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Unformatted text preview: Matt Hart Dr. Styles BRAE 433 30 Nov 2009 Formatted: Font color: Background 1 Acid Etching Comment [mjh1]: Pay attention to the teacher’s instructions about formatting. This is the wrong font, spacing, and has no page numbers. INTRODUCTION TO ACID ETCHING Acid etching is the process of applying an acid solution to a concrete surface, allowing the acid to react with the concrete (Oman). Acid etching can be used for many applications including surface preparation for the application of epoxy coatings, adding in colors, or cleaning a concrete surface so that repairs can be made. Figure 1 is an example of the addition of stains using acid etching. Concrete acid etching works by applying a strong acid to the surface of cured concrete. Figure 1. Concrete bench project The acid reacts to the lime in the concrete and can be used in many different applications. ACID ETCHING The details of acid etching boil down to chemical reactions between the acid and the cured concrete itself. The hydrochloric acid solution (HCl) reacts with the calcium hydroxide (Ca(OH2)) to form the precipitate (CaCl2) that is easily flushed away (Xiong et al). Figure 2 is an example of the reaction seen when the acid is poured onto the concrete surface. This process leaves the surface of the concrete more porous and capable of accepting treatments such as stains or allowing better bonding with epoxy coatings and repair materials. Depending on the application there Figure 2. Reaction of the acid on are different acid types that can be used to etch the concrete surface concrete. Hydrochloric is the most common and easiest to acquire but also has very strong fumes. An alternative would be phosphoric acid, which can be used around carbon or galvanized steel and has less noxious fumes. Finally, sufamic acid comes as a powder that can be mixed with water and is the least dangerous to work with but naturally is less reactive (Oman). Comment [mjh2]: Refer to Dr. Styles’ instructions for a link to a website showing proper in‐text citations. You need the author(s) and publication year. Deleted: , Deleted: t Deleted: , Deleted: h PROCEDURE The procedure for acid etching concrete essentially breaks into three parts: preparation, application, and neutralization. Preparation Surface preparation is the most important step in the acid etching process. Without a clean surface the acid will not penetrate into the pores of the concrete and its effectiveness is greatly reduced. Grease and oils on the surface of concrete must be removed either with chemical de‐ greasers or in extreme cases shot‐blasting. The unofficial test is that if water beads on the surface it is not adequate for acid etching and further preparation is required. According to Delano (2009), “The first thing you do when you get to a jobsite is to clean everything off the slab and degrease everything, but every slab is different. There are a hundred different cleaners for a hundred different stains.” If a repair or coating is the reason behind the etching of the surface must also be completely removed from the concrete, migrating moisture can cause weaker bonding to the etched surface, therefore concrete should be allowed to cure for the minimum of 28 days prior to any surface treatment application (Oman (b) ). The concrete surface should be cleaned and rinsed with water and detergent and buffed with a machine. The final surface finish should be free from streak marks, footprints and all residues (Nasvick (a)). Application There are a variety of techniques used for the application of the acid to the surface of the concrete as well as the amount of time that the acid is allowed to react with the concrete. For a concrete repair the strongest repairs resulted from the use of a 5% hydrochloric solution that was brushed on with a nylon brush and allowed to react for five minutes (Xiong et al). The dependency of the acid strength and the duration of application is demonstrated in figure 3. The longer the acid is applied the more penetration occurs and the greater the effect is. The figure shows how Figure 3. Effects of acid on concrete pores, top‐untreated. 5‐
min treatment (left) and 10‐min treatment(right) (Xiong el Deleted: ” (Delano) Deleted: Deleted: e Deleted: t Deleted: , left‐ Deleted: Deleted: , Deleted: right‐ Deleted: removing the acid too quickly doesn’t allow the penetration into the concrete surface and therefore results in less than optimal pore availability, while over‐exposure causes a small surface layer to form that is weaker than the rest of the concrete, shown by the deep cracks. For the application of a stain using acid etching a sprayer is often used in combination with brushing. The sprayer allows even distribution of the acid‐stain solution, but the brushing is required to ensure the stain is worked into the surface of the concrete. On larger slabs the person brushing wears spiked shoes in order to not leave footprints in the stain (Delano). After brushing, the surface can be re‐sprayed lightly to remove the brush marks (Nasvick (b)). Unlike etching for bonding stain acids are left on the concrete for up to 24 hours. The stain can be diluted or water sprayed onto the surface to create depth and different color patterns. Often the acid concentration is varied to provide different levels of color intensity. Care should be taken when staining concrete that all surfaces are masked and protected since the results of the reaction is irreversible and difficult if not impossible to remove. Neutralization Even after the reaction is complete the acid is still active enough to react with clean concrete. A mixture of detergent, water, and a little baking soda is sprayed on the surface to neutralize the remaining acid. Figure 4 is an example of what can happen if the acid is not neutralized and