CEMENT and CONCRETE RESEARCH. Vol. 12, pp. 87-92, 1982. Printed in the USA. 0008-8846,/82/010087-06503.00/0 Copyright (c) 1982 Pergamon Press, Ltd.

DAMAGE OF CONCRETE SLEEPERS BY CALCIUM CHLORIDE

L. Berntsson and S. Chandra Division of Building Materials

Chalmers University of Technology S-412 96 GSteborg, Sweden

(Refereed) (Received Sept:. 14, 1981)

ABSTRACT

High strength concrete sleepers damaged by calcium chloride have been studied here. Chloride content is estimated at different depths qualitatively by the classical wet method and quantita- tively by ion selective electrodes. The samples were analyzed by x-ray diffraction spectroscopy and were studied under micro- scope. The damage of concrete is attributed to be a combined effect of chemical, mechanical and physical processes. Calcium chloride is found to be very detrimental to concrete. Special care should be taken for its use.

En undersSkning har utfSrts med avsikt att fSrklara anmgrk- ningsv~rt allvarliga skador p~ sliprar av hSgh~llfast betong. Genom anv~ndning av olika analysmetoder, s~som v~tkemisk ana- lys, jonselektiva elektroder och rSntgendiffraktometri kunde bl a konstateras att kalciumklorid nyttjats som tSsalt. Skade- mekanismen utgSres sannolikt av en kombination av kemisk, fysi- kalisk och mekanisk p~verkan. Kalciumklorid i koncentrerade 18sningar har konstaterats ge upphov till kemisk nedbrytning av betong under kort rid, varfSr sErskild f~rsiktighet ~ir att rekommendera vid anv~ndning av kalciumklorid p& oskyddad betong.

Introduction

Sleepers of high strength prestressed concrete used for crane tracks have shown severe damage within a couple of years service. The type of damage under this work was studied mainly from the practical point of view considering physical, mechanical and chemical effects on concrete material.

Much of the research work on the influence of salts on the durability of concrete has been done in the laboratory scale. It is a common trend to use deicing salts for melting ice and snow on roads and streets during wintertime.

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88 V o l . ] 2 , N,,. L. Berntsson, S. Chandra

The most used deicing salt in Sweden is rock salt because of its low price. This salt contains mainly sodium chloride. Calcium chloride has some advan-

tages in comparison with the other deicing salts, such as efficiency at extremely low temperature and creation of high icemelting rate. The most common use of calcium chloride is as a dust binder on roads because of its hygroscopic character. Other use is as an accelerating admixture in con- crete (I). Normally the calcium chloride used is of technical quality and contains about 90% calcium chloride.

Materials and methods

The composition of concrete for sleepers and its compressive strength are shown in table I.

Table I

Concrete Composition and Compressive Strength

Standard portland cement

Sand (granite 0-4 mm)

Gravel (granite 4-16 mm)

W/C

Compressive strength (cylinder ~ 67 mm)

Air content

465 kg/m 3

880 "

810 "

0,45

> 60 MN/m 2

1,5 %

Mean value of chloride content in concrete sleepers from the surface to 10 mm down as well as at the depth of 45-55 mm was estimated qualitatively by the classical wet chemical analysis and quantitatively by ion selective electrodes. The samples were taken out by dry boring. The material was thoroughly grounded to get homogeneity. 5 g of material was used in the ana- lysis. The concentration of free chloride ions was determined and the results are shown in table II. The samples were also analyzed by X-ray diffraction spectroscopy.

Table II

Chloride Ion Concentration as Measured by Ion Selective Electrodes

Sample C1 mol/l C1 % by weight C1 % by weight in concrete of cement

SL-I 4.90"10 -4 0.035 0.2

SL-2 1.48-10 -4 0.011 0.06

SL-I is from 1 -10 mm down the surface SL-2 is from 45-55 mm depth in concrete measured from the surface

Specimens were cut and polished for photography and for microscopic studies. The micrographs are presented in the figures I-6.

Results and discussions

The concentration of chloride ions decreases with depth into concrete. Near the surface the free chloride content was found to be 0.2% of the

Vol. 12, No. i 89 CHLORIDE, CHEMICAL CORROSION, PHYSICAL PROCESSES, MECHANICAL DAMAGE

FIG. i

Polished surface of high strength concrete

FIG. 2

SEM-photo from concrete near the surface of steel reinforcement bar

FIG. 3

Crystals inside an air- pore in the zone of corroded concrete

90 Vol. 12, No. i L. Berntsson, S. Chandra

i 14

FIG. 4

Surface on aggregate with precipitated

crystals

FIG. 5

Replica of an aggregate in the cement paste

FIG. 6

Precipitated crystals on the wall of an air- pore at about 70 mm depth from the concrete surface

Vol. 12, No. i 91 CHLORIDE, CHEMICAL CORROSION, PHYSICAL PROCESSES, MECHANICAL DAMAGE

cement weight. This agrees with limits cited by Schorr (2). He reported a limit of 0.2% CI- referred to the weight of cement for prestressed concrete and 0.4% CI- for normally reinforced concrete. These limiting values for the chloride concentration are referred to the corrosion of reinforcement and not to the damage of concrete itself.

