Concrete Moisture Testing: F1869 vs F2170

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A concrete slab can be strong enough to build on and still be too wet for the floor covering scheduled next. That is the decision at the center of this lesson. Strength progress and moisture readiness are not the same decision, and the two moisture tests in this lesson do not answer the same question.

ASTM F1869 uses calcium chloride to evaluate moisture vapor leaving the slab surface. ASTM F2170 uses an in-situ sensor to evaluate relative humidity inside the concrete. One looks at surface emission during the test period. The other looks deeper into the slab. If I treat those results as interchangeable, I can approve flooring with the wrong evidence.

The practical rule is simple. I identify the exact floor covering and adhesive, obtain the current written installation requirements, verify which test method the manufacturer accepts, and compare the documented result with that product's stated limit. California does not provide one universal moisture number that makes every flooring system ready. The product, the adhesive, the test method, and the written acceptance limit have to match.

That distinction matters because a finished floor changes the slab's drying condition. Before the floor goes down, moisture can leave through the exposed top surface. After a relatively impermeable floor covering is bonded over that surface, evaporation can slow sharply. Moisture that was deeper in the slab can redistribute toward the covered surface. A surface reading taken before that change cannot automatically describe the condition the adhesive will face after the change.

Curing is not drying. Curing describes cement hydration and the development of the concrete matrix and strength. Drying describes excess mix water leaving the slab. Those processes overlap, but they are not synonyms.

I put the distinction into a short comparison chart. The first row is curing: hydration and strength development. The second row is drying: excess mix water leaving the slab. The point at the bottom is the one worth remembering. A familiar 28-day strength checkpoint is not a moisture-test result.

That means age alone cannot release a slab for flooring. A contractor may know the pour date, the specified compressive strength, and the cylinder or field-test history, yet still lack the moisture information required by the flooring system. I do not convert elapsed time into an assumed pass.

Think of a thick piece of bread left uncovered. The outside can feel dry while the center still holds moisture. Put a low-permeance cover over it, and moisture inside begins to redistribute. Concrete is not bread, but the analogy keeps the supervision point straight: a dry-feeling surface is not proof of a dry interior.

The concrete mix contains water needed for workability in addition to the water involved in hydration. As the exposed slab dries, the top can become drier sooner than the material deeper down. Building conditions matter as well. Air movement, temperature, and conditioning can change surface evaporation. None of that makes a calendar useless. The calendar helps schedule work. It simply is not the instrument that measures flooring moisture readiness.

ASTM F1869 is commonly called the calcium chloride test. Under the method described in the supplied technical material, a technician places a preweighed dish of anhydrous calcium chloride on a prepared concrete surface and seals it under a cover. The dish remains protected for the prescribed test period, described in the source material as 60 to 72 hours. Its weight gain is then used to calculate the moisture vapor emission rate.

The result is expressed as pounds of moisture emitted per 1,000 sq. ft. during 24 hours. That unit tells me what the method is observing: vapor leaving the surface over time. It is not an internal relative humidity percentage, and it is not a direct measurement of moisture throughout the slab thickness.

The method has a useful, defined output when it is performed under the applicable protocol. The mistake is asking that output to prove more than it measures. A favorable calcium chloride result does not, by itself, prove that moisture deeper in the slab satisfies an internal relative humidity requirement.

Surface preparation and environmental conditions are part of a valid test process. I do not improvise the procedure, move a test because it is inconvenient, or accept a number without the supporting record. I confirm that the person performing the test followed the current standard and the product requirements. If the flooring manufacturer accepts this method, I compare the moisture vapor emission result with that manufacturer's limit stated in the same units.

Some product literature may show a limit such as 3 lb/1,000 sq. ft./24 hr. That is an example of how a limit may be expressed, not a universal California pass number. A different adhesive may permit a different value, impose extra conditions, require another test, or reject the method entirely. I use the actual written instructions for the specified assembly.

ASTM F2170 takes a different approach. The technician drills into the slab, installs an in-situ sensor in the prepared hole, allows the test location to equilibrate as required, and records internal relative humidity. The result is a percentage, so it cannot be compared directly with pounds of surface emission.

For a slab drying from one side, the supplied technical material places the sensor at 40% of the slab thickness. On a 4 in. slab, 40% is 1.6 in. That calculation is useful for understanding the method, but it is not permission to run the test from memory. The technician still needs the current standard, accurate slab-thickness information, suitable equipment, and the complete protocol.

