An In-Depth Look at the Benefits of Using the Calcium Carbide Bomb Moisture Test

The ancient world didn’t communicate with a maximum of subtlety, but it understood something about the broad strokes. Take for example the common belief that four elements form the world: earth, water, fire, air.

Keeping a proper balance between these elements is both the intent of construction and its downfall.

Controlling these elements requires an understanding of the amounts present in a structure. Moisture testing is necessary to avoid a laundry list of costly problems.

With so many ways to perform a moisture test, which one does the job right? This guide will walk you through the most popular method.

Moisture Testing Basics

The first thing to understand about why you need concrete moisture testing are the myths about matter. Going back to science class, you know that matter comes in forms like gas, liquid, and solids (and also superstates like plasma).

Speaking of superstates, they are more common than you might think. Glass, for example, is sometimes a liquid and sometimes a solid. It is actually an amorphous solid, a state denoted by its lack of crystal structures.

Concrete also functions as an amorphous solid, but a porous one. Porous solids seek to absorb and house water. Think of concrete as a type of slow sponge.

Testing for moisture in concrete, then, is about finding close to nothing, not zero.

Calcium Carbide Bomb Moisture Tests

This test, abbreviated as CM testing hereon, checks a material sample for water molecules. This test is a destructive test, meaning that it takes a physical amount of the surface targeted for testing and destroys it.

The following will break down the process of the test. Then we’ll go over the variables that can cause issues with the test. Finally, we’ll talk about how well the CM test fares in comparison to other concrete moisture tests.


You perform CM testing using a sealed flask with a gauge. The gauges measures build up of gas within the flask. As long as the pressure within the flask doesn’t move the gauge past 4%, the concrete is curing properly.

You remove a sample from the target area. You perform multiple tests for larger areas with a breakdown of 3 tests per 300 SM.

Pull samples from screeds (the technical term for surface). The CM test provides accurate results up to 50% depth (horizon) in a screed. You would core a sample anywhere between 40 and 50% to get the best results.

Samples get pulverized and weighted on a scale. You only place exact, known amounts in the flask. This ensures the accuracy of the test.

Once the sample material is in the flask, you also place quantities of calcium carbide inside. Seal the vessel and agitate for a few minutes. Then read the gauge for a final result.

Pressure builds up from an interaction of the calcium carbide and water in the sample. Chemically, H20 (water) bonds with the calcium carbide (CaC2).

When water is present the calcium carbide bonds as follows CAC2 + 2H2O becomes C2H2 + Ca(OH)2. The C2H2 is a gas called acetylene. The rest is a byproduct (calcium hydroxide).

The acetylene gas pushes out (as gasses do) which creates the pressure variable measured by the gauge.

Repeat tests to find an average for a screed area. A single test may give a false positive for a higher or lower number based on specific conditions.


It is possible to get various readings from the same test. These variations occur from differences in ambient humidity. The depth of the horizon pull and failure of moisture control measures also effect results.

Improper measuring of the amounts of calcium carbide and sample material obviously cause issues. Dirty flasks create a problem of contamination.

Given that testing is destructive, avoiding retesting is important. Carefully check for and eliminate as many of these variables as possible when performing a test. Trained and certified testers know to clean gear between tests and to check in an evenly spaced area.

Benefits Over Other Tests

Several other concrete moisture tests exist. Each has some pros and cons. We’ll give a quick rundown of those here.

Bond Testing

This test places an adhesive strip under a sample of material. Usually a square meter of material in places around the site, again following the roughly 1 per 300 m2 ratio.

After 72 hours the adhesive strip is checked. If the bond of the adhesive was strong and showed no accumulation of moisture the test was successful.

This test suffers from a lot of subjectivity in the administration and a lack of viability to extrapolate conditions a whole floor.

Sheet Tests

More of a visual demonstration, perhaps, than a test. You stretch plastic sheeting across the surface and tape it along the edges. After a day the sheet is checked for any kind of moisture. Dark spots and visible water droplets clearly disqualify the area.

The issues with this test are legion. Any change in the dew point or even the temperature change overnight creates droplets. Other tests calculate an average rather than an acute failure.

Moisture Meter

Place probes on the test area. These can be on the surface or in drilled holes. An electrical impulse is then sent between the sensors.

The resistance of the material between is calculated.

This test has high accuracy for the areas it tests but tests in a thin line. Materials above or below the test area may have hidden water pockets.

Internal Relative Humidity

RH testing is an old test which still has a lot of traction, especially in the United States. Developed by the Portland Cement Association (PCA), this test fixes the ap issues from the moisture meter.

This test also provides information of resistance between the probes. This testing has high accuracy but is expensive as it requires frequent calibration of equipment.

Do Yourself a Solid

Fortunately, you don’t need to know how to do moisture testing to see the results. A good flooring company knows how to perform testing and ensure the quality of the surfaces they lay down.

To get started on your own construction project contact us for estimates.

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