Durability of concrete exposed to dilute sulphuric acid

BuildingandEnvironment,Vol.19,No.2, pp. 75 79, 1984. PrintedinGreatBritain.

036(~1323/84$3.00+0.00 © 1984PergamonPressLtd.

Durability of Concrete Exposed to Dilute Sulphuric Acid P. S. N. RAJU*

P. DAYARATNAMI" The paper presents an experimental investigation of concrete exposed to dilute sulphuric acid at 0.1, 1 and 5% concentrations. A method of estimating the depth of penetration of sulphuric acid, and the strength and weight reductionfactors for given sulphuric acid exposure conditions are presented. The expressions developedfor strength and weight reductionfactors help the designer to design a member for a specified life and environment. Similarly the expression developed for estimating the depth of penetration of acid,facilitates design of proper cover.

INTRODUCTION

investigation, cement from the same lot was used to avoid the variation in cement quality. Locally available fine aggregate and granite base coarse aggregate were selected as per IS 383-1970. Basically, the aggregates used were assumed to be of non-reactive type. All the quantities were weight batched, and thoroughly mixed manually. Cubes were compacted using a table vibrator. Cubes were demoulded after 24 h and placed in water for curing. After 28 days of water-curing, the cubes were placed in polythene drums of 90-1. capacity having sulphuric acid at 0.1, 1 and 5% concentration. Cubes were staggered in the drums to maximize the exposed surface. The strength of acid was measured regularly and the depleted acid was replenished to maintain the concentrations at the same level throughout the investigation. In volumetric analysis, potassium hydroxide, a strong base, was used as a titrating medium to measure the strength of the acid. The strength of the potassium hydroxide was determined using standard oxalic acid before determining the strength of the acid. In order to determine the strength of both acid and base, phenolphthalein was used as an indicator. This changes from colourless in acid medium to pink in alkaline medium. The strength of base used was of the same order as that of the acid, In determining the strength of acid, three samples were analysed and a mean value was taken. If Co is the initial concentration and Ct is the concentration after time t, and V0 is the volume of acid required initially for concentration Co, then the amount of acid depleted will be (Co - Ct) Vo/Co. This quantity of acid is to be added to maintain the strength. The strength and weight of the samples were measured at specified intervals. At ! and 5% concentrations the surface was irregular because it has been dissolved, so capping of the cubes was

CONCRETE is susceptible to strong mineral acids such as sulphuric and hydrochloric acids. Unlike other acids, sulphuric acid attacks concrete by causing both dissolving and swelling. DePuy [1] has investigated the relative efficiency of polymer impregnated concrete compared with ordinary concrete in a sulphuric acid environment. DePuy [1] and Pietrzykowsky [2] have reported loss of weight of concrete when exposed to 5 and 15% sulphuric acid and 15% hydrochloric acid. Toshio and Yoshiko [3] have investigated the resistance of autoclaved concrete and polymer impregnated autoclaved concrete to sulphuric acid. Hughes and Guest [4] have investigated the relative rate of damage of limestone aggregate and siliceous gravelstone aggregate concretes exposed to sulphuric acid at concentration 0.00164).02%. Some authors have suggested estimating the rate of deterioration of concrete with time in different levels of exposure. According to Browne and Baker I-5] the design life should be less than the sum of the time taken for penetration of marine environment to a critical point in concrete and the time taken for a significant change in material or structure. Glenn William DePuy [1] has suggested equations to estimate the deterioration rate, deterioration index and performance index, which include the loss of weight, compressive strength, pulse velocity and dynamic modulus of elasticity, with and without exposure to sulphuric acid, for both ordinary and polymer impregnated concretes. After investigating various structures in a number of ports, Seki 1-6] suggested a generalized equation for calculating the deterioration factor. This equation includes a constant which depends on quantity and quality of materials of concrete, age of concrete, cement content and water-cement ratio.

Table 1. Chemical composition of cement SiO 2 CaO Fe2Oa AlzO3 MgO MnO SO 3 Loss on ignition

EXPERIMENTAL DETAILS The chemical composition of the cement under investigation is given in Table 1. Throughout the *Department of Civil Engineering, Andhra University, Waltair, India. t Department of Civil Engineering, LI.T., Kanpur, India. 75

27.24 53.30 4.30 6.80 0.95 0.40 0.13 6.50

76

P, 5'. N.

