Building Construction and Drawing 1906

A Textbook on the Principles and Details of Modern Construction Stages 2, 3 and Honours Courses

Charles F. Mitchell, Stephen J. Scaysbrook, Stephen J. Scaysbrook

• Editore: Routledge
• Anno: 2022
• ISBN:
  9781000536164
• Edizione: 1
• Argomento: Architecture
• Categoria: Architecture General

Originally published in 1881, but here reissuing the 1906 edition with a new introduction by Stephen J. Scaysbrook, the Mitchell's Building and Construction Stage 2, 3 and Honours book offers an unparalleled insight into historic construction techniques and materials. Originally written to provide a concise handbook and guide for students and for practitioners, this reissue of Mitchell's 1906 Advanced and Honours edition now provides a valuable addition to building pathology, allowing students and practitioners to research construction methods and materials pertinent to the period.

Advanced and Honours Building Construction CHAPTER I MATERIALS.

Limes and Cements.

Limestoms.—Stones consisting entirely or in a great part of carbonate of lime, CaCO₃, are known as limestones, and are quarried chiefly in the form of chalk, which is a soft white rock of almost pure calcium carbonate, limestones, and marbles. For descriptions of the latter two, see Chapter on Stones.

The resultant of limestones after burning is lime, the latter is used in building operations (a) as a matrix for concrete, (b) in the preparation of mortar for bedding bricks or stones in walling, and (c) as a cementing material in the plaster used for the covering of walls.

Classification.—Limes are classified under the following heads:—

1. Pure or rich limes.
2. Poor limes.
3. $\text{Hydraulic}\{\begin{array}{l}\text{Feebly}\text{hydraulic}\\ \text{Hydraulic}\mathrm{.}\\ \text{Eminently}\text{hydraulic}\mathrm{.}\end{array}$

Limestones are never found absolutely pure in nature, b.c. but are either mixed with impurities or in combination with them, which impurities, if soluble in acids, are useful in accelerating the setting action, but if insoluble in acids are valueless for that purpose.

Rich Limes.—Limes are said to be rich or pure when the impurities insoluble in acids do not exceed 6 per cent, of the whole mass.

For plastering, rich, pure, or fat limes only should be used, because of their readiness to slake, and their consequent non-liability to blister as compared with hydraulic limes.

Poor Limes are those containing from 15 to 30 per cent, of impurities insoluble in acids. They possess the general properties of rich limes, but in a less degree. They take longer to slake, and do not increase in bulk to such an extent as the rich limes. They do not take such a large ratio of sand, owing to the foreign matter they already contain.

Hydraulic Limes contain a quantity of combinable substances other than lime, such as silica and alumina,* which on being burnt form calcium aluminates and calcium silicates, together with a portion of lime, the measure of these bodies up to a certain point being the measure of the hydraulicity. These bodies render the limes independent of external agents for their setting properties.

Limes containing 6 to 16 per cent, of these useful substances are termed feebly hydraulic, those 16 to 26 per cent, hydraulic, and those from 26 to 36 per cent, eminently hydraulic.

Hydraulic limes only should be used as the matrix for lime concrete, and they are most suitable for constructional work.

From the time limestone is quarried to the setting or the lime in the work, four processes are performed, viz., burning, slaking, mixing with sand, and setting.

CaCO₃ is a substance insoluble in pure water, and is unable to combine with carbonic-acid gas when in this form, but is rendered suitable by calcination, which drives off the carbon dioxide (CO₂), leaving calcium oxide (CaO) together with any other impurities contained in the stone.

Burning of Limestones —Limestones are burnt in kilns of various shapes, the most common being a cylindrical brick or stone casing lined with fire-brick, having a draw-hole at one part to apply the fuel; this arrangement is termed a flare kiln. A rough dome is built with the limestones, forming a chamber to contain the fire; the kiln is then filled about the dome, the top of the kiln being usually covered by a shed to protect it from the weather.

In burning the limestones, the heat should be applied gradually, otherwise the separation of the carbon dioxide from the stones takes place with such rapidity that they crumble to pieces.

