Concrete Is Not Completely Waterproof

Hardened concrete contains a network of pores, capillaries, cracks, joints, and microscopic pathways. Their size, number, and continuity depend on the concrete mixture, water-cementitious-material ratio, placement, consolidation, curing, finishing, age, cracking, and exposure history.

Moisture can occupy these pathways as liquid water or water vapor. It can move from within the slab, enter from the soil beneath it, leak through a building component, or condense on a cool surface.

A coating contractor does not need to become a concrete scientist, but the contractor should understand the basic movement mechanisms. That knowledge helps explain why a surface can appear dry while the slab still presents a moisture risk.

The Four Primary Movement Mechanisms

Bulk Water Flow

Bulk water is liquid water moving because of gravity, drainage conditions, a leak, or physical pressure. Common sources include broken plumbing, roof and wall leaks, flooding, rain entering an unfinished building, failed drains, and groundwater.

Liquid water will follow available pathways. Cracks, construction joints, penetrations, honeycombed concrete, wall-to-floor intersections, and poorly sealed drains can all provide entry points.

Bulk water is generally the first moisture condition that should be addressed. A coating or mitigation membrane should not be used as a substitute for correcting an active leak unless the complete system has been designed and approved for that specific service.

Capillary Action

Capillary action allows liquid water to move through small concrete pores. The same basic action can be observed when a paper towel draws liquid upward. In concrete, capillary suction can move moisture vertically, horizontally, or through walls.

A slab placed over moist soil can draw water from below if there is no effective underslab vapor retarder and capillary break. Below-grade walls may also absorb water from surrounding soil.

Capillary water can carry dissolved salts and alkaline compounds. When the water reaches a surface and evaporates, salts may remain as efflorescence. When the surface is covered, moisture and dissolved materials may accumulate at the coating or adhesive bond line.

What Influences Capillary Movement?

• The continuity and size of pores within the concrete
• The water-cementitious-material ratio
• Concrete consolidation and curing
• The presence of cracks, joints, or porous repairs
• Contact with wet soil, water, or saturated construction materials
• The presence and condition of an underslab vapor retarder
• Drainage and groundwater conditions surrounding the structure

Water-Vapor Diffusion

Water-vapor diffusion is the movement of water molecules in vapor form through a material. The driving force is a difference in vapor pressure. Vapor generally moves from an area of higher vapor pressure toward an area of lower vapor pressure.

In an air-conditioned building, warm moist soil beneath a slab may have a higher vapor pressure than the cooler, drier interior. If the slab lacks an effective vapor retarder, moisture may move upward through the concrete toward the occupied space.

When the surface remains exposed, some moisture can escape into the building air. Installing a low-permeability coating, membrane, adhesive, or floor covering changes the drying path. Moisture may accumulate beneath the new system and increase the risk of blistering, adhesive degradation, alkalinity-related damage, or delamination.

Moisture Carried by Air

Air can carry water vapor through cracks, openings, joints, and penetrations. When humid air enters a cooler area, the water vapor may condense on surfaces that are at or below the dew-point temperature.

Air leakage is generally much faster than vapor diffusion through solid materials. Although this mechanism is often discussed as part of building-envelope control, it can also affect coating work around exterior walls, loading docks, refrigerated spaces, mechanical rooms, penetrations, and transitions between conditioned and unconditioned areas.

Moisture Gradients Within a Slab

Moisture is not necessarily distributed evenly through a concrete slab. A slab drying primarily from its upper surface can become drier near the top while remaining wetter deeper inside.

This difference is called a moisture gradient. The surface may therefore appear dry and may produce a relatively low surface reading while significant moisture remains below.

After an impermeable or low-permeability system is installed, the moisture within the slab can redistribute. Moisture from deeper areas may move toward the surface until a new equilibrium develops. This is one reason internal relative-humidity testing is used to evaluate concrete floor slabs.

Why Surface-Only Checks Can Be Misleading

• Air movement may temporarily dry only the upper surface.
• Heating may change surface conditions without drying the entire slab.
• Grinding can expose concrete with a different immediate moisture condition.
• Direct sunlight may produce localized drying.
• Dehumidification may affect the surface before deeper concrete reaches equilibrium.
• A previous flooring system may create different moisture patterns across the slab.

Moisture Usually Moves Along the Easiest Path

Concrete is not uniform. Cracks, joints, penetrations, transitions, patching materials, and areas of poor consolidation may transmit moisture differently than the surrounding slab.

Moisture may enter at one location, travel laterally, and become visible somewhere else. A blister or wet area does not always identify the original water source. Contractors should investigate surrounding walls, drains, equipment pads, doorways, plumbing, expansion joints, and exterior conditions.

The Role of an Underslab Vapor Retarder

An underslab vapor retarder is installed beneath a concrete slab to limit moisture vapor transmission from the soil or fill into the slab. Its effectiveness depends on the material, placement, seams, penetrations, continuity, protection during construction, and connection to adjoining moisture-control components.

A missing or damaged vapor retarder can create a continuing moisture supply. However, contractors should not assume that a vapor retarder is present merely because the building is relatively new. Available drawings and construction records should be reviewed when possible.

