acid-resistant floor construction
TRANSCRIPT
ACID-RESISTANT FLOOR CONSTRUCTION
by Donald J. Kossler and Walter Lee Sheppard, Jr.
CCRM Inc., Havertown, Pa.
The importance of a properly designed floor in the metal-finishing industry cannot be overemphasized. Complex systems and a wide variety of chemicals, all under one roof, are commonly found in metal-finishing plants. The finishing plant floor is subject to punishment because of the continual handling of parts, which results in drips, spills and splashing; the presence of excessive moisture and humidity; and the destructiveness of the thermal shock, mechanical impact, cleaning methods and vibrations involved.
Where spillage and drip occur and adequate protection is not provided, concrete floors are attacked unevenly, and the surface becomes rough. Supports for tanks resting on such floors also become uneven, and tanks may tend to change position slightly, perhaps tipping out of a true vertical position. In shops with automatic plating machines, any such movement can cause the equipment to misalign, scrape the tank sides or hang up.
Three types of protection systems can be specified, depending on the service: corrosion-resistant brick or tile floors, monolithic resinous floors or polymer concrete.
BASE FLOORS
Almost all industrial floors that must support heavy traffic and heavy structural loads are constructed of concrete. The concrete must be structurally sound and strong enough to support the full operating loads. It must be of high strength (generally 3000 psi or better), steel reinforced and homogeneously poured. It must be moist cured (if new) a minimum of 10 days prior to installation of corrosion proofing. It must have a slope toward drains of at least 1/4 in./ft, and it must have a screed or wood float finish with a minimum of surface laitance (i.e., a minimum accumulation of fine particles, usually lime or portland cement, on the surface of the concrete).
Old concrete floors must be thoroughly cleaned and free of uneven, broken and badly cracked areas. Chemicals, oils and other contaminants can be removed by detergent acid washing, chipping or sandblasting. Cracks and breaks must be repaired properly before corrosion proofing is started and all weak areas replaced.
Floor drains of appropriate design should be installed or adjusted to the correct elevation with respect to the finished floor (see Fig. 1). The concrete must also be provided with expansion joints in accordance with sound engineering techniques and precautions taken to isolate vibrations, structural members, pump foundations and the like.
CORROSION-RESISTANT BRICK FLOORS
Corrosion proofing a floor by using "acid brick" and a corrosion-resistant mortar is a highly reliable method for protection. This is especially true in places where heavy traffic, harsh chemicals and general abuse of the floor are common.
Impervious Interliner The first step in construction of a good corrosion-proof brick or tile floor is the interiiner
or membrane. Tile must be a minimum of 1 in. thick when laid over a membrane. The interliner must be impervious to chemical attack and liquid penetration for the anticipated
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B\--~ / - D / -G
i Fig. 1. Typical f loor drain construction: (A) 1/4-in. asphalt interliner with glass cloth reinforcing and concrete primed with asphalt primer; (B) chemically resistant mortar joints; (C) "Josam" or similar type o f f loor drain; (D) acid-proof brick, top surface o f drain and cover should not be less than 1/16 in. below the surface o f surrounding floor.
service. Most such interliners are made of a special hot-applied asphalt. On floor surfaces the interliner is 1/4 in. thick and is reinforced with a single ply of asphalt-impregnated glass cloth. It is wise to use two layers of glass reinforcing throughout all trenches, sumps and pits and to keep the membrane in such areas to a minimum thickness of s/s in. All membranes must be continuous. An interruption in the membrane will permit liquid to penetrate the substrate at that point.
The clean, dry concrete surface is first brush primed with an acid-resistant compatible asphaltic primer to carry asphalt particles onto the concrete surface to provide better bonding. The primer is allowed to dry for 4 hr or more undisturbed by traffic. The membrane material is then poured hot (cold-applied water emulsions or solvent systems are porous, more permeable and not normally recommended, and this includes trowelled urethane and urethane asphalt putties) over the primed surface and spread with squeegees (not mops) in multiple layers to secure the thickness desired (see Fig. 2). Reinforcing is imbedded between the layers of asphalt. The asphalted surface is kept closed to all traffic until the brick or tile is emplaced.
