The Preparation of Concrete for Structural Use

New Zealand Journal of Agriculture, Volume 79, Issue 5, 15 November 1949, Page 449

The Preparation of Concrete for Structural Use

By

H. W. T. EGGERS,

Engineer,

Department of Agriculture, Wellington

/CONCRETE as a construction material is easily made and, having a high degree of workability, can be used in such a variety of ways that its use on the farm provides a means of construction that is invaluable to the farmer. Its properties make it an ideal material for every phase of farm construction. This article, the first of a series which aims at giving the farmer a better understanding of concrete so that he will be able to use it to best advantage, deals with the properties of concrete and methods of working it. Later articles will discuss reinforcement and forms, concrete bricks, pipes, and posts, concrete for foundations, yards, paths, retaining walls, and dams, and general uses of cement and concrete.

TO obtain the most effective use from any material a thorough understanding of the properties of that material is necessary. Construction in wood, metal, concrete, or any other material differs greatly and depends entirely on the properties of the material used.

Wood, being relatively soft, can be worked entirely by hand tools, and structures can be built in timber 'without any mechanical aids.. Metal, however, cannot be easily worked without mechanical aids, and structures in metal require the use of machine tools to cut, drill, and shape the sections for fabrication.

The design of structures also depends on the properties of the material used. Timber, having a much lower tensile strength than steel,' must be used in larger sections to carry an equivalent load in structural work.

The difference between tensile and compressive strength of steel makes it equally suitable for use for either type of stressing, provided suitable sections are used in each case, but the same difference with concrete makes it more suitable for compressive than tensile stressing. Concrete, as a structural material, is worked' and formed in a plastic state and can therefore be moulded to any shape required. When the chemical action that produces hardening is complete the moulds or forms are stripped off, leaving a solid material with the qualities of stone and possessing properties which will be described later in this article. Composition of Concrete Concrete is made by mixing cement and an aggregate composed of hard inert particles of varying size, such as a combination of sand or broken stone screenings, with gravel, broken stone, cinders, ' broken brick, or other material and reducing the mixture to a plastic condition by mixing it with water. Compressive strength is generally accepted as the principal measure of the quality of concrete, and with a mixture of substances depending on chemical action to form the final product it will be readily understood that several factors will influence the quality and characteristics of that product. The most important of these factors are: 1. The proportions of mix of cement and aggregates, 2. The nature of aggregates and grading, x 3. The water-cement ratio, and 4. The type of cement.

Proportions of Mix

The proportions of mix of cement and aggregate are usually designed for a particular condition, the general method for farm work being arbitrary selection based on experience and common practice, such as 1 part by volume of cement, 2 parts of sand, and 4 parts of stone (referred to as a 1:2:4 mix). Though this method is uneconomical and does not give the best results with a given aggregate, it is quite satisfactory for all normal farm concrete work.

Nature of Aggregates

Because the nature of aggregates affects the strength of concrete considerably, the particles of all materials must be sound and strong with no flakiness. As the fine materials, including the cement, enter more or less into the voids of the coarse aggregate, materials must be suitably graded to occupy these voids and be clean and absolutely free from organic impurities.

The strength of concrete depends on the bonding together of the particles and the solidarity or density of the mixture. The strength increases with the quantity of cement in a unit volume, with the decrease in the quantity of mixing water, with the density of the concrete, and with the size of the coarsest aggregate.

Unless voids in the aggregate can be completely filled by particles of less size, density is reduced and the concrete is weaker in proportion to the reduction in density.

Angular aggregates such as broken stone produce stronger concrete than rounded gravel. Specially-graded mixtures of aggregates produce concrete of higher strength. Strength is decreased by an excess of sand over that required to fill the voids in the stone and give sufficient workability.

These points can be applied to the choice of aggregate; for example, if beach sand is available, it is preferable to obtain it from below high-water mark, as any wind-blown sand available from, say, sand dunes has not the same angular particles as freshly- . deposited sand. Water-cement Ratio The importance of the water-cement ratio depends on the principle that the strength of the concrete with given aggregates and cement bears a direct relation to the ratio of the volume of water to the volume of cement. The smaller the ratio of the volume of water to the volume of cement, as long as the mix is workable, the higher is the strength of the resulting concrete. Therefore, the reduction of the mix to a plastic condition by the addition of water should be carried only as far as necessary to produce reasonable workability.

