2. The use of hollow tiles.
3. The application of plaster directly upon the con crete surface, and of a damp-proof material on the out side walls.
The use of metal furring was precluded on account of the difficulty of attaching it to the concrete surface, no provision having been made for it in the wall design.
The use of hollow tiles against the concrete would have taken up four inches of space for the entire depth and height of the building, and, as the location was very costly, such loss of space was extremely undesirable. Furthermore, in this case, a damp-proof course would have been advisable between the concrete and tile.
The only feasible solution remaining was to damp proof the outside of the building and apply the plaster directly upon the concrete.
The question then arose as to the adhesion of plaster to smooth concrete.
Had this question been considered in the design and construction of the walls, the latter might have been left sufficiently rough to leave no room for doubt as to such adhesion, or recesses and projections could have been specially provided for the keying of the plaster. This not having been taken into account, and the wall being dense and smooth, in order to eliminate any possi bility of a separation from the plaster it became evident that in order to remove the glaze the sand-blast must be resorted to, and, for pitting the surface, it would be necessary to make use of the pneumatic punch, with a consequent very considerable increase in the cost of the work.
The question also presented itself as to the actual necessity of damp-proofing the exterior, it being said that the concrete was in itself so dense as to be practically water-tight. This opinion, as well as that concrete was not porous, was held by the builders; and only a complete discussion of the physical phenomena of capillarity, with evidence of the absorptive power of concrete and its permeability under pressure, served to convince him of the possible danger of injury to the plaster from the infiltration of water through the concrete.
Some of the reasons for this conclusion are sufficiently interesting to deserve repetition; they are : 1. That concrete has never been prepared so as to be absolutely water-tight, particularly under pressure.
2. That no rule has yet been formulated for pro portioning materials so as entirely to eliminate the voids in concrete.
3. That, as shown by experience, the mixture of 1 part cement to 4 sand; which was used in this case, con tained about 5 per cent to 10 per cent less cement that that required to produce theoretically impervious con crete.
4. That even if the sensible voids are closed, the ag gregate itself may absorb as much as 1 per cent or more of its weight of water, on account of its own porosity.
5. That the minuteness of the pores does not tend to retard the passage of water; on the contrary, the well known law of capillarity is that the finer the pores (all else being equal), the greater the capillary attraction.
6. That checks unavoidably occur in concrete work, owing to settlement, shrinkage, expansion, contraction, etc., and water will find its way through them.
7. That in excessively high windstorms the raindrops are hurled at the building with a high velocity and are forced into the pores under pressure.
°S. That practical imperviousness of concrete in reser voirs results from the closing of the pores by deposit of sediment, and the conditions are totally different in con crete walls exposed to the air and to rapid changes of temperature and humidity.
9. That slight seepage, while unimportant in the case of reservoirs, may be fatal in the case of buildings.
Not only did these arguments indicate that the proper method of damp-proofing the structure was from the outside, but a comparison of cost of the several methods considered also showed that this was the most economical method.
Thus, the cost per square yard of wall surface was as follows: Damp-proofing film between wall and plaster 20 cents Metal furring between wall and plaster 30 cents Hollow tiles, excluding damp-proofing 63 cents Hollow tiles, including damp-proofing 75 cents Outside coating, applied under pressure 30 cents Cost of preparing concrete surface 25 cents The amount of wall surface to be damp-proofed, in case the outside method were adopted, was only 60 per cent of the work required for the interior damp-proofing, as some of the walls were built directly against the adjoining property, and therefore not exposed. Thus, the total cost of the exterior damp-proofing methods was less than that of any of the other methods.
Had the question of damp-proofing and plastering been given due consideration before construction was undertaken, the difficulty could have been avoided in one of three ways: First, by properly preparing the concrete surface to receive the plaster; second, provision could have been made by recesses or projections for efficient application of an interior damp-proofing course to which plaster would have thoroughly adhered; third, provision might have been made for holding metal or other furring to the wall to receive the plaster.
Too much detailed study cannot be given to these questions by architects, engineers, and others in charge of designing reinforced concrete work.