Raju and P. Dayaratnam

I~ig. 1. Photograph of concrete surface exposed to 0. I'>,>.

done to make the contact surface plain. The effect of continuous water-curing and continuous air-drying inside the laboratory, after 28 days of water-curing, were also studied. RESULTS A N D D I S C U S S I O N S While titrating to determine the strength of the acid, the yellow colour appeared prior to the pink which indicated the formation of sulphates during the sulphuric acid attack. The sulphate formation on the surface (needle shaped crystals) of concrete exposed to 0.1 ~0 sulphuric acid for 250 days, is shown in Fig. 1. In the case of exposure to 1 and 5~o sulphuric acid, the dissolving attack was so rapid that the surface dissolved quickly and no fins were observed on the surface. However, similar colour formation during titration was usually observed. The consumption of acid per unit surface area of exposed concrete can be expressed as :

react with 0.1 kg is ,4 = 0.1 x 434 x 0.72 = 31.2 l/m 2 (A=301/m 2

and

q=434kg/ma).

B and c are determined using the experimental values and method of least squares fit for a given value of A. If V is the a m o u n t of acid consumed, then the error at any point (#/)is '1 = ( V,- V',)

q2 = ~ [V~--A + A

exp (--B~)] 2

(2)

i where ti = ith time in years, and vi = acid consumed up to the qth year. Equation 2 is differentiated with respect to B and c and equated to zero to obtain B and c for a m i n i m u m least squares error. ~2 ?B - ~ [ V~- A + A exp ( -

Bt~)]

i

V ' = All--exp(--BtC)]

(1)

where V ' = acid consumed (l/m 2 of surface area of concrete), t = time in years, and A, B and c are constants. From equation (1), V' = 0 at t = 0, and for very large values oft, V' approximately equals A. Therefore, the value of A is calculated approximately, assuming that the member has failed if the concrete is fully reacted up to a depth of 100 mm. In the calculation of A, the type of acid is essential rather than the concentration of acid to which the concrete is exposed. Because the assumption is based on the saturated reaction of a given quantity of cement where the stoichiometric equations of a given acid are involved, the quality and quantity of cement is reflected in the calculation of A. The total quantity of cement in 1 m 2 of surface area and 0.1 m depth is 0.1 q kg, where q is the quantity of cement in kg/m 3. Sulphuric acid of volume 0.72 1. [-7] is required for saturated reaction with 1 kg of cement. Therefore, the total quantity of sulphuric acid required to

xexp

(-Bt~.)t~ =

0

(3)

x exp (-Bt~)~ In (t,) = 0.

(4)

}~q2 = 5" [VI-A+A exp (--B~)]

&

"7

Simplification of the above two equations gives

AIZt ,

exp ( - B t , ) - ~ e x~p ~

(-2Bt,)]

-~

Viiexp (-Bt~)t~ = 0

(5)

i

x exp ( -

2Bt~) In

( t i ) ] - - ; Vit~exp (-- B~) In (tD

O.

/

(6)

Concrete Exposed to Dilute Sulphuric Acid

77

Table 2. Values of A, B, c and ~h Concentration

A = 30.0

A = 30.0 and c = 0.5

(%)

S

c

~

B

~

0.1 1.0 5.0

0.01875 0.04604 0.10042

0.49839 0.54004 0.3380

0.0035 0.0015 0.0207

0.01882 0.04293 0.13530

0.0035 0.0035 0.3906

The two non-linear equations (5) and (6) were solved using an iterative technique. Regula falsi [8"1 method was used wherever a non-linear equation with a single variable was to be solved. For simplicity of illustration let c = 0.5. The above equations reduce to a single non-linear equation with B as an unknown :

.o

2'0

0

1'5 . . . . . .

X ..... X

C

0'5

~

(7)

~'o

z'o

3~

6'0

~'o

days. There has been an increase in strength in the case of air dried and water saturated (0.0~o exposure) concretes. Considerable loss of strength is observed in 1 and 5 ~ acid exposures. But in the case of 0.1~ there is an increase in strength compared with the strength at 28 days. However, its strength is lower with respect to the water-cured or airdried concrete. If the strength of concrete exposed to acid is compared with the strength of air-dried or water-cured concrete, consistent deterioration can be observed irrespective of the concentration of acid (Table 3). The loss or gain of strength of concrete with the amount of acid consumed is shown in Fig. 4. This can be represented by a second order polynomial as shown by equation (8):

Lo

rl =f~o = l'14--0"5741V+0'l163V2

....-o

oi...~o

E 1.o

8o

I

8

~o

Fig. 3. Variation of strength of concrete (normalized with strength at 28 days) with age at different exposures.

