Moisture testing of concrete: Methods, importance and regulations in the construction industry

Introduction

A moisture test for concrete is one of the most important measurements in the construction industry. Moisture in concrete can have a significant impact on the durability and performance of structures, so its control is critical to ensure the quality and longevity of any construction project. Inadequate moisture content can compromise the strength of concrete, the adhesion of coatings and, in general, the stability of the building.

This article focuses on analyzing in detail the importance of concrete moisture testing, the main measurement methods, the factors that influence the moisture of the material and the applicable international standards. We will also discuss the technological advances that have improved the accuracy and efficiency of these tests.

What is concrete?

It is a construction material composed mainly of a mixture of cement, water, fine aggregates (sand), coarse aggregates (gravel or crushed stone) and, in some cases, special additives. Its strength and durability make it one of the most widely used materials in the construction of infrastructure, buildings and pavements.

During its setting and hardening process, concrete undergoes a chemical reaction known as hydration, in which cement reacts with water to form a solid and resistant structure. However, the composition of concrete can be improved with admixtures that optimize its properties according to the specific needs of the project, but technical standards and specifications such as ASTM C494 and EN 934-2, which regulate the use of admixtures in concrete, must be complied with.

Some of the most common additives include:

• Plasticizers and superplasticizers: They improve the workability of concrete without the need to increase the amount of water, which helps to maintain its strength.
• Setting accelerators: They reduce the hardening time, useful in cold climates or when a quick start-up is required.
• Setting retarders: Delay the hardening of concrete, allowing more time for placement and finishing, especially in hot climates.
• Air incorporators: Introduce microscopic air bubbles into the concrete, improving its resistance to freeze-thaw cycles.
• Sealers and moisture repellents: They reduce water absorption and improve the durability of concrete in humid environments or environments exposed to chemical products.
• Anti-cracking additives: Reduce shrinkage and cracks that may occur during drying.

Moisture process in concrete

Moisture content in concrete goes through different stages, from its initial mixing to its final use in the structure. This process is described below:

• Mixing and water addition: In the initial stage, water is combined with cement and aggregates to form a homogeneous mixture. The amount of water used influences the consistency of the mix and the strength of the concrete. At this point, admixtures can be added to modify moisture retention, such as plasticizers, superplasticizers and water reducers, which improve flowability without compromising strength.
• Cement hydration: During setting, cement particles react with water in a chemical process known as hydration, forming crystalline compounds that give the material strength. Some of the water is absorbed into the cement structure, while the excess begins to migrate to the surface.
• Concrete curing: Curing is essential to control water evaporation in the first weeks after concrete pouring. Techniques such as water curing, curing membranes or sealers are used to prevent premature moisture loss and allow complete hydration. The addition of evaporation retarders can prolong curing in hot climates.
• Evaporation and drying: As concrete gains strength, excess water gradually evaporates. The rate of drying depends on factors such as ambient temperature, relative humidity and ventilation of the environment. Drying too fast can cause shrinkage cracking, so setting accelerators or shrinkage controllers are sometimes used.
• Hygrometric equilibrium: After several weeks or months, depending on the mix used and the environmental conditions, the concrete reaches a state of equilibrium in which the internal residual moisture no longer affects its mechanical properties. It is at this point that moisture testing is recommended before applying coatings or finishes.

Importance of concrete moisture testing

Concrete is a material composed of cement, aggregates and water, whose hardening process occurs through a chemical reaction known as hydration. During this process, part of the water is incorporated into the molecular structure of the cement, while the excess water evaporates over time. However, the presence of residual moisture in concrete can generate problems such as:

• Detachment and deformation of coatings and adhesives: High humidity can prevent the proper adhesion of coatings such as paints, epoxy resins and flooring adhesives. This can lead to long-term aesthetic and structural problems.
• Mold and mildew growth: Moisture in concrete can encourage the proliferation of microorganisms, affecting indoor air quality and creating health risks.
• Structural weakening: In extreme cases, excess moisture can alter the strength of concrete, reducing its load-bearing capacity and increasing the risk of cracking and structural failure.

For these reasons, measuring moisture in concrete is critical before proceeding with any finishing, sealing or coating application.

Moisture measurement methods in concrete and their relation to ASTM standards

To determine the moisture level in a concrete slab and avoid problems such as bond failure or premature deterioration, there are several methods and moisture meters for concrete, each regulated by specific ASTM standards.