Even when an effective vapor retarder is present, a new slab still contains mixing water and requires time to dry. A vapor retarder controls moisture entering from below; it does not instantly remove moisture already contained within the concrete.

Temperature Changes Moisture Behavior

Temperature affects relative humidity, vapor pressure, evaporation, and condensation. Changing the HVAC system or opening a building to outside air can change moisture conditions at the concrete surface.

Testing should be performed under the service conditions required by the applicable standard, project specification, and manufacturer. Results collected in an open, unconditioned building may not represent the conditions that will exist after the building is occupied.

Condensation Is a Surface Event

Condensation occurs when the concrete surface temperature reaches or falls below the dew-point temperature of the surrounding air. A slab can have acceptable internal moisture conditions and still become wet from condensation.

Before coating, measure and record ambient temperature, relative humidity, surface temperature, and dew point. Follow the coating manufacturer's required safety margin between the surface temperature and dew point.

How Coatings Change the Drying Path

Every coating and flooring system has some resistance to moisture-vapor transmission. Highly permeable materials allow more vapor to pass, while low-permeability materials restrict movement.

Restricting vapor movement is not automatically harmful. Properly selected moisture- mitigation systems are specifically designed to manage elevated slab moisture. Problems occur when a system is installed outside its published limits, over improperly prepared concrete, or without addressing active water intrusion and other unsuitable conditions.

The complete assembly must be considered. A moisture-tolerant primer does not automatically make every patching compound, adhesive, coating, or floor covering above it suitable for the same conditions.

What the Contractor Should Investigate

• Whether the concrete is above grade, on grade, or below grade
• The age, thickness, and construction history of the slab
• The existence and condition of an underslab vapor retarder
• Current and previous leaks, flooding, or rain exposure
• Exterior grading, drainage, irrigation, and groundwater conditions
• Cracks, joints, penetrations, drains, and wall-to-floor transitions
• Previous coatings, flooring systems, adhesives, and repair materials
• Building temperature, relative humidity, surface temperature, and dew point
• Required moisture tests and manufacturer acceptance limits
• Whether active liquid water or hydrostatic pressure is suspected

Testing Must Match the Question

Different tests provide different information. In-situ relative-humidity testing evaluates the internal relative-humidity condition of the slab at specified depths and locations. Calcium-chloride testing evaluates moisture-vapor emission from a defined surface area under the test conditions.

Non-destructive electronic meters may be useful for comparative surveys and locating areas requiring further investigation. Unless specifically approved, they should not be used as substitutes for the quantitative test method required by the specification or manufacturer.

The plastic sheet method can indicate that moisture is present beneath the sheet under the test conditions. It does not provide an internal relative-humidity value or a moisture-vapor-emission rate.

Contractor Field Checklist

• Inspect the concrete for dampness, staining, efflorescence, and previous failures.
• Ask whether moisture conditions change after rain, cleaning, or HVAC shutdowns.
• Identify cracks, joints, drains, penetrations, and other likely pathways.
• Confirm that the building is at the required service conditions before testing.
• Verify the test methods required by the project and product manufacturer.
• Map the test locations instead of relying on one convenient area.
• Record ambient and surface conditions at the time of testing and application.
• Determine whether the condition involves vapor, liquid water, or condensation.
• Obtain written approval for the proposed mitigation or coating system.
• Retain test reports, photographs, product data, and written communications.

Safety During Moisture Investigation

Drilling concrete for in-situ probes may expose workers to respirable crystalline silica and hidden utilities. Use the required dust controls, HEPA-filtered collection equipment, personal protective equipment, and approved scanning or utility-location procedures.

Wet surfaces can create slip and electrical hazards. When investigating an active leak, protect the work area and determine whether energized equipment, wiring, or machinery could be affected.

Knowledge Check

Technical References

Consult the current edition required by the project and the current instructions issued by the specified system manufacturer.

• ASTM F2170 - Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using In Situ Probes.
• ASTM F1869 - Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride.
• ASTM F2659 - Standard Guide for Preliminary Evaluation of Comparative Moisture Condition of Concrete, Gypsum Cement, and Other Floor Slabs and Screeds Using a Non-Destructive Electronic Moisture Meter.
• ASTM D4263 - Standard Practice for Indicating Moisture in Concrete by the Plastic Sheet Method.
• ASTM E1745 - Standard Specification for Plastic Water Vapor Retarders Used in Contact with Soil or Granular Fill Under Concrete Slabs.
• ACI 302.1R - Guide to Concrete Floor and Slab Construction.
• ICRI Guideline No. 710.3 - Guide for the Mitigation of Moisture in Concrete Floor Slabs.
• U.S. Environmental Protection Agency - Moisture Control Guidance for Building Design, Construction and Maintenance.
• Current technical data sheets, installation instructions, and safety data sheets issued by the specified coating, flooring, adhesive, and moisture-mitigation system manufacturers.

These references provide technical guidance but do not replace the project specification, applicable regulations, manufacturer requirements, or evaluation by a qualified design professional. Final system selection and application must be based on documented site conditions and written project requirements.