Made by Cement Australia
Concrete · Posts
Rapid Set Concrete 20kg
Fast setting post mix concrete for fence posts, hoists and letterboxes. Pour in, no mixing; hardens in 15 minutes.
Comes on pallets of 48 or 84 × 20kg bags.
1 bag (less than a pallet).
Lead time: Approx. 1 to 2 working days
Code: 200842029
Available to order from Allmat. Click and collect or delivered across Adelaide.
How many bags do I need?
Work it out with the free Allmat concrete calculator.
About Rapid Set Concrete 20kg
Rapid Set, also called quick set, fast setting or post mix concrete, goes into the hole without mixing: pour in the water, then the bag, and it hardens on its own. It suits fence posts, non-load-bearing pergola posts, clothes hoists and letterboxes.
- Hardens
- Within 15 minutes
- Supports off
- After 1 hour
- Water
- 2.0L per bag
- Mixing
- None, just add water
- Drainage
- 100mm of gravel under the post
- Load the post
- After 24 hours
Best for
- Fence posts
- Non-load-bearing pergola posts
- Clothes hoists and letterboxes
- Other non-structural jobs around the home and garden
Not for
- Any structural work
- Placing below 10°C
- Aluminium posts or poles, unless primed or sealed to the post maker's instructions
- Batching in a barrow, bucket or concrete mixer: it sets hard in the container
- Adding extra cement, sand, aggregate or admixtures
- Playgrounds, except below ground level under a soft fall cover
How to use
- Dig the hole 100mm deeper than the post needs and put 100mm of gravel in the bottom for drainage.
- Pre-soak the hole with clean water. In sandy or fast-draining soil, a plastic liner stops the water draining away.
- Stand the post in the hole and brace it so it cannot fall over.
- Measure the water and pour it into the hole: start with 2.0L per bag, and add 0.3 to 0.5L only if needed. Excess water lowers the final strength.
- Straight away, pour in half the bag evenly around the post and tamp it with a rod to push out air pockets.
- Add the rest of the bag and tamp again. Add a little more water only if the mix is not wet right through. For a bigger hole, repeat with another bag straight away.
- Leave the supports on for at least 1 hour, depending on the temperature, and check the post is firm before taking them off. Wait 24 hours before fixing anything to the post.
Storage & safety
Keeps about 6 months from manufacture, sealed, dry and off the ground. Do not use a bag that has gone hard. Wear dust-proof goggles and PVC or rubber gloves. Each 20kg bag is a two-person lift. Storing and handling bagged cement.
How many bags?
About 2 × 20kg bags per post hole, Cement Australia's own estimate. A wider or deeper hole takes more.
Not sure this is the right concrete? Compare the four bagged concretes.
Documents
The manufacturer's own sheets. Open one to read it on the page, share it or download the PDF.
Rapid Set Concrete questions
It depends on the width and depth of the hole and the size of the post. The How many bags? line on this page gives the figure for a typical post hole, and the fence post calculator, linked below these questions, works out wider and deeper holes.
It starts to harden within 15 minutes of the water going in, and can take as little as 5 minutes in warm weather. Leave the supports on for at least 1 hour, and wait 24 hours before fixing rails or anything else to the post.
No. Rapid Set is not designed for any structural use. Concrete Mix suits small footings and pads around the home. A slab is usually a ready-mix truck job, and where a job calls for a set concrete strength, Cement Australia advises ready-mixed concrete from a truck.
No. It hardens in the barrow or bucket before you can tip it out. Pour the water into the hole, then the bag, and tamp it in place.
Only if the posts are primed or sealed to the post maker's recommendation.
Yes. For higher compressive strength, Cement Australia recommends PROSTRENGTH Rapid Set Concrete, which also sets in the hole without mixing.
Linked from these answers: Fence post calculator
Sources and guidance
The product information on this page comes from Cement Australia's published product data sheets, safety data sheets and website (see Documents above). It is general guidance only. Other manufacturers may give different advice for their own products, so always follow the instructions on the bag and the product's data sheet. Structural work, such as footings for buildings, must follow your engineer's design and the relevant Australian Standards.
You may also need
Often ordered with this product. Use + Add to pick the option and quantity and put it on the same quote.
PROSTRENGTH Rapid Set Concrete 20kg
High strength rapid set concrete for gates, professional fencing and signs. Supports off after 30 minutes.
Concrete Mix 20kg
General purpose concrete for small jobs: posts, small footings, pads and garden edges.
Also for this job: H4 treated pine posts