Advances in Ultra-High-Pressure Waterjetting · Ultra-High-Pressure Waterjetting by Richard Schmid,...

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brasive blasting has domi-nated the surface prepara-tion industry for most of

this century, primarily because of its economic and performancebenefits. High productivity rates,particularly for large area surfaces,have reinforced abrasive blast-ing’s popularity with contractors. Industry-wide acceptance of a sur-face prepared by grit blasting andfamiliarity with the process werealso factors.

However, external factors such as environmental and occupationalregulations have prompted the surface preparation industry to seek alternatives. Concerns aboutthe effects of air-borne dust on the public, the environment, andworkers have resulted in the regulation of open abrasive blast-ing, especially for hazardous paint removal.

Contractors now must meet strin-gent requirements for containingabrasive and paint debris duringblasting operations to protect the environment and the public, andthey must assure that their workershave the proper training and protec-tive equipment to reduce occupa-tional exposures to lead, silica sand, and other dusts generated during blasting.

One alternative to abrasive blast-ing, ultra-high-pressure (UHP) wa-terjetting, has evolved as a viabletool for industrial applications. SSPCand NACE International define UHPwaterjetting as 25,000 psi (172 MPa)or above; the present article definesit as above 35,000 psi (241 MPa).

In the past 2 to 3 years, the accep-tance of UHP waterjetting for surfacepreparation has grown rapidly. Be-fore this period, waterjetting wasused only on unique or very chal-lenging applications. It is widelyused today on many kinds of sur-face preparation projects, such ascoating removal on bridges; ships;storage tanks; and large, complexsteel structures.

This Maintenance Tip identifiesadvances in waterjetting equip-ment, coatings technology, and the understanding of chloride conta-mination that have made water-jetting a more viable option for surface preparation compared toseveral years ago. The article alsoidentifies the 2 major uses of waterjetting today in maintenancecoating operations.

Evolution in Waterjetting TechnologyDevelopments in Pumps andNozzles Increase ProductivityProductivity levels of older, hand-held UHP lances were once onefourth to one third that of hand-heldabrasive nozzles. For Near White(SSPC-SP 10) and White Metal(SSPC-SP 5), coating removal ratesfor UHP waterjetting generally fellinto the range of 20 to 30 sq ft/hr(1.8 to 2.7 sq m/hr) per hand-heldUHP tool, compared to productivityranges of 90 to 120 sq ft/hr (8.1 to10.8 sq m/hr) per nozzle for abra-sive blasting. UHP productivitytherefore limited the commercial via-bility of the method.

The 2 main reasons for the lowerproductivity of UHP were the limitsof the hydraulic intensifier pump

A

82 / Journal of Protective Coatings & Linings

Advances in Ultra-High-Pressure Waterjetting by Richard Schmid, Flow International Corporation

Shipyards make up the largest group of users of UHP waterjetting for surface preparation.Courtesy of Flow International and Todd Shipyards

Copyright ©1997, Technology Publishing Company

and the limits of waterjet nozzle ro-tation rates. The intensifiers werenot capable of achieving pressuresabove a range of 30,000 to 35,000psi (206 to 241 MPa), which limitedcleaning rates. Moreover, until re-cently, nozzle rotation rates werenot attainable above 3,000 rpm,which also restricted cleaning rates.

However, technology has rapidlyadvanced between 1992 and 1996 tomeet the production rates demandedby the surface preparation industry.Hydraulic intensifier pumps havebeen replaced by positive displace-ment (plunger) pumps capable ofachieving pressures of 40,000 psi(276 MPa) in the field. Positive dis-placement pumps performing in the10,000 to 20,000 psi (69 to 138 MPa)range had been available in the fieldfor a number of years. The evolutionin the capabilities of these pumpshas been the most significant equip-ment advance in waterjetting tech-nology in the past 4 years. Higheroperating pressures mean fastercoating removal rates. Nozzles havealso improved in the past 4 years.Higher nozzle rotation speeds of3,000 to 3,500 rpm are now possi-ble. The higher the rotation speed ofthe multi jet nozzle means the fasterthe worker can move the wandacross the surface to be cleaned andachieve full coating removal.

Higher pressures and nozzle ad-vances have pushed productivity torates comparable to hand-held abra-sive blasting. For Near White andWhite Metal, typical removal ratesare 80 to 100 sq ft/hr (7.2 to 9 sqm/hr) for UHP compared to 90 to120 sq ft/hr (8.1 to 10.8 sq m/hr) forabrasive blasting and compared toearlier UHP rates of 20 to 30 sq ft/hr(1.8 to 2.7 sq m/hr).

