Response of Concrete to Sulfuric Acid Attack - … - ~ ~ * 1988 ~ + -... '" ., ...

, ACI MATERIALS JOURNAL TECHNICAL PAPER Title no. 85-M46

Response of Concrete to Sulfuric Acid Attack

by Emmanuel K. Attiogbe and Sami H. Rizkalla

The response oj four different concrete mixes to sulfuric acid attack was evaluated in an accelerated laboratory test program. Small test specimens cut from standard concrete cylinders and a 1 percent sulfuric acid solution with a pH oj J were used in the test program. Changes in weight and thickness of the test specimens were used as physical indicators 0/ the degree of deterioration, while increase in sulfur content of the test specimens was used as a chemical indicator of the degree of deterioration.

The study shows that all three indicators of deterioration are effective measures oj concrete response to the acid attack. However, the study suggests that the increase in thickness (expansion) of small specimens (with large surface area-to-volume ratios) may be a more consistent measure than the weight loss of larger specimens when comparing the effects of different sulfuric acid concentrations on concrete. Photomicrographs of the concrete microstructure show that the concrete deterioration storts from the acid-exposed surface and progresses inward. The degree of concrete deterioration is increased by alternate wet-dry cycles of exposure to sulfuric acid. The rate of concrete deterioration along the penetration depth of sulfuric acid could be described by a variation in sulfur concentration with the depth of acid penetration.

Keywords: acids; chemical attack; concretes; deterioration; electron microscopes; expansion; measurement; microstructure; pH; sulfurj sulfuric acid; thickness; weight (mass).

Concrete is susceptible to attack by sulfuric acid produced from either sewage or sulfur dioxide present in the atmosphere of industrial cities. This attack is due to the high alkalinity of portland cement concrete, which can be attacked by other acids as well. Sulfuric acid is particularly corrosive due to the sulfate ion participating in sulfate attack, in addition to the dissolution caused by the hydrogen ion.' Since sulfur compounds are formed as a result of the sulfuric acid-cement paste reaction, the increase in sulfur content of concrete specimens could be used as a measure of the chemical manifestation of deterioration.

In previous studies, weight loss, reduction in compressive strength, and change in dynamic modulus of elasticity were used to evaluate the extent of concrete deterioration due to sulfuric acid attack. 2

.' These studies indicate that damage starts at the surface of the concrete and progresses inward. However, the extent of damage along the depth of penetration of acid is not

ACI Materials Journal I November-December 1988

clearly defined. This information is necessary to accurately estimate the minimum thickness of the concrete cover in reinforced concrete structures or to adequately design for sacrificial layers in concrete structures exposed to sulfuric acid solutions .

As expected, the previous studies have generally shown that weight loss of the test specimens increases with a decreasing pH level of the acid solutions . However, in a recent study,' a solution with a pH of 3 produced a greater weight loss than one with a pH of 2. This apparent anomaly needs to be resolved for an adequate comparison of the resistance of different concrete mixes to acid attack.

The present study is aimed at evaluating the response of different concrete mixes to sulfuric acid attack, using both physical and chemical indicators of the degree of deterioration . An accelerated laboratory test program was conducted. The program involved alternate acid immersion and drying of test specimens, as well as continuous acid immersion of other test specimens. Changes in weight and thickness of the test specimens were used to evaluate the physical degree of deterioration of the concrete, while the increase in sulfur content of the test specimens, as measured with a scanning electron microscope (SEM) equipped with an energydispersive x-ray analyzer, was used to evaluate the chemical change in the concrete. Photomicrographs of the test specimens were used to study the extent of the acid attack.

RESEARCH SIGNIFICANCE This research sheds new light on the mechanism of

sulfuric acid attack on concrete . The experimental techniques used in the study provide information on the extent of damage that occurs in concrete as a result of acid attack. Photomicrographs show the nature of the

Received Oct. 14, 1987, and reviewed under Institute publication policies. Copyright © 198B, American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion will bc published in the September-October 1989 ACI Materials Journal if received by June I, 1989.