The consistency to be used will depend on the character of the structure. Medium or quaking concrete is adapted for ordinary mass concrete such as foundations, heavy walls, large arches, piers, and abutments. Mushy concrete is suitable for rubble concrete and reinforced concrete such as that used for thin building walls, columns, floors,

conduits, water troughs, and tanks. Dry concrete may be used in dry locations for mass , foundations which must withstand severe compressive strain within a month after placing, provided it is carefully spread in layers not more than 6in. thick and is well rammed.

A medium or quaking mixture is of a tenacious, jellylike consistency which shakes on ramming. A “mushy” mixture will settle when dumped in a pile, and will flow very sluggishly into the form or round the reinforcing bars. A dry mixture has the consistency of damp earth.

The proportion of water in the mix is of vital importance, a very wet mix being much weaker than a dry or mushy one.

For farm concreting operations mixing is usually carried out in small batches, and uniformity of mix is difficult to obtain. An easy method of testing each batch for uniformity is known as the “slump test,” which will be described fully in a later article in this series. Types of Cement The type of cement used does not affect the quality as much as the characteristics of the concrete. Normal cement used for practically all purposes is known as Portland cement. Other cements obtainable are: — Waterproofed cement, for use where a waterproof or water-repellent concrete or mortar is particularly desirable. High-early-strength cement for use where high-strength concrete is required in 1 or 2 days. Plastic cement, for use where a particularly workable and fat mortar or concrete is desired, such as for masonry work. White cement, for use in architectural or ornamental work. Natural cement, for use as a common mortar for brick or stone work. A knowledge of Portland cement will help in the understanding of its behaviour in the making of concrete. Chemical Action of Cement Portland cement is made from a mixture of about 80 per cent, of carbonate of lime (limestone, chalk, or marl) with about 20 per cent, of clay in the form of clay, shale, or slag. After being intimately mixed the materials are finely ground by a wet or dry process and then calcined in kilns to a clinker. When cool the clinker is ground to a fine powder. This powder, which is the finished product, contains silica, alumina, certain metallic oxides, and some alkalis in varying proportions, depending on the raw materials used. '

When the cement powder is mixed with aggregates and water chemical action takes place between the cement and water. The aggregate, which occupies most of the volume of the hardened concrete, is inert and the chemical action which results in the hardening of the cement paste binds all into a homogeneous mass.

This chemical process, called hydration, causes the generation of large quantities of heat, rapidly at first and gradually decreasing as the curing of the concrete takes place.

As cement depends on water to bring about its chemical change, normal atmospheric moisture will cause the' change to take place with stored cement. For this reason-it is essential to keep cement dry up to the time of use. Cement which has partially set in the bags has lost a considerable part of its cementing properties and concrete made with recrushed, lumpy, or hardened cement will be a failure.

Similarly the remixing of mortar or concrete after the setting action has started is extremely detrimental to the final soundness; by breaking up and retarding the consolidation of the elements pockets are produced where moisture cannot reach entirely, preventing completion of chemical action, that is, completion of hardening. Properties of Concrete The properties of any material used for construction are the deciding factor in the choice of application of that material to construction. The properties of concrete for consideration as a construction material are: — ' 1. Strengthcompressive, tensile, and shear. 2. Watertightness. 3. Immunity against fire. 4. Workability. Strength

The compressive strength of concrete is very high and, being dependent on the type of aggregates used and the proportion of mix of aggregates and cement, can be regulated to suit the requirements of the particular construction.

Table 1 gives the compressive strength of different mixtures of concrete in pounds per square inch 28 days after use. From this table it is apparent that with a weak mixture of 1 part of cement to 9 parts of aggregate of sand and soft cinders a strength of 4001 b. is obtained, as compared with a strength of 33001 b. when 1 part of cement to 3 parts of aggregate of sand and hard granite rock are used.

The tensile strength of concrete is of less importance than the crushing or compressive strength, as the former is seldom relied on and any members with tensile stressing are built with steel reinforcing placed in the tensile

part. The true tensile strength is about 10 per cent, of the compressive strength.

Shear strength: The strength of concrete in direct shear is * relatively high, as distinct from indirect shear such as in a beam with diagonal tension where the concrete may break with a shearing stress equal to a much lower value.

