Organic Coatings for Corrosion Control - American Chemical Society

Chapter 21

An Aspect of Concrete Protection by Surface Coating

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J. B. Johnson and B. S. Skerry

Downloaded by UNIV OF OTTAWA on March 18, 2013 | http://pubs.acs.org Publication Date: March 30, 1998 | doi: 10.1021/bk-1998-0689.ch021

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Johnson-Thomas Associates, 36 Cedarway, Bollington, Cheshire SK10 5NS, United Kingdom Cleveland Technical Center, The Sherwin-Williams Company, 601 Canal Road, Cleveland, OH 44113

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The effectiveness of polyisocyanate and alkoxy-silane coatings on concrete is investigated. Good and poor quality concrete samples were evaluated with or without surface chemical contamination (representing sea/road salts or acid rain conditions) after exposure to a natural outdoor environment cyclic test for 3 years. Concrete degradation was assessed visually and by; surface wetting; leaching of calciumfromthe cement; and depth of carbonation. The most significant factor was the quality of the concrete. Contaminants also decreased concrete performance in the order H SO >> Na SO > NaCl. To some extent, both coatings ameliorated concrete degradation even when pre-contaminated. Here, the alkoxy-silane was somewhat more effective. In conclusion, when protecting concrete by coatings, all surfaces and sub-surfaces should be contaminant free. The effectiveness of remediation of poor quality or contaminated concrete may be limited. 2

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Concrete, a building material composed of cement, stone, sand and water, has been in use since Egyptian and Roman times. In the 20th century, it has been used as an economical alternative to materials such as natural stone, brick and wood. For many years now, steel reinforcing bars have been molded into concrete to increase its structural strength. Unfortunately, a common type of failure of steel reinforced concrete is cracking and spalling loss of the concrete mainly due to the corrosion of the steel reinforcement inside the concrete. Galvanized steel or steel encased in epoxy based coatings have been attempts to isolate the reinforcing bars from the ingress of aggressive chemical agents. Another important aspect about the preservation of concrete is the complementary use of coatings on the outside surfaces of the concrete. Acrylic, epoxy, silane and

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©1998 American Chemical Society

In Organic Coatings for Corrosion Control; Bierwagen, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.


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polyurethane coatings have all been used for this purpose. The idea is that if access to and ingress of contaminants into the concrete can be minimized, then the useful life of a reinforced concrete structure can be increased (7,2). Thus, coatings may inhibit the direct interactions between environmental components and a concrete substrate surface as well as slow the rate of penetration of any such components within the concrete. Despite the obvious apparent advantages of coating the surfaces of concrete structures, several factors affecting concrete durability can still be overlooked. For example, whether the coating is compatible with the chemical nature of the substrate as well as whether it is stable in sunlight (i.e., whether is it alkali gndUV radiation resistant). Further, is the coating maintainable, repairable, renewable and cost effective? (3) Yet another complication arises when, as is becoming increasingly commonplace, a concrete structure of several years standing requires maintenance, repair and a new surface coating. In this case, the question arises as to what is the likely efficacy of the protection of concrete by the surface coating if contaminant saltsfromthe environment are already on the surface of the concrete, or worse, penetrated into the concrete prior to the coating application. Even if a specification calls for 'blast-back' to sound concrete, sub-surface permeation may make it difficult to achieve a 'clean' surface for painting. The practical requirement is, therefore, that a coating should remain on a concrete surface and be effective against soil, water and atmospheric reactivity even when applied to a surface already contaminated and deteriorating. Most laboratory evaluations of concrete and concrete coatings have used noncontaminated and good quality concrete. The purpose of this work has been to investigate four factors of practical significance, (i) concrete quality, (ii) chemical contamination, (iii) type of protective surface coating and (iv) different environmental exposure conditions, and to determine how these factors influence the likely long term performance and durability characteristics of coated concrete.

Experimental Concrete Preparation. Two grades of concrete (i.e., good and poor quality) were produced by using washed sand, limestone aggregate (graded between 5 and 15 mm), ordinary portland cement and water in the following ratios; (2:4:1:0.5 [good quality] and 2:4:1:0.65 [poor quality J). Slabs of these mixtures were made in plastic molds (approx. 70 cm long χ 35 cm wide χ 5 cm thick). The slabs were allowed to cure at a temperature of 8-13°C protected from sunlight and wind for 35 days. Each slab was then cut across the longitudinal axis into 6 samples, each approximately 11 cm wide. A total of 48 samples were made; 36 representing good quality and 12 representing poor quality concrete. Surface Contamination. Three pre-treatments for concrete contamination were investigated. These treatments were intended to represent the effects of sea or road salts (chlorides and sulfates) and of natural acid rain (dilute sulfuric acid). The 48 concrete samples were numbered and divided into four groups of 12, each group having 3 good quality and 1 poor quality concrete samples, and were then treated as follows: i) Primary controls - not contaminated with chemical treatments.


