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Heading: 1.1.1.1.3.2.3.2.1Concrete Test Reports
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There is a natural tendency to point fingers whenever concrete strength test results are lower than expected. The concrete supplier and testing laboratory often point at each other. Sometimes the contractor is blamed for moving test samples or inadequate curing. The admixture company, cement manufacturer or aggregate supplier usually comes next. The situation is exacerbated when the test results arrive late and without notice that they fail to meet a specified strength level. Finally, everyone’s opinion of what is acceptable is asked, and usually answered incorrectly.

I believe one answer to this problem is to report concrete test results as efficiently, accurately and completely as possible, including cautionary notices at early ages and non-conforming notices at specified ages. Also, data should regularly be analyzed statistically for compliance to ACI-301 Sections 1.6.7 (Acceptance of concrete strength) and 4.2.3.6 (Revisions to concrete mixtures), and standard deviation and within-test coefficient of variation should be readily available for troubleshooting problems. This information would be invaluable for early and measured response to individual low test results or downward trends in test results rather than an emotional reaction from a defensive posture.

Comments?



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david breitfeller


David points to a problem which I turned my attention to solving more than 20 years ago but which, unfortunately, remains to become official policy in USA.
There are at least 3 aspects to the answer:
1)Results should be analysed by multivariable cusum graphs, in addition to direct time line, multivariable graphs (multivariables should at least include density, temperature and slump). Such graphing will normally cause any testing procedural deficiencies to "stand out like a sore thumb" and will usually reveal the cause of any genuine downturn. I have recently been requested by TAC to review the latest draft ACI 214 proposals for the evaluation of test results and TAC have required a re-write as a result of my recommendations.
2)Certainly results should be promptly reported. They should appear on the control graphs within 24 hrs of any test taking place. Impractical? I was doing this as an independent service to consulting engineers in Melbourne, Australia in the early 1980s. Properly approached, testing laboratories are pleased to save time, effort, and expense by electronic reporting rather than preparing and posting paper reports. Generally a strength downturn should be detected from a very few 7day (or earlier) results long before any low 28day results are obtained. Confirmation that the downturn is genuine is usually obtained from seeing the cause of the downturn on the graphs of other variables long before it can be statistically confirmed by analysis of the strength results themselves.
3)As a result of the system I originated, Australian concrete producers have been carrying out their own testing and QC for more than 20 years, including amending mixes as necessary. The practice has spread to many other countries and hopefully even USA will eventually realise how much more effective this is.

The above is of course far too brief for full comprehension by the uninitiated but is explained in great detail in 3 editions (1995, 1999 and 2006)of my textbook. However the explanations are available at no cost (along with details of the book!)on my website www.kenday.id.au.
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Ken Day


Dave,

You are exactly right. It is critical that 28 day test data is shared as early as it becomes available. There are internet based tools that are available for this - CMATS originally developed Ontario ready mix association is one and there are others.

Another critical area is early age (1 to 7 day) testing and estimating whether 28 day breaks will be low and if so adjusting the mix design through a lower w/cm. Unfortunately this practice of adjusting mixes is not allowed in most US specs which require 28 day test data before making even small adjustments to mix proportions. I think the solution is either performance based specifications or at the very least bands around submitted mixture proportions expressed as +/- certain percent of cement or water weight for example.

Inappropriate initial curing is an important reason for low strength problems. So it should be made clear in appropriate ACI, ASTM standards who is responsible for maintaining the initial curing temperatures between 60-80 F at the jobsite for the first 24 to 48 hours (implication in most specs is that it is the responsibility of the testing lab). ACI 301 is addressed to contractor and if I understand it correctly it only states that contractor should provide space for initial curing specimens.
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Karthik Obla


I think it is important for the ready mix industry to proactively educate engineers as to the significance of cylinder results. If you ask an engineer why cylinders are required, they sincerely believe they are testing to evaluate the individual building element. Most engineers I talk to in educational programs do not understand the purpose of cylinders. The few sets of cylinders made - sampled, cast, cured selectively and distinctly different from the building - are for the purpose of judging potential, not reality. It is the intent of the project concrete testing program to develop data for the overall evaluation of concrete. "If you want to know the quality of the actual concrete, take core samples."
This briefly describes the problem. The solution is education on engineers, one by one.
ACI 318 gives the statistical requirements the cylinders must meet. The testing agency must provide everyone involved with the appropriate data analysis early on for proper QC. Today, it should be available online to project principals for immediate review.
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Roy Keck


Roy - Actually, CTL Engineering has a proprietary in-house program that has been very well received. The progam is web-based and follows requirements of ACI 301 in Sections 1.6.7 and 4.2.3.6 for reporting and statistical analysis of test results. It also has automatic e-mail notification of cautionary or non-conforming test results to a password-protected distribution list. It is really set up for testing laboratories like ours to inventory compressive strength samples, test them and report the test results. Our project managers review and approve reports online. Everything but the field report is paperless, including the final report. We don't pay postage and our clients don't wait for the mail. All reports can be indexed and downloaded for saving rather than filing paper reports. We are trying to decide whether to market our program for others to use.
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david breitfeller


Mr. Keck brings up a very good point that the ready mix industry needs to educate not only the engineers but also project managers and owners as to the importance of test reporting. Over the years we have encountered testing firms that under the direction of one of the aforementioned do not send test reports to the producer. In my opinion as a producer we will be more proactive in helping correct possible problems if we know about them upfront instead of 28 days or later. I'd be interested in knowing if other producers have experienced this and how they handled it.
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martin sublett


How about we start with sampling, fabrication, and handling first? There is overwhelming evidence all around that the industry has not achieved sufficient, substantial compliance with ASTM C 31 requirements.

Far too many agencies believe C 31 is not "Standard Practice for Making and Curing Test Specimens in the Field", but rather "Suggestions for Making and Curing Test Specimens in the Field".

When we get that right, perhaps we can have a serious discussion of data analysis.
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Thomas Adams


Tom is absolutely correct. The biggest problem we have as redi mix producers are the bone heads coming out of the so called test labs. Most of these people have no training and are the cause of most low cyl breaks. The earlier comment about cores for acceptance is on the mark as long as the concrete element was also cured under proper conditions. I am in a fight at present in which the cyl were low and cores were then taken. The same lab that screwed up the cyl is the one who did the cores. The cores are within 240 psi of acceptance, but the fight is on. The average low temps for the first 2 months were 45 degrees f. But of course the eng doesn't want to address that. The entire testing and acceptance arena is a mess.
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William Haas Jr


William has made an excellent observation regarding coring. When concrete strengths are being questioned the supplier and testing agency are generally in opposition. It is in the owner's best interest to get an opinion from a party that does not have a vested interest in the resolution of the dispute. If cores are to be extracted and tested,the cores should be cut by a firm whose business is to cut and saw concrete as their main business, not an ancillary activity. It is a matter of having the right equipment and trained personnel who not only know how to cut but how to do it safely. The cores should then be tested by a third party - a firm with no connection to the project. Costs for the coring and testing should be born by the responsible party.
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Thomas Adams
 
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