NOVEMBER, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
Literature Cited (1) Andre and Francois, Compt. rend., 185,387 (1927). (2) Boeseken and Belifante, Rec. trav. chim., 45,914 (1926). (3) Bijeseken and Elsen, Ibid., 48,363 (1929). (4) Bull, H. B., “Chemistry of the Lipids”, 1938; Lea, C. H., Dept. Sci. Ind. Research (Brit.), Special Rept. 46 (1938). (5) Deatherage and Olcott, J. Am. Chem. Soc., 61, 630 (1939). (6) Ellis, Biochem. J., 26, 791 (1936). (7) Franke and Jerchel, Ann., 533,46 (1937). (8) French, Olcott, and Mattill, IKD.ENQ.CHEM.,27, 724 (1935). (9) Hamilton and Olcott, Ibid., 29, 217 (1937). (10) Hilditch, J . Chem. SOC.,1926, 1828. (11) Org. Syntheses, 15,51 (1935).
1431
(12) Pigulevskii and Petrov, J . Russ. Phys. Chem. SOC., 58, 1062 (1926). (13) Powick, J. Agr. Research, 26, 323 (1923). (14) Raymond, J. chim. phys., 28, 480 (1931). (15) Rosohen and Newton, Oil & Soap, 11, 226 (1934). (16) Scala, Staz. sper. agrar. ital., 30, 613 (1897). (17) Skellon, J. SOC.Chem. Znd., 50, 131T (1931). (18) Ibid., 50,382T (1931). (19) West, Hoagland, and Curtis, J . BioZ. Chem., 104,627 (1934). (20) Yasuda, Zbid., 94,401 (1931-32).
FROMa portion of the thesis presented by F. E. Deatherage to the faaulty of the Graduate College of the State University of Iowa in partial fulfillment of the requirements for the degree of doctor of philosophy.
Pentachlorophenol for Wood Preservation
T. S . CARSWELL AND IRA HATFIELD Monsanto Chemical Company, St. Louis, Mo.
Pentachlorophenol, which is now available commercially, has been intensively studied and found to be a valuable woodpreserving chemical. Its physical and chemical characteristics make it especially suited for this purpose, and its fungicidal potency gives it a high rank as a toxic against wood-rotting and wood-staining organisms. It has also been demonstrated to be effective in preventing termite attack, and its use for preventing marine borer and powder post beetle attack is strongly indicated. Because of its lack of color and objectionable odor, it is useful where a “clean” treatment for wood and fibers is desired. The fact that it can be formulated so that treatment with the material does not impare puttyability, paintability, and further finishing is of utmost importance in connection with its use as a wood preservative.
LTHOUGH the use pf chemicals for the preservation of
\A
wood and wood products is not new, modern conditions of use have created a wider demand for preservatives as well as a need for improved properties over those possessed by chemical treatments formerly available. Marked differences exist between virgin growth wood formerly used in construction and in second growth wood more commonly used today. Greater demands are continually being placed on wood by changes in architectural design, by the trend especially in the North toward heated and finished basements, and by the advent of modern air conditioning. All of these changes create a more favorable atmosphere for decay or termites. Much of the future for wood in certain industrial applications is dependent upon the development of effective and permanent “clean” treatments which, while protecting the wood against degradation, will not alter its characteristic
feel or appearance nor interfere with subsequent fabrication or application of surface coatings. Failure on the part of wood producers and fabricators to recognize the value of proper preservative treatments can result only in the ultimate displacement of wood in favor of substitutes of greater permanency, even though such substitution may involve the sacrifice of the esthetic value man has always placed upon wood. Obviously creosote and many of the older treating materials cannot meet the requirements imposed by these new demands. This does not mean that creosote will not continue to be a favorite for railway ties, for piling, and for other such uses. B u t for millwork and much of our structural building materials, most of the older treatments are inadequate either because of their color, odor, or lack of permanence, or because of adverse effects upon the appearance of the treated products. Hence, a definite demand has developed within