NACE 2 (Chapter 15: Coating Concrete and Inspection)

Apply elastomeric polyurethane coatings

(100% solids) with plural-component spray equipment. Apply in multiple passes to 0.25 in. (6.3 mm), if required.

Generally novalac epoxies are

100% solids materials. Apply them using the airless spray method. These materials bond well to concrete, and develop a tight, dense film; they are also very acid-resistant.

coating system that is considered relatively thick on steel,

20 mils (508 µm)

In general, allow poured concrete to cure for a minimum of

30 days at temperatures above 70°F (21.1°C) before coating.

where acid etching is in progress.

Always use goggles, rubber gloves, and rubber boots

Concrete Cure Process

At least 28 chemical reactions take place in concrete as it cures, which makes it a very complex process.

The lime content of the cement is the source of concrete's high alkalinity.

Concrete's pH can be as high as 13.

Concrete and other cementitious surfaces are coated for a variety of reasons.

In architectural service, the color and appearance of architectural features may be an essential element of the design of a building or structure.

NACE No. 6/SSPC-SP 13,

Joint Surface Preparation Standard for Surface Preparation of Concrete, is attached at the end of the manual.

How Concrete is Made

Mix Portland cement, aggregate, and water to make concrete.

Concrete Block — Surfaces Poured Using Forms may cause

Pinholes, rock pockets, air pockets, cavities, tie holes from tie wires, and other imperfections in the surface may develop

High-Pressure Water Washing

Power washing at 3,000 to 4,500 psi (21 to 31 MPa) is frequently used for poured concrete surfaces.

• ASTM D 4262,

Standard Method for Testing pH of Chemically Cleaned or Etched Concrete Surfaces

• ASTM D 4259,

Standard Practice for Abrading Concrete

• ASTM D 4260

Standard Practice for Acid Etching Concrete

• ASTM D 4261,

Standard Practice for Surface Cleaning Concrete Unit Masonry for Coating

• ASTM D 4258,

Standard Practice for Surface Cleaning Concrete for Coating

• ASTM D 4263

Standard Tests Method for Indicating Moisture in Concrete by the Plastic Sheet Method

Coating Benefits

Steel Reinforcement Decontamination Abrasion and Erosion Protection Contents Protection Improving Cleaning Skid Resistance

laitance

The results in a weak, sandy surface, creating a fragile layer of sand and cement This condition can occur at both the upper surface and the concrete/form surface or interface.

It is important to perform a "water-break" test on the concrete surfaces.

The way the water reacts on the surface indicates the way a coating will react with the surface. If the water penetrates into the surface, the coating should, too.

Elastomeric Polyurethane

These materials are usually applied over an epoxy-based primer. They are used for secondary containment and quite often to coat concrete sewer pipe.

Efflorescence:

This condition is caused by moisture passing through the concrete and carrying soluble concrete salts with it to the surface. The salts react with carbon dioxide in the atmosphere, creating a fluffy white crystalline deposit on the surface.

to avoid solvent entrapment and subsequent blistering.

This occurs when the surface of the coating film dries while there are still solvents in the pores of the concrete surface.

Acid Etching:

This process uses a dilute acid solution to remove laitance and roughen the concrete surface.

The specification may require visual inspection for pinholes. Use of a holiday detector also may be specified.

Use of a low-voltage wet-sponge-type holiday detector and/or high-voltage DC type may be required. A DC pulse holiday detector is not the best instrument for use over concrete.

Laitance:

Weak surface layer of water-rich cement mixture on the surface of fresh concrete caused by the upward movement of water.

Concrete is sometimes applied over steel to prevent corrosion.

When concrete is dense and poured well, it is one of the most corrosion-resistant coatings available for steel. It provides a thick, dense, water-resistant barrier, and creates an inhibitive atmosphere that prevents steel from corroding.

Acid Etching (ASTM D 4260) Acid etching uses

a dilute acid solution to remove laitance and roughen the concrete surface

Novalac epoxy is

a more recent addition in the coating industry. It is comparable to an epoxy phenolic and exhibits some characteristics of both materials.

• Laitance

a weak surface layer of water rich cement mixture on the surface of fresh concrete caused by the upward movement of water

Concrete is made with many different types of

aggregates, ranging from river sand to granite, including various fibrous aggregates, such as glass or asbestos Each concrete and aggregate mix creates a different surface.