481

ACI member Emmanuel K. Attiogbe is a research associate at the University 0/ Manitoba, Canada. He received the BSc degree from the University of Sci· ence and Technology in Ghana and his MS and PhD degrees in civil engineering from the University 0/ Kansas. His research interests include durability and micromechanics of concrete.

A CI member Sami H. Rizkalla is an associate professor and the head of the Structures Division of the Civil Engineering Department at the University of Manitoba, Canada. He received the BSc degree/rom Alexandria University in Egypt and MS and PhD degrees from North Carolina State University. He is a member of ASCE, PCI, and CSCE. He is also a member of the Executive Committee of the Structures Division Of the Canadian Society of Civil Engineers. His research activilies are in the areas of reinforced and prestressed concrete structures .

Table 1 - Characteristics and composition of the concrete mixes

Mix numher Concrete components M1 M2 M3 M4

Coarse aggregate, lh/ ydl 1600 1600 1600 1600 Fine aggregate, lb/yd! 1400 1400 1400 1400 Water, tb/ ydl 310 310 310 310 Type 10 cement, lh/yd! 540 540 540 -(ASTM Type I) Type 50 cement, lh / yd! - - - 540 (ASTM Type V)

Water·reducing agent, oz - 22 - -Superplasticizer, oz 65 65 65 65 Air-entraining agent, oz - - 7 -

Slump, in. 7.0 8.0 7.0 6.5 Air content, percent - - 6.0 -Average 28 day compressive 5640 4670 5220 6230 strength, psi Note: 1 Ib/yd! _ 0.593 kg/ m!; I oz = 29.6 mI; I pSI - 6.895 kPa; I m. -

25.4 mm.

acid damage to the cement paste matrix of concrete. The study shows that the increase in thickness (expansion) and the increase in sulfur content of concrete specimens are good indicators of the degree of deterioration. The effect of wet-dry cycles of concrete exposure to sulfuric acid was investigated.

EXPERIMENTAL PROGRAM Materials and test procedures

The experimental program utilized four different concrete mixes. Three of the mixes were made with CSA Type 10 (ASTM Type J) portland cement, while one mix was made with CSA Type 50 (ASTM Type V) portland cement. A water-reducing agent was used in one mix and an air-entraining agent was used in another mix. A superplasticizer was used in all mixes to increase slump. Siliceous fine aggregate and a blend of calcareous and siliceous coarse aggregate were used. The characteristics and composition of the different concrete mixes are shown in Table 1.

Standard 152 x 305 mm (6 x 12 in.) concrete cylinders were made from each concrete mix. The cylinders were moist-cured for 28 days. At the end of the curing period, two cylinders were used to determine the compressive strength of each concrete mix. To induce stresses similar to those that exist in concrete under service load conditions, a third concrete cylinder for each mix was loaded to 40 percent of the compressive

482

strength. From this cylinder, small test specimens were cut and used in the acid attack test program.

Test specimens-The test specimens for each concrete mix were obtained as follows. A disk approximately 6 mm (\I.i in.) thick was cut near the midheight of each cylinder. Each disk was then cut into small 12 mm (y, in .) wide specimens . The final prismatic test specimens, approximately 100 mm (4 in.) long x 12 mm (V, in .) wide and 6 mm (\I.i in .) thick, were used to evaluate the sulfuric acid resistance of the concrete mixes. A total of six test specimens were used for each concrete mix: two for periodic physical measurements, two for continuous immersion in the acid solution, and the remaining two for microscopic examination. The test specimens were oven-dried to a constant weight for 24 hr at about 100 C, and their weights and thicknesses were measured prior to the start of the acid attack test program. The weight was measured using a balance with a 0.01 g sensitivity; the thickness was measured with a pair of calipers . The average thickness for three locations along the length was recorded for each specimen. These locations were marked with waterproof ink for subsequent thickness measurements following periodic immersion of the specimens in the acid solutions . The marks were retouched after taking measurements.

