Advanced Structural Designs

ACN 097 789 87 92 Vasey Cres CAMPBELL ACT 2612

Telephone 61612171 Facsimile 61612170

Email mal@structuraldesigns.com.au

Cracks Occurring Before the Concrete has Reached Final Set

These can take a couple of different forms that are generally easy to tell apart but can, at times, act in unison. These are either settlement cracks or plastic shrinkage cracks.

Settlement Cracks

These cracks are most commonly found on the sides of narrow columns at tie locations, or where sudden changes in the depth of slabs and bands occur. They can also be seen to mirror the pattern of the top reinforcement in deep bands. The normal cause of the problem is poor compaction but it is often wise to form the bottom half of very deep beams to slab soffit level as a separate pour. This is also true of columns, which need to be poured separately to the slabs they support.

Plastic Shrinkage Cracking

Loss of water from fresh concrete where not prevented can cause cracking. Most commonly the problem is surface cracking that results from the loss of water to the atmosphere. It is worth remembering that water can also be lost to the sub-grade which can exacerbate the problems on the surface by preventing bleed water coming to the surface. The sub-grade or sub-base should be soaked a day before the pour and again on the day if a membrane is not being used. If you are using a plastic membrane about half of the literature suggests that this worsens the problem but I have yet to read a satisfactory reason as to why this may be the case.

Canberra tends to have a few more problems with this than other capital cities due to our relatively low humidity and high summer temperatures although wind speeds are also a major factor. If you are operating on a dusty site controlling the dust with water tankers on the day you pour is also important as the dust also soaks up bleed water as it hits the slab and makes finishing difficult.

For an indication of exactly how much bleed water you are likely to loose on any particular day you can look up a chart where air temperature, wind speed, concrete temperature and relative humidity form the variables. Once you have an evaporation of more than 0.6 litres/m^2/hr you have the possibility of plastic shrinkage cracking and need to use some aliphatic alcohol to control the situation. Shading and wind breaks or fog sprays are also excellent solutions where practical.

The aliphatic alcohol needs to be applied both after bull floating and again as required during the finishing process. If evaporation losses are predicted to be greater than 1.5 l/m^2/hr serious consideration needs to be given to putting off the pour until the weather improves. All of this guidance is very general, as sub-grade conditions, membrane usage, oxide types, inclusion of super-plasticiser, aggregate grading etc all influence the amount and timing of bleed water to the surface.

It is sometimes possible for experienced concreters to revibrate the concrete after plastic shrinkage cracking has occurred. This can be done provided the vibrator can still sink into the concrete under its own weight and must be carried out to the full depth of the crack.

Blistering

If you are concreting in these conditions you might like to check out the question on blistering of the concrete surface. This is another possible problem you may encounter and you will also find an evaporation chart there.

Mix Changes

If you are looking to change the concrete mix to help you out, there are a few things you can do vary the bleed rate but this represents a whole new topic.

Further Information

If you are in any doubt or require an on site assessment by a structural engineer in Canberra ACT call Mal Wilson from Advanced Structural Designs (ph 02 61612171) about your particular problem, especially if you are pouring coloured concrete in summer as this summary doesn’t scrape the surface of what you need to know.

home FAQ's