When a test result falls outside of the specified limits or other specification requirements aren’t met, engineers typically have three options for dealing with such non-compliance issues:
- Remove and replace
- Repair and restore
- Use as is
Remove and replace is often threatened as a scare tactic, but repair and restore is a more common requirement, and use-as-is seems to be losing ground as an acceptable resolution for specification non-compliance issues.
The Introduction to the Commentary for ACI 318-14, “Building Code Requirements for Structural Concrete,” states that “The Code and Commentary cannot replace sound engineering knowledge, experience, and judgment.” The Commentary for Investigation of “low strength-test results,” further states that: “The building officials should apply judgment as to the significance of low strength test results and whether they indicate need for concern.” Although ACI 318-14 includes the word “judgment” 12 times, this recommended application of judgment seems to be increasingly rare when specification non-compliance is dealt with. At the American Society of Concrete Contractors (ASCC) Hotline, concrete contractors describe nearly unbelievable experiences on their projects. Here are just a few samples of decisions regarding non-compliance with specifications when use-as-is could have resolved the issue had engineering judgment been applied. We believe licensed design professionals may not be using engineering judgment because lawyers have discouraged this on the basis of increased liability.
Slump Too High
Specifications required a slump of five inches plus or minus 1½ inches. Slump was measured and reported at 6¾ inches when test cylinders were cast, as required by specifications. The sample had been taken from the middle third of the truckload in accordance with ASTM C172, and by the time the tests were completed all of the concrete had been pumped into place. Upon receiving the test lab report showing that the 28-day compressive strength of the concrete from that truck met specifications, the engineer saw the “high” slump-test result and demanded in-place strength testing of concrete delivered by that truck. The obvious disposition of this non-compliance is use-as-is. Here’s why.
Slump values were once a rough measure of water content. If all other mixture variables were kept constant, a higher slump indicated a higher water content and potentially lower strength. All other mixture variables, however, are seldom kept constant, and with today’s concretes slump can vary because of changes in admixture dosage, or effectiveness of the admixture at different concrete temperatures, that have no effect on strength. Consider also that the slump test itself is a rather crude test. Slump is measured to the nearest quarter inch, and research has indicated that the within-batch coefficient of variation—an indicator of variability due to testing—can be as high as 10 percent to 15 percent.1 But let’s suppose that a retest had been conducted immediately, as required by ACI 301, and the retest confirmed the 6 ¾ inch slump. What would be the effect on the concrete?
An old rule-of-thumb was that adding one gallon of water to a cubic yard of typical 4000-psi concrete will increase slump by one inch and decrease strength by five percent.2 So if we assume a linear relationship between slump and the properties affected, the ¼-in. increase in slump might decrease the strength by perhaps 50 psi. But the measured strength-test results for the concrete met specification requirements, thus strength was a non-issue.
Concrete More Than 90 Minutes Old
In another case, a truckload of concrete was not fully discharged until 100 minutes after water had been added at the batch plant. The contractor noted that the concrete was still plastic without water being added, but to be safe, the contractor asked the inspector to make test cylinders before placement. Also, the inspector observed the placing and consolidation process and did not observe any problems. When tested, the cylinders indicated that the 28-day strength was acceptable. However, the engineer insisted that the concrete should be removed and replaced. His rationale? Concrete reaches initial set at precisely 90 minutes after addition of water, and placing it after initial set interferes with the hydration process. As has been noted, the 90 minute limit is a controversial requirement. Both field and laboratory data demonstrate that concrete strengths tend to improve with time, but only when water is not added. The key is whether or not the concrete can be placed and consolidated without any further addition of water.3 The vibration limit for concrete occurs before initial set. If the concrete has set, it can’t be vibrated. But in this case the contractor could vibrate the concrete and that process was observed by the inspector. In addition, if the concrete had set, the concrete in the cylinders could not have been fully consolidated by rodding 25 times. Thus the cylinder strengths indicated that consolidation was not detrimental to the concrete strength. In this case, the measured compressive strength showed there was no issue, thus use-as-is would have been the best decision.
As-placed Concrete Temperature Too High
Many specifications require that for mass concrete, the concrete temperature after placement shall not exceed 160-degrees Fahrenheit. The reason: delayed ettringite formation (DEF) has been noted at about this temperature, and may damage the concrete.4 Sensors are placed at the center of largest portion of the placement to monitor the maximum temperature to an accuracy of plus-or-minus two-degrees Fahrenheit. But if the sensor indicates a maximum temperature of say, 164-degrees Fahrenheit, what’s the proper disposition of this specification non-compliance? Based on the currently available research, we would argue that the 160-degrees Fahrenheit value is approximate, and that damage due to DEF isn’t necessarily a certainty. In an article describing construction of mass concrete elements for bridges in California, peak temperatures ranging from 162-degrees Fahrenheit to 190-degrees Fahrenheit were noted. Steps were taken to use cooling pipes in addition to cooling the concrete at the plant, but there was no mention of the elements with temperatures exceeding 160-degrees Fahrenheit being removed and replaced.5
As noted earlier, many decisions regarding specification non-compliance issues may be based on fears of liability rather than sound engineering judgment. Inspectors observe, measure, and test to ensure that the specifications are being met. They report their results but often do not have the education or experience to decide what to do with the results. Nor should they. That’s the engineer’s job. And we know that all is not black and white on any job site. Judgment is needed. We may not see engineering judgment as often as we used to because now the engineers are told to contact their lawyers before making a decision. And once they do, we don’t know of many lawyers who see the need for engineering judgment. That’s why engineering judgment may be dying.
References
1. Hover, Kenneth C., “Observed Variability in Tests of Fresh Concrete Properties from the FHWA Highway Materials Engineering Course,” Transportation Research Record No. 2342, Transportation Research Board, Washington, D.C., 2013, pp 61-75.
2. “The Effects of Water Additions to Concrete: ‘What’s a little water going to hurt?’” CEMEX USA Technical Bulletin 9.4, 2003, 5 pp. (www.cemexusa.com)
3. Lobo, Colin, and Gaynor, Richard, Gaynor, “Ready-Mixed Concrete, ”Significance of Tests & Properties of Concrete and Concrete-Making Materials", ASTM STP 169D, 2006, p.542.
4. “Optional Requirements Checklist, Section 8.1.2,” Specifications for Structural Concrete, ACI 301-10, 2010, p.65.
5. Maggenti, Ric, “From Passive to Active Thermal Control,” Concrete International, Nov. 2007, pp. 24-30.
Note: ASTM and ACI publications cited can be purchased at www.ASTM.org and www.concrete.org, respectively.