Additional test specimens for Mix MI were covered with two coatings and used in the acid attack test program . These coatings were applied to the test specimens with a brush and allowed to air dry for 2 days before being immersed in the acid. The first coating, a methyl methacrylate sealant, provided a visible layer about I mm (0.04 in) thick on the surface of the test specimens, while the second coating, an acrylic sealer, penetrated without any visible layer on the surface of the test specimens .

Sulfuric acid solutions-The test specimens were immersed in jars filled with 'equal quantities of I percent sulfuric acid solutions (pH = I). This concentration of sulfuric acid is representative of that found in sewers that are in the process of deterioration.'·7 The pH levels of the acid solutions were monitored at an average interval of 2 days with a portable pH meter. All solutions were replaced with fresh I percent solutions if the pH of any solution exceeded a value of 1.1. All solutions were also replaced prior to reimmersion of the test specimens after taking measurements. The periodic use of fresh acid solutions along with the use of small test specimens provided an accelerated evaluation of the acid resistance of the concrete mixes . The test program was concluded after about 10 weeks when some of the test specimens completely disintegrated in the acid solutions.

Test measurements- Measurements of the test specimens were performed after selected periods of immersion in the acid. The test specimens for weight and thickness measurements, as well as those for SEM examination, were removed from the acid, immersed in fresh water several times , and oven-dried to a constant weight for 24 hr. Upon removal from the oven, the two test specimens of each mix that had been marked for


Fig. 1 - SEM specimen as mounted on stud

measurements were lightly brushed, weighed, and their thicknesses measured.

Scanning electron microscope (SEM) examinationFor the SEM examination of each concrete mix, small fractured specimens, approximately 6 mm (\4 in.) thick x 12 mm (\I, in.) long x 6 mm (\4 in.) high, were prepared from one of the test specimens that had been marked for microscopic examination. A typical specimen, as mounted on a stud, is shown in Fig. I. To obtain clear photomicrographs, the top surface of the fractured specimens was rendered conductive with a layer of aluminum about 0.02 p.m thick. No conductive coating was used on specimens for the energy-dispersive x-ray analysis .

Photomicrographs were taken of both unattacked and acid-attacked specimens. Quantitative elemental analysis of the specimens was performed at a magnification of 500X, using computer software available on the energy-dispersive system. Changes in.sulfur content were of primary interest in this analysis. The elemental analysis was performed at three locations along the length of each specimen. At each location, the analysis was performed on the cement paste matrix within an area approximately 0.25 mm' (0.01 in.'), and three adjacent areas along the thickness of the specimen were analyzed.

EXPERIMENTAL RESULTS AND DISCUSSION Expansion of test specimens

Percentage change in thickness of the test specimens versus immersion time in the acid solutions are shown in Fig. 2 through 5. Immersion time as defined in these figures does not include the time used in drying the specimens. Each data point is an average of two specimens of the same mix. The data for the individual specimens varied less than 5 percent from the average values. The increases in thickness indicate that the specimens undergo volume expansion or swelling as a consequence of the sulfuric acid attack.


~ 15 b. Mix M I : No Woter - Reducing Admixture

g 0 Mix M2 : Includes a Woter - Reducing .~ Admixture o Q. 10

" !:!:!

J ~ 5

.50

g-o ~

u 0 10 20 30 40 50 60

Immersion Time, days 70 80

Fig. 2 - Expansion oj specimens jor concrete mixes with and without a water-reducing admixture

~ 15 • l:l. Mix M I : No Air Entrainment

g * Mix M3 : Air Entrained 'in " 8. 10

o!l

.E CI)

'" 6 ~

u 10 20 30 40 50 60 70

Immersion Time J days 80

Fig. 3 - Expansion oj specimens jar concrete mixes with and without air entrainment

of!. 15

" .2 :g 8. 10 o!l

.E CI)