Brick The type of brick used depend s on the service to which the floor will be exposed and
must be completely resistant to the full range of chemicals and temperatures anticipated, including all cleaners. The masonry does not in itself act as an impervious barrier to chemical environments. Its primary function is to protect the impervious asphalt interliner from mechanical shock, to prevent continuous flow of chemicals over the interliner and to act as a thermal insulator (see Fig. 2).
Floor brick can be either the buff colored, hard burned, high-silica fire clay type or the red shale, clay type. The buff type has better thermal and mechanical shock resistance and is more porous. Red shale brick is more dense but is not as resistant to the various shocks inherent in heavy-abuse areas. In special instances, where hydrofluoric acid or other acid fluoride compounds are present, carbon brick is recommended.
Corrosion-Resistant Mortars There is no one mortar suited to all exposures. The mortar must be selected with a good
deal of care because, in combination with the brick, it forms the "first line of defense" for the
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corrosion-proof floor. Today there are seven general types of corrosion-resistant mortars: furfuryl alcohol, phenolic, modified phenolic, silica and silicate, sulfur, epoxy and polyester. (Vinyl ester mortars have chemical resistance roughly similar to that of polyester. Acrylics have similar but not as good chemical resistance as polyesters. Urethanes have resistance to mild acids, mild alkalies and some organics.) Table I shows the mortar types and their resistance to various chemicals common to the finishing industry. The mortar should be used both as the side and bed joints, as in Figure 2. Keeping the brick even and the mortar joints as narrow as possible (on average, Vs in.) will produce a better floor. Be careful, however, to leave absolutely no air voids. The curing of the mortar before placing the floor into service should be in accordance with the manufacturer's directions. Note also that the chemical limitations of the mortar should be agreed on between supplier and customer and should be tested according to recognized testing procedures (such as those procedures covered in the standards published by the American Society for Testing and Materials, volume 04.05) to qualify a given specification. Note also that the cure of chemical mortars is affected by moisture and temperature.
Expansion/Contraction Joints All of the considerations enumerated above will come to naught if expansion/contraction
joints are ignored in a brick or tile corrosion-proof oversheating. It is a known fact that acid brick tends to swell in wet services, such as in plating rooms. Therefore, expansion joints are a must if heaving of the floor is to be prevented. Expansion and control joints in the concrete subfloor must, of course, be matched in the brick work. Additional expansion/contraction joints through the brickwork to the asphalt interliner must also be provided so that expansion/contraction joints are not more than 20 ft apart, and use % in. wide over the entire floor and around all fixed points on the floor, such as equipment foundations, except for drains. Where wheeled traffic and/or drum rolling over a brick floor is expected, harder chemically-resistant, expansion/contraction joint material must be used to prevent breaking down the edges of the brick at the joint. Where traffic is no problem, softer expansion/contraction materials may be used, but care must be taken that the chemical expansion/contraction joint material must also be capable of extrusion from the joint if the brickwork swells. Asphaltics are not recommended for use in expansion/contraction joints because of poor resistance to solvents and chemicals common to the finishing industry and poor adhesion to brickwork. In addition, although they squeeze out when the joint closes, they do not drop back into the joint when it opens up again.
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Fig. 2. Acid-proof brick f loor with impervious inter- liner. (A) l/4-in, asphalt in- terliner with glass cloth re- inforcement and concrete primed with asphalt primer; (B) properly selected, chemically resistant 1~8-in. mortar side and bottom joints; (C) cover brick or "soldiers"; (D) acid-proof brick; (E) properly selected, load supporting-type expansion~contraction joint at 3/8 in. wide; (F) cap with mortar or expansion joint sealant to assist drainage.
