What pH Measures

pH describes whether an aqueous solution is acidic, neutral, or alkaline. The scale commonly runs from 0 to 14. A value of 7 is neutral, values below 7 are acidic, and values above 7 are alkaline.

A pH test on concrete does not measure the pH of dry concrete itself. It measures the pH of a liquid solution formed when water contacts the prepared concrete surface and dissolves available alkaline compounds.

Why Concrete Is Alkaline

Portland-cement concrete develops an alkaline pore solution as cement hydrates. This high alkalinity is normal and helps protect reinforcing steel within sound concrete.

At an exposed surface, carbon dioxide from the air reacts with concrete in a process called carbonation. Carbonation can reduce the pH near the surface while deeper concrete remains more alkaline.

Mechanical preparation may remove the carbonated surface and expose concrete with a higher pH. Testing before preparation may therefore produce a different result from testing the surface that will actually receive the coating or flooring system.

How Moisture Brings Alkalinity to the Bond Line

Moisture moving through concrete can dissolve alkaline compounds and soluble salts. As the moisture approaches the surface, it carries those materials with it.

If the surface remains open, moisture may evaporate and leave salts behind as efflorescence. If a low-permeability coating, adhesive, or flooring material covers the concrete, moisture and dissolved alkaline compounds may accumulate at the interface.

The resulting alkaline solution may attack materials that cannot tolerate the condition. This can contribute to softening, discoloration, adhesive deterioration, loss of bond, osmotic blistering, or failure of patching and underlayment materials.

Why the Bond Line Matters

The bond line is the interface where a coating, primer, adhesive, patching material, or membrane contacts the concrete. This very thin zone must remain stable if the system is to stay attached.

A coating may have excellent chemical resistance on its exposed face while remaining vulnerable to alkaline moisture attacking it from below. Product performance therefore depends on conditions at both sides of the coating film.

Materials That May Be Affected

• Flooring adhesives
• Water-based primers
• Patching and leveling compounds
• Resinous floor coatings
• Moisture-mitigation membranes
• Underlayments
• Resilient flooring
• Carpet backing and adhesive systems
• Wood-flooring adhesives
• Joint fillers and sealants

Possible Signs of Alkaline Moisture

ASTM F710 and Surface pH

ASTM F710 addresses the preparation of concrete floors to receive resilient flooring and includes procedures associated with evaluating surface pH. Project specifications and manufacturers may reference this practice even when other floor or coating systems are being considered.

The current governing procedure should be consulted for surface preparation, water placement, exposure time, test paper or instrument requirements, test quantity, and reporting.

Because test methods and product limits can change, contractors should not rely on an old field card, memory, or a generic internet instruction when performing a formal acceptance test.

Preparing the Test Area

The test should represent the concrete surface that will receive the installation. Adhesive residue, curing compounds, sealers, coatings, dust, cleaners, and other materials can interfere with the result.

If mechanical preparation will remove the existing surface layer, pH evaluation may need to occur after representative preparation. Follow the project requirements and product manufacturer's written instructions.

Conditions That Can Distort the Result

• Testing over an existing coating, sealer, or adhesive
• Testing through dust or loose debris
• Using contaminated water or equipment
• Using an incorrect amount of water
• Reading the test too soon or too late
• Allowing the test solution to contact cleaning chemicals
• Testing only an unusually clean or dry location
• Using expired or improperly stored pH paper
• Using an uncalibrated electronic pH instrument
• Recording a color estimate without adequate lighting

pH Paper Versus Electronic Instruments

Use the method required by the specification and manufacturer. A more expensive instrument does not correct an improperly prepared surface or an incorrect procedure.

Basic Field Procedure

1. Review the governing documents. Identify the required test method, number of locations, acceptable range, and tester qualifications.
2. Select representative locations. Include typical areas and suspect locations near walls, drains, cracks, joints, wet areas, and previous failures.
3. Prepare the surface. Expose the concrete condition that will receive the proposed system.
4. Clean without leaving residue. Follow the required procedure and avoid contaminating the test area.
5. Use the specified water and quantity. Distilled or deionized water may be required by the applicable procedure.
6. Allow the required contact time. Do not estimate or shorten the prescribed interval.
7. Measure the solution. Use the specified pH paper, test strip, or calibrated electronic instrument.
8. Record the result immediately. Include the location, time, method, surface condition, and environmental information.
9. Compare with the correct limit. Use the written requirement for the complete proposed system.