Plunger Pumps Reduce CostsIn addition to their restrictions onpressure, hydraulic intensifier pumpswere suited to shop work but not to

an abrasive blasted surface cancause blistering of coating films after a few weeks of exposure to condensing humidity.1 It hasbeen well known for many yearsthat soluble salts still remain at these levels on steel surfaces afterabrasive blasting.2,3

UHP is capable of removing solu-ble salts. One coating manufacturerfound that if potable water is usedfor UHP waterjetting, surface saltlevels below 7 µg/sq cm can be ob-tained on old rusted and pittedsteel.4 SSPC and NACE have pub-lished this level in their joint stan-dard, SSPC-SP 12/NACE 5, “SurfacePreparation and Cleaning of Steeland Other Hard Materials by High-and Ultrahigh-Pressure WaterjettingPrior to Recoating.” Furthermore,following upon the work of a majorcoating manufacturer that publishedvisual standards for waterjetted sur-faces, SSPC and NACE are develop-ing consensus visual standards forcoated steel surfaces prepared bywet methods.

Coatings Developed for Flash-Rusted SurfacesA waterjetted surface will show signsof light flash rusting shortly afterpreparation. Until recently, ownersand coating manufacturers havebeen hesitant to apply coatings overa flash-rusted surface. It is usuallythe coating manufacturer who takesfinal responsibility or warranty forthe long-term performance of thecoating. Recognizing the benefits ofUHP waterjetting, major coatingmanufacturers have developed sur-face-tolerant coatings that can be ap-plied directly over a waterjetted sur-face.5 The draft NACE/SSPC visualstandards for surfaces prepared bywet methods also provide referencephotos for degrees of flash rusting.

Most generic types of surface-tol-erant coatings on the market can be

field work. They could not with-stand the demands of harsh environ-ments such as those found in ship-yards and bridge sites. Attempts to convert hydraulic intensifierpumps from factory to field applica-tions usually failed because the in-tensifiers were unreliable and com-plex, and they required cleanoperating conditions not generallypossible in the field. If used in thefield at all, they needed a well-trained staff to operate and maintainthe equipment.

Not only do positive displacementpumps increase productivity, butthey are also much easier to main-tain and do not need the clean oper-ating conditions that intensifiers re-quire. Ease of maintenance makespositive displacement pumps a com-mon choice for work in harsh indus-trial environments.

Operation of UHP systems nowdiffers little from low pressure waterblasting, a staple for decades in theautomotive and marine industries.Users do not need to hire or trainspecialists to operate and maintainthe equipment. Operators familiarwith lower pressure water blastingprocedures and safety requirementsfor waterjetting can now operateUHP systems.

Advances in the Coatings IndustryUnderstanding of Chloride Contamination, Cleaning, andCoating AdhesionAn abrasive blasted surface is im-pacted with many small, discreetabrasive particles that flatten out thesharp peaks created from originalprofile and also create new peaks.1

On a microscopic level, abrasiveparticles will actually fold over andtrap contaminants such as chloridesand sulfates, affecting the quality ofthe finished product. Chloride levelsas low as 10 µg/sq cm and sulfatelevels higher than 20 µg/sq cm on

FEBRUARY 1997 / 83continued



applied over tightly adhered flashrust. These coatings are specificallydesigned to go over less than opti-mum surfaces and still provide ex-cellent adhesion.

Major Types of UHP WaterjettingApplicationsDrydocks and ShipyardsEnvironmental attributes of UHPhave broadened markets for theequipment. Shipyards, with theirproximity to environmentally sensi-tive areas, make up the largestgroup of users of UHP surfacepreparation equipment.

For paint removal and surfacepreparation on ships, water is typi-cally allowed to flow down the sideof the ship and drop into the bottomof the dry-dock, where it is collectedand pumped to a central filtrationsystem that is in place to processrun-off waters from rain and wash-ing operations.

Compared to dry blasting, there isno risk of abrasive contamination toneighboring residential or industrialareas. Also, there is no risk of abra-sive contamination to surroundingareas of a ship repair project, whichallows for other repair processes tobe conducted simultaneously. Forexample, repainting or equipmentrepair work can go on adjacent to awaterjetting project. Concurrent re-pair work significantly reduces thetime in dry dock, contributing tocost savings for both the vesselowner and the shipyard.

Waterjetting provides several othercost savings benefits to the shipyardowner and vessel owner. Becauseabrasive is not used, there is no costof grit, nor cost to collect and disposeof grit. The water used in the processgenerally can be filtered and disposedof through the sewer system whilethe paint chips are disposed of as aseparate waste. Waterjetting can be

used adjacent to sensitive deck equip-ment, such as winches, electric mo-tors, and cranes, without the need formasking or tarping.

Lead-Based Paint Removal fromSteel StructuresThe second largest application ofUHP waterjetting is removal of lead-based paint from steel structures.UHP surface preparation generatesno air-borne dust, eliminating thenecessity to construct costly contain-ment systems. It reduces workerprotection costs and improves work-er efficiency. Blasters can often wearlightweight face masks in place ofcumbersome and expensive air-sup-plied respiratory equipment.

In fact, UHP waterjetting has actually simplified containment.Most UHP waterjetting projects, even lead-based paint removal, aredone without the use of negative air containment.

