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Posted: Sun Nov 13, 2005 7:41 pm
by GaryN
Tony,
A lot of the contractors my way (Essex) are using lean mix concrete as a sub base for block paving. Seems a bit over the top and expensive to me.
Any views on this?
Regards
Gary
Posted: Mon Nov 14, 2005 12:16 pm
by danensis
I thought the whole point of block paving was that it was flexible? Ain't nothing flexible about concrete. If its so lean it cracks, what's the point of adding the cement?
Posted: Mon Nov 14, 2005 12:39 pm
by Tony McC
Lean mix does seem to be popular in SE England and a couple of contractors have given me the same explanation - DTp1 is relatively expensive in that part of the country (because there are no natural sources for a primary Type 1 material) and a lean mix (CBM2) sub-base helps reduce any potential problems with channelisation and/or settlement.
Some contractors tell me that they're paying 30 quid per tonne for Type 1 which is roughly 66 quid per cubic metre. They can get a lean mix for around 75 quid per metre, so they fork out the extra few quid and then make a big feature of it to the customers, claiming (with some justification) that their work is less likely to suffer any problems over the longer term.
There are, however, a few potential problems in using a rigid sub-base. In wetter parts of these islands (which means everywhere other than Kent and Essexcestershire) a CBM sub-base can prevent adequate drainage of the laying course material, resulting in liquefaction of the sand which leads to channelisation. In some cases, a CBM is being used to overcome really crap ground, but instead of using a flexible sub-base with geotextile reinforcement as an 'improvement layer' beneath a CBM sub-base, the CBM is just spread over the poor sub-grade in the certain knowledge that it will function fine until the cheque has cleared but in the medium-to-long term, it will probably fail and, once it has cracked or degraded, it can't act as a load-bearing sub-base, so you get even bigger problems.
There's also an environmental consideration. Cement is an energy-hungry product. It takes a lot of energy to produce each tonne of cement, and it also pumps out a lot of the dreaded CO2 that is helping to warm-up the planet. Although a typical driveway on a Redrow estate isn't likely to have too much of a global impact, if a larger number of driveways and other pavements were to adopt this construction model, it would make a significant contribution to the nation's carbon emissions. Quarrying on its own is environmentally naughty, but cement production is quarrying plus cooking, so it's environmentally very naughty.
And this leads on to the next problem: the price of cement is racing upwards at a frightening pace. Some of the predicted price rises for next year are very worrying and the model may change so that CBM can no longer compete, in economic terms, with granular materials.
There is a good case to be made for using a CBM sub-base for block paved areas, but it's not the only option, and it's not always the best option. However, as the PIC crowd are using the settlement/channelisation issue as the Achilles' heel of the block paving industry when pitching their product to potential customers, use of a CBM sub-base does seem to be the favoured rebuttal.