Typical problems associated with coating and lining old concrete usually involve porosity including;

air pockets; surface irregularities such as construction joints; expansion joints, control joints; and cracks; concrete strength; contaminants left on the concrete surface; and problems associated with ground water.

Apply bituminous cutbacks

alone, or, when used as waterproofing for the exterior of concrete structures, apply as built-up membranes of several coats and include glass fibers as reinforcement.

Expansion joints in concrete are

always a serious concern, and the methods of treatment depend on the severity of the service environment.

These liquid epoxy-based coatings can be applied to

an original concrete surface. They penetrate the surface well and serve as a base for other epoxy topcoats.

Apply a thick epoxy,

applied by trowel, spray, or a combination of the two, directly to a clean but otherwise unprepared concrete surface.

The hydration (curing) process begins

as soon as water contacts the Portland cement and continues for an extended length of time.

Bituminous coatings are also available

as water emulsions. Application specifications to apply bituminous emulsions on concrete may require the surface to be dampened before coating application.

Coal-tar epoxies are used extensively

as coatings for concrete in waste treatment facilities.

Coatings can enhance chemical resistance. This is important

because concrete is a very reactive material. Chemicals, mineral acids, food acids, carbonic acid solutions, pure water, and climate all take their toll on uncoated concrete

Take great care to ensure that the mortar is correctly proportioned, mixed, and cured before coating.Remove fins and projections

before sacking when the concrete is very green (uncured).

Coal-tar epoxy combines the properties of

both coal, tar, and epoxy, and is one of the few coatings that withstands the corroding action of domestic wastes.

It is also very important to get a suitable ground. Do this, when using low voltage,

by connecting the ground of a detector to rebar, or by placing a bag of wet sand over the ground wire positioned on the concrete surface. The concrete in contact with the ground wire should be wetted down.

Concrete needs protection against freeze thaw cycles that

can cause it to crack and break up. Concrete, with its water and moisture content, is very susceptible to damage caused by freeze-thaw conditions.

Waterproofing exterior surfaces (buried or above ground)

can help prevent water or moisture from passing through the concrete.

Excessive trowelling,

can result in a too smooth, shiny surface that may need to be roughened before coating.

Since concrete is nonmagnetic, magnetic test instruments

cannot be used to measure DFT. Estimate the DFT of coatings on concrete either from WFT readings, by calculations based on the quantity of coating applied to a given area,

Efflorescence

caused by moisture passing through the concrete and carrying soluble concrete salts with it to the surface. The salts react with carbon dioxide in the atmosphere creating a fluffy white crystalline deposit on the surface.

To surface and fill voids, use either

cementitious materials (with sacking, stoning, or steel trowelling), or use synthetic putties or grouts, such as epoxies and urethanes.

The concrete may have degraded due to

chemical attack that progressed through a pinhole or discontinuity in the original coating.

Because of their resistance to ultraviolet light,

chlorinated rubber coatings may be applied as a final topcoat when solventbased bituminous coatings are applied to exterior surfaces.

Chlorinated rubber coatings are used extensively for

coating concrete water tanks and swimming pools. They perform well in high humidity.

Concrete coating projects for immersion service are classified in two primary categories:

coating old concrete (concrete with a service life of less than five years) or coating freshly-placed concrete.

Holiday detectors can detect pinholes in

coatings on concrete and cementitious surfaces because the concrete normally contains enough moisture to be conductive.

Vinyl coatings are also used on

concrete in a wide range of situations. Vinyl systems usually consist of a vinyl primer, thinned per the specification, to a point where it penetrates the concrete surface and provides a good base for subsequent applications of regular vinyl coatings.

Proper placement,

consolidation and vibration can help alleviate these issues.

Glass-fiber cement products

contain glass fiber as reinforcement.

Electronic devices based on ultrasound are also used to

determine the DFT of a coating on concrete.

Abrasive blast cleaned concrete leaves an anchor pattern

different from abrasive blast cleaned steel. It is vital for inspectors to clearly understand the degree of cleanliness required in the specification.