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Copyright ©1997, Technology Publishing Company FEBRUARY 1997 / 85

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Typical to a project is a simpletarp laid out just below the removalarea with an earth or timber berm towork as a collection “pond.” Wateris pumped from the simple poly tarppond to settling tanks where paintchips and water are separated. Insome cases, additional tarps areused to shroud structures like a cur-tain and direct the water to thetarped area on the ground.

Waterjetting also eliminates thecost of disposing of lead-contaminat-ed abrasive. Abrasive blasters use 4to 10 lbs of abrasive/sq ft (5 to 9kg/sq m). In contrast, waterjettinguses approximately 2 to 3 gal. ofwater/sq ft (83 to 124 L/sq m). Thissmall amount of water is easily collected and treated if necessary for disposal or recycling. Theamount of solid or hazardous wasteis greatly reduced; without spentabrasive, only the paint debris needsto be disposed of as a solid or haz-ardous waste.

Limitations of WaterjettingDespite advances in the technologyand in the industry, waterjetting isnot suited for every type of project.It is not appropriate for projectssuch as the following.• Maintenance projects requiring aprofile of the steel: Waterjettingalone will not profile the steel struc-ture like dry abrasive blasting.Therefore, UHP waterjetting is nottypically used when removing paintfrom non-profiled steel such as thaton many bridges built before 1975.In some instances, however, such aswith lead-based paint, the paint isremoved with waterjetting, and dryabrasive blasting is used as a sec-ondary process to profile the steel.This combination of methods elimi-nates the disposal of grit contaminat-ed with lead-based paint. • New construction: Waterjetting istypically not used in new steel con-struction because it is not capable of

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providing a surface profile. Surfacepreparation is typically done in pro-duction plants with large recyclingshot blasters.• Areas where there are no environ-mental regulations: In some areas,open air abrasive blasting is still vi-able. Dry abrasive blasting projectsare being drastically reduced, but onthose projects where it can still bedone, abrasive blasting is sometimesmore cost effective than waterjetting,especially if collection and disposalof grit are not issues.

ConclusionAdvances in waterjetting equipment,coatings technology, and environ-mental protection have made UHPwaterjetting a more viable optionthan it has been in the past. Its useneed not be inhibited by equipmentthat is difficult to use and maintainin the field or by flash rusting overclean steel.

References1. D.G. Weldon, “Salts: Their Detec-

tion and Influence on CoatingPerformance,” in Proceedings ofthe SSPC Annual Symposium,Cincinnati, OH, May 22-23,1985 (Pittsburgh PA, Steel Struc-tures Painting Council, 1985),pp. 1-18.

2. Gordon H. Brevoort, “AbrasiveBlasting and Salt Contamina-tion: A Case History,” JPCL (May1988), 24, 71.

3. Wallace P. Cathcart, “Non-visibleContaminants in Railcar Interi-ors: Their Significance and Re-moval,” JPCL (December 1987),6-10.

4. Douglas Gilbert, “New Hydrob-lasting and Slurryblasting Stan-dards Issued,” JPCL (January1995), 64-69.

5. John Kelley, “There is More thanOne Kind of Rust,” Marine LogShip Repair Conference, Hous-ton, TX, May 1996 (New York,NY: Marine Log, 1996).

Two types of water transmissionproblems are associated with fail-ures of slab-on-grade flooring ortopping systems:• hydrostatic water pressure, whichis the static condition of moisture ina slab; and• water vapor pressure, which is thedynamic condition of moisture inthe form of vapor emanating from a slab.

This article explains hydrostaticand water vapor pressures, describestheir adverse effects on coated con-crete, identifies corrective measures,and describes methods of detectingand measuring moisture in concrete.

Two Types of Water Transmission ProblemsHydrostatic water pressure is de-rived from water in the liquidphase.1,7 It is sometimes known ashead pressure (Fig. 1). Hydrostaticpressure is the result of force exert-ed by a column (head) of water.The pressure is equal to 0.43 psi per linear foot of water height (9.76kPa per linear meter).

This force is the result of the dif-ferential between the highest eleva-tion of a water column and the low-est physical point of a structure.Hydrostatic water pressure problemsare generally associated with below-grade slab or areas subjected to highwater tables. Other water liquidphase pressures and dynamic forcesassociated with hydrostatic pressureresult from• capillary pressures developedwithin the concrete matrix porespaces and up through the subsoil(Fig. 2), and • osmotic pressures developed with-in the concrete matrix and across

the coating membrane to its top sur-face (Fig. 3).

Water vapor pressure, or water ina vapor (gaseous) phase, acts andresponds in an entirely different waythan water in a liquid phase, i.e., hy-drostatic water.1,7 Water vapor has adensity of only 1⁄250,000 that of water ina liquid phase. Vapor can readilypass through most concrete sub-strates that have capillary spaces assmall as 5 micrometers.

Water vapor pressure is the combined result of the dynamic effects of ambient environmental parameters: • the amount of available liquidphase water (moisture content) pre-sent to become vaporized,• temperature, and• relative humidity.

