an introduction to coating concrete floors

8/8/2019 An Introduction to Coating Concrete Floors

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Copyright 2001, Technology Publishing Company

ingredients in concrete for floors are aggregate, Portland ce-

ment, and water. The concrete hardens by hydration. (The

Portland cement reacts with water.) The hydration of the

Portland cement glues the aggregate particles together to

form the slab. More water than is needed for the hydration

reaction is used in the mixture for pouring and workabili-

ty of the wet concrete. Expansion joints, construction

joints, and control joints are placed in the floor.

The wet concrete is consolidated with vibrators placed

into the mixture while it is wet to eliminate air voids. The

surface is then finished by floating the concrete. This con-

sists of working the surface with large, flat tools such as

wood boards or steel trowels to smooth out the surface so

that none of the large aggregate particles are sticking out of

the concrete. The top 36 mm ( in.) ends up being a ce-

ment mortar layer. A smoother, harder surface

can be obtained with a steel trowel finish.

The concrete is then cured for a pre-de-

termined time or until it reaches the design

strength requirements. It is very important

that water be kept in the concrete during

these early stages because it is needed to re-

act with the Portland cement to form the

binder. Various methods are used to prevent

the loss of moisture from the concrete. The

most common methods are applying con-

crete curing compounds to the surface to

slow evaporation, and keeping the concrete

moist with ponded water.

The concrete must be dried after the ini-

tial curing is complete. The chemical reac-

tions that occur in the curing process can

take years to complete, but most specifica-tions for concrete made with Portland ce-

ment have 28-day strength requirements.

The amount of water added to the concrete is far in excess

of what is needed for the hydration reactions. Some water

will also remain within the pore structure of the dried con-

crete. The excess water must be allowed to escape before

any polymer flooring or coating system can be applied.

Concrete cannot be coated or topped with an impervi-

ous material such as coatings, linings, tiles, epoxy terraz-

zo, carpeting, etc., until there is no movement of water in

he most common substrate forindustrial floors is concrete made

of Portland cement. Concrete floors are

coated for many reasons. Some of the

factors include aesthetics, visibility

(i.e., light reflection), ease of cleaning

and decontamination, and protection from aggressive

chemicals. Other factors are resistance to impact, abrasion,

and erosion. Coatings for floors come in two types, rein-

forced and non-reinforced. This months Applicator Train-

ing Bulletin will

examine concrete floors from a coatings perspective,

discuss the special requirements for concrete floor coat-

ings, and

present general information on preparing and installing

floor coating systems.

Concrete Floors

Concrete floors are constructed by pouring and finishing

concrete on a prepared base. Various factors must be con-

sidered in designing concrete floors. Con-

structing the floor starts with a foundation

or soil support system. Depending on local

conditions, either the soil is compacted or a

mixture of fine and coarse aggregates is

placed on top of the local soil and compact-

ed. The end result is an even surface de-

signed to support the intended weight. The

walls and roof are then constructed so the

ground underneath the roof can dry. A

vapour barrier system is installed over the

portion of the prepared ground where the

slab will be poured. The vapour barrier sys-

tem is an impervious material such as 250-

micron (10-mil) polyethylene sheeting that

is overlapped and sealed. The purpose of the

vapour barrier is to prevent the concrete

from curling up at its edges during curingand drying. The barrier provides a more uni-

form release of the excess water. It also pre-

vents water from entering the slab from beneath, which

can cause osmotic blistering in the flooring material when

it is installed on top of the concrete. A sand layer is then

put on top of the vapour barrier. Be aware, however, that

some designers are of the opinion that the sand layer

should be placed below the vapour barrier in some or all

situations.

The next step is pouring the concrete. The three main

JPCL December 2001 PCE40

ApplicatorTrainingBulletin

An Introduction to

Coating Concrete Floors

This months installment of the Applicator TrainingBulletin was written by Lloyd Smith, Ph.D., Corrosion

Control Consultants and Labs, Herndon, Virginia, USA.T

Industrial plant floors are coated for

many reasons, including visibility and

protection from chemicals.

(Photos courtesy of Stonhard,

part of the StonCor Group)


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Copyright 2001, Technology Publishing Company JPCL December 2001 PCE 41

the slab. This movement is commonly referred to as the

moisture vapour transmission rate. The actual amount of

water that remains in a concrete slab is of no importance.

