How Good is Your Cylinder Test for Acceptance
Testing compressive strength has been a time-honored method of identifying quality concrete for as many years as we have been making concrete. However, we must also recognize that the cylinder is not the best way of testing for quality acceptance. Because it is so economical, the concrete cylinder has become the default method to test concrete quality. Historically, the 6" x 12" cylinder was the norm, but more recently, the 4" x 8" size is becoming more prevalent. Both give reasonable results when prepared and cured properly.
Therein lies the problem; in order to take advantage of the economy of the cylinder test, they must be properly prepared and cured to be useful. If this is not done properly, it is a surety that the compressive strength of the sample will be less than one properly cared for. Improper curing makes the use of the compressive strength test completely useless for acceptance. History has shown and the American Concrete Institute has recognized that improperly prepared and cured cylinders are invalid means of accepting concrete mixes for payment.
So what makes a cylinder test invalid for acceptance? One of the most egregious of issues occurs when the field samples are left in the field beyond the ACI standards for time. To be valid, the cylinder must be removed from the field, stripped and placed in a curing chamber or tank within the prescribed time limits, usually about 24 to 48 hours from casting. If this is not done, the ability of the cement (cementitious) to hydrate and develop strength is impaired, resulting in compressive strength characteristics inferior to the actual concrete placed in the pavement or forms. This, then, is the basis for potential rejection of concrete if cylinder breaks are less than specified. Rejection through low cylinder breaks starts in motion a very expensive process for the ready mix supplier to prove, in fact, that the quality of the mix provided is not inferior in quality to that specified.
We see some testing reports coming through with field cure ages between 2 and 7 days and occasionally up to 15 days. A source of problem could be the competitive bid process for securing field testing services. Fortunately, the problem seems to occur with just a few of the testing companies here in North Dakota. A couple of the excuses we hear include:
1. We didn't bid enough to pick up the cylinders within 24 hours of casting.
2. We forgot to pick them up. Sorry.
It may only be a couple of the testing companies who cause these kinds of headaches, but it certainly makes running a profitable ready mix operation difficult. Plus, it creates all kind of headaches for the contractor involved, and the owner of the project who likely sees his project delayed during problem resolution or litigation. In the middle of it all is the mess for the engineer or architect in charge of payment for concrete materials.
The answer to the problem is to ask the testing companies who have these kinds of quality control issues to step up and improve their service quality. It becomes, in fact, an ethics issue when their service contracts imply that ACI standards will be met, but then are ignored. Any professional engineer involved with these testing quality matters is open to professional ethics charges to the Board of Registration for Professional Engineers and possibly other violations of professional engineering conduct. In any event, the breakdown in the partnership with the concrete industry and the testing community is very unfortunate indeed.
So What does a Cylinder Test Really Measure?
The foreign environment of the cylinder sleeve is only partially conducive to the proper hydration of the cementitious in comparison that the environment expected in a structural concrete beam or pavement with nominal curing. Experience and research has shown that the cylinder will provide a reasonable duplicate of the material placed into the field member if the cylinder can be protected for the first 24 hours or so at a temperature between 60 and 80 degrees, and then removed from the field to a curing chamber or water tank until testing. It is this experience and research results that has provided the background for the American Concrete Institute to set the guidelines currently in practice for cylinder testing. Without following the ACI guidelines, the cylinder becomes useless for acceptance and payment for materials due to the degraded condition of the concrete sample. No reasonable litigation effort will fail to recognize the need for following the ACI guidelines, if the testing of cylinders is to be used for materials acceptance. If the ACI guidelines cannot or will not be followed, the use of field secured cores should be specified by the engineer in lieu of cylinder preparation.
For information or further discussion, contact Dave Sethre at 701-371-4497