[Brian Bines]

There are many different concrete mixes varying with the ratio of the fine aggregate, coarse aggregate and cemant used and also with the cement/water ratio in the mix. It would be interesting to know the specification of the concrete mix used. Were the runways constructed with a strong reinforced mix or with a leanmix (low cement content). The leanmix type would tend not to break its back if subject to ground movement due to wet/clay type conditions which are subject to a large temperature variation. But would tend to undulate with ground movement. Was the concrete laid some 150mm below ground level and then the runway bought back to ground level with the timber decking, this would have given the concrete some protection from the elements and provided good surface drainage. If a leanmix type was used this would have minmum curing time before use, as opposed to the use of a stronger reiforced type of concrete needing 28days+. If a runway had to go down fast a leanmix type with a timber decking could be constructed much faster than a traditional reinforced concrete one.