Direct shear strength is from 50 to 60 per cent, of the compressive strength, whereas an indirect shear equal to 5 to 10 per cent, may cause fracture. Watertightness Concrete can be made practically impervious to water by proper proportioning and mixing and placing. Leakage through concrete walls is usually caused by poor workmanship and occurs at the joints between two successive days’ work and through cracks caused by contraction. New concrete may be bonded to old by wetting the old surface, plastering it with neat cement mortar, and then placing the concrete before the neat cement has set. Contraction cracks are almost impossible to prevent entirely, though a sufficient amount of reinforcement may reduce their width to permit only seepage of water.

To get the best results either a quaking or mushy consistency should be used, the concrete must be placed carefully to leave no visible stone pockets, and the entire structure should be made without joints and preferably in one continuous operation. A very wet mix will cause porous concrete.

The best waterproofing agent is an additional proportion of cement in the mix. For maximum watertightness mortar and concrete may require more fine material than would be used for maximum strength, though too much fineness will give porous concrete unless the cement content is increased. Gravel produces more watertight concrete than broken stone under similar conditions.

Patented compounds are available for producing watertight concrete, but under most conditions results as good may be obtained for less cost by increasing the percentage of cement in the mix.

Membrane waterproofing, consisting of asphalt or tar with layers of felt or tarred paper, may be advisable in certain cases. Immunity Against Fire The immunity of concrete against fire is apparent from its non-combust-ible nature and its low value of heat conductivity. Being non-combustible, it can be used where fire risk is great (for example, for petrol stores) and its low heat conductivity makes it useful for the protection of combustible material from a source of . heat. Workability Any material used for construction must be readily workable. Concrete is particularly suited in this respect, as it is worked in a plastic state, the particular properties of the finished product being brought about by chemical change. Effect of Oil Mineral oils applied externally do not injure concrete. Animal fats and

vegetable oils, however, tend to disintegrate concrete unless it has thoroughly hardened. Concrete resists the attack of diluted acids after it has thoroughly hardened, but is disintegrated by strong acids. Green concrete is injured by manure, but is not affected after it has thoroughly hardened. Electrolysis injures concrete under certain conditions, and electric currents should be prevented from reaching it.

Sea water attacks cement and disintegrates concrete unless the concrete is made with the very best materials under the best conditions. Deleterious action is greatly accelerated by frost. To prevent serious damage the concrete must be made with a rich mix (not leaner than 1:2:4) and with ex-ceptionally-good, well-graded aggregates and must be allowed to harden thoroughly before it is touched by sea water.

Though there is no essential difference in the strength of concrete mixed with fresh or sea water, the latter tends to retard the setting slightly and may increase the tendency of the reinforcement to rust. Fresh water should be used where possible and in every case mixing water must be clean.

After the setting and curing period concrete continues to harden ana does not attain full strength until nearly a year old. Table 2 shows the strength of ordinary Portland cement concrete at various ages.

Methods of Working Concrete

The methods of working concrete should be arranged so that they interfere in no way with the chemical action which forms the finished concrete.

If concrete is to be used as a construction material, the nature of the structure will be the deciding factor in the proportion of mix, the choice of aggregate, and the method of moulding the plastic concrete. The proportion of the mix will vary with

the strength requirements of the structure. The choice of aggregate will depend on whether the finished structure is moulded in thin or thick sections and whether the concrete is reinforced or mass.

The method of moulding the plastic concrete will depend on whether the structure is above or below ground and whether an arch, a slab, or unit articles such as blocks, fence posts, troughs, etc., are , being made. This also influences the state of plasticity in which the moulding can be carried out. Details of the best mixes for specified usage will be given in later articles in this series.

Whatever the construction and the corresponding variation of mix, aggregate, and method of moulding, the method of working the concrete is the same in every case and may be done either by hand or by machine.

As the hardening of the plastic mixture is dependent on chemical action, the mixed ingredients must be uniformly distributed to ensure that action is uniform throughout, and this can be accomplished only by adequate mixing. Hand Mixing Hand mixing should preferably be carried out on an even, non-absorbent surface. A concrete floor or slab offers an ideal surface, but if there is not one available, a surface, of timber can be constructed. The area chosen for mixing must be twice the size of the area required to accommodate one mix.

