Measuring the early-age strength of concrete is an important step in discerning its strength. The challenge lies in finding a way to obtain concrete strength data in a simple, yet fast, and efficient manner. With sensors, measuring concrete strength, also referred to as “maturity”, is a non-destructive method that can greatly optimize your jobsite schedule.
Maturity is a non-destructive approach to testing concrete that allows you to estimate the early-age and compressive strength of in-place concrete in real-time. Adopting the maturity approach in your jobsite eliminates the need for concrete cylinder break tests, allowing you to greatly optimize your schedule.
ASTMC1074, the standard practice for maturity, defines the method as “a technique for estimating concrete strength that is based on the assumption that samples of a given concrete mixture attain equal strengths if they attain equal values of the maturity index.”
In other words, maturity is a value that represents the progression of concrete curing. The maturity index value considers concrete temperature and curing time. As a result, mix calibration is required to implement this concept in a project. The goal of the calibration is to determine a relationship between maturity and strength for a specific mix.
Using a concrete maturity sensor allows you to collect such data. The sensor works by measuring the temperature of the concrete and then calculates the concrete’s strength/maturity through the calibration data previously inputted by the user. Doing so replaces the need for break tests. These wireless maturity sensors, like SmartRock®, eliminate the use of cumbersome wires. Furthermore, it can connect to any smart mobile device and transmit data instantly without the use of a data logger, which is often expensive.
Measuring the early-age strength of concrete is an important step in discerning its strength. The challenge lies in finding a way to obtain concrete strength data in a simple, yet fast, and efficient manner. With sensors, measuring concrete strength, also referred to as “maturity”, is a non-destructive method that can greatly optimize your jobsite schedule.
The goal of the maturity calibration is to determine a relationship between maturity and strength for a specific mix. This calibration can be used to determine the in-place strength of the concrete and evidently replace the need for field-cured cylinders. To perform a maturity calibration, the ASTM C1074 standard must be followed.
Strength=a+b LOG (maturity)
Eq.1: Formula for calculating the maturity value of concrete
Ready-mix customers can save even more time on their job site by purchasing SmartRock® Plus sensors. Contractors benefit from SmartRock® Plus because the mix is pre-calibrated by the concrete producer, saving them significant time by having calibration data readily available via their iOS or Android app.
Structural integrity is at risk when strength measurement data is inaccurate. This shortens the life cycle and decreases the strength of the mass concrete element. That is why receiving accurate and timely data that allow construction workers to move forward with form removal and
post-tensioning is crucial. Although the use of break tests has been common practice in the construction industry for decades, it does not mean that this is the most accurate and reliable method in obtaining strength data.
Lack of an accurate estimation of strength at early ages of construction is twofold:
In adopting the use of wireless sensors on-site, contractors can collect more accurate data and ensure the safety, efficiency, and reliability of their structure.
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Information is gathered through the casting of cylinders taken from the pour and crushed in a compression machine. Information is gathered by embedded sensors recording temperature and strength in real-time. Results may be affected by improperly prepared, handled, and/or tested cylinders. The data is logged without interruption, so the results are generally more consistent. It takes time to send samples to the lab and retrieve results, causing delays onsite. Strength results are collected in real-time. Technician costs to cast, collect, deliver, and test results, then repeat the process. Up to 50% in direct test cost savings for determination of in-place strength of concrete done by on-site team members.
It takes time to send samples to the lab and retrieve results, causing delays onsite. Strength results are collected in real-time. Additional labor costs due to uncertainty in project scheduling resulting from delays in receiving lab reports. Up to $10,000 in labor savings as a result of more accurate job-site planning for each floor of a high-rise building.
Break Test(Destructive Test) | Maturity Test(Non-Destructive Test) |
|---|---|
| Test Procedure | |
| Information is gathered through the casting of cylinders taken from the pour and crushed in a compression machine. | Information is gathered by embedded sensors recording temperature and strength. |
| Testing time could be too early or too late. | Data is logged and/or retrieved by an external wireless device in real-time. |
| ASTM C39 | ASTM C1074 |
| Reliability | |
| Results may be affected by improperly prepared, handled, and/or tested cylinders. | The data is logged without interruption, so the results are generally more consistent. |
| Cylinders have small volumes but large surface areas, so they retain less heat which results in low breaks. | The maturity method predicts the actual in-place strength of concrete. |
| Temperature history for cylinders may differ due to curing conditions causing a different rate of strength gain which results in low or high breaks. | Maturity shows local variation in strength for different structural locations. |
| Speed | |
| Takes time to send samples to the lab and delay to receive results from the lab. | Strength results are collected in real time. |
| Cost | |
| Technician costs to cast, collect, deliver, and test results, then repeat the process. | Up to 50% in direct test cost savings for determination of in-place strength of concrete done by on-site team members. |
| Additional labor costs due to uncertainty in project scheduling resulted from delays in getting the lab report. | Up to $10,000 labor saving as a result of more accurate job-site planning for each floor of a high-rise building. |
| Extra financial costs due to late completion time in projects. | Significant financial savings as a result of early completion of the project. (Actual savings vary depending on the size of the project). |
While on a jobsite, engineers want to know as much information as possible to help guide their decision-making throughout the duration of a project. In most construction sites, field-cured concrete samples are tested for strength at various ages during the first week to decide when formwork should be removed. ASTM C31 section 10.2 defines field curing as a condition that “involves subjecting the specimens to the temperature and humidity that the actual structure experiences.” Usually, if the break tests show that the concrete reaches 75% of its designed strength, the structural engineers allow for the stripping of forms to take place, and the project can go on to the next steps.