'" " o ~ u

to. Mix M I : Type 10 (ASTM Type I) Cement

+ Mix M4 : Type 50 (ASTM Type,,) Gement

20 30 40 50 60

Immersion Time J days 70 80

Fig. 4 - Expansion oj specimens jor concrete mixes made with Types 10 and 50 (ASTM Types I and V) portland cements

Fig. 2 shows percentage change in thickness versus immersion time for the concrete mixes without and with a water-reducing admixture, Mixes M 1 and M2, respectively. The test specimens for Mix M2 expand more

483

~ 15 o

.S;;

g-o .t:: U a

~ Uncoated Specimen

x Cooted Specimen - No Visible Protective Surface Loyer

<> Coated Specimen - With Protective Surface Loyer

10 20 30 40 50 60 70 80

Immersion Time • days

Fig. 5 - Expansion of uncoated and coated specimens

than the specimens for Mix Ml. This larger expansion of Mix M2 is probably due to its lower compressive strength compared to Mix MI (Table I). The lower compressive strength is thought to be due to an excessive amount of air entrainment caused by the addition of the water-reducing admixture.

In Fig. 3, concrete mixes with and without air entrainment are compared. The air-entrained concrete has a slightly smaller expansion. Air entrainment improves the chemical resistance of concrete insofar as it helps to produce a more uniform, well-compacted, and, hence, denser concrete.s

Fig. 4 shows that beyond about 20 days' immersion in the sulfuric acid solution, the test specimens for the concrete made with sulfate-resistant Type 50 (ASTM Type V) cement expand more than those for the concrete made with Type 10 (ASTM Type I) cement. Thus, in the long run, sulfate-resistant cement does not appear to provide concrete a better resistance to sulfuric acid attack than that provided by normal portland cement. This observation is consistent with the findings of previous studies'·' and is explained by the fact that sulfate attack is only one aspect of sulfuric acid attack on concrete. I

As shown in Fig. 5 for test specimens of Mix Ml, coated specimens expand less than uncoated specimens. Also, the coated specimens with a protective surface layer expand less than those without a visible protective surface layer. These results support the well-known fact that measures aimed at making concrete more impermeable to ingestion of chemical solutions are the most efficient in providing resistance to chemical attack.

The preceding discussions show that expansion of the small test specimens used in this study provides a consistent measure of the deterioration of concrete due to sulfuric acid exposure.

Wet-dry cycles versus continuous immersion- Fig. 6 compares the expansion of test specimens that were alternately immersed in the acid solutions and dried to that of specimens that were continuously immersed until the end of the test program. Specimens that underwent alternate immersion and drying showed a greater

484

;fl 15

co .11

I ., co &. 10

" W

., ., 0> co

1 -'" ~ u

5 ~ .S;;

t:; Wet - dry Cycles of Acid Exposure 0>

'" o Continuous Acid Immersion co 0

.t:: a u a 2 3 4

Mix Number

Fig. 6 - Expansion of specimens that were subjected to wet-dry cycles of acid exposure compared to expansion of specimens that were continuously immersed in the acid solutions

expansion than those that were continuously immersed. This suggests that wet-dry cycles of exposure to sulfuric acid solutions increase the degree of concrete deterioration. The increased permeability of concrete due to drying cracks would lead to a greater volume of material being attacked by the sulfuric acid.

Weight loss of test specimens Percentage changes in weight of the test specimens

are shown in Fig. 7 through 10. Fig. 7 shows that weight loss is greater for the test specimens of Mix M2, which contained a water-reducing admixture, than for the specimens of Mix Ml. Weight loss of the test specimens for the air-entrained concrete, Mix M3, is smaller than that of the specimens for the non-air-entrained concrete, Mix M2 (Fig. 8). As shown in Fig. 9, the weight loss is slightly greater for the test specimens of Mix M4 with Type 50 (ASTM Type V) cement than for the specimens of Mix MI with Type 10 (ASTM Type I) cenient. The effect of using surface coats is shown in Fig. 10 by the smaller weight loss of the coated specimens in comparison to the uncoated specimens. The weight loss is smaller for coated specimens with a protective surface layer than for those without a visible protective surface layer. These results (Fig. 7 through 10) are consistent with those discussed in the previous section based on the expansion of the test specimens.