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Make drains the midpoint, or point of no movement between expansion joints. Try to place expansion joints near a high point and crosswise, not parallel, to the direction of drainage.
RESINOUS MONOLITHIC FLOORS
Recent developments in plastic technology have spawned some newer materials for use in corrosion proofing a floor. These are thermosetting plastic resins, usually epoxy (epoxy monolithics should not be exposed to sustained temperatures over 140°F or intermittent temperatures above 160°F), vinyl ester or polyester (acrylics and urethanes are also available but are not, in authors' opinion, as suitable for application in plating shops), which are highly filled with siliceous or carbon fillers and are applied by spray or trowel to the concrete subfloors. Such flooring costs, in place, approximately two-thirds of the cost of an acid brick floor and is suitable for use in many areas of the finishing plant. Monolithic floorings have been actively marketed for about 25 years; however, where they are used, great care must be exercised to prevent "puddling" or standing liquids. In continually wet environments, monolithic floors tend to saturate. In acid conditions this can lead to penetration to the substrate and Ioss of bond.
A resinous monolithic floor is usually applied in a nominal thickness of V4 in., with a minimum thickness of I/8 in. Do not accept thicknesses less than 1/8 in. The concrete subfloor must of necessity be relatively smooth (wood float finish), have very high strength, and be pitched toward drains and trenches, with a minimum slope of 1/4 in./ft and be free of laitance. Minimum strengths for the concrete should be 3000 psi compressive strength and 300 psi tensile bond at the surface prior to application of the monolithic floor. The concrete must also have a very sound surface because the whole success of application of a resinous monolithic topping rests with the bond of the monolithic to the surface of the concrete. No additives to the concrete, such as curing agents or air-entrainment agents, should be employed without the specific consent of the manufacturer of the monolithic topping. Construction or mechanical joints in the concrete must be carried up through the resinous topping, and a tough, flexible, chemically-resistant expansion joint sealant installed.
Occasionally one notes the offering for finishing room application of a silicate-type monolithic topping. This is not recommended at this time because silicates do not offer dependable protection to concrete floors when exposed to live steam or alkaline materials or to large quantities or running water. They also have high absorption and, if applied without a membrane, can be penetrated by chemicals and the substrate attacked.
Expansion, control joints and other points of movement in the slab must be mated to similar joints in the topping. In addition, stress-relief joints through the topping to the slab must be provided at not more than 20-ft intervals.
The chemical resistance of available monolithics varies greatly. These products can be formulated to have very high or low resistance to chemical attack. The same holds tree for their thermal shock and heat resistance.
In the epoxy monolithics there are two general classes. That suitable for platers is the polyamine-cured rigid system, which has a high degree of chemical and wear resistance but low temperature resistance (heat limit approximately 160°F). Polyester and vinyl ester floorings can vary greatly, and systems are being marketed that are flexible enough to withstand heat and thermal shock and still have good chemical resistance, although with narrower limits than epoxies. The writers do not at this time recommend urethane or acrylic toppings.
'Epoxies resist nonoxidizing acids, alkalies, salts and solvents. Polyesters will resist oxidizing agents, most all acids, moderate alkali concentrations and salts. Carbon fillers must be used in hydrofluoric acid or acid fluoride compound environments. Care must be exercised in the selection of the right monolithic, because these products have been developed for many industries, but only a few are suitable for the strong chemical conditions in the finishing
655
industry. "Or equals" should be verified through recognized testing procedures, as is done with the chemically-resistant mortars previously mentioned. Consideration of whether to use a monolithic or brick floor will depend on how severe the service and what kind of traffic loads the floor is exposed to.
The resinous monolithics should not be used for trenches, pits or sumps, especially where there is any standing "head" of chemical solution. The very nature of a highly filled monolithic does not lend itself to being impermeable enough for this service. The acid brick/membrane construction should be employed in these areas.
