Contractors in Colorado have been using intelliRock concrete maturity loggers in shotcrete for several years. In one instance in 2008, winter arrived and the contractor was concerned about construction speed and heating costs. The contractor developed a maturity calibration curve then place intelliRock maturity loggers along the retaining wall being built. Once the concrete was placed the wall was covered and the heaters were turned on.

In general, a typical Colorado winter spec requires the contractor to maintain the placement above 50 F for 7 days or until there's proof that the placement has achieved 75% of the required concrete strength. In this case, the contractor used intelliRock to determine that the placement achieved 75% strength well in advance of the turn-time on previous walls - and during this one it was snowing! The savings on the heating costs alone made the owner extremely happy, not to mention the time savings.

C-DOT has since added concrete maturity to its construction specifications under section 641. The 2009 revision can be found at http://www.dot.state.co.us/DesignSupport/Construction/Recently%20Issued%20Specs/2009-11-05/601641s.doc

ASTM C 1074 "Standard Practice for Estimating Concrete Strength by the Maturity Method" is the basis for virtually all concrete maturity specifications in the U.S.  The document provides procedures for estimating concrete strength using a maturity index as either a “time-temperature factor” or “equivalent age.”   The resulting strength information can be used to allow the start of construction activities such as:

1.    Removal of formwork
2.    Post-tensioning
3.    Cold weather protection termination
4.    Opening roadways to traffic

When using maturity on workflow-related activities maturity is replacing or enhancing information typically given by field-cured cylinders.  It is important to realize that maturity does not replace all usage of concrete test specimens (cylinders or beams).  The maturity method is based on information from cylinders or beams and destructive testing of specimens must continue for quality control purposes, to ensure consistency of the concrete mix.

The overall procedure is comprised of the following steps:

CALIBRATION
1.     Select a concrete mix design
2.    Prepare test specimens (beams, cylinders, cubes, etc).   At least qty 15.
3.    Embed a maturity sensor in the center of two test specimens.
4.    Cure the specimens.
5.    Perform breaks, typically at 1, 3, 7, 14, and 28 days and read the specimen’s maturity from the sensors in the instrumented specimens.
6.    Compile a strength vs maturity calibration curve from t he data.

ESTIMATING IN-PLACE STRENGTH
1.     Embed a maturity sensor either before, or immediately after concrete placement.
2.    Begin logging temperature and maturity information
3.    As the concrete cures, monitor the maturity reading until the maturity index indicates that the target strength is attained.  The target strength is typically 75% or 100% of the specified strength.
4.    Convert the maturity reading to compressive or flexural strength as needed.
5.    Validate the delivered mix to be sure the delivered concrete is consistent with the expected mix design.  There are multiple ways to accomplish this step, but you do NOT need to wait on test specimens to reach target strength.
6.    If the mix is validated, the strength reading based on the maturity index can be used for timing construction operations.

For more information, review the current version of ASTM C 1074.  You can purchase a copy online at: http://www.astm.org/Standards/C1074.htm