If a slab has a different drying configuration, I do not assume that the same depth applies. I verify the current method. That is an important General B boundary. I need enough knowledge to coordinate testing and recognize whether the documentation makes sense, but technical familiarity is not a substitute for performing the standard correctly.

I put the two methods side by side. ASTM F1869 uses a calcium chloride dish under a sealed cover and reports surface moisture vapor emission. ASTM F2170 uses a sensor inside a drilled hole and reports internal relative humidity. For the one-sided drying condition covered here, the internal sensor is placed at 40% of slab thickness. The final row gives the supervisory warning: a surface result does not prove internal dryness, and an internal relative humidity result still has to be compared with the manufacturer limit.

You may see product limits stated as 75%, 80%, or 85% relative humidity. Those are examples found in product requirements, not a single statewide approval threshold. The number matters only when it belongs to the selected flooring and adhesive system and the accepted test method.

Why can the tests disagree? The exposed slab can develop a moisture gradient. The surface loses moisture to the room, while concrete deeper in the slab remains wetter. A conditioned building can make the surface look especially favorable because moving, drier air supports evaporation at the top.

Now suppose that surface is covered with resilient flooring and adhesive. The escape route changes. Moisture deeper in the concrete can redistribute as the slab moves toward a new equilibrium under the covering. The adhesive is no longer dealing only with the surface condition that existed while the slab was open to the room.

That is the central why. The covering changes the boundary condition. A test focused on present surface emission and a test focused on internal relative humidity are looking at different parts of that moisture behavior. I remember it this way: surface emission describes what is leaving now; internal relative humidity helps describe what is held inside.

Neither phrase means that I can ignore the manufacturer's instructions. If the manufacturer calls for the surface method, I document the surface method. If it calls for internal relative humidity, I document that method. If it requires both, I coordinate both. If two results point in different directions, I do not choose the friendlier number. I stop and reconcile the methods, conditions, product criteria, and test records.

Moisture can also interact with alkalinity and adhesive chemistry. Depending on the specified system, excessive moisture can contribute to softening, loss of bond, bubbles, lifting, staining, or other finish failures. I avoid saying that every adhesive fails in exactly the same way. The defensible conclusion is narrower: the manufacturer's moisture limits exist for the assembly, and exceeding them can place the installation and warranty at risk.

California's indoor-air-quality rules also affect finish-product selection. The supplied research identifies California Green Building Standards Code requirements tied to adhesive and sealant volatile organic compound limits and references South Coast Air Quality Management District Rule 1168. The same research lists 50 g/L for indoor carpet adhesive and 100 g/L for wood-flooring adhesive. Those chemical limits do not select the slab-moisture test. California Green Building Standards Code does not turn ASTM F2170 into the automatic test for every floor. The floor-covering and adhesive requirements still determine the accepted method and limit.

Subslab moisture control is a separate layer of the decision. A vapor retarder limits moisture movement from the ground toward the slab. A capillary break interrupts the upward movement of ground moisture through small connected spaces. Neither feature removes the original mix water already inside fresh concrete, and neither feature replaces testing before moisture-sensitive flooring is installed.

I put the supplied 2022 California requirements into a reference table. For a residential slab on grade, California Residential Code Section R506.2.3 identifies a minimum 10-mil vapor retarder conforming to ASTM E1745 Class A, with joints lapped not less than 6 in. For a nonresidential slab on grade, California Building Code Section 1907.1 identifies minimum 6-mil polyethylene, again with joints lapped not less than 6 in.

The third row covers the capillary break in California Green Building Standards Code Section 4.505.2.1. The supplied source describes a minimum 4 in. base of clean aggregate sized 1/2 in. or larger, with the vapor retarder in direct contact with the concrete. These requirements come from the code edition identified in the source package. I verify the adopted code edition, local amendments, approved plans, and project conditions before treating the table as a project instruction.

Notice what the table does not say. It does not say that a vapor retarder guarantees a dry slab. It does not say that 4 weeks, 8 weeks, or a hot summer guarantees readiness. It does not say that the subslab assembly creates a universal flooring pass. The barrier addresses moisture from below. Testing addresses the condition of the slab before the selected finish system is installed.

