Fireproofing - mortar, cement, building, fireproof, bricks, buildings, heat, materials, sand and modern

FIREPROOFING In the modern improvements of fireproof building material, masonry plays an important part. Not infrequently it is considered of sec ondary importance by builders and structural engineers; but in the various large fires of our cities the condition of the masonry after collapse of a building has determined to some extent the relative value of the mortar employed. In the Baltimore fire, a good many collapsed buildings indicated that inferior mortar was used; and the weakness of the buildings was due to this, fully as much as to the quality of bricks, stone, or tiles.

Problems for the Mason.

In the employment of cement and other mortars for building fire proof structures of the highest grade, the mason has problems to solve that require the very best of ability. The functions of mortar, as have been described, are to unite building materials in such a way as to give the maximum surface of friction to the blocks or bricks, with the mini mum amount of mortar. In other words, too much mortar is as bad as too little. Mortar, when employed between bricks or stones, sets much harder than when tested alone. This makes it easier for the expert mason to supply cement mortar which will set as hard as the building materials used. Unless mortar thus forms a compact wall as hard as the blocks or bricks, it becomes the weakest part of the struc ture. In fireproof buildings, many defects have been found in the mortar, simply through the neglect of this point. In the old buildings where little attempt was given to fireproofing, it was sufficient that mortar should be strong and adhesive.

But in modern buildings the mortar must be fireproof as well as hard as the building mate rials. It must resist temperature of a very high degree, and also refuse to disintegrate and crum ble when a stream of water is played upon it. The fireproof qualities of the best Portland ce ment are well known, and in this we have the basic principle of a cement mortar which will resist fire. But much depends upon the quantity and quality of the cement, the methods of mix ing, and the skill of the masons.

In regard to the first, we find that cements show a variation in grades according to the fine ness of grinding and the method of burning. Even the coarse parts of the best Portland are inert and do not adhere to sand or broken stone when moistened. It is thus necessary that grinding should be perfect, as well as burning; and to secure the best results, very uniformly powdered Portland cement should be used. A too large mixture of sand with the cement to form concrete for binding purposes, weakens the fire-resisting quality of the mixture. Likewise mixing requires expert workmanship, for a too free use of water weakens the concrete to a low point of adhesiveness.

In building walls of fireproof buildings, ma sons accustomed to handling tiles and burnt bricks of a high grade are desirable. A good many fireproof buildings have had their weak ness intensified by bad masonry. Unless all the spaces between the tiles or blocks are completely filled with fireproof cement mortar, the heat from a fire may penetrate through these air spaces to the ironwork inside or even to the inte rior of the building. If the weakness of the walls depends upon the masonry work, it is evident that the very best skilled labor is none too good for laying the courses of tiles, bricks, or concrete blocks.

The relative expansion and contraction of mortar with heat and cold is a point that inter ests the mason of to-day fully as much as any question in modern building operations. The cement mortars of the highest grade are rela-e tively stronger in their adaptation to the chang ing effects of heat and cold in solid walls than any of the cheaper grades. The proper crystal lizing and hardening of Portland cement de pends upon the mixing and amount of water used; but in using such mortars with tile and porous terra-cotta, any excess of moisture is gen erally absorbed by the building materials. This leaves the mortar in the very best condition for setting permanently. It also accounts for the firmer setting of mortar when placed between two bricks or tiles than when left exposed on a surface. After the proper crystallizing of the cement, the quicker it sets the better must the results prove.

Destruction by Heat. The destruction of mortar by heat is due more to the action of the fire upon the sand and broken stone incorpo rated with it than upon the cement itself. This has been demonstrated many times in tests and in actual experience. Where there is an excess of sand or broken stone mixed with the cement, there is sure to be a lower degree of fireproofing in the mortar. This fact is not always suffi ciently emphasized. In the effort to cheapen the cost of the mortar, workers are tempted to add sand too freely in proportion to the amount of cement used, but not to such an extent as to actually weaken the strength or adhesiveness of the mortar. Those not accustomed to dealing with fireproof work may not realize the enor mity of their sin. The fact that they must con sider the fireproof feature of their mortar as well as its strength, should indicate to them the duty of maintaining the exact proportions of sand and cement. Concrete properly made is absolutely fireproof, provided it is thoroughly dried and contains no moisture.

Hardness and Resistance.

It has become an axiom of modern building laws that mortar should approximate the same resistance to heat and pressure when hardened and set as the build ing materials which it binds together. This in effect produces one compact mass in a wall. Mortar, when employed between bricks, stones, or building blocks, sets much harder than when tested alone. This fact makes it easier for the worker to supply cement mortar which will set as hard as the building materials used.