Water vapor movement throughconcrete requires the developmentof pressure as the driving force due to a differential in vapor pres-sure between the above- and below-slab ambient environments, i.e., tem-perature and humidity (Fig. 4).Vapor pressure differentials are de-veloped due to differences of rela-tive humidity and temperature aboveand below the slab (Fig. 5). Watervapor pressures are related tochanges in temperature and relativehumidity differentials.

Adverse Effects of Hydrostaticand Water Vapor PressureHydrostatic and water vapor pres-sures have a variety of detrimentaleffects on slabs on grade, coatings,and flooring systems.2,4,7

Effects on Slabs on GradeHydrostatic and water vapor pres-sures may transport water-soluble

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How Water Transmission Affects Slab-on-Grade Flooring Systemsby Charles Hall and Scott O’Connor, Phoenix Engineering Services, Inc.


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minerals from both the subsoil andthe slab concrete towards the slabsurface. Concentrations of the miner-als at or near the slab’s surface occurwhen the water/moisture has evapo-rated, thereby recrystallizing theminerals in place. Crystallization of

minerals is an expansive physical reaction that can exert strong disrup-tive forces within the concrete matrix, causing weakening or disin-tegration and subsequent break-up of the slab itself. Waterproofingprevents water in its liquid phase

from entering into and through con-crete from hydrostatic pressures.Moisture vapor controls are used toprevent water escaping in itsgaseous state and emanating from aslab’s surface.

Effects on Coatings and Flooring MaterialsHydrostatic and vapor pressuresbuild up on the negative side of animpermeable (non-breathing) mem-brane, coating, or flooring material.There are positive and negativesides to all membranes with respectto moisture vapor pressures. Thepositive side refers to the side inwhich contact from the moisturesource is being made. The negativeis opposite the moisture source.Higher pressures are found on thepositive side. If the pressures are allowed to accumulate, or are notdissipated or relieved by venting,they may develop sufficiently to disbond the coating or flooring system from the slab. Bond failurewill occur when the hydrostatic orvapor pressures exceed the adhesivebond strength of the coating orflooring system to the parent con-crete substrate.

Breathable (high permeance) coat-ings and flooring systems are far lesssusceptible to the effects of hydro-static or vapor pressures than non-breathing coatings or flooring sys-tems. Breathable materials typicallyexhibit U.S. perm ratings of greaterthan 3.0 perms. (Perm = grains ofmoisture per hour per sq ft per inch-es of mercury [P=1 gr/hr/sq ft/in.Hg].)* Concrete (3,000 psi [21 kPa]

continued

Fig. 1 - Hydrostatic pressure as exerted by a column of water. Formula: an arbitrary columnheight of 700 ft (212 m) head x 0.43 psi (2.9 kPa)/ft (m) head = 300 psi (2,070 kPa) head pressure. In this example, the bond strength of the topping exceeds the head pressure of the column of water.Figures courtesy of Phoenix Engineering Services, Inc.

Fig. 2 - Capillary pressure moisture transmission. The dark spots represent aggregate.

* Current metric formula for report-ing permeance is

57.2 • 10-12 kg/sec • sq m • Pascals.

Previously used (S.I.) formula for reporting permeance is

57.2 nanograms/sec • sq m • Pa.


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compressive) typically exhibits permratings of 20 to 30. Cross-linked,chemically cured epoxy coating andflooring systems typically exhibitvery low perm ratings of 0.15 perms.

Moisture emission from slab-on-grade concrete is quantified as the

pounds of water emitted per 1,000sq ft (90 sq m) per 24-hour period.Moisture emissions are the com-bined total of moisture from hydro-static and water vapor pressures.Moisture emissions as dictated andrecommended by industry associa-tions should not exceed 3.0lbs/1,000 sq ft/24 hrs (1.5 kg/100 sqm/24 hrs).10

Corrective Measures and Remedial ActionsDesign and ConstructionCorrective measures may be includ-ed during the design and construc-tion phases of slab-on-grade con-crete.3,4,7,9 Some easily controlleddesign and construction parametersand practices used to curtail or elim-inate slab-on-grade moisture prob-lems are listed below.

Gravel Capillary BreaksThe use of 1⁄4 to 3⁄8 in. (6 to 10 mm)of gravel is effective as a capillarybreak between the concrete slabbottom and the underlying subsoil.Gravel layers may be 8 to 12 in. (200to 300 mm) deep. Gravel breaks

up capillary pressures due to liquid phase moisture transmissionfrom wet or damp subsoil to theconcrete itself.

Unlike concrete itself, coarse sand,fine sand, and fine silt and clay subsoils, gravel completely lacks

the interconnected network of microscopic size pore spaces need-ed to effectively transport water upby capillary pressure. Subsoil is an excellent medium for capillarytransfer. Moisture content of soils isproportional to the fineness of thesoil particles.