272 ii) Contaminated with chloride ions by immersion in 2wt% NaCl for 21 days, then removed and exposed for drying in ambient air for 7 days. The immersion was repeated with a fresh solution of NaCl. Samples were then allowed to dry again and any surface salts brushed off. iii) Contaminated with sulfate ions by the same procedure as for chloride ions but substituting a 2wt% solution of Na SO .10H O. iv) Contaminated with dilute sulfuric acid at pH 3 by immersion for 2 days in a fixed volume of acid (so as to produce a concrete surface loading of 3 mis acid per 1 cm surface area), followed by removal and drying by outdoor exposure for 5 days. This procedure was repeated 10 times. 2

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Surface Coatings. All the concrete samples were again separated; this time into three groups and treated as follows; i) The first series was left uncoated as secondary controls. ii) Samples in the second group were triple coated on all exposed surfaces by alkoxy-silane [R]Si[OR] (where R, for example, may be a methyl group); prepared as a 20-30wt% solution in ethanol. iii) Samples in the third group were triple coated on all exposed surfaces by polyisocyanate prepared as a 20-30wt% solution in butyl acetate 3

H [R] -N-Ç-0-[R] O n

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where R, for example, may contain ether or ester groups. In all cases, the coatings were applied by a hand held V" broad brush using approximately 0.02 ml/cm of the alkoxy-silane or polyisocyanate solution per application with 4 hours air drying allowed between first, second and third coats. All the coated samples were then left standing in a fume hood to cure for 21 days. 2

Exposure Test. Test exposure was in a natural outdoors (-rural) environment. Samples were half buried in soil (i.e., to a depth of approximately 17cm) facing southwest. After six months exposure, they were removed from the soil and immersed to a depthrôf approximately 4 cm (along the vertical length) in de-ionized water also facing south-west for a further six months. This sequence was repeated for a total period of 3 years. Samples were therefore exposed to varying environmental factors including wind, rain,frost,pollutants and sunlight whilst being partially buried in soil or partially immersed in de-ionized water. Available or measured environmental conditions pertaining to the exposure test are noted in Table 1. Within the overall matrix of test panels in this program, for each test condition and surface state, good quality concrete samples were tested in triplicate and poor quality concrete samples were tested as single samples.


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Table I. Exterior Exposure Program, Soil and Atmospheric Environment Characteristics A) Soil 25 - 35 mg/g S0 25 - 35 mg/g Water 20 -30wt% For a 1 liter d.i. water extraction from 100 g dry soil; pH = 8.2; conductivity = 85 - 87 μ8 (20°C) CI"

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B) Atmospheric Conditions Annual sunlight: 750 - 850 hrs, Temperature: -4°C to 25 °C, Average rainfall: 800 mm/yr (pH 4.5 - 5.6), Humidity range; 55 - 100%, Pollutants; S0 5 - 20 ppb; N0 5 - 15 ppb 2

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Concrete performance assessment methods. Concrete performance was investigated by the following methods; i) Visual observations. All samples were inspected visually and photographed prior to and again after the 3 year exposure test. ii) Water surface capillary rise measurements. Selected samplesfromthe program were immersed to a depth of 4 cm in de-ionized water at 25°C both before and after the 3 year exposure test. Any linearriseof water on a sample surface was measured with respect to time up to a period of 3 hours duration. The objective of this test was to assess the relative susceptibility and affinity to water ingress for the different test sample conditions both before and after exposure. iii) Calcium leachability. Other selected samplesfromthe program were immersed to a depth of 4 cm in de-ionized water (i.e., 120 cm exposed surface area in 600ml H 0) at 25°C for two separate periods of 36 hours after which, each water sample was analyzed for calcium content by atomic absorption spectroscopy. These measurements were made both before and after the 3year exterior exposure program. The objective was to assess the effectiveness of the coatings in inhibiting concrete degradation through loss of calcium from the cement. iv) Depth of carbonation. After the 3-year exterior test program, representative samples were cross-sectioned and sprayed with a 1 vol% alcoholic solution of phenolphthalein. The depth of the non-colored band was taken as the depth of carbonation. The objective here was to assess the effectiveness of the coatings in preventing concrete degradation by acidification caused by diffusion of environmental C0 combined with water. 2

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Results Visual Observations. Prior to exposure testing, other than the presence of one of the surface coatings where present, all the samples were equivalent in appearance. After the 3-year exposure, clear visual differences became apparent. However, the blocks which had most obviously deteriorated were all those mixed using the 'poor concrete' recipe. Representative degradation is illustrated in Figure 1 (a) for the poor quality concrete sample contaminated with dilute H S0 and treated with the polyisocyanate coating. All poor quality concrete samples showed significant deterioration of this nature, irrespective of contamination or surface coating type. In contrast, Figure 1 (b) shows the equivalent result for a sample of good quality concrete also contaminated with H S0 and coated with polyisocyanate which remained in remarkably good condition at the end of the 3-year exterior test. This suggests that concrete recipe is a more important factor than either damage due to contamination or protection via surface coating type. However, significant differences in properties likely to affect long term durability can escape detection by simple visual inspection as reported below. 2

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Water Surface Capillary Rise Measurements before Exterior Exposure. Figure 2 shows the measured surface capillary waterriseresults before environmental exposure testing for all clean and contaminated samples (both good and poor quality concrete) which were not surface coated.

