5 Things You Need to Know About Moisture in Concrete

As a general contractor, you’re expected to know every aspect of the construction project. Of course, you need to trust the trade specialists on the team, but having key information can be crucial to your role in overseeing the schedule and budget of the project as a whole.

• 1. Moisture Matters in Concrete
• 2. Moisture Moves in Concrete
• 3. Concrete Can’t Be Rushed
• 4. Moisture Must Be Measured in Concrete
• 5. RH Testing: Have the Right Tool for the Job
• BONUS: Excess Moisture Can Be a Problem

Nowhere is this more important than in the area of moisture testing in concrete. Concrete provides the foundation for key systems in most modern building projects, so understanding the need for accurate moisture testing can provide the basis for better schedule management, and prevent future problems down the road.

Moisture Matters in Concrete

It may be stating the obvious to say that moisture is part of concrete, but considering the billions of dollars in damage that moisture intrusion or moisture-related problems cause each year in the building and construction industry, it’s worth saying again: concrete has moisture.

For concrete, water is a key ingredient in the hydration process that sets and cures the cement portion of any concrete blend. Even with the rapidly changing innovations in admixtures and concrete science, water still is part of every batch mix.

Why is that so important? Because if that initial water isn’t given time to leave the slab, it can cause damage ranging from roof system and floor adhesive failures to health hazards from mold and mildew growth. Moisture cannot be ignored in any construction project.

Moisture Moves in Concrete

From the initial batch mix, concrete hydrates (or cures) and then must release any excess moisture through the drying process. During that process, a large percentage of the excess moisture in the slab must be able to reach the surface and evaporate.

Moisture within a concrete slab follows a number of small pathways, or capillaries, that form as it combines with the initial mix elements: cement, aggregate, and any admixtures.

As it evaporates, more moisture moves up through these pathways in a cycle and forms a gradient effect within the slab. Until the concrete is sealed, this cycle generally means that the moisture level is typically higher deeper in the slab. This is crucial to understand when choosing a concrete moisture test method.

It’s also important to understand that this process can reverse if the ambient conditions around the slab have higher humidity levels than are present in the slab. Those pathways go two directions so that unsealed concrete can also absorb moisture, whether through contact with a direct water source (like groundwater or leaking plumbing) or through ambient moisture.

Once the concrete has been sealed, moisture continues to move within the slab until it reaches a state of balance referred to as equilibration.

Concrete Can’t Be Rushed

It’s all well and good to apply a rule of thumb to drying concrete (a typical one is one month of drying time for each inch of slab depth), but the reality of concrete is that too many factors come into play to assume you can predict when it will be dry.

There are a number of products or processes that are intended to speed the drying time for concrete. Self-desiccating products in the initial mix, dehumidification, controlling ambient conditions, and a lower water content in the mix are all intended to speed up drying, either by reducing the amount of water in the slab or by helping it leave faster.

However, if not incorporated correctly, they can cause more problems than they correct.

Mechanical processes can also impede drying time. A rushed or misjudged troweling job, a surface curing compound, or any other process or material can minimize the moisture’s ability to leave. If these pathways are blocked for any reason, or if they aren’t well dispersed through the slab, the time it takes for moisture to move out of the slab will increase significantly.

Ambient conditions also have a very significant impact on concrete drying times, so any change in relative humidity (RH), air movement, temperature (as it relates to ambient RH), or HVAC impact will ultimately affect the final drying schedule.

Try to rush it, and the performance of the slab can be compromised; too slow and your schedule is in danger.

Moisture Must Be Measured in Concrete

Ultimately, the amount of moisture in the slab must be the final guideline for deciding when the next step in the building process can take place. This requires accurate concrete moisture measurement.

What are the options?

There are two main standards in the industry right now: calcium chloride (CaCl) testing and RH testing. Also known but less common in use are the hood method, plastic sheet method, and concrete moisture meters. What is the difference?

CaCl testing (which measures the moisture vapor emission rate, or MVER) uses an anhydrous calcium chloride desiccant on the surface of the concrete slab and then seals it with a cover intended to prevent ambient conditions from influencing the test results.

The theory is that the amount of moisture absorbed by the desiccant will reflect the amount of moisture evaporating from the slab surface, and the moisture conditions of the slab as a whole.

The challenge with any surface-based tests, like CaCl testing, the hood method, or plastic sheet method, is that they can only be considered reliable for the surface of the concrete.

In understanding how moisture moves and forms a gradient in concrete, it’s obvious how a surface-based test method cannot be an accurate indicator of internal conditions. In fact, the CaCl standard, ASTM F1869, says the test shall not be used on floors containing lightweight aggregate.

RH testing uses in-situ sensors placed in holes drilled into the slab to measure the relative humidity at a specific depth (40% of the slab thickness for a slab drying from one side, 20% for a slab drying from two sides). That depth indicates the moisture level the slab would reach if the surface were sealed at that point.

ASTM International provides guidelines for RH testing in its standard F2170. RH testing puts the test in the right spot to give accurate concrete moisture measurement.

RH Testing: Have the Right Tool for the Job

When your project and your reputation are on the line, you always want the right tool for the job. In understanding the realities and challenges of concrete moisture testing, it doesn’t take much to realize that RH testing is the best choice for monitoring drying concrete and managing a building project.

RH testing has decades of scientific testing and field experience behind it to prove that it provides accurate, actionable information to trade professionals and specialists. For more than just flooring professionals, RH testing provides the foundation for a cooperative approach to moisture management.

BONUS: Excess Moisture Can Be a Problem

Water, so vital for strengthening concrete as it cures, is no friend of flooring. Excess moisture that gets trapped beneath a sealant or other finished floor product can be catastrophic.

As excess vapor struggles up to the surface, it can lead to osmotic blisters in the flooring or sealant. The chemicals it carries might delaminate the floor or prevent the sealant or adhesive from adhering. When moisture carries salts up from the concrete, it can leave a whitish-gray residue (efflorescence) on the surface. If wood flooring is installed above the concrete slab, the excess moisture can cause it to crack and buckle.

Excess moisture in concrete can lead to many unsightly or dangerous defects in the floor. Your best defense is knowing the RH within the concrete slab so you don’t seal it before the slab has dried sufficiently. The best way to know the slab’s RH is by performing an in-situ RH test at the right depth in the concrete slab.

Curing and drying concrete for a durable floor takes time. But you don’t want to guess when the excess moisture has left the slab. With an in-situ RH test, you don’t have to.

The Rapid RH^® makes accurate in-situ RH testing affordable and easy on the job site. Its Smart Sensors are simple to install, and Touch-n-Sense™ technology takes a reading as soon as the reader touches the sensor.

In addition, with the DataMaster™ app, recording and reporting are easier for job site documentation and ASTM F2170 reports.

As every general contractor knows, time is money, and so is lost time. With Rapid RH, you can manage each building project’s schedule with accurate, actionable test results.

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Last updated on October 14th, 2026