Fine soils less than 0.002 mils(0.05 micrometers) particle diameterat a 55 percent concentration maygive off 12 gal. of water or 99lbs/1,000 sq ft/24 hrs (51 L [50kg]/100 sq m/24 hrs). Capillarywater from the water table may rise2.5 ft (0.8 m) through coarse sand,7.5 ft (2.3 m) through fine sand, and 11.5 ft (3.5 m) for fine silt andclay subsoils. Capillary transfer ofwater through clean, graded gravel is negligible.

Low Perm Vapor BarriersThe use of low perm vapor barriers(sheet materials) under the slab andover the gravel or subsoil layer is ef-fective for hydrostatic and vaporpressure. Commonly used sheetstock vapor barrier materials arepolyethylene (4 to 10 mils [100 to

250 micrometers]), polyvinyl acetate-reinforced polyethylene (4 to 10 mils[100 to 250 micrometers]), roofingfelt (55 lbs [25 kg]), asphalt-impreg-nated fiberglass, and polymer-modi-fied paper. Reinforced 10-mil (250-micrometer) thick polyvinyl acetatesheet seems to offer the best overallphysical properties for slab-on-grademoisture control.

Construction damage (punctures),unsealed seams, pipes, and otherobstructions breach the monolithicquality of vapor barrier sheets andsignificantly reduce their effective-ness. Some proprietary coating andflooring manufacturers offer a “com-plete systems approach” by provid-ing integrated seam seals and bootsto seal around pipe and conduit slab penetrations.

Empirical testing has shown thatthe thickness of the membrane sheethas little or no significant impact onits effectiveness. The inherent per-meability of the membrane materialitself is the determining factor.Thicker films are, however, less like-ly to puncture or tear.

Secondary Capillary BreaksA secondary capillary break consist-ing of coarse sand 4 in. (100 mm)deep may prove beneficial whenplaced on top of the sheet barriermaterial. This break may prevent the capillary wicking of moistureinto the concrete slab where the in-tegrity of the sheet barrier has beencompromised. The synergistic com-bination of capillary break gravel,vapor barrier sheet, and secondarycapillary break sand has proven toafford the highest resistance to mois-ture and vapor transmission intoslabs on grade.

Insulation To Reduce CondensationThe use of insulation under a slaband around its perimeter edge great-ly reduces water condensation onthe slab surfaces. Insulation materi-

continued

Fig. 3 - Osmotic pressure


als must not be damaged by contactwith water. The insulating propertiesmust be capable of withstandingcyclic wetting and drying, andshould be highly resistant to pests,termites, fungus, and mildew.

Condensation is defined as water

vapor changing from a gas to a liq-uid state. Condensation occurs whenthe surface temperature of the slabis equal to or below the dew pointtemperature of the ambient air. Insu-lation aids in keeping the slab’s surface temperature above the ambi-ent dew point temperature. Insula-tion will reduce or eliminate thecondensation (sweating) or watervapor on the perimeter or undersidebase of the concrete slab. Watercondensation on a base concretesurface will be transmitted into, andthrough, the concrete primarily bycapillary pressure.

Reduced Water-to-Cement RatiosWater-to-cement (w/c) ratios in con-crete slab mix design play an impor-tant part in controlling moistureproblems.4 Concrete mixes havingw/c ratios greater than 0.50 requiresubstantially and progressivelylonger wet curing times to form rela-tively impermeable cement paste.The longer curing time lengthensthe waiting time required to installlow permeability flooring systemsover new (green) concrete. The

lower the w/c ratio, the denser theconcrete. The denser the concrete,the higher its resistance to moisturetransmitted from hydrostatic andwater vapor pressures. High w/c ra-tios produce porous concrete. Loww/c ratio concrete slabs reduce the

evaporable mixing water (waters ofconvenience), the curing period,and the moisture permeance of the concrete.

The use of high range water-re-ducing agents or super plasticizersin the concrete mix greatly aids inachieving low w/c concrete of lessthan 0.40 while maintaining flowa-bility and slump characteristics. Poz-zolans have also proved very effec-tive. These materials—such as flyash, silica fume, and somemetakaolin clays—can be mixedwith water in the presence of Port-land cement to form additional highquality calcium silicate hydrate cement paste. This paste can resultin concrete with higher densities,better physical properties, and lowerwater/gas permeability.

New concrete slabs on gradeshould be allowed sufficient time toform impermeable cement paste be-fore flooring materials are installed.Usually 2 or more months are need-ed for cure before the application ofcoatings or finished flooring (28-dayabsolute minimum).

Flooring problems occurring over

newly placed “green” concrete are most likely due to the excesswater of convenience not used up in the hydration process. Only0.3 lbs of water is required to completely hydrate 1.0 lbs of Port-land cement.

A w/c ratio of 0.57 requires ap-proximately 135 wet cure days toobtain impermeable cement paste. Aw/c ratio of 0.52 requires only 50wet cure days. A w/c ratio of 0.45requires fewer than 14 days. The re-maining water is for the conve-nience of placement flowability.