Figure 1(a). Poor Quality Concrete - Preconditioned with Dilute H2SO4, Polyisocyanate coated.



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Figure 1(b). Good Quality Concrete - Preconditioned with Dilute H2SO4, Polyisocyanate coated.

- · — G o o d ; Clean -a—Good ; Na2S04 - • - - P o o r ; Clean •-•---Poor ; Na2S04

• Good ; NaCI — * — Good ; H2S04 - - • - - P o o r ; NaCI Poor ; H2S04

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Poor Concrete

150 + 100 450 +

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Time (min)

Figure 2. Surface Capillary Water Rise before Exposure - No Surface Coatings.


276 Here, the strong dependence on concrete quality recipe can be seen. Figure 2 shows that the poor quality concrete samples had surface capillary water rise values approximately 3x the rate of the good quality samples. Also apparent from Figure 2 is that, irrespective of concrete quality, the dilute acid treated samples were rather more wettable than the chloride, sulfate or non-contaminated samples. Almost none of the alkoxy-silane or polyisocyanate treated samples showed any signs of water wettability before environmental exposure. The only exceptions to this were some of the samples contaminated with dilute H S0 which wetted very slightly, but not enough to register significantly on Figure 2. 2

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Water Surface Capillary Rise Measurements a/ter 3 Years Exterior Exposure Testing. Results obtained after 3 years of environmental exposure testing for good quality concrete samples are shown in Figure 3(a).

250 200 150

• Clean, No Coating

- - · - - NaCl, No Coating

- Na2S04, No Coating

- - * - - H2S04, No Coating

- Clean, Polyisocyanate Coating

— · — NaCl, Polyisocyanate Coating

- Na2S04, Polyisocyanate Coating

H2S04, Polyisocyanate Coating

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Good Concrete

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Figure 3a. Surface Capillary Water Rise after 3 Years of Exposure - Good Quality Concrete.

These data are plotted on the same scale for direct comparison with Figure 2 (before exposure). Only the alkoxy-silane coated samples retained their complete water nonwettability after exposure, insufficient to register above the x-axis minimum on Figure 3(a). All other good quality concrete samples, including those coated with polyisocyanate were found water wettable. Poor quality concrete sample results after exterior exposure are shown in Figure 3(b).


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- - • - -Clean, No Coating 250

--•--NaCI, No Coating

---*--Na2S04, No Coating

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- - ·* - -H2S04, No Coating

— x — C l e a n , Polyisocyanate Coating

— · — N a C I , Polyisocyanate Coating

— · — N a 2 S 0 4 , Polyisocyanate Coating

H2S04, Polyisocyanate Coating

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Poor Concrete

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Figure 3b. Surface Capillary Water Rise after 3 Years of Exposure - Poor Quality Concrete.

Again, all the alkoxy silane treated samples retained their non-wetting characteristics. All other samples, whether coated with the polyisocyanate or not coated, permitted water wetting as shown, Figure 3(b), to a considerably greater degree than for the equivalent good quality concrete samples, Figure 3(a). From Figures 2 and 3, it is clear that surface capillary water rise measurements on exposed samples show increases over the initial values obtained before environmental exposure. The effects of surface pre-contamination were also significant. Water rise measurements rose in the following order; non-contaminated (least detrimental effect), NaCI, Na S0 and finally dilute H S0 (most detrimental effect). Further, whereas the alkoxy silane coated samples showed no surface capillary water rise either initially or after exposure, the polyisocyanate samples did exhibit water capillaryriseafter exposure testing. 2

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Calcium Leachability before Exterior Exposure. Calcium leaching data before test exposures are shown in Figure 4(a) for clean concrete and those pre-contaminated with NaCI or Na S0 , and in Figure 4(b) for the samples contaminated with dilute H S0 . (These data are plotted separately because of the much larger extent of Ca leaching caused by the dilute acid contamination treatment.) 2

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Figure 4a. Calcium Leachability before Exposure - Clean Concrete and NaCl/Na S0 Contamination. 2

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Figure 4b. Calcium Leachability before Exposure - H S0 Contamination. 2

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Again, concrete recipe quality is clearly an important issue, as is the deleterious effect of pre-contamination with H S0 . However, before exposure testing, these two factors were mitigated by use of the alkoxy-silane coating. Within the sequence, clean, NaCl and Na S0 , the more severe effects were caused by Na S0 rather than by NaCl. 2

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Calcium Leachability after 3 Years Exterior Exposure Testing. After exterior exposure testing, Figures 5(a) and (b), broadly similar results were obtained. Thus, similar conclusions were drawn regarding the effects of concrete quality, NaCI, Na2S0 and H S0 contamination. Again, the alkoxy-silane coating proved effective.

