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I'm having odd difficulty in tracking down an answer to this question. Searches for "weak concrete" tend to cover weak sand/cement mixes (but no gravel), typical strong sand/cement/gravel mixes - and a lot of pages about avoiding weak concrete by not adding too much water!

The situation here is, I've taken up a strip of paving slabs at the edge of a paved patio, about 20-30 cm by 3m, replaced a drain + gully trap with usual 110mm plastic (resting on the solid foundation), and filled most of the trench to about 10cm below finished level using 10mm compacted gravel.

I now want to place weak concrete around the "bottle" of the gully trap, so it's held in place but isn't embedded in dense rock-like concrete if anything ever needs doing in that area in future, and lay about 50mm of weak concrete on the compacted gravel, to act as an unmoving substrate to put sharp sand + slabs back into, so the slabs won't subside if the gravel settles slightly. Nothing else, and nothing structural, is resting on the slabs, and the area is very lightly trafficked.

The nearest I've seen for the task is a sand+cement mix, at about 1:10 to 1:12, but I'd like sand/cement/gravel, which seems more likely to be robust against ordinary events, while still being weak enough to break out, if works are ever needed.

I'm tempted to guess at a mix of about 1:3:6 but I can't find anything to confirm if that's reasonable or if there is a more usual standard mix for the purpose. Hints appreciated!

  • For info, the cement is sulphate resisting (ground, rain, soil, proximity to foul water). It doesn't affect the strength but if I don't mention it, someone else will :) Also I want to lap some waterproof cement (sika-1) from under the slabs, a little up the adjacent wall, to prevent foul water if the drain blocks/overflows, hence a second reason why I want a firm fixed substrate below the paving slabs.

Update: photo of gully trap

enter image description here

The "bottle" is the bit where the trap is, under the grid

  • Bottle of the gully ?? – Alaska Man Nov 25 '18 at 19:37
  • Added photo to help. Its a standard Floplast 110mm gully trap – Stilez Nov 25 '18 at 20:54
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I think you’re confusing “weak concrete” with paving substrate.

If you use a “weak” concrete mix, it will harden. Then, with any settlement it will crack and pieces will shift.

However, if you use a “crushed” rock gravel fill, it will bind together yet remain flexible so it doesn’t crack and be displaced over time from loads, etc.

Both can be removed in the future when your plans change.

I’d recommend a 3/4” minus crushed rock. This gravel is comprised of various size rock that has been crushed from what can fall through a 3/4” sieve down to sand. These particles will bind together like a “solid rock”, yet will flex.

When you install the “mix” make sure you compact it in about 6” thick lifts or less.

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This page lists a number of common mixes and their strengths. You could use this as a guideline, and make it weaker by adding excess water so the mix is very runny.

The weakest mix in the list is 1:5:10 cement:fine aggregate (sand):coarse aggregate (gravel). It has a compressive strength of 5 MPa (725 psi).

  • I'm not sure this helps? I need to identify a mix that is appropriate for a very specific task, in which the concrete needs to hold in place for usual purposes and light foot traffic, but not be rock-like and hard to break apart, if ever needed. I'm not sure if compressive strength is a good guide to that (as opposed to say, brittleness), but there is surely good knowledge out there, as this kind of work is very common, so there should be a "typical/recommended mix" used for it, in practice? I definitely wouldn't want to use excess water instead of the right mix to weaken it - too uncertain. – Stilez Nov 25 '18 at 21:14

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