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Much of the concrete industry’s sustainability discussions have focused on mixture optimization, reducing portland cement content and increasing the use of supplementary cementitious materials (SCMs). While these approaches are necessary, they can overlook a significant and correctable source of embodied carbon: variability in acceptance testing caused by improper initial standard-curing of concrete strength specimens per ASTM C31 Standard Practice for Making and Curing Concrete Test Specimens in the Field.
Multiple studies show that concrete cylinders initially cured in conditions which did not comply with ASTM C31 can reduce measured compressive strength by as much as 28 percent compared with proper standard curing. In response, ready-mixed producers provide additional cement in the mixture designs to account for testing uncertainty, which results in higher average strengths and embodied carbon. An understanding and consistent enforcement of ASTM C31 and ACI 301 initial standard-curing requirements and responsibilities by all members of the ownership-designer-testing lab-contractor community would allow ready-mixed producers to reduce cement content associated with the risk of low breaks caused by improper initial standard-curing.
Two industry panels, each with 9-10 participants, were assembled to discuss the state of concrete cylinder initial curing as it relates to structural engineers, testing labs, contractors, ready mixed concrete production and concrete consulting. The representation in these discussions included:
| Structural engineering firms | 4 |
| Concrete testing laboratories | 4 |
| Ready-mixed concrete producers | 4 |
| General contractors that self-perform concrete | 2 |
| Concrete specialty subcontractors | 2 |
| Associations, ready-mixed and concrete contractor | 2 |
| Owner and former testing tech | 1 |
| Concrete consultant | 1 |
From the professional engineer’s perspective, it seemed that the most surprising realizations were:
- There can be a gap between what is specified, ACI 301 Specifications for Concrete Construction, which references ASTM C31 initial curing, and what the project owner accepts from the low bidder testing lab, which might exclude ASTM C31 initial curing due to the higher cost of providing this service and the uncertainty of having access to water, power, and site security.
- Since they do not have visibility to the contract between the owner and the testing lab, the design and construction teams are unaware that the testing lab excluded the initial curing until the concrete pre-construction conference or after it is not being done on site.
- Owners do not realize the extent that poor initial curing could skew the concrete test data.
In an attempt to reduce risk, contractors, the testing lab, or ready-mixed producers may provide a cooler without the ability to cool or heat the cylinders. While well intentioned, in almost all cases a regular cooler does not meet the standard for curing temperatures and is a common cause of reduced strength. - The Colorado Ready Mixed Concrete Association has a program, CTAC, which makes thousands of observations per year in 10 states that document whether concrete testing and initial curing are being done correctly. The most frequent issue is initial curing.
Inaction can lead to investigations and finger pointing when low breaks result. - Project scale can determine the approach to testing and initial curing. Figure 1 is derived from comments by participants.
Structural engineers have several opportunities to make a difference:
- Enforce specifications by requiring testing agencies to comply with ASTM C31 and contractors to comply with ACI 301. This can be reiterated by participating in the preconstruction meeting and raising this point.
- Inform owner and design team members that the project team will need to make a commitment to ASTM C31 initial curing to be able to rely on concrete break results.
- State the ASTM C31 temperature ranges explicitly in the project specifications. History has demonstrated that only providing a direct or indirect reference is not working.
- Create a project mechanism for live reporting of the initial curing conditions 24/7 with sensors and a web portal.
In this context, ask contractors and ready-mixed producers to provide leaner mixes with less overdesign to achieve project sustainability goals.
In summary, structural engineering firms have an opportunity to improve project sustainability and the reliability of concrete testing by asserting their influence towards enforcement of the existing ASTM C31 standard. ■
What Does ASTM C31-26 State?
Standard Practice for Making & Curing Concrete Test Specimens in the Field
3.2 Definitions of Terms:
3.2.1 Acceptance Test Specimens—standard-cured test specimens intended for evaluating whether the supplied concrete complies with the specification.
3.2.2 Field Curing—storing test specimens in the field under an environmental temperature and moisture environment similar to the in-place concrete.
3.2.3 Initial Curing—the storage of test specimens in the field after casting and before transporting the specimens to the laboratory.
4.2 Uses of the test results of standard-cured test specimens include the following purposes:
Acceptance testing for specified concrete strength
4.3 Uses of test results of field-cured test specimens include the following purposes:
- Estimation of the in-place concrete strength.
- Form or shoring removal time requirements.
- Post-tensioning.
10.1.2 Initial Standard Curing—Store standard-cured test specimens for a period up to 48 hours after molding to maintain the specified temperature and moisture conditions described in 10.1.2.1 and 10.1.2.2.
10.1.2.1 For concrete mixtures with a specified compressive strength less than 6,000 psi (40 MPa) , maintain the initial standard-curing temperature 60F - 80F (16C - 27C).
- For concrete mixtures with a specified compressive strength of 6000 psi (40 MPa) or greater, maintain the initial standard-curing temperature 68F - 78F (20C - 26C).
- Record the minimum temperature and maximum temperatures achieved for each set of test specimens during the initial standard-curing period.
NOTE 13— At later ages, strength test results may be lower if specimens are exposed to initial curing temperatures higher than the specified range.
10.1.2.2 Store the specimens in an environment that controls the loss of moisture.
What Does ACI Say About This?
From ACI 301-20
- 1.7.2.2 (c): the Contractor is responsible for providing secure locations and sources of water and electrical power on project site acceptable to Owner’s testing agency for initial curing of concrete strength test specimens as required by ASTM C31.
