Advanced Structural Designs


ACN 097 789 87    92 Vasey Cres CAMPBELL ACT 2612

Telephone  61612171         Facsimile 61612170







What are the potential consequences of my slab having been poured in the rain?


The first thing to note about this, is that pouring a slab when rain is forecast is a conscious decision to put the quality of final product at risk.  A moderate chance of prolonged showers may pose a higher risk to the final outcome than a high risk of intermittent light showers as the slab can be protected for short intervals providing adequate covering and manpower is at hand.  We suggest that concreters read their insurance policies carefully before making the final decision.


The risk with rain-affected slabs is normally that the surface of the slab will collect too much water during the finishing process and result in a relatively soft, weak surface that is prone to dusting. The depth of this weak surface is often only 2 to 5mm but can be as much as 10 mm in extreme cases.


This problem is extremely important in industrial applications where surface wear is an important serviceability consideration.  A weak surface can also cause problems in residential construction where direct stick parquetry or tiling can delaminate the top 1 to 3 mm of the slabs surface.  This problem is caused by differential thermal, shrinkage or expansion stresses and can sometimes occur in a matter of weeks rather than years. 


The extent to which the surface of the slab may be damaged during rain depends on


*   Intensity of the rain.

*   Timing of the rain relative to the initial and final setting of the concrete.

*   Efforts made to remove excess water.

*   Efforts taken to protect the work.

*   Whether vibration or power toweling took place


The time between pouring and initial setting of the concrete is generally around 2 to 3 hours and is the most critical in terms of ensuring a hard durable surface.  Every effort should be made to remove any additional water from the surface before finishing of the slab by rolling or dragging a hose over it.  If showers are intermittent it is sometimes possible to protect the work for a period to enable finishing in dryer weather.  Under no circumstances should cement powder be used to soak up the additional water.


More superficial damage can be done in the following 4.5 to 5 hours between initial and final set so protecting the surface before leaving the site is often a good idea.


If a builder wants to argue that the surface of the concrete is fit for purpose we generally score it with a coal chisel and compare the mark with a hard concrete surface.  This might not sound very scientific but there is no standard test for surface hardness.  Old fashion Schmidt Hammers used to give some indication if the problem was severe but over the last 10 years or so these have more of an indication of the density of the top 100mm.


To achieve high surface densities it is necessary to vibrate the concrete during placing.  Whilst this practice is universally accepted throughout most sectors of the building industry, residential builders often ignore this requirement as it is not expressly required by AS2780 unless the site is class H or E or if the slab forms part of the termite protection system. Further hardening of the surface is achieved during the power trowelling operation and you can expect weaker surfaces where this operation is not carried out.


How you fix the problem depends on the severity of the damage and the likely traffic and or coverings.  For example with superficial damage to a floor to be tiled we recommend water blasting at 3500 psi to remove the weak laitance and treating with a concrete hardener such as Construction Chemicals Concrete Hardener.


In similar cases where linoleum is the finished surface we often recommend the same high pressure cleaning followed by Adure K15 (self levelling floor underlayment) over Ardion P51 (primer).


We cannot responsibly recommend any treatment without first looking at the problem so for a full structural engineering assessment in Canberra ACT or the surrounding area call Mal Wilson  from Advanced Structural Designs on Ph 61612171 if you need a definite answer and full specification.  


PS If you need advice on rain-affected coloured concrete please call and discuss, as there are many more variables involved.


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