What is important is if the water is moving into, and out

of, the surface. Think of moisture vapour transmission as

evaporation. Take a plate, wash it, and leave it by the side

of the sink. The plate will be dry with time, depending on

the temperature and relative humidity. The water did not

boil away; it evaporated. This is the same thing that hap-

pens with concrete. The water in the pore structure of the

concrete rises to the surface and evaporates until an equi-

librium condition is established.

A possible consequence of applying an impervious film

to concrete if water is moving into the concrete is osmotic

blistering. The osmotic pressure that develops can be

tremendous. The author has witnessed 0.3-metre (1-foot)

blisters in a 9.5-millimetre- (38-inch-) thick epoxy terraz-

zo floor that was the result of osmotic pressure. Water shot

up like a fountain when a hole was drilled through the ter-

razzo.

A common myth in the painting industry is that con-

crete can be painted 28 days after it has been poured. This

certainly is not true for floors. The amount of time it takes

for the water to evaporate from the slab depends on

many variables such as thickness of the slab, water-to-ce-

ment ratio of the concrete, air temperature, concrete tem-

perature, relative humidity, and wind speed. It may take 60

days or more before the floor is ready to be coated.

Detecting Moisture Movement

The various methods for detecting moisture movement inconcrete are described in ASTM E1907, Determining Mois-

ture-Related Acceptability of Concrete Floors to Receive

Moisture-Sensitive Finishes. The most commonly used

methods when applying a floor topping to concrete pre-

sented in this standard are the plastic sheet test, anhydrous

calcium chloride test, and hygrometer (relative humidity)

test. The plastic sheet method consists of taking a 460 x

460 mm (18 x 18 in.) piece of transparent polyethylene

sheeting that is at least 0.1 mm (4 mils) thick and tightly

attaching it to the floor with duct tape on all edges (ASTM

D4263, Standard Test Method for Indicating Moisture in

Concrete by the Plastic Sheet Method). The sheeting iskept in place for at least 16 hours. It is then removed, and

both the underside of the sheet and the floor surface are

examined. Any signs of moisture on the underside of the

sheet or on the floor (as evidenced by a darker grey colour)

mean that moisture is moving through the concrete, so it

would not be ready for coating.

The quantitative anhydrous calcium chloride test con-

sists of placing a cup with a known, weighed amount of

anhydrous calcium chloride on the floor, covering it with a

plastic canopy, and sealing the canopy to the floor with a

moisture-tight sealant (gun-grade) or sealant tape. The unit

is left on the floor for 60 to 72 hours. The sealed container

with the calcium chloride is then weighed after the test. The

weight gain in relation to the time of the test and the sur-

face area covered give the moisture emission rate. The max-

imum moisture emission rate most commonly required by

manufacturers of floor toppings is 15 g/m2 per 24 hours (3

lb/1,000 ft2 per 24 hours). The anhydrous calcium chloride

test is more popular in the United States than in Europe.

The quantitative method most widely used in Europe is

the hygrometer test. This method consists of measuring the

relative humidity of a pocket of air underneath a canopy

sealed to the concrete surface. The canopy acts as a vapour

barrier. Suitable materials are sheet metal, glass, or 2-mm

(0.080-in.) acrylic or polyvinyl chloride sheet. Insulation is

also needed to isolate the pocket of air from the surround-

ing environment. The hygrometer can be either a dial type

or a probe type. The canopy is sealed to the floor with a

gun-grade sealant or tape-type sealant in such a mannerthat allows relative humidity readings to be taken without

breaking the seal. Measurements are made for a sufficient

length of time for the entrapped air to reach moisture equi-

librium with the floor. Experimental evidence has shown

that when the moisture has evaporated from the coarse

pores of the concrete, the relative humidity falls to 80 per-

cent. It is reasonable, therefore, to require a relative hu-

midity of 75 percent or lower before the floor can be con-

sidered acceptable for installation of the flooring system.

The most detailed information on this test can be found in

British Standards Institute BS 5325:1983 British Standard

Code of Practice for Installation of Textile Floor Coveringsand BS 8203:1987 British Standard Code of Practice for In-

stallation of Sheet and Tile Flooring.

