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Embedded versus non-embedded Bluetooth transmitters for monitoring concrete temperature and maturity.

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 onsite 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 2.4-GHz 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

Using a non-embedded Bluetooth transmitter, data is collected by the sensor, sent to the non-embedded Bluetooth transmitter through a direct connection, and then wirelessly transmitted to an app.

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.

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. However, users should consider the following severe limitations when evaluating systems with embedded Bluetooth transmitters:

More expensive — 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 (two to four 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

The benefit of using embedded Bluetooth transmitters is that no wires extend out of the concrete.

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:

Less expensive — 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. Users also have an option of locating transmitters farther from the concrete pours if desired, increasing accessibility even more. In any case, there is no need to worry about data loss due to burying a 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 such as COMMAND Center (www.commandcenterconcrete.com) offer a backup indirect download option. If users are unable to wirelessly access concrete data, they can press a save button to store data onto the Bluetooth transmitter and send it to an iOS device for analyzing at a later time. If the transmitter is damaged, users can swap it out with another transmitter and proceed, with all the sensor data still intact.

The only limitation when using a non-embedded Bluetooth transmitter instead of embedded Bluetooth transmitters is that some wire will extend 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.

What does all of this mean for your concrete project? You have to decide 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 about three times the overall cost?

Information provided by The Transtec Group, Inc. (www.thetranstecgroup.com).