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RegSenseSafety & HealthConstruction SafetyOccupational Safety and Health Administration (OSHA), DOLSteel ErectionSpecialized IndustriesCompliance DocsStandard InterpretationEnglishFocus AreaUSA
Field-cured concrete sample cylinders must be subjected to similar conditions as the footings walls; acceptable use of the Windsor Probe Pin testing method.
Standard Number: 1926.752(a); 1926.752(a)(1)
October 27, 2003
Andrew G. Forrest, Jr.
Lincoln Construction Inc.
4790 Shuster Road
Columbus, OH 43214-1997
Re: Whether a controlling contractor may heat concrete cylinder samples in performing concrete strength testing under 29 CFR 1926.752(a)(1); whether a controlling contractor may use a Windsor Probe and Pin test to meet the requirements for testing concrete footings, piers, and walls prior to authorizing commencement of steel erection under 29 CFR 1926.752(a)(1).
Dear Mr. Forrest:
This is in response to your letter dated December 19, 2002, to the Occupational Safety and Health Administration (OSHA) in which you ask about substituting Windsor Probe and Pin tests for the testing methods required by OSHA's steel erection standard, 29 CFR Part 1926 Subpart R. We apologize for the delay in providing this response.
We have paraphrased your questions as follows:
Question 1: Under §1926.752(a)(1), before approving the commencement of steel erection, the controlling contractor must notify the steel erector that the concrete1 in the footings/walls has attained a specified strength based on an ASTM test for field-cured samples. We are having difficulty in preventing field-cured concrete samples from freezing. Is it permissible to store the samples in a trailer or other protected/heated environment to prevent them from freezing?
Answer:29 CFR 1926.752(a) states:
- Approval to begin steel erection. Before authorizing the commencement of steel erection, the controlling contractor shall ensure that the steel erector is provided with the following written notifications:(1) The concrete in the footings, piers, and walls and the mortar in the masonry piers and walls has attained, on the basis of an appropriate ASTM standard test method of field-cured samples, either 75 percent of the intended minimum compressive design strength or sufficient strength to support the loads imposed during steel erection. [Emphasis added.]
The ASTM method for using field-cured samples requires that the samples be subjected to conditions that are similar to those faced by the footings/walls. The purpose of the field-cured sample is to provide an accurate representation of the condition and strength of the actual concrete footing/wall. ASTM C31 "Standard Practice for Making and Curing Concrete Test Specimens in the Field" states that:
- * * *
9.2.1 Cylinders - Store cylinders in or on the structure as near to the point of deposit of the concrete represented as possible. Protect all surfaces of the cylinder from the elements in as near as possible the same way as the formed work. Provide the cylinders with the same temperature and moisture environment as the structural work.
If the cylinder samples are freezing but the footings or walls are not, then the cylinders are not in a similar environment (including temperature and moisture) to the footings/walls. For example, concrete in footings typically are warmed to some degree by virtue of being in the ground. If the cylinder sample were left on top of the ground, it might well be subjected to lower temperatures than the concrete in the footing. In that case the cylinder must be warmed to the same temperature as the concrete in the footings.
Another example is the case of an above-ground wall where the ambient temperature is below freezing. In that situation, standard industry practice calls for heating the concrete in the wall to a specified temperature range. In this example the cylinder sample would also have to be heated to a similar extent.
In sum, the concrete in the sample cylinder must be subjected to similar conditions as the footings/walls. In freezing weather, that sometimes requires warming the cylinder in the field in order to mimic the conditions of the footing/wall - but only to the same temperature (and using the same moisture environment) as that of the concrete in the footings/walls. Taking such measures in the field would be consistent with the ASTM standard, and therefore consistent with §1926.752(a)(1).
Question 2: May Windsor Probe and Windsor Pin testing of concrete be used instead of ASTM field-cured sampling tests for authorizing the commencement of steel erection under Subpart R?
Answer: As noted above, §1926.752(a) requires that, prior to authorizing the commencement of steel erection, the controlling contractor must ensure that the steel erector is notified that the concrete in the footings, piers, and walls has attained the required strength, "on the basis of an appropriate ASTM standard test method of field-cured samples."
Windsor Probe/Pin testing
The Windsor Probe/Pin testing method ("Probe/Pin test") uses a testing device to test the concrete that is in the footings or walls, rather than to test samples. In this respect there is an advantage over testing field-cured samples: it eliminates problems with ensuring that the conditions to which the field-cured samples are subject are equivalent to those faced by the concrete in the footing or wall.
However, the accuracy of the testing device used in the Probe/Pin test depends in large measure on calibrating it accurately for the concrete mixture being used in the footings/walls. ASTM has established a protocol for using this type of device, ASTM C803/C803M Standard Test Method for Penetration Resistance of Hardened Concrete. The ASTM standard states that:
- 4.2 This test method may be used to estimate in-place strength, provided that a relationship has been experimentally established between penetration resistance and concrete strength. Such a relationship must be established for a given test apparatus (see also 8.1.5), using similar concrete materials and mixture proportions as in the structure. Use the procedures and statistical methods in ACI 228.1R for developing and using the strength relationship. [Emphasis added.]
To satisfy the ACI methods, the resistance/strength relationship must be established using concrete specimens "having similar compaction and maturity as the field concrete."2 Further, the strength relationship must be established.
- 4.2.1 - ... by a laboratory testing program that is performed using the in-place test method in the field. The testing program typically involves preparing test specimens using the same concreting materials to be used in construction.... At regular intervals, measurements are made using the in-place test techniques, and the comprehensive strengths of standard specimens are also measured. The paired data are subjected to regression analysis to determine the best-fit estimate of the strength relationship. [Emphasis added.]
These tests will result in a penetration resistance/concrete strength correlation chart designed around the specific concrete make-up being used in the field. While the manufacturer of the Windsor Probe/Pin test may provide its own correlation chart, that chart would not be tested to the specifications of the concrete actually being used in the construction. As such, it would not conform to either the ASTM or ACI standards.
With this laboratory-tested, concrete-specific correlation chart, the Probe/Pin test may be used pursuant to the ASTM C803/C803M and the ACI 228.1R protocols to determine whether the concrete in the footings, piers or walls has attained the requisite strength specified in §1926.752(a)(1).
If you need any additional information, please contact us by fax at: U.S. Department of Labor - OSHA, Directorate of Construction, Office of Construction Standards and Guidance 202-693-1689. You can also contact us by mail at the above office, Room N3468, 200 Constitution Avenue, NW, Washington, DC 20210; although, there will be a delay in our receiving correspondence by mail.
Russell B. Swanson, Director
Directorate of Construction
1 The standard also requires mortar to be tested. However, there is no ASTM standard test method for field-cured testing of mortar. As the Agency stated to Mr. Purcell in a letter dated December 27, 2002, OSHA is not currently enforcing the §1926.752(a)(1) mortar test notification requirement:
- Since there currently is no ASTM standard test method to test the compressive strength of field-cured mortar, OSHA will not enforce the requirements of §1926.752(a)(1) until such time as we are able to define an appropriate substitute or until an appropriate ASTM test method is developed.
2 ACI 228.1R In-Place Methods to Establish Concrete Strength.
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