Summary

The total cost to build a Lora sensor node, to deploy, for instance, at several locations on a fresh concrete slab, is around €50. It can be used many times until someone breaks it. The only maintenance requirement it has is a new battery at every new installation during each concrete casting and a new thermocouple sensor.

Alternatively, the sensor node, can be built instead using a Wifi or Bluetooth for data transmission. If you don’t have a monitoring system, the construction site logistics platform available to download for a preview on AeonLabs webiste. Here’s the video tutorial for the Foreman Android App , available for any Android 8 smartphone or tablet device.

Contact me at mtpsilva@gmail.com or @AeonLabsS on Twitter if you want to learn how to build and deploy this type of IoT sensor nodes or If you simply don’t have time for a DIY and want to buy ready to install and deploy.

An Overview of the Concrete Maturity Method

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.

What is Concrete Maturity?

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.

The Maturity Method

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.

Strength=a+b LOG (maturity)

With sensors, measuring concrete strength, also referred to as “maturity”, is a non-destructive method that can greatly optimize your jobsite schedule.

The Benefits of Concrete Maturity

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.

Optimize Your Jobsite With the Maturity Method

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.

Engineers

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.

Project Managers

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.

Field Personnel

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.

Arduino based Sensor Node using LoRa RF Communication

LoRa is a wireless data communication technology that uses a radio modulation technique that can be generated by LoRa transceiver chips. This modulation technique allows long range communication of small amounts of data (which means a low bandwidth), high immunity to interference, while minimizing power consumption. So, it allows long distance communication with low power requirements. LoRa uses unlicensed frequencies that are available worldwide. These are the most widely used frequencies:

868 MHz for Europe
915 MHz for North America
433 MHz band for Asia

Because these bands are unlicensed, anyone can freely use them without paying or having to get a license. Check the frequencies used in your country.

Point to point communication
Or build a LoRa network (using LoRaWAN for example)

Point to Point Communication

In point to point communication, two LoRa enabled devices talk with each other using RF signals. For example, this is useful to exchange data between two ESP32 boards equipped with LoRa transceiver chips that are relatively far from each other or in environments without Wi-Fi coverage.

Unlike Wi-Fi or Bluetooth that only support short distance communication, two LoRa devices with a proper antenna can exchange data over a long distance.

You can easily configure your ESP32 with a LoRa chip to transmit and receive data reliably at more than 200 meters distance (you can get better results depending on your enviroment and LoRa settings). There are also other LoRa solutions that easily have a range of more than 30Km.

LoRaWAN

The LoRaWAN protocol is a Low Power Wide Area Network (LPWAN) specification derived from LoRa technology standardized by the LoRa Alliance. We won’t explore LoRaWAN in this tutorial, but for more information you can check the LoRa Alliance and The Things Network websites.

Figure 1: LoRaWAN network high level architecture1

The architecture of a LoRaWAN network is shown in Figure 1. The end devices – or nodes – transmit their payloads in what is known as an uplink message, to be picked up by a LoRaWAN gateway. A key aspect of LoRaWAN is that a device is not associated with a single gateway and its payloads may be picked up by any number of LoRaWAN gateways within the device’s range. In theory an entire city could be covered by a just a handful of gateways and messages from devices belonging to any user could be detected anywhere in the city.

The gateway then passes messages to its associated network server, usually carried out over Wi-Fi or Ethernet. However, cellular and satellite gateways are available to enable very remote solutions without relying on an internet connection. There are various options for network servers, for example third-party public network servers are provided by The Things Network (TTN) and Helium, among others. For more information about choosing your network server refer to this guide.

The LNS receives the messages from the gateway and checks whether the device is registered on its network; the LNS also carries out deduping, since the message may be received and uploaded by multiple gateways. The message is then forwarded to the application server on which the device is registered.

Communication is also possible in the other direction, where a message is sent as a downlink to the node. The downlink message is sent from the application server to the network server. The network server then queues the downlink, and when the device next sends an uplink message, the network server will pass the downlink message to the nearest gateway which received the uplink. The gateway then broadcasts the downlink message for the device to receive.

Common uses for a downlink message are to update a device’s broadcast settings, for example, to trigger updates more or less frequently or trigger some other action, such as causing a device to open, closing a water valve or turning an A/C unit on or off.

For a detailed look at LoRa and LoRaWAN including RF modulation, more information on architecture including the technical detail on how devices join the network using a Join Server, and different classes of devices, refer to Semtech’s technical paper on 'What are LoRa and LoRaWAN?'

Deploy a sensor node at any construction site

On this website you will find a good tutorial on how to Build a Simple Arduino LoRa Node In 10 Minutes. It contains all the schematics, electronics BOM and Arduino code ready to deploy on a Arduino shield board like the one on the photo below. And therefore i won’t go into details on this article.

In summary, in the tutorial provided is shown the basics of LoRa technology:

LoRa is a radio modulation technique;
LoRa allows long-distance communication of small amounts of data and requires low power;
You can use LoRa in point to point communication or in a network;
LoRa can be especially useful if you want to monitor sensors that are not covered by your Wi-Fi network and that are several meters apart.

Is also shown a step-by-step tutorial on how to build a simple LoRa sender and LoRa receiver. These are just simple examples to get you started with LoRa.

Next is going to be need to add a common temperature sensor to the Arduino board and also a humidity sensor. These sensors are quite cheap, ideal for usage at a construction site and can be thrown away at the end.

1PCS 100cm Length Wire Temperature Test K-type Thermocouple Sensor Probe Tester Electrical Equipment & Supplies bridgin Wire & Cable Connectors — example of a termopar at sale on aliexpress.com starting at 0.57 USD

Several types of converters are available on Aliexpress to connect thermocouples to an Arduino board. The shield module MAX31855, enables to send data via the SPI bus. Features:

14-bit ADC.
Apart from the definition of the breakage, the thermocouple output is further determined by VCC or GND.
The range of measured temperatures from -270 to +1768 oC (max ranges).
The price is about 1.9 USD.

You can find a small tutorial with ready to deploy code, using this temperature shield on this website. Additionally one can also include, a humidity sensor for improved calculations of concrete maturity and strength.

The total cost to build a Lora sensor node, to deploy, for instance, at several locations on a fresh concrete slab, is around €50. It can be used many times until someone breaks it. The only maintenance requirement it has is a new battery at every new installation during each concrete casting and a new thermocouple sensor.

Alternatively, the sensor node, can be built instead using a Wifi or Bluetooth for data transmission.

Contact me at mtpsilva@gmail.com or @AeonLabsS on Twitter if you want to learn how to build and deploy this type of IoT sensor nodes or If you simply don’t have time for a DIY and want to buy ready to install and deploy.

If you don’t have a monitoring system, the construction site logistics platform available to download for a preview on AeonLabs webiste. Here’s the video tuturial for the Foreman Android App , available for any Android 8 smartphone or tablet device.

An insightful Saturday to all.

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1

https://lora-developers.semtech.com/library/tech-papers-and-guides/building-a-lora-based-device-end-to-end-with-arduino/

2

this article took approx. 1h to produce. This document is a draft bibliographic research. Only basic revisions were made to the text. Original contents can be found on web links provided throughout the text.

Concrete maturity related contents can be found here: https://www.giatecscientific.com/strength-maturity/

Remote Monitoring of Concrete Casting at a C…

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