In the past few years, premolded plastic trenches have become available in a variety of sizes and are manufactured from a number of different resins, including furans, epoxies, vinyl esters and acrylics. If one of these is used, care must be taken to examine the design of the joints between the sections to ensure that the joints can be made totally liquid-tight and that the design of the top of the walls will provide a liquid-tight seal with the membrane of the floor. Failure to seal either of these two joints completely will result in the eventual undermining of the floor and costly major repairs and downtime before one is even aware of the damage. With regard to selection of the best plastic for plating room service, the furan type has the widest applicability and chemical resistance and should provide the longest life.
POLYMER CONCRETE
There has been much in print about "polymer concrete," and it is likely that some of these materials will be offered to platers for use in floors and trenches. It is therefore important for platers to be aware of both the strengths and weaknesses of these materials.
First, there are basically two different types of materials that are often confused:
1. Polymer concrete is a mixture of a resinous or sulfur binder with a silica sand filler and graded quartz (silica) aggregate, containing no portland or calcium aluminate or other hydraulic cement and no calcium or alumina products. 2. Polymer cement concrete is a regular concrete composed of portland, calcium aluminate or other hydraulic cement, sand and aggregate of any type, to which has been added a sulfur or resin binder.
The first of these materials has excellent chemical resistance in the range indicated for the resin or sulfur binders (see Table I). Methacrylate polymer concrete in general would parallel the resistances indicated for polyesters but at a somewhat reduced level. The second material is designed primarily for physical strength and resistance to freeze/thaw, but with little difference from regular concrete in chemical resistance: Some suppliers believe that the addition of the resin or sulfur will protect the portland cement in the concrete. Although there is some evidence of an improvement in resistance to weak vinegar and other organic acids, there is insufficient evidence of any substantial difference or improvement in the resistance of the plain concrete to strong inorganic acids, such as hydrochloric, nitric and sulfuric.
In the case of sulfur concrete, platers should be aware of an additional problem. Sulfur cements and sulfur-bearing mortars are not recommended for exposure to solutions of copper and beryllium salts. In such exposures a very slow, but progressive, chemical reaction can take place, continuing over several years. Copper and beryllimn salts react very slowly with the sulfur in the cement or concrete to form complex and growing sulfides that slowly expand and eventually can disrupt the sulfur mortar or concrete. Test exposures of one- or two-months or even a one-year duration are not meaningful to demonstrate satisfactory service under such exposures.
Two additional limitations on the use of polymer concretes should be kept in mind:
1. They have rather high cure shrinkage characteristics and are very likely to develop cracks, which may be simple crazing or which may extend all the way through the polymer concrete. 2. They have a measurable absorption. Therefore, if a vessel or pit is formed completely
656
of the polymer concrete, and if it is kept at all times full of liquid, with no chance to dry out, the polymer concrete will eventually saturate, and what is on the surface inside will also be on the surface outside the polymer concrete as a "sweat." This will take a rather long time to occur if there is no cracking (only a very small vessel would not show some cracking), and if the polymer concrete is given periodic dry rest periods, it may never happen. Normally, polymer concretes are installed without a membrane under them; however, because of the tendency of the material to develop shrinkage cracks, perhaps as lol)g as one year after installation, the plater who considers such flooring would be well advised to require that a liquid-tight membrane, such as described above, n o t a
roofing or waterproofing felt or a polyethylene sheet, be first provided by the installer. Some single-component, water-mix inorganic cements have the capability of being used as an acid-resistant brick mortar, poured or troweled monolithic surfacing or as a pneumatically applied gunite. These cements are silica based. They have exceptional adhesion to concrete, brick and steel and will resist any concentration of any acid (except hydrofluoric and acid fluorides) up to boiling. They should not be exposed to live steam or constantly running neutral or alkaline waters or alkalies. The use of such inorganic cements in monolithic applications may be seen occasionally in all-acid exposures. They are more porous than resin monolithics and can be penetrated by chemicals. These products should be used with some discretion and should not be used in areas where they are exposed to strong alkalies. The manufacturer must be consulted for proper recommendations and guarantees. Furan resin '!concrete" has been employed in chemical service for the past 10 years, including a few areas involving metal finishing. The principal problem noted with this new material has been shrinkage cracking. Applica- tions made over a membrane will resolve fluid penetration; however, the method of sealing cracks as they occur still requires study.
