If this is all too much you could always purchase a pre-mixed dry bay from the merchant; remember however, that these bays are expensive and they dont go far. If separate ingredients are to be used the proportions should be measured by volume. Use a simple thing such as a dry bucket or similar but remember that for each measure you should fill the container to the same level. Find a clean outside area such as some old paving or better a large piece of timber boarding. It is vital that all the ingredients are mixed together in a dry format first. Be extra careful to make sure the cement is not blown away by strong winds. When mixed dry create a large mound and scoop out the centre to create a space for the water. Now it is not necessary to measure out an amount of clean water before mixing as the amount needed will depend on the porosity of the ingredients. Add the water slowly to the mix and turn over with a clean shovel. When to stop mixing? Well you will know, just make sure that you do not over do the amount of water as this will weaken the mix.

Factory and shop lumber is graded with reference to its use for doors and sash, or on the basis of characteristics affecting its use for general cut-up purposes, or on the basis of size of cutting. The grade of factory and shop lumber is determined by the percentage of the area of each board or plank available in cuttings of spec- ified or of given minimum sizes and qualities. The grade of factory and shop lumber is determined from the poor face, although the quality of both sides of each cutting must be considered. Hardwoods. Because of the great diversity of applications for hardwood both in and outside the construction industry, hardwood grading rules are based on the proportion of a given piece that can be cut into smaller pieces of material clear on one or both sides and not less than a specified size. Grade classifications are therefore based on the amount of clear usable lumber in a piece. Special grading rules of interest in the construction industry cover hardwood interior trim and moldings, in which one face must be practically free of imperfections and in which Grade A may further limit the amount of sapwood as well as stain. Hardwood dimension rules, in addition, cover clears, which must be clear both faces; clear one face; paint quality, which can be covered with pain; core, which must be sound on both faces and suitable for cores of glued-up panels; and sound, which is a general-utility grade. Hardwood flooring is graded under two separate sets of rules: (1) for maple, birch, and beech; and (2) for red and white oak and pecan. In both sets of rules, color and quality classifications range from top-quality to the lower utility grades. Oak may be further subclassified as quarter-sawed and plain-sawed. In all grades, top-quality material must be uniformed in color, whereas other grades place no limitation on color. Shingles are graded under special rules, usually into three classes: Number 1, 2, and 3. Number 1 must be all edge grain and strictly clear, containing no sapwood. Numbers 2 and 3 must be clear to a distance far enough away from the butt to be well covered by the next course of shingles. The principal destroyers of wood are decay, caused by fungus, and attack by a number of animal organisms of which termites, carpenter ants, grubs of a wide variety of beetles, teredo, and limnoria are the principal offenders. In addition, fire annually causes widespread destruction of wood structures. Decay will not occur if wood is kept well ventilated and air-dry or, conversely, if it is kept continuously submerged so that air is excluded. Most termites in the United States are subterranean and require contact with the soil. The drywood and dampwood termites found along the southern fringes of the country and along the west coast, however, do not require direct soil contact and are more difficult to control. Teredo, limnoria, and other water-borne wood destroyers are found only in salt or brackish waters. Various wood species vary in natural durability and resistance to decay and insect attack. The sapwood of all species is relatively vulnerable; only the heartwood can be considered to be resistant. Table 4.15 lists the common species in accordance with heartwood resistance. Such a list is only approximate, and individual pieces deviate considerably. Preservatives employed to combat the various destructive agencies may be subdivided into oily, water-soluble salts, and solvent-soluble organic materials. The principal oily preservatives are coal-tar creosote and creosote mixed with petroleum. TABLE 4.15 Resistance to Decay of Heartwood of Domestic Woods Resistant or very resistant

