Surface Blistering or efflorecence on concrete surface

Advanced Structural Designs



ACN 097 789 87   92 Vasey Cres CAMPBELL ACT 2612

Telephone 61612171         Facsimile 61612170

Email   mal@structuraldesigns.com.au

Blistering or Efflorescence on the Concrete Surface

During the Power Trowelling Operation

Background

Concreters generally screed and bull float the concrete and wait for the bleed water to dissipate before they attempt to power trowel the surface of the concrete. In normal conditions this often results in the power trowelling commences as soon as the concrete is capable of supporting the machine.

In hot, dry or windy weather when evaporation rates are above 1.0 litre per m^2 per hour it is often difficult to see the bleed water as it evaporates as quickly as it arrives and it is easy to misread the correct time to finish the surface. The situation can be deceptive, as the rapidly drying surface is normally hard enough to support the power trowel machine (i.e. helicopter) well before all of the bleed water has reached the surface.

When concreting in hot weather it is extremely important to commence finishing the concrete at the correct time. This is because commencing the finishing too early seals the surface of the concrete before all of the bleed water has risen to the surface. This situation often results in small blisters of concrete around 20 to 30 mm in size and 1 mm deep (sometimes referred to as laitance) peeling off during the power trowelling operation. In some cases you will see larger delaminations but the depth is always around 1 to 3 mm. Another problem can be the discolouration of the surface of coloured concrete as the bleed water brings deposits of calcium carbonate and other salts to the surface.

Using Aliphatic Alcohol

In circumstances where evaporation rates are high and pouring must proceed we recommend the use of aliphatic alcohol to control the evaporation of bleed water. The alcohol is generally applied after the screeding and bull floating operations. When the alcohol is applied the bleed water can be seen to build up under the surface of the aliphatic alcohol. The primary reason for its use is to prevent plastic shrinkage from occurring but it can also help with the finishing of the concrete by preventing the surface becoming too dry.

After the bleed water appears to have dissipated, the surface of the concrete should be poked with a finger to see if there is more bleed water just below the surface. This water may be as far as 5mm or more below the surface. If more bleed water is present, this normally indicates that the aliphatic alcohol has evaporated or runoff a sloping pavement and more needs to be applied. If there is no bleed water beneath the surface it is generally a sign that power trowelling may commence.

Other options for controlling evaporation such as fog sprays, wind breaks and shading should also be considered where they can be achieved cost effectively.

Early Start

If the concreter has started the power trowel operation too early it is sometimes possible to flatten the angle of the blades to help release the bleed water and to steepen the angle on subsequent passes where a burnished surface is required in the specification. It is far preferable of course is not to start power trowelling to early.

Mix Design

High amount of cement paste, fly ash, oxide colouring, fine sand or even entrained air in the mix can contribute to the rapid sealing of the surface during the power trowelling operation and it may be possible to reduce one or more of these by adjusting the mix design. The use of a dry shake to colour the concrete can be particularly troublesome, as it will tend to seal the surface and soak bleed water at the same time.

If the mix is using a super-plasticizer the problem can often be severe as the bleed water has a tendency to arrive late and fast. Whenever a super-plasticizer is used the concreter needs to be involved in the decision and informed as to the likely behaviour of the mix as he may need to wait longer and have more finishers available.

The Weather

When forecasts indicate evaporation losses higher than about 1.2 l/m^/hr it is generally preferable to postpone the pour until the weather is more conducive to attaining a high quality finish. Below is a chart (based on ACI 305) that you can use to estimate likely evaporation rates for any given relative humidity, air temperature concrete temperature and wind speed.

Seriously consider wetting down the surrounding area where dust is a problem as dust landing on the slab can rapidly soak up any bleed water and contribute to the problem. Also ensure that the sub-grade beneath the slab (if pouring slab on ground) has been soaked but has no free water on the surface. This will provide more bleed water to the surface and help with the finishing.

In a Canberra summer concrete temperatures at delivery rarely go above 30 degrees Celsius (typically only 3 to 5 days) but you could probably add 2 degrees for concretes carrying over 5% black oxide as the heat gain in the sun can be significant. We have a copy of CSR plot of air temperature and concrete temperature if you are interested, but your supplier should have batching temperatures of your mix.

With the high moisture retention rates being exhibited by some powered oxides (they are not all the same), there is even an argument for limiting pouring to days where evaporation rates are less than 1 l/m^2/hr when pouring coloured concrete. Any such decision would of course need to fit within programming constraints.

All of the above information relates to blistering in hot weather which is when the problem is normally at its worst in Canberra. If you are having this problem in cooler weather ignore the above advice and call to discuss.

The above is but a brief summary of what is a very complex topic, so if you have any further queries or need an on site appraisal in Canberra ACT by an experienced structural engineer please do not hesitate to call Mal Wilson from Advanced Structural Designs Ph 02 61612171.