It is important to point out that any test on moisture

movement is only indicative of what is occurring at the time

of measurement. If the moisture vapour barrier was not in-

stalled properly or was ripped when pouring the slab, and

the test was performed in dry weather (i.e., the ground was

not saturated), then the tests may show that there is no

moisture movement. Similarly, a water pipe rupturing be-

Continued

For successful floor

coatings work, cont-

aminants must be

removed.


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JPCL December 2001 PCE42 Copyright 2001, Technology Publishing Company

low the slab at a later date can cause water to enter the con-

crete, resulting in the formation of osmotic blisters at coat-

ed areas. Another common cause of moisture moving

through concrete is a change in ambient conditions. For ex-

ample, performing the moisture test when the slab is ex-

posed to the outside atmosphere, then heating or air condi-

tioning the room to a relative humidity lower than when the

flooring system was installed can cause water to accumu-

late beneath the coating, resulting in osmotic blistering.

Surface Preparation

All contaminants must be removed before applying a coat-

ing or lining to concrete floors. Common contaminants to

be aware of are concrete curing compounds, laitance, efflo-

rescence, and chemical contaminants.

The possible presence of concrete curing compounds is

mainly related to new construction. The use of concrete

curing compounds should be avoided when a coating or lin-

ing will be applied. Information should be obtained fromthe concrete subcontractor on whether or not a curing com-

pound was used.

Laitance is a thin, weak, brittle layer of cement and ag-

gregate fines on the surface of the concrete. Technically, lai-

tance is not a contaminant because it is a natural product

of cement hydration. But it is a weak layer that does not

support flooring systems or coatings. Laitance arises from

concrete finishing operations that cause weakly adherent

fines and cement paste to float to the concrete surface.

Efflorescence is a white crystalline or powdery deposit

on the surface consisting of lime or calcium hydroxide that

has leached out of the concrete and has carbonated (react-ed with carbon dioxide in the air). It is a loose material, so

it does not make a good base for coating.

Chemical contaminants are mainly associated with old

floors. Contaminants can include chemicals that have at-

tacked the concrete. Oils, grease, or other organic materials

are of concern especially in machine shops, food processing

facilities, and manufacturing facilities. Visually examine the

concrete for integrity. Tapping it with a hammer or picking

at it with a knife will reveal weak areas that may have un-

dergone chemical attack. Also look for dark stains, espe-

cially around equipment where lubricants or other materi-

als may have spilled. If oil or grease contamination is

found, its depth of penetration must be determined. This

may include the need to take core samples. Surface oil and

grease can be removed by scrubbing the surface with a

strong, low-foaming detergent and then thoroughly clean-

ing it with water.

Organic contaminants such as non-visible oil, grease,

concrete curing compounds, or other materials can be iden-

tified with a water bead test. For this test, a drop of water

is placed on the surface. If it beads, an organic contaminant

is present. Be aware that if the floor is coated, this test will

also be positive. It may be possible to remove oils and

grease by scrubbing the surface with commercial de-

greasers or floor strippers. If not, it will be necessary to

physically remove the contaminated concrete layer. Con-

crete is alkaline, so surface pH measurements could be in-

formative for other types of contaminants such as those

formed from acid attack.SSPC-SP 13/NACE No. 6, Surface Preparation of Concrete,

presents various methods for decontaminating and cleaning

concrete prior to coating. The most common method used

for floors is abrasive blasting. The intent of abrasive blasting

is to remove any loose surface materials and roughen the

concrete surface. The standard recommends a surface profile

appearance of fine (150 grit) sandpaper for light service

where there is minimal foot traffic, chemicals, or changes in

temperature. An appearance of coarse (60 grit) sandpaper is

recommended for severe service.

Dry or wet abrasive blasting can be performed. Cen-

trifugal blasting, discussed in the July 2001 ApplicatorTraining Bulletin, is also appropriate for floors.

Another surface preparation method is acid etching.

