Wireless Concrete Monitoring: Embedded vs. Non-Embedded Bluetooth® Transmitters
The maturity method, often simply referred to as maturity, is a way of evaluating new concrete’s in-place strength by relating time and temperature measurements to actual strength values. Typically, in-situ strength estimation is achieved through standard test methods which involve flexural or compressive tests of beam or cylinder specimens. However, concrete in the field does not always gain strength at the same rate as test specimens. Maturity testing addresses this guesswork and can prove if on-site concrete has achieved required strength and whether or not formwork removal and load application is feasible.
In the past, maturity testing has been performed by placing sensors into fresh concrete and then connecting a data logger or other maturity reader directly to each sensor to retrieve and analyze the data. In recent years, Bluetooth® technology has been incorporated into concrete maturity systems to ease data collection and allow for wireless sharing.
Bluetooth provides the ease of immediate connection between electronic devices such as mobile phones, computers, and wired sensors. However, Bluetooth does have some limitations. Bluetooth uses a 2.4GHz frequency to transmit data and frequently competes with other radio waves for transmitting information to its receiving device. Other devices that operate under a similar frequency include microwaves, wireless speakers, satellite dishes, monitors, cameras, cordless phones, power lines, and power stations. Water is an additional substance that can reduce the Bluetooth signal, so weather disturbances and even humans (with a significant volume of water in our bodies) standing between the transmitter and the receiver will slow down or inhibit the connection.
Wireless Bluetooth maturity systems are typically available as one of two options: with a transmitter that is completely embedded within the concrete, or with a transmitter that sits outside of poured concrete. Both options have benefits and limitations to consider before choosing a maturity system for your project.
Embedded Bluetooth Transmitters
An embedded Bluetooth transmitter is a sacrificial device that transmits data wirelessly via Bluetooth and is secured in place within concrete just below the surface. It’s usually attached to a sensor that extends below the transmitter and further into the concrete to a typical maximum length of 10 feet. Data is collected by the sensor and transmitted through concrete to an app.
No Wires Extending from Concrete
The only benefit of using embedded Bluetooth transmitters instead of non-embedded Bluetooth transmitters is that there are no wires extending out of the concrete.
There are several severe limitations users should consider when evaluating systems with embedded Bluetooth transmitters.
Using a sensor with an embedded Bluetooth transmitter means you are burying the expensive technology in the concrete with each sensor that is installed. These sensors are significantly more expensive (2 to 4 times the cost, or more) than sensors that utilize non-embedded, reusable transmitters.
Degraded Bluetooth Signals
Bluetooth signals are compromised by objects that lie within the signal’s line of sight, so dense material, particularly concrete, is extremely difficult to traverse and requires embedded transmitters to be placed within a few inches of the concrete surface. Even when the transmitter is placed this close to the surface, the transmitter’s ability to relay data becomes degraded and optimal Bluetooth range (33 feet assuming typical Class 2 transmitters) will be decreased significantly. If the transmitter is buried deeper than the manufacturer specifies, the signal may not be strong enough to transmit data beyond the concrete surface.
No Alternative Data Collection Options
For embedded transmitters that only transfer sensor data via Bluetooth and do not offer a backup data collection option, a compromised Bluetooth signal, a transmitter that was buried more than a few inches deep, or damage to the sensor or transmitter means there is no way to retrieve the data that is locked away underneath the concrete surface.
Non-Embedded Bluetooth Transmitters
A non-embedded Bluetooth transmitter is a reusable device that transmits data from sensors wirelessly via Bluetooth and is not embedded within concrete. This type of transmitter connects to sensors that extend from poured concrete and can be placed almost anywhere at a job site. Data is collected by the sensor, sent to the non-embedded Bluetooth transmitter through a direct connection, and then wirelessly transmitted to an app.
There are several significant benefits for using non-embedded Bluetooth transmitters.
Because they are not buried in the concrete, non-embedded Bluetooth transmitters are reusable and are a one-time purchase, resulting in significant cost savings for ongoing and future projects. There is also no need to buy a Bluetooth transmitter for each sensor, because multiple sensors can be connected to each transmitter at once. This translates into significant savings because the sensors themselves are much less expensive, and an entire project can be performed with as few as one Bluetooth transmitter.