When concrete is relatively new,

efflorescence is highly likely. Although it is highly alkaline when first placed, the alkalinity depletes by moisture passage, and the onset of surface corrosion may be accelerated by moisture passage.

The surface resulting from hand or power tool cleaning varies

from great roughness with open voids, to not much more than dust removal.

The physical forces of ice are

greater than the strength of the concrete, and cause concrete to spall and shatter.

The brick

grinds down surface imperfections, opens up surface cavities, and works the mortar into the cavities.

Poured (Wet-Cast) Concrete

has a high moisture content that allows it to flow into the form Laitance, holes, and air bubbles are commonly encountered in wet-cast concrete, even on vertical surfaces.

Asbestos cement products have

higher tensile strength compared to other concretes, but may be brittle.

The most common acid used is

hydrochloric acid. Several etchings may be required to do the job.Other acids, such as phosphoric, citric, or sulfamic, may be used, but they are less widely encountered than hydrochloric.

Coal-tar epoxy is particularly useful

in environments where water can permeate the concrete and cause the coating to blister.

It is possible to repair some cracks by

injecting 100% solid epoxies or urethane resins to help restore the monolithic character of the concrete

Shotcrete

is a dense mixture of cement and relatively small aggregate with a low moisture content. A filling agent is frequently added to help hold the shotcrete in place until it cures.

Guniting

is the process of spraying or slinging shotcrete onto a surface as a coating

Sacking is

is the technique of scrubbing a mix- ture of cement mortar over the concrete surface using a cement sack, gunny sack, or sponge rubber float.

If water infiltrates the concrete,

it can disbond coatings on the structure's interior.

High-Pressure Water Washing generally

it generally does not open up subsurface voids and pockets, or provide a profile on sound, dense concrete as well as abrasive blasting does. But, if too much pressure is used, the waterjet stream may actually cut the concrete. Wet abrasive blasting is another possible choice.

One reason for the wide-spread use of concrete is that

it is an extremely durable material.

"suck" in water from the coating as well as some resin along with it,

leaving only a powdery, chalky film of pigment on the surface. This condition could affect adhesion of any subsequent coats applied.

Using hand or power tools removes

loose, powdery, and weak concrete at the surface, but this method is slow and does not open air pockets as well as abrasive blasting.

Use a steel trowel to

move mortar over the surface to fill holes, and to provide a reasonably pore-free surface to apply coatings

Concrete and concrete products usually are made locally because

of their heavy weight and high transportation costs. This reason contributes to the wide range of aggregate materials used.

Relatively thin epoxy coatings are applied

over thoroughly prepared concrete surfaces.

Blisters occur frequently on coated concrete since concrete is

porous; it holds air which expands when the concrete heats up.

Coating concrete helps

prevents absorption of contaminants. As stated before, concrete is porous and tends to absorb contaminants readily.

Abrasive blast cleaning

provides a roughened, irregular surface and removes laitance. Abrasive blasting opens holes and voids so they can be sealed more effectively.

differences between coating old concrete versus new concrete is that

repairs often are necessary before top-coating old concrete.

When acid etching is completed,

rinse the surface to neutralize acidic deposits. Use pH paper to determine whether the surface is alkaline or acidic

a shotcrete surface is unusually

rough and dense, but has few pinholes, air pockets, or subsurface cavities.

Because the wood trowel has a relatively rough surface,

sand grains are brought to the surface creating a granular surface. Wood floating may also create more laitance on the surface

Stoning is

similar to sacking, except that the process uses a carborundum brick, or other appropriate abrasive block, to smooth the surface of the concrete

Steel Trowelling

smooths the surface. A sand/mortar mix can be applied to the poured surface before trowelling to provide a very smooth, hard, dense surface.

Epoxies for concrete are usually

solvent based and use relatively high molecular weight resins similar to epoxies used for steel.

Bituminous cutbacks are

solvent solutions of coal tar or asphalt, both of which are used extensively on concrete.

Acid etching is difficult to use on vertical surfaces because

the acid can run off before it has had time to thoroughly react.

