The Problem

Breaking concrete test specimens to determine concrete strength dates back to the early 1900s. Although standard cured cylinders are valuable tools for determining the potential strength of a concrete mix design or as-delivered concrete, field cured cylinders are commonly used to determine in-place concrete strength at early ages (e.g. 1 to 7 day breaks) during construction. Are field cure cylinders – cylinders cured in the same environment as the concrete placement – accurate representations of the actual placement? What if they’re not?

TIME - Are you needlessly waiting for cylinders to achieve target strength? What if your placement was ready hours ago? What’s an hour costing you? Or even days! Now multiply that cost by the number of pours!

MONEY - Is that high-early mix design really necessary? What if you could save money on every cubic yard of concrete by using a standard mix? It is very often possible!

SAFETY – Is it really safe to strip those forms? Are those columns ready to load? Maybe not!

The Reason
Since cement hydration is slow at low temperatures, and fast at high temperatures, the strength of concrete at any moment is very much dependent on that concrete's age and temperature history. This means that the temperature history of cylinders used to represent in-place concrete strength must be of the same age, AND the same temperature history in order to accurately represent the in-place concrete's strength.

Even if the curing environment of the placement and test specimens are similar, as is the case with field cured cylinders, the temperature of each may be drastically different! The primary reasons for this difference are size and geometry. Heat that is generated by hydrating cement can easily and quickly escape test specimens due to their small size. However, as placement dimensions increase to "feet", it takes days for heat to escape. This causes temperatures to rise, and rising temperatures mean faster hydration and faster strength gain.

The graphs to the right show an example of this behavior in a concrete wall. The top graph shows the temperature of a field cured specimen versus the actual temperature of the placement. Note that the exothermic reaction causes the placement temperature to rise. This is because the heat is not able to escape through the concrete mass and formwork. However the field-cured cylinder, in the same curing environment, doesn’t retain heat and is also strongly affected by the ambient temperature. The bottom graph shows the resulting strength gain over the first 24 hours. The placement achieves 4500 PSI while the companion cylinder only reaches 1500 PSI. As you can imagine looking at these graphs, if the formwork was able to be stripped at 3000 or 4000 PSI one would be unnecessarily waiting for days for the cylinders to reach target strength, while the placement was ready in only 24 hours.

intelliRock - How it works

intelliRock uses the procedures defined in ASTM C 1074 to determine concrete strength based on the maturity method. This method combines cylinder break data and the actual placement temperature profile to calculate a much more accurate measurement of in-place concrete strength.

  • Step 1: Calibrate - Each mix design is calibrated versus the maturity reading measured and reported by intelliRock sensors/loggers.

  • Step 2: Place Sensors - Place sensors in the concrete pour. This is often accomplished by attaching the sensors to rebar prior to the pour.

  • Step 3: Read Strength, anytime! - Simply read the maturity value reported by intelliRock and convert the value to strength (PSI) using the calibration data for that particular mix design.

  • Step 4: Verify - Periodically place sensors in cylinders to monitor and ensure mix consistency.

“The information from intelliRock has been invaluable. It gave us peace of mind knowing when to jump forms. From a safety standpoint we knew the strength of the concrete in real-time, so we were not taking any chances with the lives of the men that were moving the forms. Economically, intelliRock put the project 6 weeks ahead of schedule."

SKANSKA Wesley Pickens, Quality Assurance Manager, Skanska USA Building Inc.

The benefits

What if they're wrong?

Real World Example


Specifications and References
  • ASTM C 1074 - "Standard Practice for Estimating Concrete Strength by the Maturity Method"
  • ASTM C 918 - "Standard Test Method for Measuring Early-Age Compressive Strength and Projecting Later-Age Strength"
  • ACI 228.1R (Section 2.7)- "In-Place Methods to Estimate Concrete Strength"
  • ACI 306R - "Cold Weather Concreting"
  • ACI 318 R6.2 - "Building Code Requirements for Structural Concrete"
  • NRMCA CIP 39 - "Maturity Methods to Estimate Concrete Strength"
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