Surface blemishes due to dirt or cement adhering to the form boards have no excuse if the engineer or contractor cares to avoid them. It is a simple matter to keep the lagging clean and free from such accumulations.
IMPERFECT MIXING AND PLACING.—Imperfectly mixed and placed concrete gives irregularly colored, pitted and honeycombed surfaces with here a patch of smooth mortar and there a patch of exposed stone. Careful mixing and placing will avoid this defect, or all chance of it may be eliminated by using surface coatings of special mixtures. There is no great difficulty, however, in obtaining a reasonably homogeneous surface with concrete; the task merely requires that the mixing shall be reasonably uniform and homogeneous and that in placing this mixture the spading next to the lagging shall be done in such a way as to pull the coarse stones back and flush the mortar to the surface. Spading forks are excellent for this purpose. A better tool is a special spade made with a perforated blade; this special spade costs $3.
EFFLORESCENCE.—Efflorescence is the term applied to the whitish or yellowish accumulations which often appear on concrete surfaces. "Whitewash" is another name given to these blotches. Efflorescence is due to certain salts leaching out of the concrete and accumulating into thin layers where the water evaporates on the surface. These salts are most probably sulphates of calcium and magnesium, both of which are contained in many cements and both of which are slightly soluble in water. Efflorescence is very erratic in its appearance. Some concretes never exhibit it; in some it may not appear for several years, and in others it shows soon after construction and may appear in great quantities. The most effective way to prevent efflorescence would naturally be to use cements entirely free from sulphates, chlorides or whatever other soluble salts are the cause of the phenomenon, but the likelihood of engineers resorting to the trouble of such selection, except in rare instances, is not great, even if they knew what cements to select, so that other means must be sought. The most common place for efflorescence to appear in walls is at the horizontal junction of two days' work or where a coping is placed after the main body of the wall has been completed. The reason of this seems to be that the salt solutions seep down through the concrete until they strike the nearly impervious film of cement that forms on the top surface of the old concrete before the new is added, and then they follow along this impervious film to the face of the wall. The authors have suggested that this cause might be remedied by ending the day's work by a layer whose top has a slight slope down toward the rear of the wall or perhaps by placing all the concrete in similarly sloping layers. Mr. C. H. Cartlidge is authority for the statement that this leaching at joints can be largely done away with by the simple process of washing the top surface of concrete which has been allowed to set over night by scrubbing it with wire brushes in conjunction with thorough flushing with a hose. But efflorescence frequently appears on the faces of walls built without construction joints and in which a wet concrete is puddled in and not tamped in layers, and here other means are obviously essential. Waterproofing the surface of the wall should be effective so long as the waterproofing lasts; indeed one of the claims made for some of these waterproofing compounds is that efflorescence is prevented. The various waterproofing mixtures capable of such use will be found described in Chapter XXV. Failing in any or all of these methods of preventing efflorescence the engineer must resort to remedial measures. The saline coating must be scraped, or chipped, or better, washed away with acids.
Efflorescence was removed from a concrete bridge at Washington, D. C, by using hydrochloric (muriatic) acid and common scrubbing brushes; 30 gals. of acid and 36 scrubbing brushes were used to clean 250 sq. yds. of concrete. The acid was diluted with 4 or 5 parts water to 1 of acid; water was constantly played with a hose on the concrete while being cleaned to prevent penetration of the acid. One house-front cleaner and 5 laborers were employed, and the total cost was $150, or 60 cts. per sq. yd. This high cost was due to the difficulty of cleaning the balustrades. It is thought that the cost of cleaning the spandrels and wing walls did not exceed 20 cts. per sq. yd. The cleaning was perfectly satisfactory. An experiment was made with wire brushes without acid, but the cost was $2.40 per sq. yd. The flour removed by the wire brushes was found by analysis to be silicate of lime. Acetic acid was tried in place of muriatic, but required more scrubbing.
SPADED AND TROWELED FINISHES.—With wet-concrete and ordinarily good form construction a reasonably good surface appearance can be obtained by spading and troweling. For doing the spading a common gardener's hoe, straightened out so that the blade is nearly in line with the handle will do good work. The blade of the tool is pushed down next to the lagging and the stone pulled back giving the grout opportunity to flush to the face. Troweling, that is troweling without grout wash, requires, of course, that the concrete be stripped before it has become too hard to be worked. As troweling is seldom required except for tops of copings and corners it is generally practicable to bare the concrete while it is still fairly green. In this condition the edges of copings, etc., can be rounded by edging tools such as cement sidewalk workers use.
PLASTER AND STUCCO FINISH.—The ordinary concrete surface with a film-like cement covering will not hold plaster or stucco. To get proper adhesion the concrete surface must be scrubbed, treated with acid or tooled before the plaster or stucco is applied and this makes an expensive finish since either of the preliminary treatments constitutes a good finish by itself. When a coarse grained facing is made of very dry mixtures, as described in a succeeding section, it has been made to hold plaster very well on inside work. In general plaster and stucco finishes can be classed as uncertain even when the concrete surface has been prepared to take them, and when the concrete has not been so prepared such finishes can be classed as absolutely unreliable.
MORTAR AND CEMENT FACING.—Where a surface finish of fine texture or of some special color or composition is desired the concrete is often faced with a coat of mortar or, sometimes, neat cement paste or grout. Mortar facing is laid from 1 to 2 ins. thick, usually 1½-ins., the mortar being a 1-1, 1-2 or 1-3 mixture and of cement and ordinary sand where no special color or texture is sought. This facing often receives a future special finish as described in succeeding sections, but it is more usually used as left by the forms or at best with only a troweling or brushing with grout. Engineers nearly always require that the mortar facing and the concrete backing shall be constructed simultaneously. This is accomplished by using facing forms, two kinds of which are shown by Figs. 45 and 46. In use the sheet steel plates are placed on edge the proper distance back of the lagging and the space between them and the lagging is filled with the facing mortar. The concrete backing is then filled in to the height of the plate, which is then lifted vertically and the backing and facing thoroughly bonded by tamping them together. The form shown by Fig. 46, though somewhat the more expensive, is the preferable one, since the attached ribs keep the plate its exact distance from the lagging without any watching by the men, while the flare at the top facilitates filling. The facing mortar has to be rather carefully mixed; it must be wet enough to work easily and completely into the narrow space and yet not be so soft that in tamping the backing the stones are easily forced through it. Also since the facing cannot proceed faster than the backing the mortar has to be mixed in small batches so that it is always fresh. A cubic yard of mortar will make 216 sq. ft. of 1½-in. facing. Cement facing is seldom made more than 1 in. thick. If placed as a paste the process is essentially the same as for placing mortar. When grout is used a form is not used; place and tamp the concrete in 6 to 8-in layers, then shove a common gardener's spade down between the concrete and the lagging and pull back the concrete about an inch and pour the opening full of grout and withdraw the spade. If this work is carefully done there will be very few stones showing when the forms are removed. When stiff pastes or mortars are used the contractor often places the facing by plastering the lagging just ahead of the concreting; this process requires constant watching to see that the plaster coat does not slough or peel off before it is backed up with concrete.
Fig. 45.—Form for Applying Cement Facing (Massachusetts Highway Commission).