One of the problems, however, is that cylinders that undergo a break test have a much smaller volume, but a larger surface area, compared to the in-place structure or slab. As a result, less moisture is retained than the actual structural element, making the specimen not necessarily representative of the in-place strength, often causing low breaks. Additionally, as the specimens for the break tests are being transported to the third-party lab, the construction crew remains on the jobsite awaiting data results, adding unnecessary labor costs.
Comparatively, as a non-destructive testing technique, the maturity concept is a reliable practice that can eliminate guesswork. Other onsite non-destructive methodologies in use to measure strength, such as the Schmidt Hammer or Ultrasonic Pulse Velocity techniques, are often less exact than maturity. The maturity equation is able to more accurately estimate the compressive strength of the entire structure in an objective and quantitative measurement, once the maturity curve is calculated through calibration of the concrete mix.
For jobsites to operate smoothly they need to have the right tools. Maturity sensors, like SmartRock, greatly benefit engineers, project managers, and field personnel in numerous ways.
Maturity sensors can provide engineers with real-time data that can be accessed on any mobile device and distributed to all team members through the cloud. The ability for the sensors to provide fast results allows for well-informed and quick decision-making onsite.
When compared to break tests, the maturity method provides more accurate and reliable results, effectively avoiding inaccuracies associated with lab tests. Furthermore, continuous logging of concrete temperature and strength allows contractors to reduce the possibility of liability in case of structural failure.
As a non-destructive approach, project managers value concrete maturity meters due to their ability to collect data measurements on their own. Knowing the information that is collected from these sensors is accurate, project managers can make decisions immediately.
Not having to wait for results of cylinder break tests also drastically reduces the costs associated with labor and equipment. This also eliminates the need to employ a third-party testing lab.
Forget having to untangle, cut, or fuss with wires. With concrete maturity sensors, wires are a thing of the past. Having wireless sensors also means no longer having to rely on break tests to measure the strength of your concrete, saving hours, even days, on your projects’ schedule.
Real-time monitoring of early-age concrete strength allows contractors to proceed with critical operations like formwork removal, post-tensioning, and shore stripping much sooner than if they were relying on laboratory break tests. Ultimately, this cuts days, even weeks, off project schedules. In addition to that, sensors are fully embedded in the concrete, and easy to install. Simply label the sensor, install it onto the rebar and pour your concrete.
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When it comes to the choice of maturity meter, or concrete temperature or maturity measurement sensors and equipment, contractor have many options to select based on the cost, accuracy, and ease of use, and of course how all these considerations would fit their project needs and budget. The measurement systems available in the market are categorized as follows:
A concrete thermocouple consists of two wires of different metals connected and twisted together at one end to form an electrical junction. There is a temperature-dependent voltage that is produced and measured by external equipment and is then used to estimate the concrete temperature.
Although thermocouples are relatively inexpensive, they have several disadvantages that make them not suitable for use, such as:
To address some of the deficiencies in thermocouple-based systems, wired temperature and maturity loggers were developed. These loggers and meters have an electronic circuit board that contains a coin-size battery with an onboard thermistor (typically an NTC type sensor) for temperature measurement. The measurements are recorded and stored on this circuit board at pre-defined intervals. The whole circuit board is completely sealed with a connector wire coming out to download the measurements using an external device as needed unlike the thermocouple-based sensors that need to always be connected to external data recording equipment.
The wires used for these types of temperature/maturity loggers are more rugged compared to thermocouples which makes them less prone to damages on the jobsite. The external unit is also not exposed to potential damages in a constriction environment as it is used only when downloading the data. External devices can offer various types of data analysis in the field. But, for full analysis and report generation, the data needs to be downloaded later on to a computer.
