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Debunking the 28 Day Concrete Curing Myth
  • 28 days used in testing specification is not an indicator that concrete has reached a specific point in the curing process

Published in:
  • Barger, Eric. "Debunking the 28 Day Concrete Curing Myth," Precast Solutions (Fall 2013): 28-29. [PDF]
Throughout the construction industry a common belief exists: concrete takes 28 days to “cure” and reach 100% of its strength. This myth comes from unnecessary precaution on the part of the specifier and a misunderstanding of what curing actually means. Some regulators will often characterize concrete as “green” if it has not cured for a certain time frame, leading to unnecessary delays and higher project costs. Evaluating separately the concepts of measured compressive strength and curing will shed light on the mystery that surrounds 28 days.

Specifying concrete strength is normally done with a minimum compressive strength (psi) at a certain age (days). Specified concrete compressive strength (f’
c) is the minimum compressive strength at which the concrete should fail at a maximum age (days). A typical specified concrete strength looks like the following example: 5,000 psi @ 28 days. Some specifiers go a step further and mandate concrete products cannot be installed or used until 28 days after the date of manufacture. This method of specifying concrete strength has allowed the misconception to perpetuate that concrete is not fully “cured” and has not reached the required compressive strength until 28 days has passed, which implies concrete not having 28 days to cure must be weak or inferior.

Twenty-eight days is an arbitrary age, though chosen for many good reasons, for testing the compressive strength of concrete. Specification writing authorities chose 28 days as the standard age that concrete should be tested, thus bringing consistency to the testing procedures throughout the industry.
(1) Concrete does not get weaker as it ages from 3 days to 28 days. Its compressive strength can only remain stable or increase over time. It is estimated that concrete reaches 75% of the 28 day strength in 7 days.(2) A specific ratio can be determined for each specific mix design and curing process to fine tune this estimate. Therefore, it is the mix design that dictates the 28 day compressive strength. There are mix designs that reach a 5,000 psi compressive strength in seven days or even in twenty-four hours; however, the faster concrete reaches the minimum design compressive strength, the cost of the mix design increases.

Measured Compressive Strength
Measuring the compressive strength of concrete is achieved by taking a sample of concrete at the time of placement. Cylinders measuring 6” diameter x 12” tall are compressed by a break machine that is a type of press. The machine exerts increasing force (lb) upon the cylinder until it structurally fractures. When a failure, commonly referred to as a “break”, occurs, the compressive strength is measured by taking the force measured at the time the cylinder failed divided by the surface area of the concrete sample. At the time of the break, the age is generally noted for quality assurance purposes. Therefore, to meet the specified compressive strength as noted above, the break results should calculate to at least 5,000 psi at 28 days.

When a specification has a performance criteria of 5,000 psi @ 28 days, for example, governing authorities will want a test record - two individual cylinders breaks - at 28 days to ensure conformance with the specification. Fortunately, while collecting concrete samples for testing, more than two cylinders are typically collected. Through research and empirical data collecting, projecting a 28 day strength from a three or seven day test break is easy to perform and results in an estimated strength very close to the strength measured at 28 days. With the accuracy of this early estimation, if a problem arises, it can be reviewed much sooner than 28 days.

Curing Misunderstood
“Curing is the process of controlling the rate and extent of moisture loss from concrete during cement hydration.”(3) This would refer to a great many processes that are used to cure concrete such as steam curing, moist curing, air curing, etc. It is immediately apparent the phrase “curing” can lead to different meanings to different people. When someone states the concrete has not cured, are they really referring to the process, which is the actual meaning of curing? No. Strength and age are the overwhelming misunderstood associations behind the word curing.

Digging deeper into the curing process, hydration is the chemical process that allows concrete the ability to go from a plastic state to a hardened state while gaining strength over time. Hydration occurs at a faster rate in the early stages after concrete placement and slows down after a month, give or take, goes by. Hydration reaches completion in modern day concrete mixtures around one year from manufacture but this is not always the case. Many cement pastes will have hydration stop before one year and some may continue to hydrate over the course of several years.
(4) Because of the length of the hydration process, the phrase “green concrete” is a purely subjective characterization made by someone who does not understand concrete.

Waiting for the hydration process to complete or near completion before testing the compressive strength of concrete would be impractical. An age more practical was chosen to determine concrete strength and this is where the 28 days comes in. While curing does help the hydration process, it is not an inclusive rule directly related to a specific time to produce minimum measured compressive strengths. Simply stated, as the concrete cures and hydration takes place, the concrete gets stronger. Hydration in concrete can continue long after the minimum required compressive strength is reached.

A specified compressive strength may be given in any strength and age. There is nothing wrong with specifying a 5,000 psi compressive strength at 1, 7, 11, 14, 56 days, etc. It all depends on the mix design, circumstances, and project requirements. The American Concrete Institute recognizes 28 days or at the test age designated for determination of the specified compressive strength.
(5) Therefore, for a specifier, simply specifying 5,000 psi concrete at 28 days, for example, tells the concrete producer that the mix design should be set such that a minimum of 5,000 psi is reached 28 days after manufacture.

In typical precast manufacturing, it is extremely rare that a specifier would need to make any specified requirements regarding curing. Specifiers could consider to further state that concrete can be installed prior to 28 days provided two cylinder breaks prove the minimum required compressive strength has already been reached. With a select mix design, concrete could easily reach its minimum required compressive strength the day after it is manufactured and be ready for installation.

Conclusion
Reaching the minimum required compressive strength before 28 days is most often the case in precast concrete manufacturing. Curing is a process and not a measure of concrete strength, and the use of 28 days provides a basis for comparison of compressive strength of concrete products. The 28 day timeframe is not directly related to whether a specific product meets compressive strength requirements for a particular application. As long as the minimum compressive strength is met before the product is put in service, the timeframe to reach the minimum compressive strength should not be relevant.




Pullout Bullets
  • 28-day curing myth permeates construction industry
  • Use of 28 days provides basis for comparison of compressive strength of concrete products, but is not directly related to whether a specific product meets compressive strength requirements for a particular application
  • 28 days used in testing specification is not an indicator that concrete has reached a specific point in the curing process




Works Cited

1. Cement & concrete faqs (frequently asked questions) | portland cement association (PCA); Available from: http://www.cement.org/basics/concretebasics_faqs.asp. Accessed 5 July 2013.
2. Kosmatka SH, Panarese WC, Allen GE, Cumming S. Design and control of concrete mixtures. Portland Cement Association Skokie, IL; 2002.
3. Curing of concrete; April 2006. Available from: http://www.concrete.net.au/publications/pdf/Curing06.pdf.
4. 5.1 - overview of the concrete hydration process; Available from: http://iti.northwestern.edu/cement/monograph/Monograph5_1.html. Accessed 5 July 2013.
5. Building code requirements for structural concrete (ACI 318-11) and commentary. Farmington Hills, MI: American Concrete Institute; 2011.

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