A Look at Discrepancies in Concrete Strength Testing

As a general contractor or subcontractor working on a concrete structure, one of the most important tasks is ensuring that the concrete has been properly cured and its quality has been tested according to applicable standards. This is of the utmost importance for quality control and quality assurance purposes. Equally important is selecting an appropriate and accurate method for monitoring the strength of in-place concrete. Unfortunately, popular methods of testing concrete quality, especially compressive strength, are frequently subject to discrepancies.

Cylinder Break Tests

If the project is anything like most other concrete construction sites, break tests are likely used to monitor the strength of newly placed concrete. This practice has remained mostly unchanged since the early 19th century. There are two types of specimens that field technicians collect to test the strength of concrete: standard-cured cylinders and field-cured cylinders. These samples are cast and cured according to ASTM C31, Standard Practice for Making and Curing Concrete Test Specimens in the Field, and are tested for compressive strength, most often by a third-party testing laboratory.

As the name suggests, field-cured cylinders are subject to the same temperature and relative humidity conditions that the completed structure will experience in its environment. Unlike standard-cured cylinders, field-cured specimens are kept right beside the concrete slabs on site. They are predominantly used for determining whether a structure is ready for critical operations like removing formwork or post-tensioning.

In standard or lab curing, concrete cylinders are sent to the lab where they are stored in curing tanks or rooms which are subjected to curing conditions outlined in the ASTM standard and the project’s specifications. Standard-cured cylinders are generally tested 28 days after the concrete is placed for quality control and standard acceptance purposes.

Although cylinder break tests are the most widely accepted method of compressive strength testing, they are frequently associated with testing discrepancies that are not often genuinely representative of in-situ concrete elements. Curing conditions, the surface area of the cylinders compared to the onsite concrete element, and transportation to the laboratory of field-cured specimens are all factors that can skew the setting, hardening, and strength performance of the samples in comparison to the actual structural elements made from the same concrete material.

Standard-Cured Cylinders

Even though the process of testing cylinders is fully standardized, there has been a considerable amount of “bad” or low breaks recorded when standard specifications are not properly followed on site. The American Concrete Institute’s (ACI) pertinent specifications (ACI 318-14, 301-16, and 311.6-09) state that acceptance test specimens need to be standard-cured in accordance with ASTM C31. After the cylinders are molded, ambient temperature and humidity are to be monitored and maintained. Test specimens are required to be stored in a temperature range of 60°F to 80°F (16°C to 27°C) for a period of up to 48 hours (subject to change based on the type of concrete). Moisture and relative humidity loss are prevented by storing the samples in a moisture-filled environment, which is typically a cooler installed on site. Improper temperature and relative humidity control at the initial stages of the cylinder life can result in inaccurate strength data when testing occurs at later ages. Furthermore, as standard-cured cylinders are subject to these strict curing conditions, they largely do not reflect the in-situ concrete but rather verify the QA/QC of the concrete’s mix design to ensure it meets specifications.

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