A common challenge in concrete construction is seeing 28-day cylinder results that fall below the specified strength, even when everything onsite tells a different story. The structure performs well, inspections raise no concerns, and core samples later confirm that the concrete has reached the strength it needs.
So why the discrepancy?
It often comes down to curing conditions. Cylinders stored at a steady 73°F (23°C) in a curing box follow standardized requirements, while concrete placed in the field may experience temperatures anywhere from 45°F (7°C) overnight to 95°F (35°C) during the day. In other words, the cylinders cure in ideal conditions that the in-place concrete never sees.
This difference can create confusion around acceptance criteria, lead to schedule impacts, and prompt unnecessary adjustments to mix designs.
In this blog, we take a closer look at why these gaps occur and compare the two primary approaches for monitoring concrete temperature and estimating strength: traditional curing boxes and the maturity method. By understanding how each method works and when to use them, you can make better, more informed decisions on your next project.
Understanding Curing Boxes
Curing boxes are insulated containers designed to maintain concrete test cylinders at the standardized temperature outlined in ASTM C31: 73.4°F (23°C) ± 3°F. By controlling temperature, these boxes help evaluate a mix’s potential strength under consistent laboratory conditions.
Two types are commonly used:
- Standard curing boxes: Maintain 73°F from initial curing through testing
- Field curing boxes: Aim to reflect jobsite temperatures more closely
Most boxes include insulated walls, heating or cooling components, and temperature control systems. The workflow is familiar: technicians cast cylinders, place them in the curing box, transport them to the lab, and perform breaks at scheduled intervals.
This method is widely accepted, straightforward, and essential for specification compliance. However, real-world conditions don’t always match the stable environment inside the box—and that’s where limitations arise.
When Curing Boxes Create Blind Spots
Because curing boxes hold cylinders at a steady temperature, they may not reflect how concrete behaves on site.
- Cold weather example:
A February pour in Chicago cures slowly in the field at around 35°F (2°F), even with insulation. Cylinders kept at 73°F (23°C) gain strength faster. Early test results may suggest the concrete is ready for formwork removal when the in-place material is still behind.
- Hot weather example:
Concrete placed at 95°F (35°C) gains strength more quickly than cylinders at 73°F (23°C). This can delay construction unnecessarily while teams wait for lab breaks that lag behind the structure’s actual performance.
A significant temperature difference can shift strength development timelines by several days. Logistics also introduces risk; handling cylinders improperly can reduce measured strength.
Cylinder testing remains valuable, but it does not always provide the full picture, especially in fast-paced projects or in climates with large temperature swings.
The Maturity Method: Monitoring What Happens in Place
The maturity method provides an alternative by directly measuring how the concrete in your structure behaves. Instead of curing separate cylinders, sensors track the temperature of the actual placement. Strength is estimated using the temperature history of that concrete, following the guidelines of ASTM C1074.
How It Works
- Create a calibration curve:
Technicians cast cylinders using the project mix, embed temperature sensors in some of them, cure them under controlled conditions, and test them at intervals. Strength results are plotted against maturity values to establish a curve.
- Install sensors in the structure:
Wireless sensors are embedded at critical locations during placement.
- Monitor continuously:
Sensors record temperature frequently. Software calculates maturity values, giving engineers real-time insight into estimated strength.
Modern systems provide mobile access to data and automated notifications when strength thresholds are met. The focus is simple: understand what the concrete is achieving in the field, not what it might achieve under lab conditions.
Key Differences at a Glance
To help clarify where each method shines, let’s break down the key differences that influence strength monitoring and decision-making in the field.