the woodusing industries for a satisfactory toxic chemical which will enable them to supply the public with properly preserved wood products. The possible value of chlorinated phenolic compounds as wood preservatives has been suggested many times over a number of years. Several factors have prevented their extensive use. I n general, the lower chlorinated phenols possess objectionable odors and are not sufficiently stable. The higher members of the series have not been commercially available until recently. Only since 1936 has pentachlorophenol been studied with sufficient intensity to permit its true usefulness as a wood preservative to be evaluated. In 1932 Iwanowski and Turski (IS)patented “ olychloro derivatives of phenol” as wood preservatives, but &e, limited their claims to “chlorinated phenol compounds containing a t least two and not more than three chlorine atoms”. Because of the high vapor pressure, high water solubility, objectionable odor, and comparatively low degree of toxicity to wood-rotting and staining fungi of some of the members, the lower chlorinated phenols are not generally satisfactory for use in wood preservation. Although pentachlorophenol was prepared as early as 1841, it was not produced commercially until 1936. In the intervening years, little actual experimental work was done upon it. In 1914 Aylsworth ( I ) patented it and other chlorinated benzene derivatives as flame-extinguishing materials. In his patent specification he said: “Fabrics or materials coated or impregnated with the improved composition in addition to having flame-extinguishing properties may likewise be given waterrepellent qualities and rendered aseptic against the ravaging of
1432
INDUSTRIAL AND ENGINEERING CHEMISTRY
VOL. 31, NO. 11
volatility of a substance. At 20" C. pentaTABLE I. SOLUBILITY OF PURE PENTACHLOROPHENOL IN ORGANIC SOLVENTS chlorophenol has a vapor pressure of 0.00017 Solubility, Grams/100 Grams Soln. mm. 02 mercury (&), which is, for instance, less Solvent O o C, 10' C. 20' C. 30' C. 40" C. 50° C. 60' C. than one twentieth of the vapor pressure of 72.0 75.5 77.5 Methanol 40.5 48.0 57.0 65.5 &naphthol a t the same temperature. Labora67.0 60.0 63.5 Anh drous ethanol 46.0 49.5 53.0 56.5 45.8 52.9 60.3 DietKyl ether (U. 9. P.) tory tests have indicated that pentachlorophenol 65:O 62:O 64:O But Butyl1ether of diethylene glycol 34:5 39.5 48.0 57.0 65.5 39.0 43.0 47.5 52.0 56.5 61.0 9 5 7ethanol 95% in wood possesses a high degree of resistance 46.6 43.0 24.5 28.0 32.0 35.5 39.0 Pine oil to leaching and volatilization, which bears out Ethyl ether of diethylene glycol .. . . 30.0 40.0 51.0 62.0 .. 27.5 52.5 62.0 64.5 .. Diethylene glycol the information obtained on water solubility and 27.0 37.5 50.0 65.0 Ethvl ether of ethvlene 1i:o .. 8 :0 - -elvcol Aoeione .. 13.5 21.5 33.4 .. vapor pressure. Linseed oil (raw) ,. 10.2 13.9 15.4 18:O 2012 The solubility of pentachlorophenol in a numDioxane .. 5.5 11.5 16.0 22.0 30.0 37:5 Tung oil ,. 3.5 1 1 2 13.5 15.7 17.8 ber of common organic solvents is given in Benzene .. 11 0 14.0 19.5 24.5 3i:5 o-Dichlorobenzene (technical) 5.5 6 : 5 8.5 11.5 15.5 20.5 26.0 Table I. These data are particularly pertinent Di entene ,. 7.1 8.4 10.3 since there are many commercial applications Etgylene glycol .. 6.0 11.5 zi:o 3i:5 38:5 Engine distillate" .. 2:4 3.6 5.0 .. .. .. where the water-soluble sodium salt cannot be Trichlorobenzene 9.5 Diesel oilb i:9 2:4 3:i 4.0 517 6:2 ii:3 used, and the free phenol in a suitable solvent Carbon disulfide .. 1.7 3.0 4.3 .. is required. When a water-soluble form is deTur entine ,. 1.6 3.0 4.4 6..3 8.'4 .. FueB oil6 .. 1.8 2.6 3.7 5.4 7.8 .. sired, pentachlorophenol is applied as the sodium Carbon tetrachloride .. 0.8 2.0 3.1 Stoddard solventd .. 0.5 1.5 2.5 3:5 5:5 +:5 salt, the solubility of which is about 26 grams per Petroleum ether (86-100" C.) .. 0.2 1.O 1.6 .. .. .. 100 grams of solution a t 25" C. 5
Obtained from the Standard Oil Company (Indiana).
b Obtained from Magnolia Petroleum Company. Known as No. 2 fuel oil obtained from Shell Petroleum Corporation.
e
d
A grade of petroleum sdlvent described in Bureau of Standards booklet CS3-28.
