Not All Sensors are Created Equal: An 8 Point Guide

Concrete curing and drying have been monitored for thousands of years, really. In the first century B.C., Vitruvius cautions about the challenges of putting flooring down on concrete in the first chapter of Book 7 of The Ten Books on Architecture. You think we would have figured things out by now!

Monitoring today can have several purposes that address everything from schedule acceleration to quality control. Concrete comes in many different mix designs (recipes), is poured in many different climates, and is formed into many different geometries. Each of these variables has its own engineering and construction challenges.

Since there is what feels like an unending number of variables, there are few people who are trained and skilled enough to pay attention to all the issues. This is where a technology solution can provide value. My hope is that this guide can help you figure out what you need.

Strength Monitoring

The traditional method to know the strength your concrete in the field has been to crush samples – either a cylinder or a cube. It is important to differentiate the standard cured cylinders vs field cured cylinders, both outlined in ASTM C31, because only field cured cylinders should be used to estimate the in place strength.

More recently, the Maturity Method (see ASTM C1074) has become an interesting no-destructive solution to monitor the field strength. In fact, it is already an accepted method in the code.

If you are interested in this option, pay attention to how you calibrate your system.

This method relies on determining a rate of curing, which is dependent on the temperature. Only testing ahead of time at multiple temperatures will allow you to handle any mix design at any temperature.

Relative Humidity

Over the years, the most frequent used testing methods have been dictated by the manufacturers. If you want a warranty, then you have to do the testing the manufacturer dictates. More recently that has been the relative humidity test described in ASTM F2170 – drill a hole, vacuum it, put a sensor in it, wait and read. The manufacturers have organized themselves around this and if you want a warranty, you should do this.

Sensing RH from inside the concrete is new and recent advances have shown promising results to a difficult task.

The value is that it is easy to install and logs a full history of the drying of concrete. This provides valuable insight to the project team to make better decisions about balancing investment in expensive mitigation vs compromising on affordable materials.

Wired vs Wireless

Sensors need to be powered and historically that has meant there is a wire that powers them. The primary advantage of wires are that they are cheaper to make. The disadvantages of wired sensors are the cumbersome installation, the chance they will be cut or damaged, data collection is manual, and you typically can’t monitor many at the same time.

Wireless sensors, while newer to the market, can have several advantages. The installation is typically easy, the software can be more versatile since a custom reader is not required, data collection can be simple, and you can usually monitor as many sensors as you want at one time.

When looking at options pay attention to range, limitations on depth of installation and data collection.

Mass Concrete

Whether you are going with a wired or wireless sensor, measuring mass concrete can have its own unique set of challenges. Mass concrete is generally defined as a concrete pour thicker than 3 feet. There is code that requires concrete monitoring and the engineer of record will typically produce a Thermal Control Plan. This means that sensors need be placed at the center of the pour and at the surface. Large differences in temperature at the center and the surface will significantly increase the risk of cracking.

Look for sensors that can be place into deep concrete pours and withstand higher temperatures. Aside from hardware performance, look at the software and make sure it can show you performance data and provide alerting. You want to know if there is a problem before it happens!

Cold/Hot Weather

In the summer and winter months, pouring concrete takes extra work and costs more money. In the winter of 2018, 98% of the country would have been pouring cold weather concrete for at least a full month. In 2016, the ACI revised their standard on cold weather concrete and noted that field cured cylinders are not appropriate indicators of in-place strength. Concrete cures differently in a cylinder than it does in the field. Instead,methods like the Maturity Method are much better at providing insight into field curing.

Real-time reporting and alerting are essential when monitoring cold and hot weather concrete. Like mass concrete, you want to know if there is a problem before it happens.

When locating sensors, pay attention to the temperature in the center, edges and corners of a pour. Identify where the pour will be most exposed to the elements.

Data Collection

When implementing a solution, getting the data will become a big part of the experience. Understand the benefits and limitations of the solution.

Look at automatic data collection vs manual collection, time required to collect data, extra software or hardware requirements, and if wireless ask about depth of installation.

All of these things will help you evaluate the total cost and time investment in the solution.

Analysis

Concrete is more complex than most people give it credit for and you want a system that will do the work for you. Do I have a problem or not?

Analysis of your data in a way that is actionable for your project and shareable with your team can make a construction manager’s life much easier.

Reporting and alerting in ways that are intuitive will make a solution shine.

Support

This is last but probably should be first item. As a construction manager, I relied heavily on my contractors and vendors to give me advise when I needed it.

Make sure you work with someone who is reachable, knowledgeable and trustworthy.

It is said that people buy from people they like, there is probably a good reason for that.