If up to f cub. yd. of materials is being mixed at one time, an area of not less than 12ft. x 12ft. should be provided; for smaller quantities the area can be proportionately less. Straight planks IJin. or 2in. thick should be laid side by side either on levelled ground or on 3in. x 4in. bearer timbers so that a firm, reasonably level surface is obtained. A shovelful of sand should be scraped over the boards to fill up any spaces between their edges.

As the proportioning of aggregates for all farm concreting operations will probably be done by volume, one or more measuring boxes will be necessary. The size of the boxes will depend on the volume of materials being mixed at any one time. If 1 cub. yd. is being mixed and the mix is 1:2:4, one box about 1/6 cub. yd. in volume would suffice. If a large amount of mixing is to be done, two boxes can be provided, one for the cement and sand and the other for the coarse aggregate. In this case the volume of the box for the aggregate would be four times the volume of the box for the cement and sand or 2/3 cub. yd. Fig. 1 shows the general construction of a measuring box.

The first step in hand mixing is to measure the coarse aggregate. If one small measuring box is being used and the mix is 1:2:4, the box is filled level with the top and emptied four times. If two boxes are used, the larger is filled level with the top only once and emptied. The boxes are placed on the mixing surface, filled more than level with the top, and the surplus brushed off with a straightedge. The box ’ is emptied by lifting and the coarse aggregate spread out over the mixing surface. Next, measure the fine aggregate, using the small box in exactly the same way as for the large aggregate and filling and emptying twice only. Spread the fine aggregate over the coarse. Measure the cement, using the small box once only. Spread the cement over the sand. These operations will result in a bed of gravel, sand, and cement spread over the mixing surface. This bed is them shovelled into a heap to one side of tne mixing surface. The heap is moved by shovelling to the other side of the surface and back again to its original position. The knack of shovelling the dry mix to ensure a uniform distribution of all materials is soon acquired, each shovelful, being spread over the heap and not dumped. As the success of the concrete depends on a thoroughly-uniform distribution of the materials, they should be turned over in the manner described until there is no doubt about their being thor- < oughly mixed. When the mixture has been turned over at least twice. the heap is again spread over the mixing surface and a ridge scraped round the edge, leaving a hollow in the middle. The water is poured into the hollow, and the ridge gradually pushed inward, the mixture being mixed all the time so that none of it remains dry. When the mix is all moistened it is turned over and puddled and sufficient water added to reduce the whole mix to the degree of plasticity required for the

work. Fig. 2 shows the various stages of hand mixing. A square-nose, long-handled (Cornish) shovel is the most suitable for mixing concrete, and should be used in the direction of the planks when mixing on a board ■ surface. • Machine Mixing Machine-mixing procedure differs from that of hand mixing because, owing to the bowl of the machine being wet from the previous mix, the ingredients cannot be mixed dry; if the materials were put into the bowl dry, they would cake on the beaters and bowl. The mixing procedure with a mixer, whether hand or power operated, is begun by placing about two-thirds of the water. required for the completed mix in the. mixer and then. a measured quantity of cement. A power-driven mixer will be revolving all, the time these operations are being carried out, but a hand mixer must be given a few turns to mix the cement and water thoroughly. Next, add about two-thirds the measured quantity of mixed aggregate and mix until all is moistened. The remainder of the aggregate can then be added and sufficient water added a little at a time to reduce the mix to the plasticity required. When all ingredients have been put into the mixer mixing should be continued until the consistency of the mix 'is uniform throughout. The mix can then be deposited in a wheelbarrow for disposal, and the operation repeated. ■ ■ As a- small mixer wffl be used for most farm concreting operations, measuring boxes are not necessary, the shovel being the most convenient measuring medium. The average 3 cub. ft. mixer will hold comfortably a mix measured by the shovel. A mix of, say, 1:2:4 will be 1 shovelful of cement, 2 of sand, and 4 of gravel, a total of 7 shovelfuls. Table 3 sets out units of weights and measures that are helpful in calculating. quantities of material to be mixed;