insects, vegetable growths and the like, because of the valuable qualities of the impregnating substance." In 1935 Hatfield (7) reported on the toxicity of a number of chlorinated phenols. He stated that neither pentachlorophenol nor its sodium salt, as then available, were sufficiently soluble in agar to permit a satisfactory evaluation of their toxic action against wood-inhabiting fungi. Hence; the chemical did not at that time receive a high rating as a fungicide. Since 1935, however, further fungicidal tests have been made by Hatfield and at several other laboratorie3, and the data obtained have shown pentachlorophenol to be one of the best of the chlorinated phenols from the standpoint of fungicidal activity. An intensive study of its physical and chemical properties has also indicated its value as a wood preservative. Bateman and Baechler (3) stated in 1937, that "of all the compounds that were found to kill our test organism, tetrachlorophenol and entachlorophenol are the cheapest sources of toxic action", d e mass of data assembled since their work, however, indicates that the physical and chemical characteristics of pentachlorophenol recommend it for the field of wood preservation. While much of the research data on pentachlorophenol is still unpublished, considerable information has been released. Hubert (10, l l , l a ) published information on the formulation of some of the newer organic fungicides into colorless oil treatments for millwork, and pentachlorophenol is considered t o be the most suitable for this purpose. I n 1938 a bulletin was issued (16)to outline the use of sodium pentachloro henate for the control of sap stain in unseasoned lumber. AltPhough applied as the sodium salt in this case, the salt is soon converted to the free phenolic form and thus in reality affords a treatment with pentachlorophenol. Carswell and Nason (6) in 1938 described in considerable detail the chemical, physical, and toxicological properties of pentachlorophenol and outlined its principal uses. Hubert's 1938 publication (10) also presents physical, chemical, and biological data on pentachlorophenol. Probably no new wood-preserving chemical has been offered t o the trade with so many physical, chemical, analytical, biological, and toxicological data available in the early stages of its development as has accompanied the introduction of pentachlorophenol. In order to make these data and research results available to those interested in wood and wood products preservation, the salient points are summarized in this paper.
Chemical and Physical Properties The solubility of pentachlorophenol in water is very slight, ranging from 5 parts per million a t 0" C. to 35 a t 50" C. ( 5 ) . This extraordinarily low solubility in water tends toward permanence of the chemical in treated wood and resistance t o loss by leaching. The volatility of pentachlorophenol is also remarkably low. Vapor pressure data are the best indication of the
Physiological Properties
Pentachlorophenol has been shown t o possess a high degree of toxicity t o fungi, bacteria, yeasts, algae, protozoa, and other microorganisms ( 5 ) . Data on its toxicity toward a number of fungi, as determined by the Petri dish-agar block method, are given in Table 11. For comparison, analogous data on P-naphthol are included. The value of pentachlorophenol as a fungicide has also been tested and compared with other chemicals by using essentially the wood block method as advocated by the National Door Manufacturers Association (17). This method was adapted by the authors for testing pure chemicals a t varying concentrations in benzene when the solutions were injected into ponderosa pine sapwood and exposed t o fungus mats. Visual results of wood block tests of pentachlorophenol on mats of Lenzites tra6ea are given in Figure 1. Comparable visual information on P-naphthol against the same organism is shown in Figure 2. Absorption and loss in weight figures for not only Lenzites trabea but also for Madi-
TABLE 11. TOXICITY OF PENTACHLOROPHENOL AND &NAPHTHO TO FUNQI -----yo
of Chemical in MediumCausing Total Inhibition Causing Death
PentaPentachloro8-Naphchlorophenol thol phenol Fungus Ceratostomella pilifera 0.006 0.006 0.02 Hormonema dematioides 0.006 0.006 0.03 0.06 ' 0.04 Hormodendrum cladosporioides 0 006 0.09 0.01 Polyporus versicolor 0 008 0.02 Polyporus hirsutus 0 00s 0.20,002 Coniophora cerebella 0 002 0.01 0.006 Fomes roseus 0.02 0 006 0,006 Madison 517a 0 002 0.02 0 002 0.02 0.004 Lentinus lepideus 0.002 0 002 0.01 Poria incraasata 0 001 0.002 Trametes serialis 0.02 0 002 0.006 Lenzites sae iaria 0.01 0.004 0.02 0 002 Lenzites tragea 0 OOlb 0.001b 0,005b Merulius domesticus 0,002 0.02 0 002 u-100 0.01 0 004 0.006 Trichophyton rosaceum . . ~ ~ 0.008 0 006 0.02 Trichophyton interdigitale 0 004 0.004 0.02 Epidermophyt,on inguinale 0.02 0 006 Aspergillus niger 0 006 0:OOS Penicillium chrysogenum 0.01 0.004 0.004 0.01 Penicillium digitatum 0.006 0.006 0.1 Alternaria radicina 0.006 0 004 0.02 Rhizopus nigricans 0.02 0.02 0 01 Fusarium vasinfectum a Formerly known as Fomes annosus. b O r less. C Obtained from R. E. Waterman, Bell Telephone Laboratories.
