When does concrete reach 5 MPa?

[Stabbsson]

I am planning to cast a slab outdoors (a foundation for an air/water heat pump), perhaps a bit greedy with the time of year as it can still be below freezing at night, but I don't want to pay much more for our electric bill!

I have read that as long as the coarse concrete reaches 5 MPa in strength, it's fine, but I can't find anything about how long this usually takes. Does anyone possibly know this?

 

[kungfuu]

What we can infer from the post is the risk of frost damage on the surface of the concrete. The thinner the casting, the more sensitive it is because the intrinsic heat in the concrete is low. A small foundation can be suitably protected with a weather tent or by applying cellular plastic, but you then risk slight marks on the surface that you have to decide if they are acceptable or not. Concrete has the absolute greatest strength development in the first few days, but temperature and amount of cement have a significant impact. I would protect the concrete the first night until above-zero temperatures and wait a few more days before putting significant load on the slab.

 

[Stabbsson]

What we can infer from the post is the risk of frost damage on the surface of the concrete. The thinner the casting, the more sensitive it is because the inherent heat in the concrete becomes small. You can suitably protect a small foundation with a weather tent or by laying down foam plastic, but then you risk a few surface marks that you have to decide are okay or not. Concrete has the absolute greatest strength development in the first days, but temperature and amount of cement have a big impact. I would protect the concrete for the first night until above freezing and wait a few more days before putting more load on the slab.

The construction itself will be 170 cm wide, about 70 cm long and 15 cm deep, 12 cm of which is above ground.

 

[mexitegel]

It's not like you're casting the Öresund Bridge .

If you're worried, you can build a small tent with a tarp and put in a car heater on a low setting, and place an old blanket or two on top.

 

[kungfuu]

Since the construction is fairly long, it's appropriate to use a centric net, but perhaps you had already considered that.
For protection against drying out, it's good to have a construction plastic that retains moisture, as well as a layer of cell plastic or, as suggested, a tent and fan. The fan should only keep it frost-free, so no building dryer or 40 degrees in the tent.

A cement mixer is good if you're going to break bags, you'll need quite a few after all.

 

[Stabbsson]

Since the construction is quite long, it's a good idea to use a centric mesh, but you might have already considered that. As for protection against drying out, it's good to have a construction film that retains moisture, as well as a layer of cellular plastic or, as suggested, a tent and a fan. The fan should only keep it frost-free, so no construction dryer or 40 degrees in the tent.

A cement mixer is good if you're going to break open bags, you'll need quite a few after all.

Of course, the mesh is included!

We say right after casting to lay plastic (to retain moisture) and cellular plastic on top to keep the heat? The idea is also to place 10 cm thick cellular plastic at the bottom of the casting mold.

 

[kungfuu]

The lower one can be discussed, as long as you have a good base to cast on (like packed macadam or gravel with geotextile between the crushed stone and concrete). The cover plastic is to prevent the concrete from drying out too quickly, which would cause cracks, and the foam plastic lid is to protect against early freezing. Both of these are just temporary measures and are sufficient. Foam plastic preferably around 50mm or so, it's not so cold anymore unless you live in the north.