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?

 

[par060]

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

 

[cvg]

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.

 

[Ron]

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.

 

[BigH]

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

 

[hokie66]

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

 

[iandig]

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!)

 

[diar87]

thank you all for your support

 

[TDAA]

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.

 

[stanford94]

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.

 

[Zambo]

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?

 

[stanford94]

was that a smart-a?! comment?

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

 

[hokie66]

stanford94,

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

 

[cntw1953]

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.

 

[concretemasonry]

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.

 

[diar87]

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.

 

[cvg]

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

 

[jputn72]

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.

 

[cntw1953]

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.

 

[diar87]

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