8-Naphthol 0.03 0.05 0.05 0.1 0.3 0.01 0.02 0.02 0.04 0.01 0.03 0.05
0.03 0.005b
0.05 0.02 0.02 0.02 0.03 0.02 0.02 0.25 0.03 0.03
NOVEMBER, 1939
IDrDUS'l'RIAL AND ENGINEEMNG CtIEMISTllY
son KO. 517 and U-10 are given in Table TIT. The tabulated data indicate the value of this method of test for comparing the toxic qualities of wood-treating chemicals. P h y s i o l o g i c a l Effects
It is also important to know the physiological effects of new products upon individuals who may handle the products. Such knowledge enables one to take necessary precautions in order to ensure safe handling of the material and to eliminate unforeseen toxic reactions among industrial workers. Toxicological dat.a have been collected by Bechhold and Ehrlich (4)and by Kehoe, Uei~hmann-Gruebler,and Kitzmiller (f4),and a summary of their data was given by Camwell and Nason (5). Briefly, toxicological studies indicate that pentachlorophenol is not a cumulative poison, but that excessive doses can cause death. An oil solution of pentachlorophonol or a water solution of its sodium salt causes irritation and dermatitis if allowed to remain on the skin a sufficient length of time. This dermatitis clears rapidly after exposure to the chemical is discontinued, and the lesions heal without scars or other residual effects. These observations indieatx the gencral effeets of the absorption of pentachlorophenol. Although it is less toxic than many chemicals in everyday use, a certain degree of potential hazard does exist in connection with it. Experience has shown that the health hazard may be rendered entirely negligible by intelligent handling and hy the use of protective garments. Application to Wood Preservation Pentachlorophenol is generally applied as such by dissolving it in suitable organic solvents. The sodium salt is applied in water solution. In general, pentachlorophenol is applied to seasoned wood as a 5 per cent solution, and because of cost considerations, petroleum solvents are generally used. Such a solution may be applied by pressure impregnation, hot and cold dipping, immersion, and hand brushing or spraying. Each method has its particular sphere of usefnlness and its limitations. Only application by pressure
1433
methods will give what approaches thorough and complete treatment of the wood, hut frequently other methods of application must be used because of the nature of the products to be treated. As Hunt (8) has pointed out: "It is probable that hrnsh treatment will usually be found the only convenient way to treat such timbers (sills and foundation timbers) on account of their size, but wherever possible better t r e a t m e n t s should be used." Hunt and Garratt (9) in their d i e cussion of brush and spray treatments state that although these two methods of treatment are not to be recommended for use when more thorougb treab m e n t s a r e aGailahle, such applications will be decidedly better than no trcatment a t all. Naturally, even the superficial protection afforded by a brush treatment will not be satisfactory unless all surfaces are treated; hence brushing a t the time of, or prior to, construction is recommended rather than treatment after the wood is in place. In commercial applications of pentachlorophenol to wood, the color and paintabiIity of the treated wood depends upon the type of petroleum carrier used in the treatment. Several petroleum oils with a boiling range below 700' F. (371" C.) have been satisfactorily used as carriers where a "clean" treatment was desired.
TABLE I11
ABSOEPTIONAXD DECAY DATAON TREATED Woon BLOCKS
'z?
Sdu-
Ciismieul
tioil
Av.
Ab- ~
sorption
'rorio
.
I.b./cu.
% Pentaeliloionhenul
0 00"
n.uaa 0.25 0.50 0.75 1 .OO 2.00
0:079 0.18
0.24 0.30
8.00
0.80 0.91
4 00 5.00
1.3 1.6
. Madison 517
u-10
%
%
%
67.8 68.1 0.48 0.44 - 0.63 - 1.02 - 0.11 0.26 0.28 0.10
30.6
1: . 8
It.
...
