Cement setting times in current weather

[Chris Green]

I've searched on the internet but I'm having trouble finding 'real world' practical cement setting times. It's fairly easy to find how long it will be workable and thus when you have to finish a mix. However finding when it will be acceptably hard and strong is more difficult.

So, at present, with outside temperatures in the 5 to 10 degrees Celsius sort of range (hereabouts anyway) how long should I be expecting my cement to take before being reasonably hard? I know it's upwards of a month before it has reached most of its final strength but I want to know when I can put weight on slabs supported by concrete and when pointing will be strong enough to walk around on.

Two to three days seems a reasonable guess but is that for warmer times?

[Chris Hogg]

[Harry Bloomfield, Esq.]

Chris Green explained :

Gently - 24 hours, 2 or 3 days it should be OK to walk on normally, providing it is under slabs.

[nightjar]

Assuming you have got the water content right: Strong enough to walk on, a minimum of one day. Two days is better. Strong enough to work on, about a week. To help curing, cover with insulation - cardboard or proper concrete blankets. That keeps in the heat of the curing and helps to protect from cold weather.

[Chris Green]

Thank you, those are useful.

[Chris Green]

Thank you. Yes, I hadn't realised that the curing process is slightly exothermic so some insulation helps things along (and stops drying out).

[alan_m]

I laid some slabs to form a step at the beginning of last week when the temperature was expected to be around 8C during the day and above 4C during the night. I put the left over sharp sand/cement mix (4:1) in two piles on a bit of plastic bag. I wasn't too happy with the strength of one of these piles after 20 hours but after 48 hours the second pile seemed really hard. The original mix was on the dryish side and the water used was warm from the hot tap. The step was covered with a tarpaulin or 48 hours.

[invalid unparseable]

Trouble is that acceptably hard and strong is a movable feast.

Do you mean hard enough so kids cant graffiti it, or strong enough so you can drive over it ? Even with that last, there is a big difference between being able to drive on it with your car to park it in the garage and big bucket truck or mobile crane etc.

Yes for walking but not for driving a mobile crane onto it.

[Andy Burns]

Almost exactly a year ago, I had some foundations poured ranging from

550 to 850mm depth of concrete, I was a little concerned that there had been some chilly mornings, the supplier said no problem it would be hard enough to walk on overnight, as it happened I didn't visit until 48 hours later and it was by then.

[Brian Gaff (Sofa]

How thick is it though? Brian

[The Natural Philosopher]

24 hours is good enough for light loads, It will be crumbly but you can wlak on it. even overnight will show some appreciable hardening.

you can test with a finger

[Andrew]

When large concrete dams are being built they have to pour the concrete in 'cubes' in such a way that excess exothermic heat does not weaken the final result.`

[Andrew]

Maximum IQ achieved after 28 days :-)

Andrew

[alan_m]

I also bricked up a chimney opening this morning after removing a gas fire. As I already had some sharp sand and cement from my previous job I used a 4:1 mix to mortar in some bricks. 11 hours later the mortar is hard, although not fully dry. I put a pile of the mortar I had left over on a sheet of plastic in the outside cold. This pile is still very crumbly so an extra 15C of temperature has made a large difference to the setting time, as expected.

[Andrew]

You should use proper sand for brick and blocklaying mortar. Sharp sand is for screeding and rendering, or so I thought.

[Vir Campestris]

Have you made provision for ventilating it? (It's normal to leave an airbrick at the bottom. For good reason)

ANdy

[alan_m]

I'm aware of this but it was to brick up a two foot by two foot hole which will be plastered over. I already had the sharp sand and cement from a previous job so I wasn't going to go out and buy another bag of soft/builders sand.

[alan_m]

Yep, all my bricked up chimneys have a ventilation grill in the room. I've used tinyurl.com/3nsk36yk

[R D S]

I rendered our yard wall yesterday afternoon, in that i've shuttered it and poured about 2 inch of concrete between the board and the wall.

Wondering when I should remove the boards... As soon as it's starting to stiffen, so it 'dries out' exposed and evenly or is it best covered for longer?

Any opinions from the group?

[The Natural Philosopher]

As long as it wont fall off makes sod all difference