when cylinders are cast onsite they are usually kept onsite for a day or so  

you are talking about curing and yes, many structures are water cured. 7-day water curing is absolutely required for many structures. 28-day curing is not generally necessary. However, your statement that 28-day test results are used in the design is not generally true. compressive strength tests are used to confirm that the supplied concrete will meet the design requirements which were set way before any testing was started.

Concrete test specimens are intended check the mix design,not the in-place concrete.  That's why they are sampled, transported, cured, and tested under STANDARDIZED conditions, so that all will be tested under very similar conditions.  

To try and test for all the variable conditions encountered in the field using a few cast cylinders of limited area and mass would be ludicrous.

There are many correlatable ways to estimate the in-place strength of concrete.  All are approximations, not actual.  Some methods are nondestructive, while others are destructive...but again, they are estimates.
 

To confirm Ron's points - the test cylinders are done to a standard methodology to confirm quality of concrete - consistency.  They are not a real measure of the concrete strength in situ.  The question to be asked - does the specified strength used in design that of the concrete in situ or the concrete tested in the standardized way??  Mmm

Specified strength = design strength, but we use strength reduction factors to allow for, among other things, lack of consistency of the in situ concrete.

Concrete test specimens are placed in water to ensure all cement is able to hydrate. Be aware that what ever you do to concrete (when not following the guidance) will normally make it weaker (apart from testing the samples too quickly).
The testing of concrete specimens is undertaken to ensure what was delivered was correct and the correct mix (Quality Assurance/QA). Anything which can reduce the strength of the concrete (workmanship, hydration etc...) should be taken into account within the factors of safety, partial factors of safety at the design stage. The testing is there as a QA check, so needs to follow a standardised procedure for routine testing. If however you want to know something else, i.e. what is the actual strength for the actual concrete under the actual conditions, then you should design a sampling, curing and testing program to suit outside of the routine QA process. There is a big difference between QA checks on what was delivered to site (cured samples, tested at 7 and 28 days etc...) to investigative testing.
Concrete companies are very sensitive to how samples are taken, cured and tested and at any opportunity will disregard anyone elses testing , especially when it 'suggests' a failure (e.g. all the correction factors applied to cores to return an equiavlent cube strength!)
 

thank you all for your support

Just to add: Cylinders = Mix Check, to see that what was specified was delivered.  The rest of the requirements in ACI dealing with curing of the structure and temperature control, etc. are there to see that your in place concrete achieves acceptable strengths compared to your mix strength.

all civil/ structural engineers should have an aci field certification. in my opinion. this educates and trains about all of the applicable astm standards / procedures.

stanford94,

I'm from the UK and working in Asia, so what advantage would there be for me to have an American Concrete Institute field certification?  

was that a smart-a?! comment?

my point was......you should have applicable training. good luck

stanford94,

I enjoyed Zambo's comment.  This is an international forum, but many US contributors do not seem to realize it.  

My introductory concrete class require lab time, though it didn't make me certifiable, but did provide the basics. However, the knowledge become dusty if not practicing it.

My concrete lab (3rd or 4th year)course while studying engineering was a real joy because it was a 5 year curriculum, so there was some "elbow room" and not as hurried.

In the class, we learned the basic principals for gradations, mix designs, sample preparation and testing procedures. The professor had a hint of mischievousness and encouraged variations in sample preparation withing the specified limits (no rebar in the the cylinders, etc.), but allowed the use of "admixtures" like sugar to recognize the effects on compressive strengths. He also explained the effects of "short stroking" and non-vertical rodding when entering OR retracting and the effects of tapping the exterior of the cylinder before finishing and putting into curing. We also got into the problems of properly handling and transporting cylinders before lab curing. It is easy for a technician to add or reduce the cylinder strength by 300 psi and still be withing the required procedures. That is why it is important to have the same technician preparing the samples (so get sloppy).

When working as a concrete technician for the DOT, it was amazing to see the abuses that were no different than what we learned as tricks and jokes.

ACI is very good, but it must be adopted by a code or referenced in a specification, enforced and recognized for what it is and how it can be legally abused. In many international locations, the standards are adopted, but rarely but into force with controlled results because the structures are not critical or high-performance.
 

Guys, first I am a recently graduated engineer (building construction), second I have no experience, and third I studied, educated and living in Kurdistan-Iraq. The point is the technique and training you get at your universities and other places is much more efficient than I get, so I appreciate your support through answering me, thanks again.

diar87 -  You might try downloading some of the engineering manuals from the Army Corps of Engineers website which are quite good.  The following would be a good one to start with:

EM 1110-2-2000 Standard Practice for Concrete for Civil Works Structures Change 2 01 Feb 94(original)

http://140.194.76.129/publications/eng-manuals/em1110-2-2000/toc.htm

Please someone correct me if I'm wrong, but compressive strength specimens do not need to be placed in water but in an area with 100% humidity. A tank of water is just one of the easiest ways to do this.

diar87:

You have raised a simple, yey good question, many of us have learned/refreshed with you together. I, for one, long forgot a lot of those basics.  

thank you cvg,it is a good manual. cntw1953, thank you for your statement

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.

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

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.

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.

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.

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.

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

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.  

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!

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 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