Temperature Representation
- Curing boxes: Maintain a consistent 73°F for standardized results
- Maturity method: Captures real onsite temperatures, whether hot, cold, or fluctuating
Timing
- Curing boxes: Strength verification occurs at scheduled breaks (7, 14, 28 days)
- Maturity method: Provides continuous updates, supporting earlier, more confident decisions
Relevance to In-Place Concrete
- Curing boxes: Cylinders cure separately under controlled conditions
- Maturity method: Sensors reflect the actual temperature history of the structure
Cost Structure
- Cylinders: $25–$50 per test, plus labor and transportation
- Maturity sensors: Higher upfront cost, but reusable practices and time savings can offset expenses, especially on larger projects
Meeting Industry Standards
- Understanding the relevant standards helps ensure that whichever method you choose—cylinders or maturity—fits within accepted industry practices and project requirements.
- ASTM C31: Standard Practice for Making and Curing Test Cylinders
- Governs how cylinders are prepared, cured, stored, and transported
- Requires maintaining curing temperatures of 73.4°F ± 3°F
- Defines initial curing duration and handling protocols
- Ensures cylinder results remain consistent and comparable across projects
- ASTM C1074: Standard Practice for the Maturity Method
- Establishes procedures for implementing the maturity method
- Details calibration requirements using the project’s actual mix
- Sets accuracy expectations for temperature measurements
- Outlines maturity calculations and guidelines for estimating in-place strength
- ACI Recognition
- ACI 318 references cylinders as the default acceptance method
- Does not prohibit alternative approaches such as maturity
- Reinforces that maturity can supplement, but not replace, acceptance testing where required
- State DOT Adoption
- Many DOTs now permit maturity monitoring for specific decisions
- Commonly used to determine formwork removal timing or when to open pavements to traffic
- Cylinder testing is typically still required for final strength verification
How to Implement the Maturity Method
Mix Calibration
Cast at least six cylinders using the production mix. Embed temperature sensors in half the cylinders. Cure all cylinders under controlled conditions. Test pairs at intervals (1, 3, 7, 14, 28 days). Record maturity at each test age. Plot compressive strength against maturity to create your calibration curve.
Sensor Installation
Location matters. Place sensors at points representing critical concrete or areas with severe exposure. For slabs, position sensors at mid-depth in the center of large pours. For walls or columns, install near the surface where cold weather impacts occur most severely.
Protect sensors during finishing. Mark sensor locations clearly. Verify wireless connectivity after placement. Most systems confirm sensor communication through mobile apps within minutes.
Monitoring and Interpretation
Access temperature and maturity data through your monitoring platform. Most systems update every 15 to 30 minutes. Compare current maturity values to your calibration curve. When cumulative maturity matches the calibration point for required strength (often 3,000 or 4,000 psi), the concrete has likely achieved that strength.
Document everything. Save temperature histories, maturity calculations, and strength estimates. Export reports for submittal to engineers and inspectors.
Cost Considerations
Evaluating the true cost of each method requires looking at both direct expenses and the broader impact on project timelines.
Curing Boxes
- Equipment: $500–$2,000 purchase or $50–$150 monthly rental
- Recurring materials: $5–$15 per test
- Lab fees: $25–$50 per cylinder
- Labor and transportation add to total cost
Maturity Monitoring
- Sensors: $100–$300 each
- Software: $50–$200 monthly
- Training or initial support for first projects
Calculating ROI
Time savings from early formwork removal put your schedules ahead. Each day saved can reduce costs on labor, materials, and unexpected delays. A single day might save $2,000 to $10,000 depending on project scale.
Break-even depends on project characteristics. Small projects with minimal testing favor cylinders, whereas large projects with tighter schedules and numerous pours recover maturity costs quickly.
Choosing the Best Approach
Curing boxes remain essential for meeting specification requirements and establishing standardized strength benchmarks.
The maturity method complements this by offering real-time insight into how concrete actually behaves onsite, especially valuable for cold or hot weather, accelerated schedules, or mass concrete placements.
Many teams use both: cylinders for acceptance and maturity for decision-making. With modern wireless sensors and cloud-based platforms, maturity monitoring has become an accessible tool for improving quality control and project efficiency.