The component parts of a fixed arch are shown in Fig. 9.57. For a discussion of the different types of arches and the stress analyses required for each, see Art. Because the depth of an arch and loading for maximum moments generally vary along the length, several cross sections must be chosen for design, such as the crown, springing, haunches, and the quarter points. Concrete compressive stresses and shear should be checked at each section, and reinforcement requirements determined. The sections should be designed as rectangular beams or T-beams subjected to bending and axial compression, as indicated in Arts. 9.82 to 9.84. When an arch is loaded, large horizontal reactions, as well as vertical reactions, are developed at the supports. For roof arches, tie rods may be placed overhead, or in or under the ground floor, to take the horizontal reaction. The horizontal reaction may also be resisted externally by footings on sound rock or piles, by reinforced concrete buttresses, or by adjoining portions of the structure, for example a braced floor or roof at springing level. Hinged arches are commonly made of structural steel or precast concrete. The hinges simplify the arch analysis and the connection to the abutment, and they reduce the indeterminate stresses due to shrinkage, temperature, and settlements of supports. For cast-in-place reinforced concrete, hingeless (fixed) arches are often used. They eliminate the cost of special steel hinges needed for hinged concrete arches and permit reduced crown thicknesses, to provide a more attractive shape. Arches with spans less than 90 ft are usually constructed with ribs 2 to 4 ft wide. Each arch rib is concreted in a continuous operation, usually in 1 day. The concrete may be placed continuously from each abutment toward the crown, to obtain symmetrical loading on the falsework. For spans of 90 ft or more, however, arch ribs are usually constructed by the alternate block, or voussoir, method. Each rib is constructed of blocks of such size that each can be completed in one casting operation. This method reduces the shrinkage stresses. The blocks are cast in such order that the formwork will settle uniformly. If blocks close to the crown section are not placed before blocks at the haunch and the springing sections, the formwork will rise at the crown, and placing of the crown blocks will then be likely to cause cracks in the haunch. The usual procedure is to cast two blocks at the crown, then two at the springing, and alternate until the complete arch is concreted. In construction by the alternate block method, the block sections are kept separate by timber bulkheads. The bulkheads are kept in place by temporary struts between the voussoirs. Keyways left between the voussoirs are concreted later. Near piers and abutments where the top slopes exceed about 30 with the horizontal, top forms may be necessary, installed as the casting progresses. If the arch reinforcement is laid in long lengths, settlement and deformation of the arch formwork can displace the reinforcing steel. Therefore, depending on the curvature and total length, lengths of bars are usually limited to about 30 ft. Splices should be located in the keyways. Lap splices of adjacent bars should be staggered (50% stagger), and located where tension is small. Upper reinforcement in arch rings may be held in place with spacing boards nailed to props, or with wires attached to transverse timbers supported above the surface of the finished concrete. Forms for arches may be supported on a timber falsework bent. This bent may consist of joists and beams supported by posts that are braced together and to solid ground. Wedges or other adjustment should be provided at the base of the posts so that the formwork may be adjusted if settlement occurs, and so that the entire formwork may be conveniently lowered after the concrete has hardened sufficiently to take its own load. (Guide to Formwork for Concrete, ACI 347R, American Concrete Institute.) Thin shells are curved slabs with thickness very small compared with the other dimensions. A thin shell possesses three-dimensional load-carrying characteristics. The best natural example of thin-shell behavior is that of an ordinary egg, which may have a ratio of radius of curvature to thickness of 50. Loads are transmitted through thin shells primarily by direct stressestension or compressioncalled membrane stresses, which are almost uniform throughout the thickness. Reinforcedconcrete thin-shell structures commonly utilize ratios of radius of curvature to thick- FIGURE 9.58 Continuous cylindrical concrete shell. ness about 5 times that of an eggshell. Because concrete shells are always reinforced, their thickness is usually determined by the minimum thickness required to cover the reinforcement, usually 1 to 4 in. Shells are thickened near the supports to withstand localized bending stresses in such areas. (See also Art. 5.15.) Shells are most often used as roofs for such buildings as hangars, garages, theaters, and arenas, where large spans are required and the loads are light. The advantages of reinforced-concrete thin shells may be summarized as follows: Most efficient use of materials. Great freedom of architectural shapes. Convenient accommodation of openings for natural lighting and ventilation. Ability to carry very large unbalance of forces. High fireproofing value due to lack of corners, thin ribs, and the inherent fire resistance of reinforced concrete. Reserve strength due to many alternative paths for carrying load to the supports. One outstanding example withstood artillery fire punctures with only local damage. Common shapes of reinforced-concrete thin shells used include cylindrical (barrel shells), dome, grained vault, or groinior, elliptical paraboloid, and hyperbolic paraboloid (saddle shape). Cylindrical shells may be classified as long if the radius of curvature is shorter than the span, or as short (Fig. 9.58). Long cylindrical shells, particularly the continuous, multiple-barrel version which repeats the identical design of each bay (and permits reuse of formwork) in both directions, are advantageous for roofing rectangular- plan structures. Short cylindrical shells are commonly used for hangar roofs with reinforced-concrete arches furnishing support at short intervals in the direction