the mix, thereby giving off heat, which leads to the evaporation or loss of water.  As water molecules are lost in the mix, it leaves voids behind.  These voids, together with the drying process, lead to stresses, which induce strains.  Strain is just a fancy word for movement, but what it means is that as the slab dries out, it will shrink as a result.   The thrust of this article, then, is about how to handle drying shrinkage cracking.   (There are other forms of cracking, such as plastic settlement shrinkage cracking, but that’s a story for another day).

Engineers and builders attempt to control drying shrinkage by managing  how the concrete will move.  We consider the slab’s thickness and the amount of reinforcement required, and we introduce joints.  These may be straight joints, or key joints, or saw-cut joints, and we’ll look at the effectiveness of some of these in a moment.  However, the main objective here is to manage and control the size and shape  of each slab panel.   If a slab (or slab panel) is reasonably square, then shrinkage will be uniform in each direction, and the risk of a crack appearing is reduced.   However, if the slab panel has a high aspect ratio, where the long side is significantly longer than the short side (i.e. a rectangle, rather than a square), then obviously the shrinkage will not be uniform.   The amount of strain in the long direction is significantly greater than the strain in the short direction, and thus there is a very high probability the slab will crack – as illustrated below.

The first step in combatting shrinkage cracking is to therefore lay out your joints in a configuration that keeps each slab panel as square as possible, i.e. avoiding panels with high aspect ratios.  The rough rule of thumb is to keep your aspect ratios no greater than 1:1.3.

The second step is to keep the overall size your slab panels down to sensible limits.  Or, in other words, to keep your joints at reasonably close centres.   Until very recently, the industry typically adopted a maximum panel size of 6.0 metres.  However, after many years of observing that pavement slabs were continuing to crack, despite “best practice” being followed, the relevant Australian Standard (AS3727) was recently updated for the first time since 1993, and the maximum size of any concrete pavement panel has been reduced to 4.5m.

What this means is that architects and builders will have to start accepting that joints will be at closer centres and may not always work beautifully with the configuration of the pavement.   For example, a driveway into a double car garage might typically be 5.5m wide.   Until recently, it would have been permissible to simply run a horizontal joint perpendicular to the driveway.   With the change to the Standard in 2017, a compliant design will now have to introduce a joint up the middle of the driveway.

The next question comes down to what sort of joints to employ?  There are many options available, such as key joints, straight joints, tooled joints, dowelled joints, expansion joints, and saw cut joints.  Of all these, saw cut joints are the most misunderstood and most abused – and are thus the least successful – joint solution.

The intention of a saw-cut joint is to force the concrete’s hand and tell it precisely where you want it to crack.  Saw-cuts are typically cut into the top third of the concrete, so if a footpath slab is 100mm thick, the saw-cutter will set the blade and cut a 30mm deep slot or groove into the top of the pavement.  Generally speaking, shrinkage cracking will almost always occur at the weakest point, so by cutting a slot into the top surface (effectively severing the slab), you are creating a major weak point, and the crack should form in a beautifully straight line directly underneath the saw-cut.   Sounds good in theory, right?

The problem is that the window of opportunity in which to form a successful saw-cut joint is extremely narrow.  In fact, it’s scientifically known and accepted that, as soon as the concrete is poured into the forms, you have just a 6-12 hour window in which to cut the slab.  (In the heat of summer or on dry, windy days in winter, that window can reduce down to six to eight hours).  If you wait any longer than this, the concrete will already start to relieve its own stresses by itself, and cracking will occur in other places.  However, it’s a tricky thing to balance, because the concrete also has to be hard enough to allow the saw-cutter to roll his equipment across the slab without damaging it.

What this means is that if your concreter finishes brooming or trowelling off the slab by 12.00 noon, then the chap with the saw-cut blade has to be back on site sometime between 6.00pm and midnight to do all the saw-cutting!   But we know what happens instead, don’t we?  The saw-cutter turns up the next morning and starts cutting some time after 7.00am – half a day after he’s missed the window!  In real and true terms, his work is now completely obsolete and a waste of time.  By the next morning, the concrete will have already decided where to crack, and there is nothing you can do now to change, influence, or reverse that.

So what are the answers here?   The first step is to acknowledge the timing involved.  If you’re in a domestic situation (such as a house project) and the DA prevents you from making noise and mess on site at 10pm, then a saw-cut solution probably isn’t the best approach.  Consider using a tooled (aka grooved) joint instead.  This works on the same theory as a saw-cut, it’s just that the groove is tooled or pushed into the slab with a straight edge whilst the concrete is still reasonably plastic.  You’ll see these a lot on Council footpaths, as shown below.