It should be noted that the test specimens initially gain weight followed by weight loss. The weight gain cannot be attributed to saturation of the specimens since the specimens were oven-dried prior to weighing. The reaction between sulfuric acid and the cement constituent of concrete results in the conversion of calcium hydroxide to calcium sulfate (gypsum) which, in turn, may be converted to calcium sulfoaluminate (ettringite). Each of these reactions involves an increase in volume of the reacting solids by a factor of about twO.1O The formation of calcium sulfate leads to softening (decrease in density) of the concrete. Since weight depends on both volume and density, the initial weight


5

.E Immersion Time I days '" - 5

~ .!: -10 ., g

.s= U -IS

-20

6. Mix M I : No 'Noter -Reducing Admixture

o Mix M2 : Includes 0 Woter - Reducing Admixture

Fig. 7 - Weight loss oj specimens jar concrete mixes with and without a water-reducing admixture

-.s=

5

'" -5 ~ .5 ~ -10

o .s=

10 20 50

Immersion Time ,doys

U -15 l:l. Mix M I : No Air Entrainment

* Mix M3 : Air Entrained

,-20

80

Fig. 8 - Weight loss oj specimens jar concrete mixes with and without air entrainment

gain of the test specimens is probably due to the relative increase in volume being greater than the relative decrease in density.

Both the increase in volume and the decrease in density of the concrete due to the sulfuric acid-cement paste reaction would be larger the higher the acidity (the lower the pH) of the acid solution. This implies that a stronger acid solution could produce a smaller weight loss in a concrete specimen than that produced by a weaker solution due to a significant increase in the volume of the concrete in comparison to the reduction in the density. This phenomenon explains the results of the study reported in Reference 6, in which the weight loss of concrete cylinders exposed to sulfuric/nitric acid solutions with pH of 2 was smaller than the weight loss of identical specimens exposed to the acid solutions with pH of 3. In the same study, the weight loss of the concrete cylinders produced by other acid concentrations increased with growing acidity, as expected.

It seems, therefore, that weight loss may not be a consistent measure when comparing the effects of different sulfuric acid concentrations on concrete. The expansion of concrete specimens may be a more consistent measure than the weight loss of the specimens since the sulfuric acid-cement paste reaction entails a volume increase. In this regard, the use of test specimens with large surface area-to-volnme ratios (such as those used

ACt Materials Journal I November·December 1988

5

:s. V· -5

~ .5: -10

g-o o -15

-20

60 70

t:. Mix M I : Type 10 (ASTM Type I) Cement

+ Mix M4 : Type 50 (ASTM Type 1[) Cement

Fig. 9 - Weight loss oj specimens jar concrete mixes made with Types 10 and 50 (ASTM Types 1 and V) portland cements

~ 0

-.s=

'" ... ;t .5 ., i? 0

.s= U

5

0

-5

-10

-15

-20

Immersion Time , days

l:::.. Uncoated Specimen

x Cooted Specimen - No Visible Protective Surface Layer

<> Coaled Specimen - With Protective Surface Layer

Fig. 10 - Weight loss oj uncoated and coated specimens

in this study) may be preferred since the acid attack is a surface phenomenon, as indicated by previous studies2~ and supported by results discussed in a subsequent section of this paper. Further investigation is needed to evaluate specimen expansion and weight loss as measures of concrete deterioration due to acids with different strengths and for specimens with different surface area-to-volume ratios. In such an investigation, more than two test specimens would need to be used for the expansion and weight loss measurements to clearly establish the statistical reliability of the tests.

