"It ought not to be necessary on first-class work to be continually patching up. Arrises and corners cannot be patched up except by cutting out large sec tions of the work. Plastering which is put on finished work will not hold unless it is put in very thin layers, and then it is doubtful. Corners can never be satisfac torily patched up by plastering. Good work sometimes may be executed on plane surfaces by first wetting the wall and then rubbing on it a coating from 1/16-in. to in thickness. It takes a good deal of rubbing to get any results; and too much rubbing generally results in hair-cracking, as excessive cement is worked to the surface. The less cement is rubbed, the less hair-cracking will result. No kind of mortar facing should be richer than 1 : 3, and when cast in place it may be as little as 1 :4 for plane surfaces; ornamental work should not be leaner than 1 : 3. If conditions are such that plas tering a veneer on finished work is necessary, I would use a mortar which was already partially set up; then add water, and retemper. The reason that plastered work breaks off, is that the mortar in setting up shrinks. The richer it is, the more it shrinks, and the shrinkage results in leaving a hair-plane between the plaster and the concrete work. The frost does the rest.
"Don't leave timber work in the concrete unless it is absolutely necessary.
"Note any movement or bulging of the form. Cor rect, if necessary, by wedging, and remodel other similar forms to overcome this trouble.
"Bear in mind that an excess of water weakens the concrete a little, increases efflorescence, and retards the time of setting.
"In work where it is necessary to throw a heavy load upon it in a short time, reasonably dry concrete should be used. Tests indicate that it is probable that wet concrete will be as strong as dry concrete in a year. However, wet concrete certainly bonds the new work better to the old work, and is particularly valuable when encasing steel, as the air is excluded and the steel becomes coated with cement. Excess of water, however, not only results in excess of efflorescence, but also in the amount of laitance or slime which appears on the surface of a day's work.
"In a batch mixer (excepting the Haines gravity mixer) it does not make any practical difference as to the order in which the material is entered, excepting that the water should be last. The mixer should be kept clean, which means that it should be cleaned every night. The water should be varied with the amount of moisture in the sand and the stone, and should be greater for porous stone. Remember always that after a rain you need less water in your concrete. The amount of sand should be varied also, so as to get a uniform product with run of crusher. This is a very vital point.
"The permeability of concrete may depend upon the cement, but it is certain that it is greater with a dry concrete, and less with a well-graded sand and with a sand containing silt. While 10 per cent and even 15 per cent of silt may be desirable for concrete having a low permeability, it is probable that any amount over 10 per cent weakens concrete, and it is certain that it increases the amount of efflorescence and slime which rises to the surface of a day's work.
"Inspectors on my work use hand-tallying machines to keep track of the number of batches, and at noon and quitting time they check their tally with the bags at the mixer.
"On finished work which is not to be tooled, the lagging should be coated with linseed oil. Glycerine leaves a fatty stain. On work which is to be tooled, it is not necessary to use oil.
"Concrete dumped from large buckets should be lowered as near to the concrete as practicable, particu larly when deposited on concrete which is not older than 48 hours, so that the bond between adjacent layers will not be broken.
"The most noticeable imperfection in concrete sur faces is the lipping between consecutive day's work.
The stiffer the forms, the less the lipping. On account of this lipping, I bring each day's work to a horizontal layer by using a straight edge. In mortar facing, I find that if extreme care is not taken the steel plate which is ordinarily used between the concrete and the mortar facing drags, leaving an unreasonably large space be tween the two; and if the rammers are not particularly careful, there will be no bond between the two, and the mortar facing will flake off.
"A great many so-called concrete workers and con crete contractors do not attempt to cast fine arrises, but assume from the start that they will fix up the work by plastering after the forms are removed. Occasionally your details will not turn out as well as they should, and under such circumstances a thin veneer of mortar is permissible, but should be applied only by an expert concrete finisher. Rubbing finished concrete with grout or painting the face with grout is deprecated. The writer once got hold of several barrels of cement rich in free lime, which he unfortunately used to paint a concrete bridge in order to give it an even finish. The results were very disastrous, and the bridge was a mass of blotches, varying between white and a dirty pink.
"Where it is necessary to cast a cornice in place, the top surface of which is comparatively flat, it is impossible to get this finish against a top form. I would leave the top form off, tamp carefully to grade, and, as soon as the concrete has a fairly good set, from 3 to 12 hours, I would rub it down to an even grade with a small flat stone or a wooden float. In my judgment, it is not desirable to add any additional mortar to make this finish. However, if you have got a poor job on this part of the work and you are practically forced to rub it up, using additional mortar, I would roughen it slightly with a mason 's ax, wet it thoroughly, and rub a coat of not more than using a 1 :3 cement mortar which has been set up three or four hours.