compressive strength of concrete 5

[diar87]

I know that concrete needs some time to hydrate; therefore (compressive strength test specimens or cubes) are placed in water for 28 days (at that time concrete reaches 90% of its total comp. strength) and then tested and the results are usually used in design, BUT the structure itself cannot be placed in water! So does anyone know if there is any scientific reasoning for that, or why we place the cubes in water while we cannot place the structure in water as well?

 

[iandig]

Regarding the curing of concrete specimens, it depends on your code but Eurocodes do allow for 1005 humidity. W e'esperimented with this idea in place of using curing tanks a couple of years ago, but there are health and safety issues . The idea was for the entire lab to be constant humidity, the downside is that a sealed building at a humidity 100% and a temperature of 20º is a great place to breed diseases such as legionella . So it is not just easier to use a tank, it was also deemed to be much safer for the techs.

 

[diggerman]

I've been asking myself that question for YEARS. I think concrete samples should represent the field conditions of the structures they represent. Great POINT

 

[Ron]

diggerman...keep asking yourself. That's not the reason for plastic concrete sampling and testing, and shouldn't be.

There are many ways to evaluate the strength of concrete in the field, almost all of which must be properly correlated to standardized sampling and testing.

There are too many variables in the field to assume the concrete sampled in the field and tested in a lab represents the in-place conditions.

 

[iandig]

Bear in mind that routine concrete sampling and testing is to make sure you got what you ordered, not a check on site workmanship. If you need to check site workmanship, this should be done seperatley to routine concrete CQA testing. No concrete supplier I know would warrenty concrete after it was placed by someone else, only that what they supplied was what had been ordered.

 

[40A]

In certain circumstances, additional cylinders are cast at the time of the laboratory specimens and allowed to cure in the same environment (exposed) as the in-situ concrete. The additional cylinders can be used as "Field Cures" to mimic the curing conditions in the field for informational purposes and even be used to determine whether forms may be stripped or a load applied. This method is only comparable to the in-place concrete, because as many have stated, the volume of concrete and the loads present may effect the curing process and strength at early ages.

With regard to the curing environment: The near 100% humidy (moist/mist room) is an acceptable method. Usually when a laboratory is set up this way, this curing environment is segregated from general work-space so as not to pose the health risks identified to employees.
The materials used to construct the moist cure room determine how much maintenance is required to keep it functional (i.e. wood shelves need replacing often, mold needs to be given attention...etc.)

Having worked with both environments, the moist room is more convenient for the technicians, where as it more costly in initial setup and maintanence. There are companies that manufacture pre-built cure-rooms.

The cure tanks pose ergonomic issues for the technicians that have to retrieve cylinders on a daily basis. If you're pulling 50+ cylinders out of tanks a day, you can feel the strain on your back - they make tools for that as well.

There are pros and cons to both environments.

 

[Ron]

I have designed and built two curing rooms so far in my career. Both were done to get away from curing tanks. While curing tanks guarantee 100% relative humidity, the temperature of the water is often hard to regulate. In a moist room, it is much easier to control the temperature and humidity, and as mentioned, it is easier on the techs.

iandig...I can't imagine why anyone would propose having the entire lab area at 100% humidity...that would be a miserable working environment and would cause problems for the equipment that wasn't designed for it (compressive strength testing machine for instance). As for legionella and other humidity related maladies, if curing tanks are kept with lime saturated water as required, you're not likely to get any of that. As for the surrounding area, reasonable ventilation will prevent the room's humidity from reaching intolerable values.

 

[iandig]

sorry, it was the entire concrete lab, the set-up was such that seperate buildings did seperate 'types' of testing, big operation.

 

[hokie66]

I concur with 40A's point about site held specimens for the sole purpose of determining form stripping times. The main advantage of this approach is that it tends to encourage the contractor to cure the slab.

 

[diggerman]

Curing tanks are ok if you do small amounts of testing and I mean small. Curing rooms are the best, however they are alot of work to maintain. By maintain I mean sweep the chunks off the floor, bleed the compressor, keep the water lines clean. But that's job security for the lab techs! I am very familiar with moisture rooms and they can be quite a handful if you don't upkeep them. You need a good dry thermometer with a wick to pick up the average humidty. You also must change the humidity records once a month and keep them on file for inspection. Like I said...they're the best for curing...but alot of maintenance!

 

[ConcreteNDT]

You can also use concrete maturity meters to correlate between field concrete compressive strength in the field and those cured in the bath.

You can find these at

http://www.ndtjames.com

IN regards to ACI certification, it is actually a very large organization with chapters all over the world, I personally am aware of many asian chapters, with certification being acceptable in Hong Kong as well as Kuala Lumpur