Publisher’s version / Version de l'éditeur:
Roads and Engineering Construction, 98, 2, pp. 68-71, 1960-02-01
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Improved quality of concrete may be answer to winter salt damage
Legget, R. F.; Swenson, E. G.
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N A T I O N A L
R E S E A R C H C O U N C I L
C A N A D A
D I V I S I O N O F B U I L D I N G R E S E A R C H
OTTAWA
FEBRUARY, I95O//'r
lmproved Quqlity of Concrete
Mqy be Answer
to'Winter Sqlt Domqge
by
R. F. LEGGET
ond E. G. SWENSON
A N A L Y X E D
RePrinted from
Roads and Engineering Consiruction, Vol. 98, No' 2, FEBRUARY, I960. P. 58-7I
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publication is being distributed by the Division of
I Building Research of the National Research Council as a
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pAVEMENT
(R) wAS MOSTLY COVERED
WITH SNOw; (L) WAS EXPOSED
TO SALT AND HEAVY
TRAFFIC.
rrlPRoYED QUAIIIV OF CONCRETE
MAY BE A]ISWER
rO WTNTER
SA1T DATIAGE
R. F. LEGGET a n d E . G . S W E N S O N D i v i s i o n o f B u i l d i n g R e s e a r c h , Nalional Research Council, Oflawa
nlTY engineers in Canada who are fltroubled with an unusual amount of spalling of concrete pavements can blame the rapid dieselization of Cana-dian railways for some, at least, of their current worries. Until a few years ago. it was common Practice for city engineering departments in Canada to obtain supplies of cinders from local railwaY Yards for use in roughening icy sidewalks and pave-ments. The railways were glad to get rid of their cinders in this waY and cities had a cheap, relatively satisfac-tory, and simPle means of coPing with slippery road surfaces in winter.
Within the last few years, however,
the supply of railwaY cinders has been almost completely cut off as the C.N.R. and C.P.R. have raPidlY switched to the use of diesel locomo-tives. This change has occurred con-currently with a continued increase in the amount of winter driving in cities and in the total number of automobiles on citY streets, thus forcing municipal authofities to use other abrasive materials and a greatly increased quantity of salts and other chemicals for de-icing Purposes.
Motorists and Pedestrians have demanded protection against slippery icy surfaces. An adequate supply of cinders has ceased to be available,
and alternatives, have had to be found. Crushed rock, has been tried without much success. Sand and gravd are frequently in short supply and when available, their use in larger cities is expensive. City engi-neers, therefore, have turned to the use of salts such as calcium chloride and sodium chloride, or chemicals such as urea, as an economical and speedy means of coPing with the problern of winter trafficability' It does nof appear to be mere coinci-dence that this is reflected by the cor-responding rapid increase in the deterioration of many sections of the exposed surfaqes gf cgncrete
pave-ntents and sidewalks in city streets. The deterioration that iS causing such concern to many city engineers in Canada takes the form of ex-cessive scaling of the surface of con-crete slabs used as sidewalks or pave-ments. The most serious trouble has naturally made itself evident in the pavements of busy streets where the intensity of traffic has had the effect of quickly removing the products of deterioration, leaving coarse aggregate exposed with a correspondingly rough surface which is objectionable to motorists. If the concrete so exposed is not itself of good quality, the deterioration rnay continue, as in-dividual pieces of coarse aggregate are "plucked" out of their matrix by the action of traffic, leading in extrerne cases to almost complete failure of the concrete slab.
Typical of the trouble as it is most commonly experienced is the roadway shown in title photo. The con-crete pavement for both roadway lanes was placed at about the same time, with the same mix using the same materials, the same placing procedure, and the same curing process. Scaling is evident in the left-hand lane but not in the right-left-hand lane, neglecting the shielding effect of the small extension of the bituminous cover from the cross lane on which the photographer was standing. The difference between the performace of these identical concrete pavements, within two years of their having been placed, is at least partially explained by the practice of snow removal fol-lowed on this particular city street. Snow was plowed off the left-hand lane, to a small extent towards the dividing strip but mainly on to the right-hand lane, which was therefore blocked by snow for the coldest part of the winter. The photographer was standing in line with traffic lights. The location shown, therefore, is that at which cars would be starting up rapidly after having stopped at the lights. It is at such locations, .and at the corresponding stopping places before traffic lights, that scaling is usually most pronounced.
