Concrete temperature/curing monitoring

[lawrie]

How often is temperature monitoring used for calculating when concrete has cured.
What are the typical type projects that this would be used for and are there any advantages in longterm temperature monitoring of concrete after the initial build?

thanks.

 

[Ron]

Temperature can be correlated to concrete maturity and strength gain, as well as measuring the heat gain during hydration that can affect the strength of concrete. This is usually done in mass concrete placements (dams, large mat foundations, large drilled shafts/piers for bridges)to check the heat gain, though it can be done in many placements if you are looking at a maturity issue rather than a thermal issue.

It is not used routinely due to the cost. Only large, longer term projects can support such effort and cost.

There is no significant reason to monitor concrete temperature for the long term, after it has gained its required strength and adequately cured.

The process is done using thermocouples attached to a monitoring or recording device (or laptop computer).

 

[jheidt2543]

We've used indoor/outdoor thermometers to monitor concrete tempertures during winter concrete placements. By putting the "tail" in the concrete and leaving the thermometer exposed to the outside air, we can check both the concrete temp. and the air temp. The intent is to keep a record that shows the concrete did not freeze. I does mean that a crew member must take a few night time readings (we used 4 or 6 hour intervals), but it is a cheap, quick way of tracking winter concrete temps. Of course the thermometer is not reusable, they are only a few dollars a piece. Just another way to get the job done.

 

[RDK]

I have to disagree with Ron. Temperature monitoring is not expensive.

All it takes is a special thermocouple wire with the last 25 mm of the wire stripped and twisted together. This is imbedded in the concrete with the other end exposed. A reusable plug ($3 CDN each but not really essential) is attached that then plugs into the monitoring equipment. The monitoring equipment can be a simple reader ($200 CDN. for the simple readers. I have even seen digital multi meters that have the capability of giving temperature readings.) that gives a digital readout of temperatures or something that records the temperature over time. The wire is about $1 CDN per m. I usually recover whatever piece is sticking out of the concrete forms and reuse it several times until its too short to be useful.

When one considers the other costs in inspecting construction and the cost of poor quality construction these costs are insignificant.

It is usually only done in mass pours where the heat gain is a concern. In smaller pours where the thickness is less than 500 mm heat gain is not that much of a concern. It might also be done in high temperature conditions in southern climates but that’s something not usually a problem here in Canada.

Since these thermocouples are left in the concrete they could be used for long term monitoring. There is no concrete reason (pardon the pun) to monitor temperatures so this would be done for some other reason such as monitoring energy efficiency or some process reason.

Heat gain (called heat of hydration) occurs due to the chemical action in the curing of the concrete and can set up some thermal residual stresses in the concrete when it cools. These stresses then take up some of the available strength or can cause cracking in the concrete.

If the concern is freezing before the concrete cures, there are special thermometers available that will record the max and min temperature. These are twin tubes filled with mercury and then have a plug that stays at the top or bottom of the travel. The plug is reset by pushing a button and taping the thermometer so that the plugs move to the current temperature. These are normally placed inside the hording and simply left there until the hording is removed. I would consider the use of these to be normal due diligence in concrete work when freezing is a concern. These are inexpensive ($20 CDN or so each) Any concrete testing firm will have a supply of these.
Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

http://www.kitsonengineering.com

 

[Ron]

Rick,
I agree with all you said, but my "expense" premise is that of the US. In Canada, inspection and testing are much more accepted as routine, even those tests not considered commonplace. In the US, it is often difficult to sell any of the project team on the benefits of such testing/monitoring as they often fall back to "I didn't do it on the last job and it didn't fall down!!". In the last few years, I've proposed on at least 3 projects for thermal concrete monitoring (and it was specified in the contract documents), only to have the contractor convince the owner that it wasn't necessary (I live in the Southeast US and hot weather concreting procedures apply often, so yes, it can be beneficial!)and the expense was too much for the project (though not great, was still more than just the compressive strength specimens!). Imagine placing a half million dollars worth of concrete in a single placement and not being willing to spend a couple of thousand for monitoring and testing...go figure!
Ron