This procedure involves applying an acid on the floor, al-

lowing it to react, then neutralising the acid by rinsing with

plenty of water. The floor must then be allowed to dry. A

normal acid etching solution is a mixture of one part of 20

Be hydrochloric acid (muriatic acid) diluted with one part

of water. Fizzing will occur, indicating the acid is reacting

with the cement. If fizzing does not occur, this indicates the

presence of a curing compound or other organic material

such as grease or an existing coating on the concrete. This

contamination would have to be removed by solvent clean-ing or physical methods (i.e., power tool cleaning), and the

area would need to be re-etched. The acid is then left on

the concrete until a roughened surface is achieved (as indi-

cated when the tops of the pea gravel are exposed). Hy-

drochloric acid is a dangerous material requiring skin, eye,

and respiratory protection. The spent solution must be dis-

posed in accordance with local regulations. This has limit-

ed the use of acid etching in the United States. Acid etch-

ing is not used in Europe, although a non-toxic acid, citric

acid, is an alternative to muriatic.

ApplicatorTrainingBulletin

Rolling sealer onto a broadcast flooring system


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Copyright 2001, Technology Publishing Company

quence and must be thoroughly mixed to wet out all the ag-

gregate particles. Some flooring materials are self-levelling

while others may require finishing, including techniques

such as trowel application or board-finishing like concrete.

Some products may need to be back-rolled to smooth the

surface or rolled with a spiked roller to remove air bubbles.

Always read the manufacturers application instructions

and talk to the manufacturer to determine the proper tools

and methods to use for a specific product.

Another variation of floor system application is to place

sand or another abrasive into the coating to reduce the pos-

sibility of slipping by increasing friction. This is referred toas a broadcast system. In this case, the aggregate is thrown

over the floor while the coating is still wet so the aggregate

(sand or abrasive) particles stick out of the coating or form

a rough surface. This is different than the flooring systems

previously discussed where the aggregate is mixed into the

coating and forms a smooth surface. A thin layer of topcoat

or sealer may need to be applied.

Conclusion

Floor coatings applied to concrete present unique situations

compared to other coating applications. Floors must be

tested for moisture movement before applying the floorcoating system. Old concrete must be examined for conta-

minants, especially in shop or plant environments or where

chemical contaminants are present. Floor coating systems

come in two main types, thin-film and thick-film. Thin-film

systems are applied by techniques commonly used for coat-

ings application. Thick-film systems may require special

application techniques, especially squeegee and trowel ap-

plication.

JPCL December 2001 PCE

Floor Coating Systems

The most common coating materials for industrial con-

crete floors are epoxies, vinyl esters, and polyesters. There

are many variations; the selection is based upon the expo-

sure environment, especially for chemical exposure. Floor

coatings can be thin-film or thick-film systems. An exam-

ple of a thin-film system is an epoxy at 250375 microns

(1015 mils) that is applied in two coats. Thick-film sys-

tems include aggregate-filled systems and reinforced sys-

tems such as those described in the November 2001 Ap-

plicator Training Bulletin. These systems normally are

applied at 1.6 to 6.5 mm (116 in. to 14 in.) thickness.

Application

The procedures described here are generalisations. Always

consult the manufacturer for specific directions.

The first step is determining what must be done to the

concrete surface after the surface preparation has been

completed. The floor may contain holes or irregularitiesthat have to be filled. The manufacturer will provide in-

formation on how deep an irregularity must be if it is to be

filled and the product to use. The next step is addressing

cracks, including stress cracks and joints. They may or

may not have to be treated, depending on the flooring sys-

tem used and the exposure environment. What is impor-

tant is how much movement will take place and not the

actual width of the crack. For example, a floor in a climate-

controlled area would not undergo thermal expansion and

contraction compared to an outside floor, so for the inside

system, joints and cracks may not have to be addressed.

The most common way to address cracks and joints ifthey must be treated is with the use of joint sealants or

bond breakers. Joint sealants are elastomeric materials

that can move if expansion or contraction occurs. Bond

breakers are materials such as vinyl tape or polyethylene

sheeting that are placed over the crack or joint. These ma-

terials will allow movement of the concrete underneath the

coating. Large joints may require a more detailed treat-

ment consisting of saw-cutting a groove, inserting a piece

of material called a backer rod into the groove, and filling

in the remainder of the groove with the manufacturers re-

quired material.

The flooring system may need a primer coat. Primersare thin-film systems (usually

an introduction to coating concrete floors

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