Many coatings fail due to the loss of adhesion between

the cementitious material and the concrete.

over-trowelling or overworking

the concrete brings the paste to the surface and affects the long-term durability of the concrete.

cement linings often do not bond to the substrate. They may have minor cracks,

the cracks tend to "heal" themselves. There are some drawbacks;

Many hand and power tool techniques are used to prepare concrete surfaces for coating,

they generally are time consuming and costly.

is not uncommon for a wax-based additive to be added

to a concrete mix to make the concrete waterproof and non-porous. This additive can completely hinder coatings application since the additive prevents the coating from wetting-out and adhering.

Concrete may be coated, or "waterproofed"

to mitigate moisture vapor transmission. Without waterproofing, concrete allows water to enter and pass through its porous structure.

Brooming

uses a stiff-bristle broom to provide a rough surface to the cement

Wood floating

uses a wood trowel to smooth the poured concrete.

Bituminous cutbacks are also used to

waterproof, particularly the exterior of underground structures. It helps prevent infiltration of water from the outside of the structure.

"penetration is to concrete

what surface profile is to steel."Coatings that penetrate the surface usually achieve an excellent bond.

Hydration occurs

when water is added to the cement/aggregate mixture. The water and the cement combine chemically, causing the concrete to set up and harden.

Hydrochloric acid should not be used

where chlorides are prohibited.

Chlorinated rubber and vinyl have been used

widely in the past, but environmental considerations now make them less attractive as currently formulated.

Coating is a practical way to maintain the concrete

with as little contained water as possible. Design considerations that allow run-off and avoid water-entrapment in depressions or crevices, also prevent freezethaw damage.

• Finishing operations: A variety of finishing operations can be used on concrete:

— Steel Trowelling — Wood floating — Brooming

The specification may call for the abrasive blast cleaned concrete to have:

• A finish coat of mortar applied by any of the methods previously discussed • The primary coating system applied directly • A sealer coat applied prior to application of the primary coating

To prepare the surface, use one of the following methods:

• Abrasive blast clean • Hand and power tool clean • High-pressure waterjet or blast • Acid etch • Stone • Centrifugal blast • Scarify

Cracks are classified as either:

• Active — these are self-made expansion joints, and because they are subject to movement, must be handled like an expansion joint. • Static — these do not move, and may be filled or covered without projecting through the topcoat.

wet Poured concrete can be affected by:

• Ambient conditions: Hot weather causes concrete to cure more rapidly than otherwise, resulting in a greater possibility for voids, and a dusty, low-strength surface. • Vibration: This is done to remove air pockets, but can cause the heavy aggregate to sink to the bottom of the form.

Several generic types of coatings are commonly used over concrete including:

• Bituminous cutbacks • Chlorinated rubber • Vinyl • Epoxy • Novalac epoxy • Elastomeric polyurethane • Sheet materials (e.g., rubber) • Glass-fiber-reinforced plastics • Furan resins

The procedure for acid etching requires the operator to:

• Carefully inspect for and remove any grease or other residues from the surface. • Apply acid to the oil and grease-free concrete surface. • Allow the acid to react with the cement until bubbling stops. The dwell time of the acid is typically 5 to 10 minutes. • Wash the surface thoroughly to remove the acid salts; use brooming if needed with the washing and flushing process.

Some of the properties that give concrete its strength and durability are listed below:

• Concrete is inorganic. • Concrete is hard. • Concrete has good compressive strength, • Concrete can improve with age.

the most common methods to apply coatings to concrete are:

• Conventional air spray or airless spray • Hand lay up (a thick mastic-type coating trowelled or otherwise spread onto the surface with or without glass fiber reinforcing mat)

During the coating operations, the inspector may be required to:

• Determine that the specified coating is used • Ensure that the coatings are stored as specified • Observe the mixing and thinning operations • Observe application operations • Monitor ambient conditions

Specific items the coating inspector may be required to inspect for, detect, record, or require correction of, may include:

• Ensure laitance removed • Ensure projections removed • Ensure hollow areas, voids, and other imperfections are remedied • Ensure by-products of acid etching are removed • Check and record pH levels • Determine if abrasive blasting was performed as specified • Ensure sand, dust, and other contaminants are removed • Test the surface for complete removal of sand, dirt, etc.