.trt requires but the simplest observa-tion of cars on winter roads to see that melted snow drips off the mud-gBafds of stationary cars at exactly suchr locations. If salts or de-icins
chemicals have been placed on ad-jacent pavements, they will be mixed with 'the melting snow and will be similarly deposited. The resulting chemical solutions will be rubbed along the roadway surface by the wheels of succeeding cars, and will naturally be absorbed into the con-crete and also forced into the slight-est cracks or irregularities that may exist ,in the pavement. In view of the high power of modern auto-mobiles, and the quick get-away habits of so many modern drivers, the resulting punishment of the surfaces of concrete pavements in winter-time is indeed severe. Scaling, and
associated deterioration. is an inevrt-able result in view of the known in-teraction of de-icing materials with concrete.
That the resulting troubles are not peculiar to Canada is shown by the published records of research work on this problem that has been con-ducted by well-known concrete re-search laboratories in other countries. In Sweden, for example, the problem has been recognized as so serious that the use of de-icing salts has been prohibited for some years. Study of the problems created by the use of de-icing materials has been carried out at the Swedish Cement and
crete Research Institute in Stockholm. A report on this work, Deterioration of Concrete Pavements Due to Salt-ing in Winter Time, by S. G. Bergstrom, Swedish Cement and Concrete Research Institute, Stock-holm; applied studies No. 3, 1959, 43 p. (in Swedish: summary in Eng-lish), has recently been published.
In Nortb America, intensive work on the same problems has been in progress at the well-known research laboratories of the Portland Cement Association at Skokie, near Chicago. In a notable pairer to the Highway Research Board at its 35th annual meeting, G. Verbeck and P. Klieger presented a report on their investiga-tion of the acinvestiga-tion of de-icing materials upon concrete. This was not in any sense a final report but a progress statement which showed conclusively that all the commonly used de-icing materials (such as those already mentioned) are capable of producing serious deterioration under laboratory freezing and thawing conditions. Deterioration was not. however. produced in any sound concrete in which proper air entrainment was used. Work at the P.C.A. laboratories is continuing. Investigation of the problem, using Canadian materials, has been started in a modest way, because of staff limitations, and of the claims of other urgent concrete research work, by the Division of Building Research of the National Research Council at Ottawa. Other laboratories, in North America and elsewhere, are undoubtedly studying the problem since it is so widespread and serious.
The evidence that the use of salts is closely allied with current difficul-ties is too strong to be ignored. Difficulties have increased concur-rently with the greatly increased use of salts. It has been shown in the laboratory that salts can produce serious scaling. The field failures in the main tend to follow closely the use of salt but there are some anomalies in the pattern, such as some observed failures with concrete in which air-entrainment has been used. None of this negative evidence contradicts the main conclusion that the use of salts is involved, but it does suggest that other factors may be involved in a minor way. There is,
for example, some evidence to sug-gest that concrete which is not per-forming well despite the use of air-entrainment may not have the proper distribution of air bubbles essential for good performance. Clearly, further study of the problem is re-quired. In the meantime, however, city engineers have to grapple with the problem as it exists and with the demand of the motoring public for effective de-icing of city streets.
One aspect of the problem, in so far as the concrete is concerned, could easily be solved by the Swedish pro-cedure of prohibiting the use of all de-icing chemicals. The dernands of the motoring public, however, must also be met. Some de-icing materials probably must be used, especially on hills and at intersections, but it can be suggested that their use elsewhere should be reduced to the absolute minimum that is practicable. There are many motorists who would welt come any diminution in the use of chemicals for they know, all too well, the serious effect of such salts upon the rate of corrosion of automobile bodies. Until non-corrosive auto-mobile bodies come into general use, reduction of corrosion provides an-other sound reason for keeping the use of de-icing salts to a minimum.
Existing concrete pavements de-mand the best of maintenance and early and competent repairs of even the slightest irregularities that develop, especially nehr intersections, in order to inhibit the start of deep scaling during winter periods. Much can be done, however, to minimize future trouble when new concrete pavements have to be placed, or when sections of existing pavements have to be renewed. There follow suggestions with regard to such new concrete work, but these will be better aP-preciated against a general back-ground of understanding of the basic problem that is involved.
