Mike Hernandez, Technical Director The Voice Newsletter October 2023
The maturity method for estimating in place strength of concrete has been around for decades. It is useful for timing the start of post-tensioning unbonded cables and for removing formwork under elevated decks. A concrete mix can be correlated to make a time-temperature strength curve. This is quite helpful in the winter months when in place strength could be substantially different than the cylinders taken on pour day and stored in a curing box at 60F to 80F or even from field cured cylinders because the cylinders have far less mass than the structure so they will generate less hydration heat. Recently, the maturity method is being used to optimize saw cutting of slabs on grade. Contractors are trying to hit the Goldilocks window of just strong enough to not ravel the sawcut edges yet fresh enough that random shrinkage cracks are not developing yet on Type IL cement concrete mixes. Knowing early age strength is a helpful tool.
My experience using maturity probes dates back to the early 2000’s. The team was placing elevated decks in Denver Colorado, from December - March while heating the formwork enclosed from below. A strength to maturity relationship or maturity index had been developed that fall using ASTM C1074 by the testing lab. They needed 15+ cylinders and a few maturity probes. With a strength to maturity relationship, our team could figure out the approximate strength of the concrete at any given time. It was a balmy 70F under the tented and heated decks that winter. Each morning and afternoon I would go out and attach a reader to the wires sticking out of the deck, which were connected to loggers inside the concrete. The maturity probes provided feedback on the current temperature and degrees C-hours number, which correlated to a given strength. Once all the probes had crossed the minimum threshold, stressing of post-tension cables began. Without the reassurance of the maturity index, it is unlikely the engineer of record, GC or owner would have allowed us to stress PT and a few days later strip decks so quickly in the dead of winter. ACI 318 and CSA A23.1 both reference this strength evaluation method in accordance with ASTM C1074 as do 30+ state departments of transportation. NRMCA Technology in Practice #15 “Estimating Concrete Strength using Maturity” is a useful resource.
By the 2010’s, radios were added to the same equipment and types of probes, which were used to monitor the temperature on hundreds of mass concrete placements our team had in bridge substructures. ACI and FDOT set limits on maximum temperature and differential between the core of the concrete and the surface. Temperature probes are the only effective way to track and record this data. Part of the challenge was to monitor the temperature every 8 hours to meet the specification and know if thermal control concrete blankets needed to be added or could be removed. With the radios in the system, we could connect to a computer brought within 300’-500’ of the probes. Still had to drive on site close to midnight, but it was better than manually plugging wires into a handheld reader.
Several years ago, the wires were deleted and replaced by Bluetooth capable devices, simplifying their use even more. Lifting tilt up panels in winter, we collaborated with our ready mixed supplier and had maturity devices in the last panels placed prior to erecting. The probes confirmed the panels were stronger than the field cured cylinders. and we were confident to start lifting.
The introduction of Type IL cement concrete or Portland Limestone Cement (PLC) concrete into ~40% of all concrete purchased in the U.S has required contractors to consider a few more things. Contractors will need to rerun maturity index strength curves using PLC mixes, they are no longer valid for maturity curves developed using I/II cement. From discussion with many contractors, the acceptance cylinders with IL cements seem more sensitive to poor initial curing than the I/II mixes.
Having probes in the concrete could resolve slow and expensive disputes about in place concrete strength. Many contractors are reporting lower early age strength compared to older I/II mixes. Using the maturity method will help keep schedules on track by being able to monitor strength continuously, not just when cylinders are broken.
Regarding saw cutting, the US Federal Highway Administration (FHWA) has a free software HIPERPAV to look at early age strength of concrete to time the cutting of control joints. Combining this with maturity probes can assist contractors doing paving work and large slabs on grade. This is one more tool to have in the box of options when trying to be a productive ASCC contractor.