allowed to run off. According to Delano (2009), “It is very important that you neutralize between every coat, the pH needs to be neutral.” The residue can then be hosed off the concrete surface and depending on the required results the surface can either be sealed or the next treatment can be applied. In some cases where salts are present the use of a Figure 4. Cal Poly 433 project runoff mineral/acid salt remover should be used. Salts can of stain onto ramp. ruin adhesion for future coatings (Oman (a)). ADVANTAGES OF ACID ETCHING Acid etching can be used for a wide variety of applications including decorative and practical. The procedure itself opens up the pores of the concrete on a microscopic level allowing improved bonding with other materials whether it be a stain or repair materials. Techniques such as shot or sand blasting, jack hammering or mechanical sanding are only effective on the macroscopic level and don’t provide the same level of cleaning. For the purpose of staining the concrete for a decorative material, acid etching is an easy cost effective technique that works well for many situations. Acid staining can yield more color variety and longevity than Deleted: Deleted: Deleted: (Delano). Deleted: Deleted: ¶ traditional flooring methods like carpet or linoleum. The trend towards radiant heating in family homes shows another advantage for concrete floors as they are the most efficient at conducting this heat effectively. Acid etched floors are impervious to water damage if sealed properly and cost about the same as tile or wood flooring (Nasvick (b)). DISADVANTAGES OF ACID ETCHING Concrete has its own problems that create difficulties for acid etching. The fact that concrete is brittle and cracks easily means that the aesthetic appeal of concrete floors isn’t high for owners living in seismically active areas. The cleaning process required for the acid to react properly over the entire surface is tedious and difficult especially if there is grease or oil contamination. There must be an existing slab for the technique to be cost effective in staining applications also the slab must be very level as pooling of the acid is not desired and can Figure 5. Concrete bench with cause major areas of discoloration. Figure 5 is an permanent brush lines . example of the permanent effect the acid has on the concrete surface. The brush marks will always be there. The sensitivity to the time that the acid is applied to the surface of the concrete has a large effect on the color of the finished product but more importantly, when doing concrete repairs over exposure to the acid actually creates a weak layer of concrete as the bonding surface. ALTERNATIVES TO ACID ETCHING Acid etching is not the only technique that can be used for altering the surface appearance of concrete. Traditional methods for adding texture and decorative inserts have been improving the aesthetics of concrete since it was invented, but these techniques must be applied to the concrete prior to curing and are permanent and even less forgiving than errors in the etching process. Nonreactive concrete stains also exist. They act by infiltrating the existing pores in the concrete and depositing the colors there. This fills the pores, reducing the effectiveness of sealers and can fade with time. However, this technique is easier than etching and provides more color variety and uniformity than regular acid etching. Traditionally, repairs to concrete involve chipping away a large enough area for the repair material to bond with and then applying an adhesive bonding agent and a repair material. This has been effective but often results in inadequate bonding to the repair material. For civil uses Comment [mjh3]: Up until this point you were doing a great job citing your sources. Don’t get tired and give up! Deleted: , Deleted: t Deleted: t such as sidewalk repair this is okay but in the domestic environment the appearance of repair materials is less than desirable. CONCLUSIONS The use of acid for the manipulation of the surface finish of concrete is a simple concept. The difficulty lies in the preparation. If the surface preparation is taken care of the rest of the process is easy. The cost for a contractor to etch a floor is right about $4.00 a square foot (Delano). There are advances in the types of acids available. Delano uses a citricacid for etching since it doesn’t kill surrounding lawns and the fumes are much more tolerable. Also, new techniques are being developed that give the surface stain the appearance of wood or marble. Acid etching is a science that will continue to develop and change as more and more people become interested, according to Delano. REFERENCES Delano, Brookins. Etching specialist. Telephone interview. 27 Nov 2009. Oman, Paul. (a) (n.d.). Acid etching. Retrieved from http://www.epoxyproducts.com/acid.html Oman, Paul. (b) "The Coatings and Preparation page." ePoxy Products. Progressive Epoxy Polymers Inc., Web. 30 Nov 2009. <http://www.epoxyproducts.com/prep.html>. Nasvick, Joe. (a) "Surface Preparation for Stained Concrete." Acid Etch Staining. 2000. Hanley‐Wood, Web. 30 Nov 2009. <http://www.concretenetwork.com/stained‐concrete/surface_preparation.htm>. Comment [mjh4]: Refer to Dr. Styles’s instructions for a link to the correct referencing format. Nasvick, Joe. (b) "Concrete Stain Application." Acid Etch Staining. 2000. Hanley-Wood, Web. 30 Nov 2009. <http://www.concretenetwork.com/stained-concrete/stain_application.htm>.
Xiong, Guangiing, Yuqing Cui, Liqiang Chen, and Hao Jing. "Influence of hydrochloric acid on bond strength between concrete substrate and repair materials." 29 oct 2002. Elsevier, Web. 30 Nov 2009. ...
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This note was uploaded on 01/13/2011 for the course BRAE 433 taught by Professor Styles during the Fall '10 term at Cal Poly.
- Fall '10