Figure I shows the photo of a specimen of not corroded concrete in a sleeper. The airpore volume is estimated to be 1.6%. Figure 2 is a cut section photographed by SEM. This shows clearly spherical airpores of different sizes in the cement paste. Figure 3 shows well grown crystals in an airpore. The length of the crystals is 50-60 ~m and the thickness about 1.5 ~m. Crystalli- zation has concentrated between the aggregate surface and the cement paste in the neighborhood of the corrosion zone. Figure 4 shows an aggregate particle with crystals on its surface. On the top righthand side corner of the pic- ture there is a contact crack between the particle surface and the cement paste. A replica of a sand particle in the cement paste is shown in figure 5. Its contact zone is totally covered with crystallized products. Another type of crystals on the walls of airpores outside the corrosion zone is shown in figure 6. The length of the crystals is about 2-3 ~m.

The damage of concrete studied here seems to have connection with chemi- cal attack. Deformations caused by moisture movement, temperature (thermal gradient:), freezing and thawing and mechanical stresses (caused by crane movement:) are also acting upon the material in the practice. Lawrence and Vivian (13) have reported that strong calcium chloride solution has given a severe damage on cement mortar, not only during drying and wetting cycles, but also during continuous storage in strong calcium chloride solution (i.~. 30%). Chlorides are found in the structure of cement paste in concrete. A complex is formed when hydrated C3A reacts with calcium chloride. The damage because of complex salt formation seems to be due to expansion and shows similarity with sulfate attack. Precipitated salt can be seen from the micro- graphs presented. Similar results are reported by K~hl (4) and Biczok (5).

X-ray diffraction analysis from the specimen near the concrete surface has shown vaterite (a metastable form of calcium carbonate), calcium chloride hydrate and monochlorohydrate with traces of Friedels salt (C3A CaCI2 • IOH20 ) with some unidentified peaks. In the sample from 50 mm depth under the con- concrete surface more Friedels salt, monochlorohydrate, calciumhydroxide and vaterite were detected. Monochlorohydrate is present in both the cases, i.e. near the surface as well as deep into concrete. It shows its stability both in Portland cement - H20 system and in Portland cement - CaCI2-CO2-H20 system. The phenomenon is well discussed by Chatterji (6). According to Richartz (7) and the literature cited by him, Friedels salt forms on the surface of c~on- crete in the first hand, but it is not stable during carbonation and forms hydrargillite and metastable ~aterite. It seems that the presence of chloride ions influences the formation of vaterite. In some other sample there was comparatively less effect of carbonation so that a bigger amount of Friedels salt was present.

It has been pointed out that concentrated solutions of calcium chloride have shown that the severity of damage on Portland cement concrete increased with decreasing temperature. The breakdown seems to be due to some comple× formation at a temperature below room temperature, for example 5°C (6).

In combination with frost damage, the effect is more complicated with both physical and chemical phenomena involved. Salt like chloride is no~ chemically damaging the concrete under normal conditions, but can cause sew~re

damage by frost action specially at low salt concentration. Tht~ frost mechanism has been described by Lea (8), Browne & Candy (9) and Fa~er]und (I0).

92 V o l . 12 , No. 1 L. Berntsson, S. Chandra

Conclusions

High strength concrete with compressive strength of more than 50 MN/m 2 has been observed to be severely damaged in a very short time by calcium chloride.

The chloride concentration decreased with the depth of concrete. Com- plexes have been formed with calcium chloride and cement paste. The damage due to this appears to be similar as due to sulfate attack.

The damage of concrete is attributed to be a combined effect of chemical reaction between calcium chloride and cement paste due to the mechanical stress developed by load and physically due to freeze-thaw cycles.

As a practical conclusion calcium chloride is very detrimental to con- crete, especially when it is exposed to the atmosphere all the time. Special care should be taken for its use.

Acknowledsements

The authors are grateful to B. Hedberg for his help in taking the micro- graphs, to miss B. Lendheim for preparation of the text and to A. Lampinen for reproduction of micrographs.

References

I. V.S. Ramachandran, Calcium Chloride in Concrete Science and Technology. Applied Science Publishers htd, London (1976).

2. K. Schorr, Chloridkorrosion in Stahlbeton. Betonwerk+Fertigteil-Technik, 3, pp. 150-152 (1981).

3. M. Lawrence and H.E. Vivian, The Action of Calcium Chloride on Mortar and Concrete. Australian Journal of Applied Science, II, 4 (1960).

4. H. KHhl, Zement-Chemie, Band Ill. Verlag Technik, Berlin (1952).

5. I. Bicz6k, Concrete Protection. Akad~miai Kiad6, Budapest (1972).

6. S. Chatterji, Mechanism of the CaCI 2 Attack on Portland Cement Concrete. Cement and Concrete Research, 8, pp. 461-468 (1978).

7. W. Richartz, Die Bindung von Chlorid bei der Zementerh~rtung. Zement-Kalk- Gips, 10 (1969).

8. F.M. Lea, The Chemistry of Cement and Concrete. Edward Arnold Ltd, London (1970).

9. F.P. Brown and P.D. Cady, Deicer Scaling Mechanism in Concrete. Durability of Concrete. ACI Publication SP 47-6, Detroit (1975).

10. G. Fagerlund, RILEM Recommendation: The Critical Degree of Saturation Method of Assessing the Freeze/thaw resistance of concrete. Materials and Structures, 10, 58 (1977).