ScannIng electron microscope (SEM) analyses Micrographs - Photomicrographs of unattacked

and acid-attacked specimens are presented in Fig. II and Fig. 12 through 15, respectively. Fig. 12 through 14 are micrographs of specimens that were not coated prior to immersion in the acid solutions, while Fig. 15 is a micrograph of a coated specimen. The changes in microstructure, as discussed later, are typical for all the concrete mixes studied . However , as discussed in earlier sections, the extent of deterioration as a function of acid immersion time varies among the concrete mixes due to differences in mix composition, such as the type of cement and the admixtures used.

485

Fig. 11 - Microstructure of an unattacked specimen

Fig. 12 - Microstructure of an acid-attacked specimen immersed for 32 days: region near acid-exposed surface of the specimen

Fig. 13 - Microstructure of an acid-attacked specimen immersed for 32 days: middle region of the specimen

A typical micrograph of the cement paste matrix on a fractured surface of matured concrete is shown in Fig. II. The predominant structure appears to be a modified Type III calcium silicate hydrate (CSH) , one of the prime hydration products of portland cement."·!' A calcium hydroxide crystal is seen in the lower right corner of the micrograph. Fig. 12 is a micrograph of a specimen that had been immersed in the acid for 32 days. A comparison of Fig. 12 to Fig. 11

486

Fig. 14 - Microstructure of an acid-attacked specimen immersed for 71 days

Fig. 15 - Microstructure of a coated, acid-attacked specimen immersed for 49 days; the coating is the dark vertical band on the left

shows that the acid reacts with the cement paste matrix reducing the matrix to a more porous material consisting of smaller particles.

Fig. 12 shows a region near the acid-exposed surface of the specimen, while Fig. 13 shows a region farther from the acid-exposed surface of the same specimen. These micrographs show that the deterioration is smaller in the region farther from the acid-exposed surface of the specimen than in the region near the acidexposed surface. This result supports previous studies and clearly indicates that the acid attack is a surface phenomenon: the deterioration starts at the surface of the concrete and progresses inward.

A typical micrograph of a specimen that had been immersed in the acid for 71 days is presented in Fig. 14. This micrograph is representative of the total surface of the specimen at this stage of the acid attack. The spherical fibrous structures that are visible in several locations on this micrograph are Type I CSH. These structures are usually not visible on the fractured surfaces of matured cement paste and concrete. The fracture path usually passes through Type III CSH (see Fig. 11 and References 13 through 15). Comparing Fig. 14 to Fig. 11 and 12, it appears that after 71 days immersion, the acid has dissolved the Type III CSH to a large extent exposing the Type I structures .

ACI Materials Journal I November·December 1988

Mg - Magnesium At - Aluminum Si - Silicon -

Ca S - Sulphur

K - Potassium - Co - Calcium -

Fe - Iron

- -

Si

-

I- Al -

-

S A Fig. 16 - Elemental energy-dispersive spectrum for an unattacked specimen

A micrograph of a specimen that was coated before being immersed in the acid for 49 days is presented in Fig. 15. The protective surface layer of the coating is shown as a dark vertical band on the left side of the micrograph. A comparison of this micrograph to Fig. 12 shows that the extent of deterioration is less in the coated specimen, even though it was immersed in the acid for a longer duration.

Elemental analysis - Typical energy-dispersive spectra obtained from the quantitative elemental analysis are shown in Fig. 16 and 17 for unattacked and acidattacked specimens, respectively. These spectra were obtained within three adjacent regions, each approximately 0.25 mm (0.01 in.) wide, along the thickness of the specimens. Since sulfur compounds are formed as a result of the reaction between sulfuric acid and cement paste, sulfur components of the spectra are of primary interest in these figures and are highlighted by the dark shadings. The figures show that the sulfur content of the acid-attacked specimen is higher than that of the unattacked specimen.