ADJACENT FLOOR AREAS
Concrete floor areas adjacent to plating operating should be given some consideration as to protection or sealing, "tracking" of chemicals via truck or foot traffic into these areas or the very real problem of dusting of the concrete, with resulting contamination of the fnishing operation. Th e following treatments should be considered:
Chemical Surface Conversion This is usually a magnesium/zinc fluosilicate or a sodium silicate solution scrubbed or
mopped into the concrete surface. The free lime in the concrete, which is the cause of dusting, is chemically "tied up," and the surface is densified. This method of protection will minimize dusting but is of no value for resistance to chemical attack. Also, alkali cleaners will damage or reverse such treatments.
Coatings (%8 in. Thick or Less) Use of epoxy, polyurethane or other chemical- and wear-resistant coatings will provide
a dust-free surface and give some chemical protection. The success of any coating is dependent of course on surface preparation, and the user should also expect to perform periodic maintenance on the floor because the coating will wear off or be chipped by traffic, etc.
Sealers Concrete sealers have been on the market for many years. Some of these are the
penetrating type; others act more like a coating. Most of the older types act primarily as
657
dustproofers and have little or no value as chemical barriers; however, there are some new penetrating sealers that have a limited degree of chemical and abrasion resistance.
The use of penetrating sealers is the best method of treating concrete floors adjacent to plating areas because they last longer, give better protection in depth and do not depend entirely on a coating surface for impact and abrasion resistance. Penetrating sealers should be applied to sandblasted or etched concrete and may penetrate up to 1/4 in. deep. Because the pores are filled, traffic is borne by the exposed concrete aggregate, whereas the portland cement matrix is covered by a thin layer of the sealer.
It must be thoroughly understood that surface treatments for concrete cannot be substituted for proper acid-resistant construction in the process areas of the plating plant.
SPECIFICATIONS AND TESTING
The original design and any refurbishing of floors should be covered by adequately written specifications. This helps prevent costly mistakes, both from the customer's and the supplier's point of view. All conditions of environment that the floor materials must withstand, such as temperature, traffic load, chemicals, cleanup materials and procedures, should be spelled out explicitly. Materials specified and all materials offered as equal should be verified as meeting the specifications. The applicator should be required to give evidence of the satisfactory completion of three similar jobs within the past two years and agree to place on the job the foreman or lead man who laid one of the reference floors.
Testing, especially for chemical resistance, should be carried out in the chemicals and at the temperature involved. Long-term chemical tests of at least 56 days' duration should be the rule. The tests spelled out in American Society for Testing and Materials (ASTM) Standards should be followed exactly. So-called chemical resistance tests that call for "spot testing" or that are of very short duration should not be considered for qualifying mortars and toppings for the severe chemical environment common to the finishing plant. Physical testing should also be carried out under procedures given in the published standards of ASTM, American National Standards Institute, National Association of Corrosion Engineers, or similar recognized organizations. Installers who request variances or substitutions for specified materials should be required to give long-term warranties for the performance of substitutes.
The Chemical Analysis of Electroplating Solutions by T.H. Irvine 182 pages $50.00
Chapters in this work are divided into groups in accordance whh the periodic table of elements. Though the procedures are traditional, theoretical aspects are included with other information. Anyone who studies this book carefully will derive a helpful understanding of what he or she is doing so that unexpected results can be searched out for causes and corrected.
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