- 1.7.3.3 (e): Owner’s testing agency will conduct concrete strength tests by making standard curing test specimens in accordance with ASTM C31.
From ACI 132-24
“Testing agency has the responsibility to provide the means and methods for initial curing unless that responsibility has been assigned to another party in the contract documents.”
From ACI 132.1-22
“The testing agency should ensure that the initial curing of test specimens complies with the temperature limits and moisture retention requirements of ASTM C31.”
From ACI 311.6-18
Can be used by the owner for testing agency scope which states, “provide and maintain adequate facilities on the project site for initial storage and curing of the concrete specimens unless otherwise specified.”
About the Authors
Mike Hernandez, PE, FACI, is a Sr. Concrete Engineer with Kiewit in Lone Tree, CO, with over 30 years of industry experience focused on concrete construction operations and construction engineering.
Bryan M Birdwell, FACI, is President, Structural Services Inc., a concrete floor design, construction, and repair consulting firm with over 30 years of concrete construction and consulting experience.
Zachoriah Ballard, PE, MCE, is a principal engineer and materials department manager at CTL Thompson, Inc., with over 20 years of experience in construction materials engineering and materials testing.
Jason Wimberly, PE, MCE, FACI, is Vice President, Technical Solutions and Innovation at AM Concrete with over 25 years experience in concrete construction, ready mix production, engineering and consulting.
Panel Participants
The following table lists participants from the two panels (listed alphabetically).
| Name | Role | Employer | Panel # |
| Bobby Dowdy | Ready Mixed / NRMCA Research & Standards Comm Vice-Chair | MMC Materials | 2 |
| Bryce Thoutt | Concrete Contractor | Thoutt Brothers Concrete | 2 |
| Bryan Birdwell | Concrete Consultant | Structural Services Inc (SSI) | 1 |
| Chris Garcia | General Contractor (Self-Perform Concrete) | DPR Construction | 1 |
| Daniel Campos | Ready Mixed Concrete Producer | Martin Marietta Ready Mixed | 1 |
| Heather Brown | Ready Mixed Concrete Producer | Irving Materials (IMI) | 1 |
| Ian McFarlane | Engineer of Record | Magnusson Klemencic Assoc. | 2 |
| Jacob Borgerson | Engineer of Record | Wiss, Janney, Elstner Assoc. | 1 |
| Jason Bray | Engineer of Record | Walter P. Moore | 1 |
| John Fultz | Testing Laboratory | ECS Southeast | 2 |
| Julie Buffenbarger | Ready Mixed & Cement / ACI 132 Responsibility in Concrete Construction Co-Chair | St. Marys Cement & United Materials | 2 |
| Kyle Kammer | Concrete Contractor | Concrete Strategies | 1 |
| Marty Martinez | General Contractor (Self-Perform Concrete) | Swinerton Builders | 2 |
| Matt Sherman | Engineer of Record | Simpson, Gumpertz & Heger | 2 |
| Mike Hernandez | Moderator / Concrete Contractor Association | American Society of Concrete Contractors | 1&2 |
| Richard Allen | Testing Laboratory / ACIL Board | Bowser-Morner | 2 |
| Todd Ohlheiser | State Ready Mixed Concrete Association | Colorado Ready Mixed Concrete Association | 1 |
| Walter Flood IV | Testing Laboratory | Flood Testing | 1 |
| Wes Monley | Owner Representative / former Materials Lab | Denver Water | 1 |
| Zack Ballard | Testing Laboratory | CTL-Thompson | 1 |
References
1. ASTM C31/C31M-24, “Standard Practice for Making and Curing Concrete Test Specimens in the Field,” ASTM International, 7 pp.
2. Montoya, R., “Hot Weather Comparison Program – Fresh Concrete Testing and Initial Curing Practices,” Report to General Membership, New Mexico Ready Mix Concrete and Aggregates Association, Albuquerque, NM, Apr. 1995, 11 pp.
3. White J.L., Fleming K.R., and Schindler A.K., 2023, “Evaluation of jobsite cylinder curing practices for the Alabama concrete industry,” ALDOT Project 931-051, 179 pp., https://eng.auburn.edu/research/centers/hrc/research/final-reports
4. Obla, K.H.; Rodriguez, F.; and Ben-Barka, S., “Effects of Non-Standard Curing on Strength of Concrete” Concrete in Focus, V. 3, No. 4, 2005, pp. 57-59.
5. Kane, K.; Laker, T.; and Morrical, S., “Improving the Quality of Concrete Testing,” Concrete International, V. 37, No. 8, Aug. 2015, pp. 33-40.
6. ACI Committee 301, “ACI SPEC-301-20: Specifications for Concrete Construction,” American Concrete Institute, 69 pp.
7. ACI Committee 132, “Responsibility in Concrete Construction—Guide (ACI PRC-132-24),” American Concrete Institute, Farmington Hills, MI, 2024, 11 pp.
8. ACI Committee 132, “Responsibility for the Care of Test Specimens for Acceptance of Concrete—TechNote (ACI PRC-132.1-22),” American Concrete Institute, Farmington Hills, MI, 2022, 5 pp.
9. ACI Committee 311, “Specification for Testing Ready Mixed Concrete (ACI SPEC 311.6-18),” American Concrete Institute, Farmington Hills, MI, 2018, 6 pp.
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