Z'0

4

£

Time in w e e k s

}5

1-5

1.0%

"~" 5.0%

i

where T~ = sq. rt t~. The above equation was solved using Regula falsi method to get B. The constants A, B and c and the cumulative least squares error (th) for each concentration are shown in Table 2. The comparison of ~ from Table 2 reveals that, if c is taken to be 0.5 to simplify equation (1) and to obtain the solution more quickly, the error involved differs more significantly in higher concentrations (20 times at 5~) than in lower concentrations (only twice at 1% and no change at 0.1%). But for the purpose of illustration, c is taken to be 0.5. Figure 2 shows the variation of acid consumed (1/m2) with time for different concentrations. It can be observed from the least squares fit values that the error at any point in early age, up to about 2 weeks (excluding the initial age ot"28 days of water-curing) is more and after that the error is of the order 0-9~, in the case of 0.1~ exposure. But at the other two concentrations the error ranges from 0 to 4.5 ~ irrespective of age. Strength of concrete at various ages under different exposures are shown in Fig. 3. The strengths at all ages were normalized with the cube strength of concrete at 28

0"I %

:~" 1.0 ='~

V~T~exp (-BT~) = 0

Air dried

/

x

-~

rec~ c~ ~ ~

12

16

20

T i m e in w e e k s Fig. 2. V a r i a t i o n o f acid c o n s u m e d w i t h t i m e at 0.], 1 a n d 5% concentration.

(8)

7~

P. S. N. Raju and P. Dayaratnam '~

1.2

.o

1.1

I£

o

o

1.0

0'9

c

"oC

:g

x ; 0.7 ~

}

L

%

>.

~o.~

~

I

I

0.2

o-6 Acid

titres/m

0.2

I

lJ.6

1~-o

consumed

0.5

I'.B z.o

i

I

I

I

1

0"6

1-0

1.4

1.8

2-0

Acid

2

consumed

litres/m 2

Fig. 4. Variation of loss or gain of strength with the amount of acid consumed.

Fig. 6. Variation of loss or gain of weight with the amount of acid consumed.

where

air drying. The loss or gain of weight normalized with airdried concrete is shown in Table 4. For ordinary concrete, a loss of weight of 33% in 30 weeks at 5% exposure and 35% in 7 weeks, at 15% exposure to sulphuric acid was reported by Glenn William DePuy [1]. The loss or gain of weight with the amount of acid consumed can be expressed as a second order polynomial:

r r = strength reduction factor, fee = strength of exposed concrete, and fco = 28 days cube strength of concrete. The percentage of error at all points is in the range 1.5-15% except at one point, where it is 31%. Figure 5 shows the loss or gain of weight of the exposed concrete with time at different exposures, and Fig. 6 shows the loss or gain of weight with the amount of acid consumed. The weight at all ages was normalized with the weight ofconcrete at 28 days. The increase in weight is 3.5% in the case of the 0.1% exposure. Only a marginal change in the weight of water-stored concrete was observed. A loss of weight of about 2% in air-dried concrete could be due to evaporation of moisture from concrete due to continuous

Wee r w --

(%)

4

0.0t 0.1 1.0 5.0

1.050 0.921 0.888 0.674

12

Age in weeks* 24 36

1.224 1.217 0.863 0.767 0.724 0.765 0.592 0.463 0 . 6 0 7 0.167

1 10%. The above observations can be used to estimate different aspects like depth of penetration of acid, strength and weight reduction factors. Let VR be the volume of acid required for saturated reaction with 1 kg of cement [7]. If V is the acid that was absorbed, then the cement affected is equal to V/VR. Assuming a linear rate of reaction taking place along the depth of penetration, with fully saturated reaction at the exposed surface and zero reaction at the depth (tp), the depth of penetration of acid is

52 0.579 0.343

t,, = 2 0 0 0

1.1

Water

cured

o

Air dried

o.~ u

V/q V.