• Contact hygrometer: It is an electronic device that measures the moisture content in concrete in a non-destructive way. This method allows obtaining a comparative moisture value without altering the structure of the material, making it ideal for preliminary evaluations. ASTM F2659 establishes procedures for the comparative evaluation of moisture content in concrete, gypsum cement and other surfaces using an electronic meter. However, this method only provides a reference and does not provide an accurate measurement of internal slab moisture.
• In-situ relative humidity method: This standard method involves the insertion of moisture sensors into the concrete to measure its internal relative humidity. It has become one of the most reliable and widely used procedures in the construction industry. This procedure is regulated by ASTM F2170, which describes the determination of relative humidity in concrete slabs by in-situ probes. It is especially useful for determining the moisture behavior of concrete prior to the installation of moisture-sensitive coatings.
• Plastic sheet method: Consists of adhering a plastic sheet to the surface of the concrete for at least 16 hours. If condensation is observed, it means that there is excess moisture. ASTM D4263 regulates this test and recommends it as a quick and easy method to detect the presence of moisture on the surface.
• Desiccation box method: Also known as the calcium chloride method, it measures the moisture vapor emission rate (MVE) of concrete. A container with calcium chloride is placed on the surface of the concrete and covered with an airtight dome to prevent interference from the outside environment. ASTM F1869 regulates this procedure and states that an emission rate of more than 3 pounds of water per 1,000 square feet in 24 hours may pose a risk to the coating installation. However, this method has the limitation that it measures only moisture in the top layer of concrete and does not detect moisture at greater depths.
• Gravimetric method: It is based on weighing a concrete sample before and after drying it in an oven at controlled temperature. The difference in weight reflects the moisture content of the material. Although it is a highly accurate method, it requires laboratory equipment and a long testing time. The gravimetric method for determining moisture content in materials such as concrete is regulated by ASTM D2216 for the laboratory determination of water (moisture) content of soils and rocks by mass.

For a complete evaluation of moisture in concrete, a combination of several test methods is recommended:

• Contact hygrometer (ASTM F2659): Rapid and non-destructive evaluation.
• In situ probes (ASTM F2170): Accurate measurement of internal humidity.
• Plastic film (ASTM D4263): Visual surface moisture detection.
• Desiccation box (ASTM F1869): Vapor emission rate measurement.
• Gravimetric method: Direct and precise measurement of total moisture.

Factors affecting moisture in concrete

• Environmental conditions: The temperature and relative humidity of the environment can affect the evaporation of water contained in concrete. In humid climates, evaporation slows down, while in dry climates, water loss is faster.
• Curing process: Inadequate curing of concrete can leave moisture trapped in its structure, affecting its strength and durability. Proper curing allows cement to hydrate and prevents premature water loss.
• Concrete composition: The water-cement ratio and the type of aggregates used can influence the ability of concrete to retain or release moisture.

Regulations related to moisture testing of concrete

There are several international standards that regulate the measurement of moisture in concrete, among them:

• ASTM F2170: Measurement of relative humidity inside concrete.
• ASTM F2659: Preliminary evaluation of the comparative moisture content of concrete slabs and floors using a non-destructive electronic moisture meter.
• ASTM D4263: Standard test method for indicating moisture in concrete by the plastic sheet method.
• ASTM F1869: Calcium chloride test method for measuring moisture vapor emission in concrete.
• ISO 11375: Procedures for the measurement of moisture content in building materials.

Conclusion

A moisture test for concrete is a fundamental phase of quality control in construction. The application of accurate and reliable methods, together with the following of international standards, allows the avoidance of problems related to excess or deficiency of moisture in the material. In addition, technological advances in sensors and measuring devices have significantly improved the efficiency and reliability of these tests.

Ensuring correct measurement of moisture in concrete is significant to ensure safe, durable and high quality structures in the construction industry.

References

1. Berger, H. (2018). “Moisture Control in Concrete.” John Wiley & Sons.
2. Kosmatka, S. H., & Wilson, M. L. (2011). “Design and Control of Concrete Mixtures.” Portland Cement Association.
3. Bautista, A. & Gómez, R. (2019). “Evaluación de la Humedad en Hormigón para la Instalación de Revestimientos: Métodos y Normativas.” Revista de Ingeniería Civil.
4. National Ready Mixed Concrete Association (NRMCA) – Technical Documents on Concrete Moisture Testing.