Advanced Structural Designs

  

ACN 097 789 87    92 Vasey Cres CAMPBELL ACT 2612

Telephone  61612171         Facsimile 61612170

Email   mal@structuraldesigns.com.au

 

 

 
                                                                                                           

                                                             

Why are there cracks in my driveway and who’s responsible?

 

This is a very common question as driveways often crack and it can be for a variety of reasons some of which are listed below.

 

*   Concrete Shrinkage

*   Plastic Shrinkage

*   Settlement cracking (settlement of concrete)

*   Excessive vehicular loading

*   Upheaval from tree roots

*   Trench settlement

Concrete Shrinkage

This is the cause of the majority of cracking in driveways.  Concrete in Canberra has a relatively high linear shrinkage of around 800 micro-strain.  This high shrinkage is due to the high shrinkage characteristics of some of our local sands.  Actual shrinkage of particular mixes will depend on aspects of the mix design such as water cement ratio and cement content.

 

In order to eliminate or control cracking it is necessary to provide:

 

*   Adequate jointing.

*   Adequate reinforcement near the surface to minimize the size of any cracks that do occur.  

 

In the case of driveways the minimum requirements are set out in AS 3727.  These include requirements to prepare subgrade and basecourse (where required) as well as requirements for adequate surface drainage.  This code also spells out minimum pavement depths and reinforcement as well as permissible jointing patterns and the ultimate resting place of this reinforcement.  This code is in direct conflict with AS3600 when it comes to concrete strength and cover, but in the case of residential pavements AS3727 takes precedence over AS3600. One disconcerting aspect of this code is that it mandates reinforcement requirements independent of the basic design parameter of concrete shrinkage.  It is worth noting that all rectangular slabs or slabs over 3m square require reinforcement and that the maximum allowable control joint spacing is 6m.

 

On occasions concreters claim to be so experienced and expert that they do not require bar chair supports for the reinforcement. We have had cause to test this theory more than once to find (to nobody’s surprise but the concreters) the fabric resting neatly on the dirt below the slab where it is useless.  Supporting reinforcement on bar chairs at minimum of 1000mm centres (we prefer 800mm) is mandated by the code).

 

If the work has been carried out recently and is cracking to over say 0.8mm in width we can come out with a cover meter and confirm that all the reinforcement is in and correctly supported and that joint patterns comply with the code.

 

Plastic Shrinkage

You will see more on this if you look up the relevant FAQ, but this refers to cracking that is relatively closely spaced and often random.  The responsibility for them lies with the builder because with good workmanship and proper precautions they should not exist.

 

Settlement Cracking

If you can see the pattern of reinforcement in cracks in the pavement it is likely there was no compaction at all in these areas.  The implication of this is that the compaction fails to comply with AS3600, a situation that can normally be proved by coring and testing.  This situation can be exacerbated by insufficient cover to the reinforcement which may have long term durability ramifications.

 

Excessive Vehicular Loading

Anyone allowing regular use of a 100 deep residential driveway by a vehicle over 3 tonnes is responsible for the damage it causes.  Even one pass of a 20 tonne vehicle such as a full concrete truck is enough to crack the pavement and this cannot be blamed on the builder.

 

Upheaval from tree roots

This can cause problems after a couple of years even with 150mm deep concrete pavements.  In the case of young trees, root guards can be placed alongside the driveway to prevent the roots growing under and lifting the driveway.  This can in turn affect the stability of the tree so it is important not to plant large trees too closely to driveways.

 

Trench Settlement

Settlement of poorly compacted trenches can cause severe cracking in slabs and no slab should be laid over poorly compacted trenches unless they are specifically designed to span over any potential subsidence.  Even boring under a driveway slab can lead to a small subsidence capable of damaging the slab.

 

Acceptable Performance Criteria

If your pavement has been down for about a year it may be found to be non code compliant due to its failure to meet the criteria for acceptable performance set out in section 5 of AS3727.

 

Some typical criteria are

Allowable

Crack widths

Relative surface levels of adjacent pavement

Chipping or spalling of concrete

Slip resistance

Ponding

 

Less than 1.5 mm

Less than 5 mm

None

To AS3661.1

No deeper than 10mm

 

 

Obviously there is a fair amount of fine print in the code so if you require an on structural engineer’s site assessment in Canberra ACT call Mal Wilson on Ph (02) 61612171.

 

New Work

You may be in the situation of building your own home and want to reduce the risk of any of the above problems by specifying a higher standard than is required by the code.  Give us a call if you need to know what additional measures represent good value for money.

 

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