That separation helps with defect investigation. If moisture results remain high, I do not immediately blame one cause. I review the subslab records, pour information, building enclosure and conditioning history, test protocol, test locations, product instructions, and any known water exposure. I distinguish verified facts from assumptions, then involve the flooring manufacturer, design professional, testing specialist, or other responsible party as the project requires.

For General B supervision, the cleanest process begins with the specified assembly. I identify the floor covering, adhesive, primer, patching or leveling material, and any proposed moisture-mitigation product. A limit for one component does not automatically approve every other component in the stack.

Next, I collect the current written installation instructions and warranty conditions. I find the accepted moisture test or tests, the maximum values, the required environmental conditions, and any substrate-preparation requirements. I make sure the units on the report match the units in the limit. Pounds of emission do not become relative humidity, and relative humidity does not become pounds of emission.

Then I coordinate the test at the correct point in the schedule. The slab, building conditions, and testing process need to satisfy the applicable protocol and manufacturer instructions. I confirm who is qualified to perform and document the work. I also preserve the report, device information, dates, locations, ambient conditions, slab information, results, and any required calibration or verification records provided by the testing party.

When the report arrives, I compare each relevant result with the correct product limit. I do not approve from the word pass alone without knowing what passed, under which method, and against which criterion. I do not average away a high location unless the governing instructions expressly allow that treatment. I do not substitute an older data sheet for the current specified product.

If a result exceeds the accepted limit, I pause the flooring decision. Possible next steps may include more drying time, investigation of moisture sources, a manufacturer-approved mitigation system, a product change approved through the project process, or retesting. The correct path depends on the contract documents, manufacturer instructions, design responsibility, test findings, and authority having jurisdiction. I do not invent a field fix and hope the finish hides it.

Documentation protects the decision chain. It shows what system was selected, what method was required, who tested, what conditions existed, what the results were, and who authorized the response. That record is useful before installation, during inspection, at closeout, and if a failure appears later.

Based on the published CSLB study outline, floor covering, project coordination, field inspection performance, and error identification are testable areas. That does not mean anyone outside PSI can predict a particular question. It means this is a sound General B supervision problem: recognize the limit of each test, coordinate the right evidence, and refuse to turn a schedule date into a moisture result.

Consider a hypothetical tenant-improvement project. The slab has been exposed inside a conditioned building, and the schedule calls for resilient flooring. The adhesive instructions accept internal relative humidity only up to 80%. A calcium chloride report shows 2.5 lb/1,000 sq. ft./24 hr. Someone calls that a pass and asks for installation to begin.

I cannot approve from that statement. The number may satisfy a surface-emission limit for some product, but the specified adhesive in this hypothetical requires internal relative humidity. The test method and acceptance criterion do not match.

Suppose the qualified testing party then performs ASTM F2170 under the applicable protocol and records 94% internal relative humidity. That exceeds the hypothetical manufacturer's 80% limit. The proper supervisory response is to pause installation, document the result, and resolve the condition through the project requirements and responsible parties. Installing anyway could lead to loss of bond or other flooring distress, but failure is not the only concern. The contractor would also be acting against the documented product limit.

Now reverse the pressure. Suppose an owner points to the slab age and says it has been down long enough. I return to the same logic. Age is not a moisture-test method. Suppose the vapor retarder inspection passed. I return to the same logic again. Ground-moisture control is not a measurement of remaining slab moisture. Suppose one test location looks good while another exceeds the limit. I follow the governing protocol and manufacturer criteria; I do not erase the unfavorable data.

This is where experienced supervision matters. The difficult part is rarely recognizing a dish or a probe. The difficult part is keeping the schedule, units, method, limit, and assembly connected when people want a quick yes.

Here is the memory check I want you to carry to the next project. Curing is strength development; drying is moisture leaving. ASTM F1869 reports surface moisture vapor emission in pounds per area over time. ASTM F2170 reports internal relative humidity as a percentage. For the slab drying from one side in the supplied material, the probe depth is 40% of slab thickness. A favorable result is meaningful only when the method, units, conditions, and manufacturer's limit all match the specified floor system.

Keep the subslab controls in their own box. The vapor retarder and capillary break manage moisture coming from below. They do not certify the slab for floor covering. Keep California's material-emission rules in their own box as well. They affect product requirements, but they do not create one mandatory moisture test or one universal passing number.

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