For small concrete jobs around the section, mixing your own is easy and far cheaper than ordering ready-mix. Daltons Builders Mix is a blend of Daltons No1 Washed Sand and screened aggregates, so all you need to add is cement and water.
What Builders Mix is good for
- Setting fence posts, gate posts and clotheslines
- Mowing strips along garden edges
- Small paths and pads, such as under a BBQ, bins or a heat pump unit
- Concrete edging to hold pavers in place
- Footings for garden walls and small structures
Because it's low in silt and clay and evenly graded, it makes good, strong concrete. For anything structural or needing building consent, follow your builder's or engineer's specifications.
Mixing ratios
The right amount of cement depends on the job. Always follow the mixing directions on your cement bag, which give ratios for different applications. As a general rule, the more cement in the mix, the stronger the concrete.
How to mix
-
Dry mix first. On a sheet of ply or in a barrow or mixer, combine Builders Mix and cement until the colour is even.
-
Add water gradually. Add a little at a time and mix thoroughly. You want a workable mix that holds its shape, like thick porridge, not a runny slurry. Too much water weakens concrete.
-
Use it promptly. Mix only as much as you can place in about half an hour, especially on warm days.
Setting a fence post
- Dig a hole about three times the width of the post and around a third of the post's above-ground height deep (deeper for gate posts and tall fences).
- Add 100mm of GAP20 to the bottom and tamp it firm. This helps drainage and stops the post sitting in water.
- Stand the post in the hole, brace it plumb on two sides, then fill with concrete, rodding it to remove air pockets.
- Slope the top of the concrete away from the post so water runs off.
- Leave the bracing on until the concrete has set.
Pouring a mowing strip
A concrete mowing strip along a garden bed saves hours of edge trimming and keeps mulch out of the lawn.
- Dig a trench about 100mm deep and slightly wider than your mower wheel.
- Set timber boxing along both sides, level with the lawn surface so the mower can run along it.
- Compact a thin layer of PAP7 crusher dust in the base.
- Pour concrete, tamp it and screed it level with the boxing. Cut control joints every metre or so to reduce random cracking.
- Finish with a wooden or steel float and lightly broom the surface for grip.
Curing
Concrete gets its strength from curing slowly. Keep it damp for the first few days by covering it with plastic or wet sacks, or by lightly misting it, especially in warm or windy weather. Protect fresh concrete from frost and heavy rain.
How much do I need?
Work out the volume of concrete (length × width × depth in metres). Adding cement doesn't add much volume, so order Builders Mix roughly equal to the concrete volume plus about 10% for waste. A 10m mowing strip 200mm wide and 100mm deep is 10 × 0.2 × 0.1 = 0.2 m³, so ½ m³ of Builders Mix covers it with plenty to spare for a few fence posts.
Quick answers
Do I need to add anything else? Only cement and water. Builders Mix already contains the sand and aggregate.
Can I use Builders Mix for mortar? No. Mortar for bricks and blocks needs a fine sand without the larger aggregate. Use No1 Washed Sand.
How long before I can walk on it? Usually after a day or two, but give it about a week before heavier loads. Check your cement bag for guidance.
Related reading
