CD Designs Blog
One of the fundamental factors about crack prevention in concrete is for the contractor to ensure there is sound firm base to lay the concrete on. Typically it is recognised that you use a type1 MOTT (Ministry Of Transport Tested) as a standard sub-base. This ensures it meets the standards which is required on public roads. Type 1 is recognised as stone, which is 1 inch in size down to dust. Once compacted the dust and stone form a solid base. With current legislation we are encouraged to use a open aggregate with out the fines, This allows water to naturally soak away under the driveway.
This is something which is quite often a cause for dispute. We aim to place and compact our sub-base in accordance with our specification. This tends to be the minimum. If our specification is not to your standard, you can alter it to suit your needs at the time of quotation. Our specification is designed to withstand normal driveway traffic up to six tonne, and is usually adequate for most domestic driveways.
If the drive was to fracture then could it be because of inadequate sub-base? If the drive has sunk or slipped away then we could argue that the base has failed. Using the base we have put down, any fractures that have occurred have remained hairline. It is more likely fractures occur due to factors out lined in, “Cracking Factors outside our control.”
Concrete (Add Mixes)
The product it self is important and the specification is laid out in our “Domestic Driveway PIC Specification”. The cement content is important since this gives the concrete the specified minimum strength. Add mixtures which are used to improve the durability of the concrete also add extra benefits.
The first add mixture is the air-entrainment, this provides protection against frost. It does this by producing lots small bubbles like the inside of an aero, just on a lot smaller scale. The air bubbles allow for the normal expansion of water which the concrete may of absorbed, especially in freezing conditions. (Read “Salt And Concrete”, an exception to the rule) This helps concrete become a lot more durable in all weather conditions. This is a secondary protective measure, because we seal all our units with an impermeable acrylic sealer. So even if the sealer was to of become worn during periods of frost the concrete is still protected.
The second add mixture is the Polypropylene Fibres. These actually help inhibit (Prevent) the formation of cracks in concrete, whereas steel mesh only has functional value after the concrete has cracked. As well as adding strength to the concrete it reduces plastic cracking and enhances surface finish. This add mixture can be used without the need of using conventional steel re-enforcement in our slabs.
Crack Control Joints
With all our driveways it is necessary to incorporate crack control joints. As we know all concrete expands and contracts we need to allow for this. We can not prevent it from cracking but we can try and control it. The industry standard for spacing of our joints is for every inch of thickness of the concrete. We should allow 3ft (Just less than 1 meter). Therefore our slabs on a driveway should be cut up into sections less than 4m2 in area. This will and does help, allow for the initial drying shrinkage and thermal expansion and contraction associated with concrete.
We also have to take into consideration any integral items such as Manholes, walls, corners of house etc. Any intrusions in the concrete such as these will affect it when it shrinks during the early days of curing. It is recognised that more cuts within the pattern is better than too few.
When the design of the layout is formed, it is important to allow for expansion and contraction against wall and fences etc. The more “boxed in” our concrete is, the greater the chance of these stresses affecting it. Please be assured everything that can be done to minimise fracture will be done.
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27th January 2011