The alternative is to replace your saw-cuts with Straight Joints or Key Joints.  Builders don’t like this because it’s more expensive and/or might force them to pour the slab in multiple pours on subsequent days.  No one thanks the engineer for that.  But no one gets any thanks when a slab cracks, either!  Pre-formed metal Key Joints (such as those produced by Connolly or Danley) at least allow the builder to pour both sides of the joint at the same time, but these products are both reasonably expensive off the shelf, and their use in aggressive environments is questionable, i.e. in Sydney’s eastern or harbour suburbs where airborne salts will accelerate corrosion to the steel form, despite it being lightly galvanised.

Of course, there are other issues and causes behind cracking in a concrete slab – and most of them are preventable.  It simply comes down to the builder and engineer working together, and the builder following the rules.  A classic example again is the use of tooled joints or saw cut joints.  In order for the joint to work successfully, it is important that the slab reinforcing mesh be cut each side of the joint.  It’s amazing how often this doesn’t happen.  If the mesh is continuous through the joint, it is harder for the concrete to shrink away from it, and a crack is likely to form elsewhere.   Alternatively, the concreter will correctly cut the mesh at the nominated locations, but then fail to mark on the formwork where he made these cuts.  The saw-cutter then turns up after all the concrete has been poured, and he doesn’t know where to cut!  If he doesn’t locate his saw-cut directly over the position where the mesh was cut, the joint will be ineffective.  How often do you see a crack appear in the concrete just 300mm or so from a saw-cut joint?  It’s frustratingly common.

The other common catalyst for cracking is a re-entrant corner.  A re-entrant corner creates a situation where the slab shrinks away from a corner in two orthogonal directions, which creates the high probability of a diagonal crack occurring.  You’ll have seen this many times.  (See below).   Engineers attempt to prevent the crack forming by specifying diagonal trimmer bars across the corner, but this is a “cross your fingers” approach, and – where possible – introducing a joint to eliminate the re-entrant corner would be a better option.

Finally, the last thing is to actually take steps to ensure the concrete is capable of moving and shrinking freely.  Most cracks in pavement slabs are actually caused because the slab “locked up” somewhere as it attempted to shrink.   It’s important that all parts of the slab are free and unrestrained, without any interruptions or obstructions such as an uneven base.  This is why engineers typically call up for the slab to be cast on to 20mm or 50mm of bedding sand.  The bedding sand helps the builder provide a smooth and level surface, which will enable the slab to move and shrink evenly.   If the surface you’re casting onto is uneven or has undulations, it’s possible for the slab to either lock up or for the undulation to act as a crack inducer.

A classic example of this occurring inadvertently is when engineers or builders provide an edge thickening around the perimeter of a slab.  Similarly, “old school” key joints were formed in timber, with the slab typically thickening from 100mm to 150mm to accommodate the timber key.   Whilst this was done with good intentions, the reality is that the concrete was restrained against the inclined soil and couldn’t shrink.   This then often resulted in a crack forming at the location of the thickening.  Infill slabs with edge thickenings sometimes behave the same way, as illustrated below.

So – as a community of professionals working together – how can engineer, architect, and builder work together to ensure the best result?  The answer is to talk and co-ordinate.  For architects, simply putting “Slab to engineer’s details” on your drawings isn’t good enough.  Take charge of the specification and liaise with your engineer to locate the joints.  Show the joints on your architectural plans, and co-ordinate with the engineer to make sure the engineering drawings show the same thing.  Prepare a specification that is appropriate to the site and the conditions.  If saw-cuts are  being employed, then the specification must  state that the joint is to be cut within 6-12 hours of pouring concrete.  Discuss this step and its timing with the builder, and if the builder feels this is unrealistic or improbable, change the specification to a different type of joint that the builder can successfully work to and achieve.

Much of the above discussion is focussed on pavements, driveways and footpaths.  We haven’t yet addressed internal floor slabs and the current fashion for polished concrete floors.  These are another beast again, with their own set of rules and pitfalls.  We’ll address these in a separate piece to be published down the track.

Until then, just remember that cracks in your concrete slabs should be preventable, provided everyone thinks about what they’re doing and then plans appropriately.  It’s not a case of leaving it up to chance or just assuming your concreter/builder will get it right.  There are guidelines and rules in place, and we’ve been pouring concrete slabs on ground for over 70 years, so we should all know how it will behave.  The trick is to be pro-active and to engineer the result.




Just say Your opinion.

  • Ray

    4 years ago

    Thanks Andrew, great article.
    There is an other think that is worth a mention.
    That is that a lot of contractors placing concrete tend to add more water to make it more workable, additional water usually results in more shrinkage, and and therefore more cracks.

  • Danny

    2 years ago

    YES!! Talking more, and coordinating better is what it’s all about. Thanks so much for this article, very well written. We need to work together to raise the bar so the standards of service are better!
    – Danny (S.A. Concreting Service)