Handling Concrete Because concrete is plastic, the ingredients of the mix can become badly disposed by vibration. If wet concrete is conveyed in a wheelbarrow, the vibration consolidates the mix, driving off the water, and if the carrying is continued for any , length of time, the larger stones and gravel will tend to sink to the bottom. If - concrete must be transported in a wheelbarrow, it should be remixed in the barrow before placing. This effect of vibration on concrete is used to consolidate a mix after placing it in moulds, as in the manufacture of concrete posts. For this application, however, the vibration is controlled and is not carried to the point where redisposition of the particles begins to take place, but is used solely for consolidation and to release trapped air. The process of tamping or spading concrete placed in forms produces the same effect as vibrating and for this reason concrete can be overtamped. Tamping is necessary with the use of forms to ensure that no gravel pockets are left and to remove all coarse material from the outside surface to produce a clean finish. When concrete is run down a chute into forms the heavy aggregate tends

Care should be taken to see that all shovelfuls are equally piled. Cement will pile higher than gravel or sand, and if it is not measured exactly, it may cause the mix to be richer than intended.

to separate out and a quantity of pebbles may arrive in the forms ahead of the more even mix. This is not detrimental where the previous mix is still plastic, but if allowed to occur on a previous mix already hardened, it will result in a gravel pocket and poor bonding. The previous application of a bonding slurry of neat cement and water will help bonding, but will not remove the gravel pocket, which must be done by tamping.

The method of handling concrete must be arranged to suit the work. The degree of plasticity can be suited to the work, but for some work where there is external water the mix can be used dry. In the construction of a bag dam (described in the article “Establishing a Farm Water Supply System,” which appeared in the February, 1949, issue of the “Journal”) the dry mix is placed in bags and the creek water allowed to moisten the mix by percolation.

Dry-mix concrete, that is, a mix with a small amount of water to produce a consistency of damp earth, is used for the manufacture of concrete blocks or concrete field tiles by machine. A normal mushy mix is used for under-water concreting and is placed by means of a tremie. If under-water concrete were placed m the normal way, the cement would be washed out as the particles settled. A tremie, which consists of a length of pipe with a funnel attached, is placed with its discharge end near the bottom of the under-water mould. Concrete is then fed into the funnel and carefully placed in the mould by the tremie, the bottom of which is kept embedded in the concrete, which displaces water without the cement being washed out.

Because of . the detrimental effects of regauging or remixing concrete, no more concrete should be mixed than can be used before the initial set commences. Special care is necessary in hot weather, when setting may occur in a much shorter period than under normal conditions. It is desirable that concrete should be placed within half an hour of the addition of water and that it should not be disturbed after that period. Concrete mixed some time before it is deposited should be used only if it can be remixed to a workable consistency without the addition of water; if this cannot be done, it should be discarded.

Concrete should never be placed in very cold weather, as hardening, is considerably retarded as freezing point is approached.

Tables 4,5, 6, and 7. contain information which will assist in the calculation of quantities of materials required for different concrete work. In the tables concrete is divided into two classes— 1, extra-strong and relatively watertight concrete; and No. 2, ordinary, good concrete. Curing of Concrete Concrete attains its best results if it hardens in a warm, damp atmosphere. If it is exposed to a hot, dry atmosphere while hardening, there is danger of the water required by the cement for hardening being evaporated and possibly preventing hardening and certainly tending to produce contraction cracks.

All cement generates heat during setting, and if this heat can be conserved in the concrete, a satisfactory means of protection against frost is produced as long as it does not cause unbalanced internal stressing. This . naturally-developed heat can be conserved in newly-laid concrete by covering the concrete with a tarpaulin or other material placed not only to exclude draughts underneath, but to „ leave a space between the concrete and the covering.

The protection of newly-laid concrete from heat and drying winds is as vital as protection against frost, as the concrete is not hard enough at an early age to resist without cracking the stresses set up by contraction. Timber shuttering is an adequate protection if left in position for at least a week; otherwise the concrete should be kept damp for a fortnight after being laid by being covered with wet sacks, damp earth, or by frequent watering. Hardeners Though several . proprietary brands of concrete-waterproofing material or hardeners are available, the best means of waterproofing is by the use of additional cement and a consolidated mix. A gqod hardener that can be profitably applied to such work as milking shed floors or cattle yards is sodium silicate. This is not added to the concrete mix, but is applied as a solution to the surface after setting. It converts the inert lime set free from the cement during setting into silicate of Time, which is a strength-giving material, thus hardening the surface of the concrete. -

NOTE: Moist sand or aggregate contains about 1 gal. of water per cubic ft.

