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Concrete is a solid material with an extremely high compressive strength. However, if concrete is suitable to pave over or not will depend on some further inspection.
If the concrete is a continuous solid mass with no sign of cracking or movement it is probably ok to pave over. If the concrete is cracked or heavily fragmented it means it is thin and poorly based. This means it is not ideal to pave over!
In this scenario you should break out the existing concrete. However it is important to realise that it is always best to break out old concrete if you can. This is because you do not know what is under the concrete.
Most commonly existing concrete slabs do not provide the ideal height you want for your new paving. However breaking and getting rid of concrete is expensive so this is a judgement you must make.
Large slabs of continuous, buried, concrete can move independently over time. This is why most concrete areas have expansion joints built in. Type 1 compactable hardcore is much better for basing patios as there is some tolerance to ground movement.
When breaking out old concrete and excavating for patios and driveways make sure you excavate to correct depths.
For driveways you should dig out a base of 225-250mm. That’s allowing 150mm of compactable sub base, 50mm of sand or mortar and 50mm for blocks or 20mm for paving.
For pedestrian traffic you will need to excavate to a depth of between 150-175mm. That’s allowing 100mm of compactable sub base, 40mm of mortar and 20mm for paving.
Can you bury broken concrete under new paving projects?
In a perfect world you want a consistent and evenly graded sub base aggregate. You can bury broken concrete under new paving projects if it is broken up and evenly graded.
This reused concrete should be buried at the very base of the excavation and topped with mot type 1. Larger sections of buried concrete can lead to settlement if not broken up well.
Only bury large sections of concrete if they have a flat surface and laid flush with a flat excavation base. This way there will be no settlement over time.
Crushed concrete verses mot type 1 which is best?
Mot type 1 is a crushed stone aggregate typically used to base hard wearing surfaces. In fact type 1 is used to base under roads, motorways and even runways. This aggregate is traditionally made up of evenly graded particles of granite and limestone.
The geological properties of these rocks crush down into angular particles with relatively flat edges. It is these angular particles which enable the aggregate to compact so effectively in a tight manor.
When type 1 aggregates are subject to a process of vibro-compaction they lock together. This is normally carried out with a whacker plate. The resulting surface has a very high compressive strength which can bite into soft subsoil’s.
Crushed concrete is exactly what the name suggests and is also used extensively for surfacing. This is popular on large sites to base routes for heavy machinery and is cheaper than mot type 1.
Crushed concrete is a much better basing material for paving when graded to a size of 40mm. However I personally find crushed concrete does not compact as well as type 1 and feels softer under foot.
For rigid paving construction bedded and jointed with mortar type 1 is a much better choice.
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Paul Nicolaides has over 30 years of recreational gardening and 20 years of professional landscaping experience. He has worked for landscape contractors including design and build practices across London and the South East. In 2006 he qualified with a BA Hons degree and post graduate diploma in Landscape Architecture. In 2009 he founded Ecospaces an ecological landscaping practice which aims to improve social cohesion and reduce climate change through landscaping. In 2016 he founded Buckinghamshire Landscape Gardeners which designs and builds gardens across Buckinghamshire and the South East. This blog aims to provide easy problem solving information to its audience and encourage others to take up the joy of landscaping and gardening.