Grading to Improve DrainageGrading the site to achieve surfacewater drainage away from the slabwill reduce the potential for hydro-static problems.3 Site grading shouldbe performed to carry water awayfrom the slab in all directions. Fin-ished grades away from the slabshould be a 12-inch (300-millimeter)drop for every 25 ft (7.5 m) in all di-rections. This is equal to a 4.0 per-cent slope. Finished grades at out-side walls should be a minimum of8 in. (200 mm) below the top sur-face of the slab. Finished gradesnext to slab-on-grade concreteshould also have a 12-inch (300-millimeter) drop for every 25 ft (7.5 m) in all directions, i.e., a 4 per-cent slope.

Corrective and Remedial Measures on Existing SlabsCorrective and remedial measuresmay be required on existing slabson grade after design, construction,and concrete placement. Some com-mon materials and practices are pro-vided below.

Liquid Silicate PenetrantsThere are 2 basic types of liquid sili-cate penetrants. The first type arepenetrants that clog and plug upconcrete slab capillaries to physical-ly block water vapor transmission.

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continued

Fig. 4 - Moisture vapor transmission (MVT)


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The second type are penetrants thatrestrict (plug up) capillaries only atthe concrete’s slab surface. Thesetypes are different chemically butsimilar in terms of performance.

Penetrants that clog and physicallyblock capillaries are based uponpotassium and sodium silicates. Thepotassium silicates are applied directly to clean, dampened con-crete surfaces by means of low pres-sure spray.

These proprietary formulas reactin situ with available calcium hy-droxide to form insoluble carbonatecompounds that fill in and block theconcrete’s capillaries. Both potassi-um and sodium silicates are usuallyapplied at rates between 200 and250 sq ft/gal. (4.9 to 6.1 sq m/L).The rate depends on the porosity ofthe concrete.

Neither the potassium nor thesodium silicate is effective enoughby itself to completely eliminate thehigher vapor or hydrostatic pres-sures sometimes incurred; however,each has been used effectively withother methods. ACI publication212.3-91 states that the use of chem-ical admixtures such as sodium sili-cate is “detrimental to concretestrength” and is “not effective or ac-ceptable in controlling moisture mi-gration through slabs on grade.”5

This statement is in reference to itsuse as an admixture and not as atopical surface treatment.

Surface Coatings Polymer materials may include various deck paints, epoxies, ure-thane acrylics, and other surface-applied liquid films. Chemicallycured materials such as epoxies andsome two-part urethanes have ex-tremely low perm rates with respectto high perm (high breathability)coatings such as water-borneacrylics. Alkyd and oil-based coat-ings should be avoided at all costsbecause of saponification problemson concrete.

Breathable (High Perm) CoatingsCoatings exhibiting high perm ratings of 3 or more have the abil-ity to “breathe” and let the mois-ture vapor pass through, therebymaintaining their bond to the concrete and allowing the release

of vapor pressures. High permbreathable coatings are not verygood moisture vapor barriers andare still susceptible to loss of adhe-sion due to hydrostatic liquid phasewater pressures.

The overall effectiveness of sur-face coatings is highly dependent ontheir tensile and bond strengths tothe concrete slab. They must alsohave good resistance to the high pHalkalinity of concrete. Generallyspeaking, low perm coatings such asepoxies and two-part urethanesshould not be used where they haveto reduce more than 50 percent ofthe moisture transmission as quanti-fied and measured by instrumentaltechniques. Given a standard of 3.0lbs of water/1,000 sq ft/ 24 hrs (1.5kg/100 sq m/24 hrs) as a maximumamount permitted, a measurement

of greater than 6.0 lbs (3.0 kg)would exceed the 50 percent reduc-tion requirement. The resultant backpressure development and some-times the surface pH increase canlead to disbondment or spalling onthe surface of the concrete slab.

Surface coatings should not beused alone to reduce or eliminatemoisture transmission through slabs. Applied to concrete slabs, sur-face coatings are usually used toprotect the concrete from its imme-diate environment, which may in-clude chemical attack and physicaldamage. Considering the above, sur-face coatings may become part ofthe moisture problem rather than a remedy.

Moisture Barrier (Low Perm)For surface-applied coatings to be effective moisture barriers, they must have the following characteristics:• low perm ratings (less than 0.5U.S. Perms);• high resistance to the effects of

continued

Fig. 5 - Moisture vapor pressure (VP) at different temperatures and relative humidities (RH)


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high pH (alkalinity), which may leadto saponification;• adhesive bond strengths to theconcrete slab greater than the com-bined pressure effects of hydrostaticand vapor pressures; and• adequate chemical resistance andphysical properties to perform intheir intended service environment.

Membrane Moisture Dispersion SystemsThese materials include any surface-applied material that allows for and accommodates moisture vaporemission absorption, wicking, ex-pansion, and subsequent evapora-tion. Two basic systems are present-ed below.

