In a recent blog titled “What Does Cylinder Strength Mean,” editor-in-chief Bill Palmer noted that the tested strength of a concrete cylinder actually represents the potential strength of the concrete as delivered — the highest strength that single concrete batch can attain under ideal conditions. Frank Kozeliski, a long-time concrete producer from New Mexico, wants every cylinder to reach its full potential, which he says can only happen if it is immersed in water for the first 24 hours. Ideally he recommends requiring all testing labs have a temperature-controlled curing box on the jobsite to store the cylinders. Do you agree? This was a hot topic, both for contractors and producers. Here’s what some of you had to say, plus a response from Kozeliski.
I find the comments on cylinder strengths interesting but more than that, concerning. It seems that we are more concerned with getting the last PSI out of a cylinder than the actual "in place" strength of the structure. The only thing that a cylinder of concrete represents is a cylinder of concrete. Controlled environments generate controlled results. In my 30+ years and hundreds of thousands of yards of concrete placed, I have never had the opportunity to place concrete in a laboratory environment. I think the industry should start to place more emphasis on the technology of "in place" testing than how to get every last PSI out of a cylinder that doesn't represent anything other than itself. Dennis Purinton
What is required is the representative strength of the concrete that is in place. In situ concrete is not submerged, therefore, the first 24-hour submersion would give a false indication of in situ performance. The added expense of a submersion box is unwarranted. What is more important is to move the test cylinders to the lab on the second day and to submerge for the balance of the testing period. R. Michael Brenneman, PE, Professor at College of Engineering, University of Nebraska-Lincoln
A TestMark Curing box costs upwards of $2,600. Who is going to bear the cost increase from testing labs as they raise prices to cover the costs of dozens of these? Bailey Buffalo
For far too long failure to maintain proper initial curing has resulted in unnecessary meetings, unnecessary testing of hardened concrete including coring, unnecessary mix adjustments, and unnecessary construction delays causing unnecessary expense. If the party failing to fabricate, handle, and test strength specimens in strict accordance with ASTM had to bear these expenses there would be a lot less discussion of this topic. Thomas Adams
This is a very big problem. Most projects do not have a curing box for the cylinders. The producers are to blame when low strength happens. Why is it the producer is held to air specifications, slump specs, concrete temperature, batch weight tickets in tolerance, and w/c ratio, but the cylinders do not need to be kept to specifications? Kevin Jones, Quality Control Manager, Builders Concrete East
My experience (45 years involved with lab testing as a PE in California) this is rarely a problem if the existing procedures in ASTM C31 are followed. Clifford Craig, P.E.
The bigger issue is a lack of recourse for poor testing practice. The lab that fails to meet standards on testing resulting in low breaks is also the lab doing investigative testing for core acceptance and structural review. Meanwhile, the producer is holding the bag for costs and schedule impacts to prove their concrete was sound. Tamson Omps
It seems the problem usually starts in the field. Poor testing and storing on the site prior to getting to the lab. I do agree the supplier is usually defending their product. Tonya K. Alexander
It's great for the concrete producer if the cylinders are made properly and handled properly. Cylinders being immersed in water do not necessarily represent how the contractor cured the concrete. A large percentage of the time cylinders are broken early to determine adequate stripping strengths. But that's just another can of worms. Blaine Bronson
In the last 23 years I have been involved in highway construction in the state of Texas . I have seen a wide variety of onsite testing facilities. In my mind these facilities’ greatest obligation is to provide consistency. I have always been involved in contractor-produced, client-tested concrete with little resources dedicated to testing provided by the contractor. I believe we are as an industry misunderstanding the goal of cylinders as they truly cannot represent the structure made from the sampled concrete for a myriad of reasons. This would be accomplished with temperature probes and maturity tests. The cylinders provide us with a general idea of how the concrete is performing as the project is being built. Many variables can occur that these specimens can alert us to, but the data is only valid if the testing and sampling is done consistently. So we are not comparing the cylinder strength to the product made from the tested concrete but to other cylinders. Michael Black Sr.
As an engineer with a ready mix background, I have mixed feelings. On the one hand testing of concrete is so poor in our industry that I always could deflect low breaks on the lab and trash them for improper procedures. That was the standard first step. This usually got me out of trouble even if I may have been at fault to some degree. On the other hand, I have paid out significant money for low breaks when it was only a testing issue because the engineer/architect/owner didn't understand concrete and the impact of improper curing in the first 24 to 48 hours (easily over 1000 PSI loss). This simple solution would eliminate all the problems with heating/cooling boxes of air which can change rapidly and how sealed the cylinder molds are. My experience with working with water cured cylinders is that in the summer it eliminates the curing problems even if the water during curing rises above 80 degrees. My apologies to the few good labs out there, obviously you are a small minority. Mike Donovan, P.E., FACI
Not only should we do a better job on cylinders taken and stored on jobsites, we also should monitor curing of slabs. Way too much good concrete has been "rejected" when quality control was followed and no issues would occur. Mike Murray, FACI
This is obviously a discussion that should continue. Kozeliski followed up with some ideas that may address some of the concerns, but we hope that the conversation continues to expand on more ideas and solutions. Here’s what Kozeliski had to say:
A Cost-Effective Solution
The fact is, we batch concrete to the exact design and water cementitious ratio with computers. When we test the concrete we have a method of varying the initial curing that can change the 28-day result by 800+ psi. The producer is penalized and told he does not have a good quality control plan and has to add extra cement for this problem. We need to get all producers to specify the initial curing method to show they are doing their part to provide quality concrete. The initial curing for testing is generally the problem of the testing.
As we get into the summer months we are reminded to take precautions to avoid the dangers of working outside. In fact, OSHA just began its summer heat safety campaign. The same precautions need to be taken with concrete test cylinders, which are on the jobsite exposed to the same heat conditions we are. They too get hot if not protected, and the results could be low strength, which can lead to loss of revenue.
We need to consider using item 1 in ASTM C-31, which states to initially cure the cylinders in water. For example, a white 5-gallon bucket with at least three inches of water over the cylinders will hold four 4x8 cylinders.
Place a high-low thermometer in the bucket and see if you are curing the cylinders over night between 60˚ F to 80˚F. Then make an adjustment to cool or heat the water between these ranges. If they are warmer than 80˚ F get some ice and cool the water. The amount is a trial and error procedure for where you live in the world.
If using an orange bucket that is exposed to the sun, you will notice that the bucket will heat up above the 80˚F most of the time. You want to keep the bucket in the shade if the air temperature is between 60˚ F to 80˚F because the heat of hydration from the concrete in the test cylinders will warm up the water in the bucket. Black buckets in the sun will cook the cylinders by heating the water above the 80˚ F.
I would be interested in seeing what the water in a bucket is when you use a high-low thermometer. This is the least expensive procedure to properly initially cure the test cylinders.
As a reminder, remember to talk to your employees about the dangers of heat exhaustion and heat stroke. Give them plenty of water and keep them cool.