Optimal Bluetooth Range
Non-embedded Bluetooth transmitters achieve an optimal Bluetooth range because the signal does not have to travel through concrete. The user also has an option of locating the transmitter farther from the concrete pour if desired, increasing the accessibility even more. In any case, there is no need to worry about data loss due to burying your transmitter too deep into the concrete, because the transmitter is external to the concrete.
Multiple Data Collection Options
If there is an issue with data collection via Bluetooth, there are alternative ways to retrieve the data. Systems like COMMAND Center offer a backup indirect download option. If you are unable to wirelessly access the concrete data, you can press a save button to store data onto the Bluetooth transmitter and send it to your iOS device for analyzing at a later time. If the transmitter is damaged, the user can swap it out with another transmitter and proceed, with all the sensor data still intact.
Wires Extending from Concrete
The only limitation when using a non-embedded Bluetooth transmitter instead of embedded Bluetooth transmitters is that some wire will be extending from the concrete. However, these cables are durable and only have to extend a couple feet from the surface of the concrete to allow user access. The minimal effect of external cables creates little to no disruption for the rest of the working process. If the cable happens to be damaged during or after installation, the user has the option of trimming down the cable and connecting closer to the surface of the concrete.
The COMMAND Center Wireless system is a non-embedded Bluetooth maturity system that consists of sacrificial sensors embedded in the concrete with cables that extend out of the concrete and connect to a Sensor Reader Module (SRM). The SRM transmits data via Bluetooth to a free iOS application.
The COMMAND Center Wireless system provides two methods for data retrieval: direct and indirect download. “Direct download” allows the user to collect data from the sensor and transfer the data via Bluetooth to an app on any iOS device (iPhone, iPad, iPod). The user can immediately view and analyze temperature and strength data in the app, which can also instantly sync data to the cloud or export data files and reports via email.
“Indirect download” allows the user to transfer and store data onto the SRM using the Save button and then transfer via Bluetooth to the app later when it’s more convenient. The indirect method offers an alternative means for data retrieval that can be particularly useful when a Bluetooth connection is not immediately available or reliable in the field or when a technician that does not have an iOS device is tasked with collecting data.
COMMAND Center Wireless is affordable and makes it very easy to get started since more costly data collection devices are purely optional. SRMs are just $99, and any concrete project needs a minimum of one. The SRM is reusable from project to project. Sensors start at $35 each, and the iOS app, as well as the compatible Windows software, are free.
COMMAND Center also offers a great deal of flexibility in that users are not limited to only using the SRM for data collection. All data is stored on the sensor and can be download via USB cable to a Windows device or via Serial cable to a Windows Mobile device. Whatever method is used to collect data, the sensors come pre-configured to the user requirements so that there is no need to initiate or turn on the sensor itself. This saves time during installation and allows the sensors to be in place well before the concrete is even poured, if desired.
What does all of this mean for your concrete project?
The non-embedded Bluetooth transmitter system that COMMAND Center offers provides multiple options for retrieving data, drastically reducing the risk of data loss and the cost per sensor installation of any given project.
Imagine you are waiting to pour the next story for a high-rise and are on a tight schedule, but you need to make sure the concrete has reached sufficient strength before performing post-tension operations. You open up the COMMAND Center app to retrieve sensor data, but Bluetooth service isn’t working. It’s been working off and on throughout the whole project so far—you think the nearby power lines are interfering with the signal. You don’t stress about the spotty signal, because you know you have alternative options with COMMAND Center. You go ahead and manually press the ‘save’ button on your SRMs. Your data is successfully retrieved, and you’ll be able to view it later when you open your app away from the power lines causing the interference.
A couple months later you’re working on a new project that will require 500 sensors. Luckily, you already invested in COMMAND Center’s affordable SRM devices and only have to purchase sensors instead of a combined sensor and transmitter package for each location to be monitored. This decision will end up saving you tens of thousands of dollars.
What is more important for your project?
Using a non-embedded transmitter and having a few feet of cable coming out of the concrete? Or, having no cable coming out of the concrete but having an increased risk of data loss, a restriction on data collection options, and in most cases, paying around three times the overall cost?