Sulfur concentrations obtained from the elemental spectra, such as those in Fig. 16 and 17, are shown in Fig. 18 versus specimen immersion time in the acid solution for the three adjacent regions analyzed along the depth of acid penetration. Each data point is an average of the sulfur concentrations for three locations along the length of the SEM specimen. Fig. 18 shows that the sulfur concentration increases with immersion time, the increase being larger in the region near the acid-exposed surface during the initial period of immersion than in the regions farther from the acid-exposed surface . As the sulfur concentration in the region near the acid-exposed surface approaches an optimum value, the concentration in the adjacent region increases rapidly, while the concentration in the region farthest from the acid-exposed surface increases less


Mg - Magnesium AI - Aluminum

l- Si - Silicon -S - Sulphur

Ca - K - Potassium Co - Calcium -Fe - Iron

- S -

- -

Si -

-

ifi Ca

Mg '-I ~~--

Fig. 17 - Elemental energy-dispersive spectrum for an acid-attacked specimen

40 --0.00- 0.25 mm from Acid - Exposed Surface ----0.25-0.5Omm from Acid-Exposed Surface

;,l! 0 35 -"- 0.50-0.75 mm from Acid -Exposed Su1 e

"i 30

c 0 25

~ <: 20

~ 0 u ~

" or .9-

" C/)

////

/ d""'- ,.....0

,,' "" .. ..".. .. .," ....... ~. .,. ... -,," ..,.. ..

" , .,-" _C1'-"-"-'-'-<Y'

10 20 30 40 SO 60 70 80

Immersion Time , days

Fig. 18 - Sulfur content of specimens versus immersion time in the acid solution

rapidly. This result suggests that the concrete deterioration starts at the surface and progresses inward, as revealed by the photomicrographs.

Fig. 19 shows sulfur concentrations versus distance from the acid-exposed surface of uncoated and coated specimens. As in Fig. 18, each ordinate is an average of the sulfur concentrations for three locations along the length of the SEM specimen. For both the uncoated and coated specimens , the sulfur concentration is higher in the region near the acid-exposed surface than in the regions farther from the acid-exposed surface. The coated specimen has a smaller sulfur concentration within each region than the uncoated specimen. The smaller sulfur concentrations for the coated specimen correspond to smaller expansion (Fig . 5) and weight loss (Fig. 10) when compared to uncoated specimens.

To accurately estimate the minimum thickness of concrete cover in reinforced concrete members exposed

487

40

~ 35r-__________ -, ~ _ 30-

.§ 25 e C 20-

~ 8 15

S 10-

--- Uncoated Specimen ..•.. - Coated Specimen

15.. ..................... _.M. __ Jl 5 ••....•........ --........... """ __ ~~_'"

O~ ______ ~~ ______ ~~ ____ ~~l 0.00 0.25 0.50 0.75

Distance from Acid - Exposed Surface, mm

Fig. 19 - Sulfur content of uncoated and coated specimens versus distance from acid-exposed surface

to sulfuric acid, the rate of deterioration along the depth of penetration of acid should be known. The minimum cover thickness was estimated in Reference 5 by assuming the degree of deterioration to be linearly related to the depth of penetration of acid, with full sulfuric acid-cement reaction at the exposed surface and zero reaction at the maximum depth of acid penetration. This relationship could be more accurately represented by the variation in sulfur concentration with the depth of penetration of acid. Further investigation is required to explore this issue.

CONCLUSIONS The following conclusions are drawn from the test

results and analysis presented in this paper. I . The increase in sulfur content of test specimens, as

measured with an SEM equipped with an energy-dispersive x-ray analyzer, is a good indicator of the extent of damage in concrete due to exposure to sulfuric acid.

2. Photomicrographs, as well as the variation in sulfur concentration with distance from the acid-exposed surface of test specimens, clearly show that deterioration of concrete due to sulfuric acid attack starts at the surface and progresses inwards.

3. The increase in thickness (expansion) of small specimens (with large surface area-to-volume ratios) may be a more consistent measure than the weight loss of larger specimens when comparing the effects of different sulfuric acid concentrations on concrete.

4. Wet-dry cycles of exposure to sulfuric acid increase the degree of concrete deterioration.

5. The relationship between degree of concrete deterioration and depth of penetration of sulfuric acid

488

could be represented by the variation in sulfur concentration with the depth of acid penetration.