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Figure 5a. Calcuim Leachability after 3 Years of Exposure - Clean Concrete and NaCl/Na S0 Contamination. 2

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Figure 5b. Calcium Leachability after 3 Years of Exposure - H S0 Contamination. 2


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Carbonation Depth after 3 Years Exterior Exposure Testing. Carbonation depths after 3 years exterior testing were obtained from cross-sectioned samples. The results are shown in Figure 6.

Figure 6. Carbonation Depth after 3 Years of Exposure.

Once again, the impact of concrete quality is clear as is the worsening effects of contaminants in the sequence; NaCl, Na S0 and H S0 . Also apparent is that both coatings types, polyisocyanate and alkoxy silane, resist the extent of carbonation for all concrete samples for both good and poor quality concrete. Overall, the alkoxy silane based coatings gave the lowest carbonation depths. 2

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Discussion There are three major considerations in this work; firstly, the impact of correct concrete recipe mix, secondly, the relative behavior characteristics of pre-contaminated samples compared to non-contaminated (clean) samples and thirdly, the relative effectiveness of two types of concrete surface coatings.The results of all the examinations, whether by water wettability (by surface capillary rise), calcium leachability or by depth of carbonation showed a general consistency. The most striking information generated in this work is of the critical importance of mixing the concrete to the correct recipe. Errors here lead clearly to decreased concrete performance properties and likely long term reduced durability. However, results obtained in this work suggest that a good surface protective coating can compensate, to some extent, for a poorer quality concrete mix. Precontaminated samples all exhibited increases in wettability (capillary rise), leachability and carbonation over the non-contaminated samples. The degree of increase was in the order; NaCl (least effect) to Na S0 to dilute H S0 (most effect). That is, the presence of inorganic salts is detrimental to the life of the concrete, perhaps 2

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281 due to decreases in surface mechanical properties and increases in permeability characteristics of the concrete. Both coating systems tended to produce reduced water wettability (capillary rise), calcium leaching and carbonation compared with non-coated equivalent samples. However, the presence of inorganic salt contaminants reduced, to some degree, the efficacy of the protective coatings, possibly due to factors such as enhanced osmosis and reduced adhesion. Of the two coatings investigated in this work, the alkoxy-silane apparently remained hydrophobic for the 3 year exterior exposure period and protected the concrete samples rather more efficiently than the polyisocyanate. However, it is important to note that generalizations regarding different coating types should not be made based on the limited data presented here. Rather, since coatings behave differently in different environments, when considering the use of surface coatings, a recommendation should be given to conduct exposure trials with a variety of generic coating types to select the most appropriate for the specific circumstances expected. Conclusions 1 ) The importance of correct recipe concrete mix is clearly a critical issue. In this work, it was the overriding factor determining performance properties of the concrete samples as tested. 2) The surface application of protective coatings reduces the rate of degradation of both clean as well as contaminated concrete samples. Here, polyisocyanate produced some benefits but not as great as those brought about by the alkoxy-silane treatment. 3) The presence of contaminating inorganic salts either on, or within the surface layers of concrete substrates reduces the protection sought by the use of organic overcoatings. This reduction in protection is particularly apparent with poor quality concrete. 4) When organic surface coatings are applied to concrete, the surfaces and subsurfaces should be contaminantfree.Any attempted use of coatings to 'cover up' poor quality concrete or to try to extend significantly the life of such concrete should be recognized as having limitations. Literature Cited 1. D. Starke & W. Perenchio, 'The Performance of Galvanized Reinforcement in Concrete Bridge Decks'. Project No. ZE-206, International Lead Zinc Research Organizations & American Hot Dip Galvanisers Association Construction Technology Laboratories, Skokie, Illinois, 1975. 2. 'Epoxy Coated Reinforcing Bars', Engineering Data Report No. 14, CRSI, Schaumburg, Illinois, USA. 3. A. Baba & Ο. Senbu, 'A Predictive Procedure for Carbonation Depth of Concrete with Various Types of Surface Layers', Proc. 4th Int. Conf. On Durability of Building Materials and Components. Singapore, 1987, Pergamon, Oxford, Vol. II, p.679.


Organic Coatings for Corrosion Control - American Chemical Society

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