\o

0.7

"~'x"~x~ o

1;

(1 O)

where q = cement content in concrete kg/m 3. This can be used for the provision of minimum cover thickness in reinforced concrete.

* No. of weeks exposed to acid. t Concrete stored in potable water.

.£

(9)

where rw = weight reduction factor, wee = weight of exposed concrete, and Woo = cube weight of concrete at 28 days. The percentage of error at all points is of the order of

Table 3. Cube strength of acid-exposed concrete with respect to that of air-dried concrete Concentration of acid

-- 1.011 + 0.08108 V-- 0.128 V 2

Wco

x~.

z'o

;o Time

io

~o

6'o

1"0%

7;

in w e e k £

Fig. 5. Variation of weight of concrete (normalized with concrete weight at 28 days) with age at different exposures.

Concrete Exposed to Dilute Sulphuric Acid Table 4. Weight of the acid-exposed concrete with respect to that of air-dried concrete Concentration of acid (%)

4

0.0f 0.1 1 5

1.000 1.015 1.020 0.865

12

Age in weeks* 36 24

52

1.005 1.025 1 . 0 2 5 1 . 0 4 6 1.061 1.062 0 . 9 8 5 0 . 9 2 7 0 . 8 6 7 0.773 0.690 0.538 -

79

Similarly strength reduction factor and weight reduction factor can also be calculated and can be made use of in the design. (c) r I = 1.14-0.5741V+0.1163V 2 = 1.14-0.5741"2.422 + 0.1163"2.422 = 0.432. (d) rw = 1.011 +0.08108V-0.128V 2

* No. of weeks exposed to acid. f Concrete stored in potable water.

= 1.011 +0.08108*2.422 -0.128"2.422"2.422 = 0.457.

Example A concrete with A/C ratio 4.5 and W / C ratio 0.45 is exposed to 0.1% sulphuric acid. Let 1 kg of cement require 0.7 1 of sulphuric acid for saturated reaction and let the cement content in the concrete be 434 kg/m 3. Find (a) the depth of penetration of acid in 20 years, (b) the quantity of cement required in 1 m 3 if the depth of penetration of acid is limited to only 25 m m in 20 years, (c) the strength reduction factor, and (d) the weight reduction factor. (a) V = 30.01-1-exp (-0.01882* x sq. rt(20))] = 2.422 1/m 2

tp =

2000V/qVR = 2000 x 2.422/0.70 x 434 = 15.9 mm.

(b) q = 2000V/tpVa

= 2000 x 2.422/25

x 0.70 = 277 kg/m 3.

The initial strength should be 1/0.432 = 2.315 times the actual design strength for the structure to survive for the above specifications.

CONCLUSIONS The experimental results on the reduction of strength and weight of concrete when exposed to sulphuric acid at different concentrations has enabled to develop an expression to estimate the reduction factors. The expressions developed to estimate the depth of penetration of acid will enable the designers to provide an appropriate cover to the reinforcement. In most of the exposed concrete, the damage starts at the surface and the life of the structure depends on the extent of surface damage and the depth of the damaged surface. The life of the structure can also be predicted by the expressions developed in this paper.

REFERENCES 1. G.W. DePuy, Durability of Concrete, pp. 233-257, SP-47, ACI, Detroit (1975). 2. J. Pietrzykowsky, Polymer-Concrete Composites, p. 4. IABSE Proceedings p-38/81. 3. F. Toshio and O. Yoshiko, Chemical resistance of autoclaved concrete with and without polymer impregnation, J. Soc. Mat. Sci. Japan 26, 1117-1123 (1977). 4. B.P. Hughes and J. E. Guest, Limestone and siliceous aggregate concretes subjected to sulphuric acid attack. Mag. Concr. Res. 30, 11-18 (1978). 5. R.D. Browne and A. F. Baker, Developments in Concrete Technolooy--I (ed. F. D. Lydon), pp. 111-149. Applied Science, Barking (1979). 6. H. Seki, Durability of Concrete, p. 301. SP-47, ACI, Detroit (1975). 7. P.S.N. Raju and P. Dayaratnam, Durability considerations of concrete in marine environment. Paper presented at workshop on hydrodynamic and geotechnical aspects of structures on marine deposits, held at Waltair, India, 29-30 January (1982). 8. F.B. Hilderbrand, Introduction to Numerical Analysis, p. 572. Tata McGraw-Hill, New Delhi (1979).