ACKNOWLEDGMENTS This research was performed at the Structural Engineering and

Materials Laboratory of the University of Manitoba. Electron microscope analysis was carried out on the JEOL JXA-840 Scanning Electron Microscope of the Department of Mechanical Engineering, University of Manitoba. The authors are grateful for the technical and financial assistance provided by Barkman Concrete Ltd., Steinbach, Manitoba.

REFERENCES 1. Mindess, Sydney, and Young, J. Francis, Concrete, Prentice

Hall, Englewood Cliffs, 1981, pp. 548-555. 2. Meyer, Alvin H., and Ledbetter, William B., "Sulfuric Acid

Attack on Concrete Sewer Pipe," Proceedings, ASCE, V. 96, SA5, Oct. 1970. pp. 1167-1182.

3. Hughes, B. P., and Guest, J. E., "Limestone and Siliceous Aggregate Concretes Subjected to Sulphuric Acid Attack," Magazine of Concrete Research (London), V. 30, No. 102, Mar. 1978, pp. 11-18.

4. Fattuhi, N. I., and Hughes, B. P., "Effect of Acid Attack on Concrete with Different Admixtures or Protective Coatings," Cement and Concrete Research, V. 13, No.5, Sept. 1983, pp. 655-665.

5. Raju, P. S. N., and Dayaratnam, P., "Durability of Concrete Exposed to Dilute Sulfuric Acid," Building and Environment, V. 19, No.2, 1984, pp. 75-79.

6. Kong, Hendrick, L., and Orbison, James G., "Concrete Deterioration Due to Acid Precipitation," ACI Materials Journal, V. 84, No.2, Mar.-Apr. 1987, pp. 110-116.

7. Sand, W., and Bock, E., "Concrete Corrosion in the Hamburg Sewer System," Science and Technology Letters, V. 5,1984, pp. 517-528.

8. Wilder, Carl R., and Spears, Ralph E., "Concrete for Sanitary Engineering Structures," Concrete International: Design & Construction, V. 3, No.4, Apr. 1981, pp. 29-34.

9. Wenger, E. C., "Concrete for Sewage Work," ACI JOURNAL, Proceedings V. 54, No.9, Mar. 1958. pp. 733-738.

10. Soroka, Itzhak, Portland Cem'ent Paste and Concrete, Macmillan Press, London, 1979, pp. 151-152.

11. Berger, R. L.; Young, J. F.; and Lawrence, F. V., Discussion of "Morphology and Surface Properties of Hydrated Tricalcium Silicate Paste" by M. Collepardi and B. Marchess. Cement and Concrete Research, V. 2, No.5, Sept. 1972, pp. 633-636.

12. Diamond, Sidney, "Cement Paste Microstructure-An Overview at Several Levels," Proceedings, Conference on Hydraulic Cement Pastes; Their Structure and Properties (Sheffield, Apr. 1976), Cement and Concrete Association, Wexham Springs, 1976, pp. 2-30.

13. Lawrence, F. V., Jr., and Young, J. F., "Studies on the Hydration of Tricalcium Silicate Pastes: Scanning Electron Microscopic Examination of Microstructural Features," Cement and Concrete Research, V. 3, No.2, Mar. 1973, pp. 149-161.

14. Attiogbe, Emmanuel K. , and Darwin, David, "Submicrocracking in Cement Paste and Mortar in Compression," 8M Report No. 16, University of Kansas Center for Research, Lawrence, Nov. 1985,439 pp.

15. Attiogbe, Emmanuel K., and Darwin, David, "Submicrocracking in Cement Paste and Mortar," ACI Materials Journal, V. 84, No.6. Nov.-Dec. 1987, pp. 491-500.

ACI Materials Journal I November·December 1988

Response of Concrete to Sulfuric Acid Attack - … - ~ ~ * 1988 ~ + -... '" ., ...

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