Cement-based coatings may bepolymer modified. The cementi-tious coating material is usuallybrushed on or applied by broom di-rectly to the cleaned concrete slabsurface. Application may be fol-lowed by placement of an imper-vious low perm vapor barrier sheetmaterial, itself followed by anothercoat of cementitious coating. Theuse of an acrylic polymer in the ce-mentitious coating increases thephysical properties, bond strength,and vapor pressure resistance of the coating.

Fiberglass wicking systems aremade with woven strand fiberglassmating impregnated with an acrylicresin binder to create a membrane-like sheet. The fiberglass is capableof transporting moisture throughwicking (capillary pressure) from theconcrete slab. The moisture is trans-ferred laterally throughout its sur-face, to be dissipated and evaporat-ed into the ambient air.

The system is effective only for the control of moisture in its vapor phase and only up to the moisture saturation threshold of the fiberglass membrane itself.This system is, however, highly effective up to its saturation thresh-old limit.

Detection and Measurement of Moisture6,7,8,10

This discussion must be pre-faced with the warning that contrac-tors planning to install low per-meance (non-breathing) flooring or coating systems to slab-on-grade substrates should perform slab moisture emission tests onlywhen the environmental conditions

closely approximate the antic-ipated in-service conditions. Mois-ture vapor and hydrostatic pressuresare not constants and are thereforesubject to significant change at any time.

The amount of water in a liquidphase within a slab is measured andquantified in percentage factors.

continued


Moisture content in slabs is quanti-fied as a direct percentage of thewater weight of the concrete.

Water vapor emanating from aslab’s surface in its gaseous state ismeasured and quantified inweight/area/time. Flooring industryconsensus is that moisture vaporemissions from slabs on gradeshould not exceed 3.0 lbs of mois-

ture/sq ft/24 hours (14.7 kg/sq m/24 hours).10 Moisture emission isquantified by the weight of wateremanating from 1,000 sq ft (90 sq m) of slab surface over a 24-hour period.

Moisture detection and measure-ment techniques performed as pre-and post-installation testing includethe following.

Methods for Measuring Moisture ContentGravimetric TestingGravimetric testing is currently themost accurate and reliable techniquefor determining the actual percent-age by weight of water (moisture) inconcrete, slabs or otherwise. Thismethod may be used to verify the 3percent maximum moisture content.Concrete samples are hermeticallysealed, weighed, and oven dried at 248 F (120 C) until a constantweight is achieved. The differencebetween the weights before andafter oven drying is expressed as thepercentage of moisture of the totalmass. Gravimetric testing is suitablefor both pre- and post-installationmoisture measurements, i.e., plasticand hardened. This method mea-sures static moisture contained inthe slab but is not suited to mea-surement of moisture vapor. Themethod is destructive.

Newly placed concrete of 3,000 to5,000 psi (21 to 35 kPa) compressivestrength exhibits moisture contentbetween 5 percent and 10 percentby weight.

Nuclear DensityNuclear density is highly accuratebut not as practical or easy as thefirst method described. This methodutilizes nuclear tagging of the hydro-gen (H) atoms in water (H2O) withthe isotope Americium 241 (berylli-um neutrons). A commercial instru-ment know as the Troxler Gage uti-lizes this nuclear technology. Thismethod provides the percentage ofstatic water in the slab by weightrelative to concrete density. Densityinstrumentation requires careful cali-bration and correction for other hy-drogen compounds that may be inthe concrete.

Radio FrequencyLike nuclear density, radio frequen-cy is accurate but not as practical or

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easy as the first 2 methods de-scribed. Several proprietary, afford-able instruments use radio frequencyenergy (RFE) as moisture detectionand measurement techniques.

RFE is propagated several inchesinto the concrete. The resultant re-flected RFE power loss or RFE ab-sorption levels are measured. Thepower loss is directly proportional to the moisture content. The instrumentation must be calibratedagainst a known concrete moisturecontent. Many readings can be takenover a large surface area in very little time. This method is also helpful in detecting and tracingmoisture intrusion sources. Thismethod does not measure mois-ture vapor.

Both the nuclear density and RFE measurement techniques arenon-destructive.

Electro-conductiveThis measurement technique indi-rectly measures moisture content ofconcrete by directly measuring theconcrete’s (dc) electrical conduc-tance, or, reciprocally, its (dc) elec-trical resistance. The moisture con-tent of concrete is related to theconcrete’s electrical conductance.The conductance is measured by theinstruments in millimeters over arange of 0 to 32. An electrical cur-rent is passed between 2 pin-likeelectrodes pushed into the concretesurfaces. The resultant reading is theconcrete’s conductance between theelectrodes.

There is no direct correlation be-tween the measured current and ab-solute percentage of moisture, butcomparisons are useful. In addition,this technique is falsely affected bythe presence of ionic salts such assodium and calcium chlorides. Thehigher the level of ionic salts, thehigher the conductance will be, re-sulting in erroneously high moisturereadings. A moisture meter is avail-able based on this principle.