Understanding the Logarithmic Scale

The pH scale is logarithmic rather than linear. A surface solution at pH 12 is not simply a little more alkaline than one at pH 10. Each whole-number increase represents a tenfold change in hydrogen-ion activity.

This is one reason small differences can be significant when a product is near its published limit. Test equipment resolution, procedure, and documentation therefore matter.

pH Results Can Change

Surface pH is not necessarily permanent. Carbonation, grinding, washing, acid exposure, moisture movement, salt transport, and installation of a covering can change the environment at the concrete surface.

A slab may show a moderate surface pH while uncovered because the upper layer has carbonated. After surface preparation or renewed moisture movement, the bond-line environment may become more alkaline.

For this reason, pH results should be evaluated with moisture tests, surface preparation, slab history, and product limitations.

Efflorescence and Soluble Salts

Efflorescence forms when moisture dissolves salts, transports them to the surface, and then evaporates. The remaining deposit demonstrates that moisture movement has occurred.

Removing visible efflorescence does not eliminate the moisture source or salts remaining within the concrete. Recoating without further investigation may result in additional deposits, loss of adhesion, or osmotic blistering.

Standard pH testing does not identify every soluble salt or measure its concentration. Where contamination or osmotic blistering is suspected, additional sampling and laboratory analysis may be required.

Acid Washing Is Not a Universal Solution

Contractors sometimes assume that an alkaline surface should be neutralized with acid. Acid washing can introduce water, leave reaction products, alter the concrete surface, attack cement paste, and create disposal and safety concerns.

Do not acid-treat concrete unless the project specification and system manufacturer expressly approve the material, concentration, procedure, rinsing, neutralization, drying, and waste handling.

Responding to an Unacceptable pH Result

Reporting the Test

• Project name, address, area, and test date
• Name and qualifications of the person performing the test
• Standard, procedure, and edition used
• Test-location identification and floor-plan reference
• Concrete surface condition and preparation method
• Type and source of test water
• Quantity of water and contact time when required
• Test-paper brand, range, and expiration information
• Electronic instrument model, serial number, and calibration information
• Individual pH result at every location
• Ambient and concrete temperatures when required
• Visible efflorescence, dampness, residue, or contamination
• Applicable acceptance range and its source
• Exceptions or deviations from the required procedure

Contractor Field Checklist

• Review the complete system's published pH limitations.
• Determine whether testing is required before or after preparation.
• Expose clean, representative concrete.
• Use the required water, equipment, and procedure.
• Check test-strip expiration or electronic meter calibration.
• Test representative and suspect locations.
• Record individual results instead of only an average.
• Compare the results with moisture-test patterns.
• Investigate efflorescence and recurring salt deposits.
• Do not acid-wash unless specifically approved.
• Notify the responsible party of results outside the permitted range.
• Obtain written approval before proceeding with a corrective system.

Safety Considerations

Concrete dust produced during mechanical preparation may contain respirable crystalline silica. Follow applicable OSHA requirements, the project exposure-control plan, and equipment instructions.

Acids, cleaners, and other chemicals can cause burns, harmful vapor exposure, and dangerous reactions. Review the current safety data sheet and use required ventilation, personal protective equipment, containment, and disposal procedures.

Knowledge Check

Technical References

Use the edition required by the project specification and follow the current instructions issued by the specified system manufacturer.

• ASTM F710 - Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring.
• 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 D4263 - Standard Practice for Indicating Moisture in Concrete by the Plastic Sheet Method.
• ICRI Guideline No. 710.3 - Guide for the Mitigation of Moisture in Concrete Floor Slabs.
• ACI 302.1R - Guide to Concrete Floor and Slab Construction.
• Current technical data sheets, installation instructions, and safety data sheets issued by the specified coating, flooring, adhesive, patching, primer, and moisture-mitigation system manufacturers.

These references provide technical guidance but do not replace the project specification, governing regulations, manufacturer requirements, laboratory analysis, or evaluation by a qualified professional. Final testing, product-selection, and application decisions must be based on current documents and documented site conditions.