-
Av. 1.0s~ in Wt. Due t o Decay'
Lamitea tTaiea
-
20.2
18.2 1.32 2.99 0.77 1.17 0.45
--- 0.54 - 0.57
--22.7 1.406 -- 200 ...619779 - 2.37 - 0.50 -- 0.7s 0 40
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Field exposures of wood treated with pentachlorophenol have been under test for three active seasons, and each y e a new tests are installed. Iii 1935 pressure-treated saplings were installed in a test ground near New Orleans. In 1936 two hundred fence posts were treated at the &re& Products Laboratory at Madison, Wis., with pentachlorophenol in spent crankcase oil, and the posts were installed under service oomditions. I n 1937 two carloads of lumber and dimension materid were treated with 5 per cent pentachlorophenol in gas oil, and this material has been placed in service tests in Texas, Florida, Mississippi, Louisiana, Illinois, and Missouri. In 1937 and 1938 pressuretreated stock was placed in marine borer control tests. All such pressure treatments have called for a minimum absorption of 10 pounds of the 5 per cent treating solution per cubic foot of wood. The amlications have been made by wrious methods in order to determine the behavior of the solution under different conditions of treatment. Steaming treatment of green stock as well as full- and empty-eel1 treatments of seasoned stock have been made and show that pentachlorophenol in oil can be applied to steam-seasoned wood as well as to air-dried wood. Inspection of all this treated stock and of other installations not mentioned here indicates that pentachlorophenol is exceptionally effectivo for controlling decay and termites in those cases where at least 0.5 pound of toxic per cuhic foot of wood has bern applied. Except for cases of too heavy absorption, the application of chemical in proper solvents has not interfered with the finishing of wood so treated. Finished millwork such as window sash,
VOL. 31, NO. 11
frames, doors, and interior trim have heen receiving a dip treatment since the impetus given the work by Hubert (IO, 11, I $ ) . Since water treatments have proved unsatisfactory for application to finished millwork a n d since creosote cannot be usrd, an oil treatment which leaves the wood unchanged in dimension and in a satisfactory condition for further processing is desirable. Pentachlorophenol i n suit,al>le earriers has hem found to furnish the greatest protection thus far produced by such a dip treatment. Accordiiig to standards for millwork preservat.ives (28)of the National Door Manufacturers Association, thesolution used for treating millwork must contain not less than 5 Der cent by weight of tlie toxicant used, and "the trcating sointion, when reduced t.o half strength by volume by the addition oE Stoddard solvent, sliall prevent decay when tested by the standard X. D. M. A. vood block method". Pentachloro-
FIGURE3. F m c n s RESISTANCE OF TREATDD WOOD
KOVEMBEK, 1939
INDUSTRIAL AND ENGIIVEERING CHEMISTRY
phenol formulated in a fuel oil type of solvent (Permatol A treating solution) has been tested by the above method, and has been found especially satisfactory. Results of such a test are shown in Figure 3. Minimum standards for treating finished millwork have been set up by the Western Pine Association (IO, 11, 12) and by the National Door Manufacturers Association (18); and if these are followed, the more vulnerable portions of sash, frames, etc., will receive an appreciable measure of protection. Paint and putty behave normally if applied according to the best practices known by the trade. That such a treatment can be given without deleterious effects to the putty or paint has been demonstrated by Gardner and Hart (6),as well as by millwork manufacturers who have been using the treatment over a period of time. Even brush treatments containing pentachlorophenol have been demonstrated t o furnish a marked degree of protection when properly applied t o nondurable wood (6). Although brush treatments are not recommended in preference t o more thorough treatments, there are cases where wood “at the job” needs treatment, and superficial methods of treatment furnish the only means of applying preservatives. Where dipping cannot be carried out, brush treatments of three coats applied a t 24-hour intervals have been recommended. Wood brush treated with pentachlorophenol before the wood is placed in service will increase the resistance of such treated wood to the attack of fungi and termites, provided the work is thoroughly and adequately done. Though not often so considered by the layman, sap stain or blue stain prevention should actually be considered a part of the preservation program. Blued wood often contains incipient decay, and such wood is being placed on the market with greater difficulty each year. Sodium pentachlorophena t e (Santobrite) has been demonstrated to give a high degree of stain control if applied to the green lumber in the properly prescribed manner. I t s value is also being demonstrated for the preservation of various types of fiberboards and insulating materials, as well as wood pulp, paper, and other fibrous products. Methods of assay and suitable methods for the quantitative determination of pentachlorophenol in oil solutions and other solvents are available (2, 16) ; i t is thus possible for the wood-preserving plant and other preservative users to check on the concentration of toxic in their treating solutions.
Acknowledgment The writers wish to acknowledge the use of laboratory data obtained by W. P. Metzner, H. L. Morrill, H. K. Nason, R. S. Shumard, and other members of the Research Department of Monsanto Chemical Company.