Even with some improvements from thermocouples, wires temperature and maturity loggers do present some disadvantages such as:
Using wired temperature and strength sensors requires expensive data loggers to retrieve this data. An individual must find each wire connected to a sensor and retrieve the measurements with the loggers. As a result, wires are often damaged or cut. Furthermore, the loggers are required to stay on-site where they can easily get damaged with exposure to humidity. When monitoring temperature at various spots for one concrete pour, the wires are accumulated in one location for ease of access. However, this can create a hassle of identifying and labeling them.
Moreover, the assembly of thermocouples requires some attention to detail! If not conducted properly, wires can cross over in the plug and cause reading errors. Once the data is logged, this information must be synced to a device, such as a laptop or desktop, where it must be analyzed by an experienced individual. That can take significant labor hours, depending on experience.
With a wireless maturity sensor, the device is fully embedded on the rebar before pouring. The installation is simple and hassle-free with no protruding wires. Data is collected via Bluetooth on a mobile device or tablet. This eliminates the need for a data logger. With SmartRock, the data collected by the sensors is updated every 15 minutes and uploaded to the iOS or Android app. This data, as well as measurements provided during mix calibration, is used to determine the maturity/ strength of the in-situ concrete in real-time. With the SmartRock app, this data can also be easily shared with team members.
Therefore, no additional labor is needed to calculate when further steps can be taken. In this way, non-destructive wireless concrete temperature sensors and maturity meters, such as SmartRock, have been developed for the concrete industry to reduce labor costs. These wireless systems can therefore significantly improve the efficiency required in fast-paced construction projects.
Learn More About Wireless vs. Wired Sensors Here
There are several options for purchasing a concrete temperature/maturity sensor for monitoring the concrete curing and hardening in your concrete project. This gives contractors a wide range of options for selecting maturity meter based on cost, accuracy, and ease of use. Check out this blog to view a list of various commercially available concrete sensors, temperature loggers, and maturity meters.
As the leaves start to change, and the temperature starts to drop, construction companies and ready-mix producers are gearing up for the colder weather. To avoid lag time or facing issues such as freezing of concrete at an early age, lack of required strength, improper curing, rapid temperature changes, and improper protection of structures, contractors must plan, plan, plan. This entails using the right protection and tools to aid in creating durable concrete during cold weather concreting.
“Cold weather,” is defined in ACI 306R-16 as follows, “When air temperature has fallen to or is expected to fall below, 40°F (4°C) during the protection period.” ACI refers to the protection period as “the time recommended to prevent concrete from being adversely affected by exposure to cold weather during construction.” Contractors must prepare long before the weather changes to adequately protect fresh concrete. Having the right equipment ready to use at the jobsite, such as tarps and blankets, can help avoid extraneous delays and unsafe concrete development.
To ensure proper strength development in these ambient conditions, some placement specifications include;
If these practices are not followed, any concrete construction project is subject to various risks. These include cracking, rapid temperature changes, and loss of durability.
The most common method for monitoring the strength of in-situ concrete is the use of field-cured cylinders. These samples are cast and cured according to ASTM C31, before being tested for compressive strength at various stages, usually by a third-party lab. Typically, if the slab has reached 75% of its designed strength, engineers will give the ‘go-ahead’ to their team to move on to the next steps in the construction process.
However, when pouring in cold weather, ACI 306 specifically recommends not using this method as field-cured cylinders “can cause confusion and unnecessary delays in construction”. This is largely because cold weather makes it difficult to maintain the cylinders in the same conditions as your structure.
It is, therefore, recommended that other in-place testing methods, like maturity testing, be used for monitoring concrete strength.
SmartRock can be highly beneficial on a jobsite when monitoring concrete during cold weather. When you use SmartRock on your jobsite, you get temperature and strength data uploaded to your mobile device every 15 minutes. This is done using a wireless signal, which means that you have to be onsite to collect this information. However, it is much easier than having to search for wires under blankets to collect data with a logger, if you were to use wired sensors. This also improves safety standards on your site, as there are no wires to get in the way of other operations. Further, these embedded sensors provide temperature and strength measurements within ± 0.1°C accuracy. This allows for close monitoring during cold weather, serving as a reminder to contractors when their concrete gets too cold and strength gain slows down. Equipped with real-time results, contractors can improve the heating process, decrease energy costs, and save time in their project schedule by knowing when to move on to subsequent construction operations, such as formwork removal or post-tensioning.
SmartHubTM is a remote monitoring system that allows you to access your SmartRock data at anytime, from anywhere. These user-friendly sensors are easily installed in the concrete formwork (on the rebar) before pouring to monitor your concrete’s in-situ temperature and strength in real-time. The Hub automatically collects this data recorded by the SmartRock sensors and uploads it to the Giatec 360TM cloud dashboard via LTE where it is synced to your team’s mobile devices in the SmartRock app. The Giatec 360 alert system sends smart notifications to let you know when your concrete reaches specific thresholds.