The coatings industry and instru-ment manufacturers have collective-ly through job site experience andtesting reached a consensus thatreadings greater than 16 are not con-ducive to proper coating or flooringmaterial application.

This technique is the least accu-rate method. The method does notmeasure moisture vapor.

Methods for Measuring MoistureEmitted from ConcreteCalcium Chloride Desiccant TestThis method is the most reliable testfor moisture content. It is easy andpractical to perform. It has beenadopted by the flooring industry tobe the standard for determining theamount, by weight, of moisture

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emitted. The technique requires littletraining and can usually be conduct-ed by field technicians. Unlike thegravimetric method, this test mea-sures dynamic moisture (vapor) em-anating from the concrete. Anhy-drous calcium chloride is weighed tothe nearest tenth of a gram and her-metically sealed to the concrete sur-face for 60 hours. It is then removed

and weighed to determine theamount of moisture absorbed. Thisdetermination is based upon theweight before and after placement,as in the gravimetric method.

The information provided by this method is the water moistureweight in lbs/1,000 sq ft/24hrs(kg/90 sq m/24 hrs). This method is non-destructive.

Calcium chloride test frequenciesare based upon the total surfacearea requiring evaluation. Recom-mended test frequencies are as follows:• 500 to 1,000 sq ft (45 to 90 sq m)= 3 tests minimum,• 1,000 to 5,000 sq ft (90 to 450 sqm) = 4 tests minimum, and• 1 test for each additional 5,000 sqft (450 sq m).

Proprietary Variation of Calcium Chloride TestA proprietary method has been de-veloped by which the calcium chlo-ride test units are placed on theslab’s surface in a measured gridpattern. The data are then automati-cally collected and downloaded intoa computer to perform transformanalysis. The transform data arecomputer generated into color-coded map representations of themoisture vapor emissions.

This technique is currently themost accurate and economicalmethod to evaluate large surfaceareas. The calcium chloride desic-cant test and the gravimetric methodare judged by the industry to be themost accurate practical methodsused to quantify moisture content in or emanating from concrete slabs on grade. Other methods maybe quicker and easier to perform but do not provide the accuracy of quantification.

Plastic Sheet Test (ASTM D 4263)This ASTM test method utilizes a 4.0-mil thick (200-micrometer) transpar-ent polyethylene sheet cut to 18 in.square (457 mm square).6 The plas-tic sheet is tightly taped to the con-crete’s surface around its perimeterwith two-inch (50-millimeter) wideduct tape, with the edges carefullysealed. The sheet is left in place aminimum of 16 hours and then visu-ally evaluated for the presence ofmoisture condensation on the un-

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derside. One sheet test should beconducted every 500 sq ft (45 sq m)of horizontal surface. The plasticsheet method provides only qualita-tive information for go/no-go floor-ing application. It may be used toindicate moisture vapor emissionfrom a slab’s surface, but only on aqualitative basis. JPCL

References1. “Hydrostatic, Capillary & Osmot-

ic Pressure on Coated Concrete,”Concrete Repair Bulletin(March/April 1995), 13,14.

2. Steven H. Kasmatka, “Don’tApply Floor Coverings Too Soon,”Concrete Construction (May1996), 472-476.

3. Staff, “Basics of Residential Slab-On-Grade Construction,”Handout, The World of ConcreteIV Seminar Topic, 1978 (Chicago,IL: The Aberdeen Group), pp. 22-24.

4. H.W. Brewer, “Moisture Migra-tion—Concrete Slab-on-GroundConstruction,” Journal of thePCA Research and DevelopmentLaboratories (May 1965).

5. “Miscellaneous DampproofingAdmixtures,” in Chemical Ad-mixtures for Concrete ACI Publi-cation 212.3-91, Section 6.9(Farmington Hills, MI: AmericanConcrete Institute, 1991).

6. ASTM D 4263, “Indicating Mois-ture in Concrete by the PlasticSheet Method,” 1995 AnnualBook of ASTM Standards, Vol-ume 06.02 (Philadelphia, PA:ASTM, 1995), p. 286.

7. Eric H. Lindholm, “Slab MoistureTesting: Is it Always Reliable?”Concrete Construction (June1996), 480-483.

8. Malcom Rode and DougWendler, “Methods for Measur-ing Moisture Content in Con-crete,” Concrete Repair Bulletin

(March/April 1996), 12-14.9. “Barrier Systems, Types and Per-

formance Requirements,” inGuide to the Use of Waterproof-ing, Dampproofing, Protective,and Decorative Barrier Systemsfor Concrete, ACI Committee Re-port 515 R-79 (Farmington Hills,MI: American Concrete Institute,1985).

10. The Rubber Manufacturers Asso-ciation (RMA), Installation Manu-al (Washington, DC: RMA, No-vember 1990), p. 1.

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Send Maintenance Tips toKaren Kapsanis, Editor, JPCL,2100 Wharton St., Suite 310,Pittsburgh, PA 15203; 412/431-8300; fax: 412/431-5428; e-mail:[email protected].

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