When inspecting coatings on concrete, the inspector may first be required to:

• Ensure that the concrete has cured for the specified time prior to coating • Determine the moisture level of the concrete. The moisture level in concrete may be inspected with the plastic sheet method (ASTM D 4263). — Moisture detector

Special Concrete Surfaces

• Gunite • Asbestos cement • Glass fiber cement products

Practical considerations for abrasive blast cleaning of concrete include:

• Hold the blast nozzle somewhat farther from the work than when blast cleaning steel. • Use pressures lower than those used on steel. • Move the blast nozzle as rapidly as practicable, consistent with the specified surface profile. Avoid gouging the surface or exposing large areas of bare aggregate. • Use a finer-size abrasive than is used on steel; coarse abrasives can remove too much concrete.

Key points in a more successful concrete coatings project include:

• Inspect the concrete and coated surface thoroughly prior to the job start • Establish the magnitude of the project • Schedule each work activity and establish a realistic completion date • Select suitable products for the application • Develop thorough specifications • Select an experienced contractor • Use experienced inspectors to ensure the specification has been followed

When coatings are applied to concrete or cementitious surfaces, the process generally includes:

• Inspect the surface before beginning; may include pre-cleaning, steam cleaning, and/ or chemical cleaning • Inspect after pre-cleaning • Prepare surface • Inspect surface preparation • Treat cracks and expansion joints • Coating application • Inspect after each coat in a multi-coat system • Inspect the completed coating system

, inspect the surface to be coated for any conditions or defects the specification requires be corrected, or that may damage the coating process. Some of the conditions coating inspectors may encounter include:

• Laitance (a weak surface layer of waterrich cement mixture on the surface of fresh concrete caused by the upward movement of water) • Pits • Voids • Efflorescence (caused by moisture passing through the concrete and carrying soluble concrete salts with it to the surface. The salts react with carbon dioxide in the atmosphere creating a fluffy white crystalline deposit on the surface.) • Projections • Porosity • Moisture content • Form release oils • Location of expansion joints: — Mark to uncover after coating — Special treatment may be required • Visible residues of dirt, chemical salts, or other foreign substances likely to cause coatings problems (e.g., poor adhesion) • Ice or ice crystals on the surface (require particular attention when coating outdoors in very cold weather) • Water on the surface

A coating inspector's responsibilities for surface preparation of concrete and cementitious surfaces may include:

• Observe surface preparation to ensure that all operations are performed as specified. • Inspect the prepared surface prior to coating to ensure that the surface is prepared as specified.

Make a visual inspection of the coated surface after each coat has been applied to check for:

• Pinholes (detected either visually or with a holiday detector) • Bare spots • Runs • Blisters

There are a wide variety of concrete and cementitious surfaces, including:

• Poured (wet-cast) • Concrete block (poured using forms) • Special concrete surfaces — Gunite — Asbestos cement — Glass fiber cement products

specification may require a number of operations:

• Pre-cleaning • Surface preparation • Surfacing/filling voids

The advantages of waterjetting and wet abrasive blasting include:

• Quickly cuts the surface • Washes dust away • Reduces abrasive and concrete particles in air

The inspector should also ensure:

• There are no ridges in the coating • The coating is cured properly by testing: — Hardness test (impressor) — Solvent wipe • The recoat time is as specified • Minimum and/or maximum DFT are achieved • No overspray or damage to adjacent areas

Avoid blisters the problem with:

• Use of a special primer. • Shade to protect the concrete surface from direct sunlight. • Use of no more solvent than necessary. • Planned timing so the application is done when ambient temperature is decreasing. This ensures the coating is "sucked" into the pores of the cement.

Coating concrete can protect it from numerous environmental hazards like:

• Water • Freeze-thaw • Chemical contamination

The sacking process generally requires workers to:

• Wet down the substrate with water to prevent the concrete from sucking all the water out of the sack coat, which makes it too dry to finish correctly. • Apply the mortar by rubbing it over the surface in a circular manner, to make sure all voids are thoroughly filled. • Go over the surface again when it is almost dry to remove as much of the sacking material from the surface as possible, without removing it from the voids as well.

Smoothing concrete surfaces may be done at any one of these times:

• When the concrete is freshly poured, before any surface preparation • After pre-cleaning and surface preparation

Concrete...

• has 28 chemical reactions • cures by hydration • pH may be as high as 13 • May passivate steel