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.