Working Strength

[j19]

I have always heard that masons or steel erectors should not start on a building for 7 days after the slab is poured. Does anyone know if this is just a rule-of-thumb, or is it in a document somewhere?

 

[Ron]

The time to continue construction depends on several factors. The one you mentioned is a rule of thumb, and an old one at that.

For conventional concrete with no admixtures (or only minor admixtures such as air entraining agent), concrete is expected to achieve about 70 percent of its strength in the first 7 days. Even that rule of thumb has gotten distorted with the use of ground granulated blast furnace slag cement and other admixtures such as fly ash.

To figure out how long you should wait before moving to the next phase of construction, you should have a strength gain curve for that particular mix design. If the loads being placed on the concrete do not exceed about 50 percent of the strength of the concrete at that point, it is usually acceptable to move on. That includes equipment, access, storage, and in-place construction loads.

For typical slabs on grade, the construction loads are usually light; however, you might have some anchorage considerations (anchor bolts will pull out of concrete with insufficient strength) or wedge anchors might damage the concrete when tightened...there are lots of considerations to make, so look at the curve, and figure out where you need to be for each case. As you get more familiar with it, you can wing it, but understand the concept before you do that.

 

[sdz]

Good structural concrete should be moist cured for 7 days but this can be achieved in several ways and might not prevent access providing Ron's limits are also followed.

 

[rscassar]

Personally I don't think it would be advisible to have the steelwork guys working on the slab within the first 7 days because of durability issues.

 

[Ron]

OOPS!! I noted in my post that fly ash is an admixture...actually it is not. It is a cementitious enhancement (somewhat pozzolanic). Sorry!

 

[rscassar]

Is there a minimum time that should elapse regardless of whether the strength attained is that required to performed the function during construction?

Reading through my specifications, I noticed that it is requested the contractor submit calculations to show the structure has attained sufficient strength during construction. That the contractor is to cure slabs for 7 days and that notice is given before loading the structure.

Because the slab is cured for 7 days I don't think it would be feasible to have other trades working on the slab withing this time period.

Because a 7 day concrete test is usually requested I would say that this is the minimum time before other trades can begin working on the slab.

 

[hokie66]

Seven day curing would be nice if achievable, but high rise structures are commonly on a 7 day or shorter floor cycle. Methods of achieving acceptable curing for the structure have to be negotiated.

 

[pwht1]

Masons shouldn't start working until all of the formwork is stripped and the backpropping has been removed. Otherwise it'll crack big time.

Steel workers should be able to work as soon as the curing compound has set. This is typically the day after pouring on most of the jobs I've worked on. Don't forget that there is formwork providing all of the load-bearing for the slab. Depending on your countries code it should be able to take 4kPa + the conc dead weight, so live load from erectors really isn't a problem. Stacked materials may be an issue though, you'll have to make sure they comply with the formwork engineers drawings.