It is imperative that the calcined stones should be withdrawn from the kiln as soon as the carbon dioxide has been driven off, this being determined as follows:—While any carbon dioxide remains in the stones they will be of a dark red colour, but when this gas passes off the colour changes to a brilliant white glow; at this point the stones should be withdrawn, or they will be overburnt, the result being limes very difficult to slake, this action often not taking place in some of the particles for a considerable time after being used for building purposes, this renders the limes unreliable for use, as they may blow when bedded in the work. A somewhat similar result occurs with underburnt limes.

Slaking.—The object of slaking lime is to form a calcium hydrate, thus rendering it quickly in a fit condition to readily combine with the CO, to form crystals of calcium carbonate, the formation of the latter being a necessary condition for strength in a mortar. Slaking is induced by adding water to quicklime; these on combining give off great heat, which generates steam, causing the lime to expand, burst, and disintegrate with a series of small explosions forming a calcium hydrate, Ca(OH)₂, a whitish-yellow powder. The slaked lime thus formed is soluble, and hence when more water is added some of this dissolves and forms a saturated solution.

The soluble lime in a saturated solution is ready for the absorption of CO₂, which always exists in the atmosphere.

If quick-lime be left exposed to the air, it absorbs CO₂, which under these conditions renders it inert, as the resulting carbonate is not crystalline.

Slaking is an important process in the manufacture of mortar, and it is imperative that every particle of quicklime must be thoroughly slaked, for if any unslaked portions are built in the work it will by its subsequent expansion disturb the rest of the work.

To obviate this failing, the mortar after mixing should always be kept for at least fourteen days before being used.

Sand is a form of silica (SiO₃); it is added to lime in the preparation of mortar, (i) to counteract the excessive shrinkage that takes place with pure lime mortar, (2) to assist in the crystallization by forming ducts through which the necessary CO₂ can have access and act upon the particles behind the surface, and (3) to increase the bulk of the mass (sand being much cheaper than lime).

The sand for mortar should be free from all earthy or clayey matter; it should have sharp angles and a rough surface.

The best sand for building purposes is that known as pit sand. The next in order is river sand, which is obtained from the banks and beds of rivers; this kind is not considered so good as pit sand, the grains being rounded and worn smooth, the adhesive value being thus reduced. River sand is, however, largely used for plasterers’ work, it being fine and of a light colour. The grains in sea sand are similar to river sand, round and smooth. Sea sand should never be used for plastering or other building work, as it effloresces, thereby causing a wall to be damp for a considerable time.

Sand should be fine for plasterers’ work, and moderately coarse for bricklayers’ work; it is usually screened and sometimes sifted to remove any large stones or shingle that it may contain.

Loamy or dirty sand should be washed before being used. This is usually effected by placing the sand in a vessel through which a stream of water is constantly passing, the sand at the same time being agitated to separate it from the foreign matter, which latter becomes suspended in the water and passes off.

Sand for all coats of plasterers’ work is better washed, although for the first coats this is often neglected.

Setting of Lime.—The setting of lime depends on the absorption of CO₃ from the atmosphere by the particles of slaked lime in solution in the mortar, the carbon dioxide being soluble in water. The Ca(OH)₉ with excess of CO₂ combine to form Ca(HCO₃)₂ on evaporation, which decomposes into crystals of CaCO₃, the H₂O helping to dissolve the next particle, forming it into a saturated solution, and putting it into a condition to take up a molecule of CO₂; this in its turn repeats the already described action, and crystals of CaCO₃ are formed. The crystals always have a tendency to adhere to something rough and hard, such as sandy particles or the surfaces of bricks; for this reason the addition of sand up to a certain ratio increases the strength of the mixture, the best ratio being one part pure lime to one of sand, the maximum being one of pure lime to three parts of sand.

A long time elapses before pure limes harden, owing to their depending upon external aid to attain this state. If lime alone were used the surface would set and form an impervious layer, and so check the CO₂ from acting on those particles below the surface, the moisture in which evaporates and leaves it in the state of a powder; and even when a large proportion of san...