The exact mechanism by which de-icing salts produce scaling of con-crete pavements and sidewalks is not known. Some authorities consider that it is a physical rather than a chemical reaction. This oPinion is based on the fact that the same result is produced by chemicals which differ widely in properties, e.9., calcium chloride and urea. The destructive
action of these de-icing materials oc-curs in conjunction with freezing and thawing of the concrete. In general, the factors that promote resistance to freeze - thaw deterioration also promote resistance to de-icer scaling. It is therefore probable that the role of the salt is to "trigger" or ac-centuate the destructive action of fre€ze-thaw cycling.
The mechanism involved in the deterioration of concrete due to freez-ing and thawfreez-ing has been postulated by T. C. Powers and others. Part of the water in the cement-paste voids freezer. The increase in volume results in a "hydraulic pressure" which can become sufficient to break down the concrete. The role played by any intentionally entrained air is essentially to reduce the distances be-tween voids and thus to reduce the pressures that tend to build up when ice forms. It is evident that any air entrained should be in the form of properly spaced bubbles of the right size. It is also evident that the per-meability of the concrete should be kept to a minimum. This is particu-larly important near the surface; it is here that the composition of the concrete can readily be affected ad-versely by excessive vibration or by over - trowelling. Although cement paste may be made relatively immune to frost action by air-entrainment, the aggregate itself must also be resistant if the concrete is to be durable. This can be assured by proper tests.
Resistance of concrete to de-icing "salt" action can be greatly increased by attention to the following points. It must be kept in mind that disregard of only one of these factors may nullify the benefits to be expected from others:
1. The aggregates and cement must be of good quality. They should meet standard specifications for such materials. Aggregates with good paformance records are to be preferred.
2. A good mix design should be developed for the particular ag-gregates and cement to be used. This is important in order to in-sure a dense, impermeable con-crete, rigid control of proPor-tioning is necessarY.
3. Water-cement ratios should be kept to a minimum, consistent
with good workability; rigid con-. trol is particularly important here. An increase in' water-cement ratios will reduce resistance to frost action and salt action. owing largely to the increase in capillary. voids. The tendency to bleeding and formation of laitance material caused by ex-cessive water will promote scaling.
4. Air-entrainment is absolutely nec-essary. The air-entraining agent selected should be one generally accepted as having a good per-formance record. The amount of air may vary from 4 per cent to 7 per cent in the concrete and this air must be properly dis-tributed throughout the concrete as evenly spaced bubbles. Air contents should be checked at the job and any concrete that does not meet the ai{ requirements should be rejected.
5. Any admixture other than a plain air-entraining agent is preferably to be avoided unless its per-formance record has proved it to be advantageous. It should be em-phasized that any extra ingredient, although it may be potentially beneficial, adds to the difficulty of mix design and control. 6. Proper compaction is most
im-portant in order. to obtain a dense impermeable concrete. If a surface vibrator is used, it is best to check the effectiveness of compaction by cutting out sec-tions of the hardened concrete and examining it for voids. In-spection and control is here ex-tremely important. Excessive vibration or tamping may produce a surface susceptible to scaling. 7. Proper surface finishing is
re-quired. A minimum of surface working is important since ex-cessive trowelling brings up water and fine material, leaving a weakened surface very suscep-tible to frost and salt action. Screeding and light wood trow-elling (or burlapping) is best. Steel trowelling should be avoid-ed. Rigid inspection is important for this phase of the work. 8. Proper curing is important in
order to produce a dense, im-permeable concrete. This means
IN AIR-ENTRAINED
CONCRETE
AGGREGATE
IS WELL COATED.
a continuous moisture supply and moderate temperatures for at least one week after placing. 9. Following the curing period, the
concrete should be permitted to dry out as much as possible before it is exposed to frost and salt action. This has been shown to have a definite influence on the resistance of concrete to salt at-tack.
10. Concrete pavements or sidewalks should not be placed after about the end of September in most parts of Canada. Concrete that has not matured properly before being subjected to frost and salt-ing will not have good resistance. The placing of concrete late in the fall will not normally permit the required drying period. I 1. Design should be such that good
drainage is obtainecl, Concrete
continuously immersed or part-ly immersed in water will become more or less saturated and have poor resistance. Concrete pave-ments and sidewalks should pre-ferably be placed on a crushed rock base to avoid wicking up of soil water.
12. Careful records should be kept of all major concrete pavement and sidewalk jobs in order to gain information for future work. Advance in the further develop-ment of sound concrete pavedevelop-ment practice in Canada, in the face of the extreme winter conditions to which such pavements are subjected in this country, can only come with the accumulation of such carefully documehted experience, coupled with the application of the results of laboratory and field research work as they become available.