Literature Cited (1) Aylsworth, 3. W., U. S.Patent 1,085,783(Feb. 3, 1914). ( 2 ) Baechler, R. H., Proc. Am. Wood-Preservers’ Assoc., 35, 361-64 (1939). (3) Bateman, Ernest, and Baechler, R. H., Ibid., 33, 91-104 (1937). (4) Bechhold, H., and Ehrlich, P., 2. physiol. Chem., 47, 173-99 (1906). ( 5 ) Carswell, T. S.,and Xason, H. K., IXD. ENG.CHEM.,30, 622-6 (1938).
(6) Gardner, H. A., and Hart, L. P., Natl. Paint, Varnish, Lacquer Assoc., Sci. Sect., Circ. 573 (1939). ( 7 ) Hatfield, I r a , Proc. Am. Wood-Preservers’ Assoc., 31,67-66(1935). (8) Hunt, G. M., U. S. Dept. Agr., Farmers’ Bull. 744 (1916, rev. 1928). (9) Hunt, G. M., and Garratt, G. A., “Wood Preservation”, New York, McGraw-Hill Book Co., 1938. (10) Hubert, E. E., IND. ENG.CHEM., 30, 1241-50 (1938). (11) Hubert, E. E., Western Pine Assoc., Tech. BUZZ.6 (1936). (12) Zbid., 6, revised (1938).
1435
Iwanowski, Waclaw, and Turski, Jozef, U. S. Patent 1,881,617 (Oct. 11, 1932). (14) Xehoe, R. A , , Deichmann-Gruebler, Wm., Kitzmiller, K. V., (13)
J . I n d . H y g . Tozicol., Z,l‘, 160-72 (1939). (15) Monsanto Chemical Co., Method for Quantitative Determination of Chlorinated Phenols in Petroleum Oil Solution”, Dec. 5 , 1938. (16) Monsanto Chemical Co., “Sap Stain Control”, Jan., 1938. (17) Natl. Door Mfrs. Assoc., Method of Testing Oil-Soluble Wood Preservatives Using Wood Blocks Uniformly Impregnated, May 17, 1939, revision. (18) Natl. Door Mfrs. Assoc., Minimum Standards for Millwork Preservatives, May 21, 1938. revision.
CORRESPONDENCE Industrial Floors of Concrete SIR: A recent article on “Factory Floors” [31, 283 (1939)l did not mention the improved methods now being used in the construction of concrete floor finishes. These methods are being adopted particularly on industrial floors where there is abrasion from trucking and other severe usage, and where it is desirable to build resistance into the floor against many substances that may be detrimental to surfaces of poorer quality. The relatively low cost of concrete floors has had a strong appeal for the prospective owner, but it is unwise to attempt t o make an already low-cost, good floor cheaper by neglecting basic principles of concrete making. Use of a mixture with too much fine material or too much water, overtroweling while the mixture is still plastic, and inadequate curing must be avoided. The architect or engineer responsible for the job must prepare his specifications t o prohibit these practices and should have a representative present during construction to see that the specifications are enforced. Correct procedure is just as easy to follow as an incorrect one and with suitable equipment the contractor’s costs will often be lower. Many examples can be cited of the long life of properly built concrete floors even when subjected to most trying conditions. Concentrated foot traffic is almost as hard on floors as heavy trucking. Probably no floors in the world have more foot traffic than the station platforms at Grand Central Station in New York. Concrete placed there 25 years ago is still serving admirably. Moving heavy loads on trucks with small steel wheels is the test for wear resistance of industrial floors. For example, the Eastman Kodak Company has concrete floors in their plant a t Rochester, over which heavily laden trucks of paper have been hauled for over 20 years. It has been the practice of some to use a very plastic mortar mixture for floor finishes consisting of cement, sand, and water. Although such a mixture is easy to spread and finish, it does not give the best results. If part of the sand is replaced by small pebbles or crushed stone graded from 1/8 to ”8 inch, and the mixture is placed rather stiff, the surface will be made up largely of the aggregate and will produce satisfactory results even under very trying conditions. Aggregates should be hard, tough and durable. They should be clean and free from dust, clay, silt, or foreign matter. Trap rock, granite, quartzite, gravel, and limestone are excellent for heavy-duty floors. The mixture should be 1 part sand, and 7.5 t o 2 parts coarse aggregate with as little water as possible. The stiff mixture recommended will require careful handling, but any extra effort will be more than compensated for by the