Proportions by parts Aggregate Proportions by parts 1:1:2 1:11:3 1:2:4 I 1:21:5 1:3:6 Granite or trap rock 3,300 2,800 2,200 1,800 1,400 Gravel, hard limestone, and hard sandstone 3,000 2,500 2,000 1,600 1,300 Soft limestone and sandstone .. 2,200 1,800 1,500 1,200 1,000 Cinders .. .. .. .. .. .. 800 700 600 500 400

TABLE I—COMPRESSIVE STRENGTH IN LB. PER SQ. IN. OF DIFFERENT MIXTURES OF CONCRETE 28 DAYS AFTER LAYING

Approximate Compressive percentage Age Compressive z Approximate percentage strength of hardness Ib./sq. in. of hardness 28 days . . 4,000 60 3 months . . 5,700 85 6 months . . 6,300 95 1 year . . 6,600 100

TABLE 2- STRENGTH OF ORDINARY PORTLAND CEMENT CONCRETE AT VARIOUS AGES (1:2:4 Laboratory Test Cubes)

1 cub. yd. = 27 cub. ft. = 27 cub. ft. 1 ton of cement = 24 cub. ft. cement = 24 cub. ft. = 18 hessian or 24 paper bags 941b. of cement = 1 cub. ft. 24 paper 1 hessian bag of cement contains If cub. ft. 11 cub. ft. 1 paper bag of cement contains 1 cub. ft. 1 cub. ft. 1 hessian bag of cement contains If bushels fl bushels i hessian bag of cement weighs 1251b. 1251b. 1 paper bag of cement weighs 941b. 941b. 4 hessian bags of cement will mix about 1 cub. yd. of concrete of 1:2:4 mix.

TABLE 3—WEIGHTS AND MEASURES

Thickness Area covered Thickness Area covered in. sq. ft. In. sq. ft. 3 . . 864 ' 4 81 ? ■ . . 648 648 41 4J 72 72 " ' 3 . 432 6 54 t .... 324 8 40.5 2 162 9 36 3 .. 108 12 27

TABLE 4—AREA COVERED BY 1 CUB. YD. OF CONCRETE OF DIFFERENT THICKNESSES

Class of work Thickness in. Quality of concrete Size of graded gravel in. Light footpaths, dairy and light shed floors, and base course for tennis courts .. No. 2 . 3 to 4 1 or less base course for tennis courts No. 2 3 to 4 1 or less Cow yards, heavy shed floors, ordinary garage floors, and drives or less floors, and drives .. 4 to 6 4 to 6 No. 2 No. 2 11 or less Floors, drives, etc., for extra-heavy wear 4 to 6 No. 1 11 or less Thick foundations and unimportant large masses such as retaining wails and thick dams (under 6ft. high) 3 or less 6ft. high) .. .. As required As required No. 2 No. 2 3 or less Thick dams over 6ft. high .. .. .. As required No. 1 3 or less Reinforced inside walls and unimportant shed walls i or less walls .. .. .. .. 3 to 6 No. 2 2 or less Reinforced important outside walls, cisterns, tanks, swimming pools, ponds, silos, and 3 to 6 No. 2 cellars .. As required No. 1 3 or less Fence posts (farm) .. .. .. .. 4 to 8 No. 2 I or less Plaster coats for paths, floors, walls, etc., 4 to 8 No. 2 I or less top course for tennis courts, and thin troughs, stucco, rough cast, and ornaments such as sundials, fountains, seats, etc. No. 1 2 tO 4 Sand only sundials, fountains, seats, etc. .. .. No. 1 to ? Sand only

TABLE 5-QUALITY OF CONCRETE AND SIZE OF GRAVEL FOR DIFFERENT PURPOSES

If aggregate used Is graded up to Cement (12411b. per hessian bag; 941b. per paper bag) lb. -XX. Sand and gravel or if already mixed (loose measure) cub. ft. Sand, moist (loose measurement) cub. ft. Gravel or metal (loose measurement) ' cub. ft. For No. 1 concrete jin. 740 131 24 27 lin. 720 13 24 27 Jin. 676 12 26 27 1 in. 640 111 27 28 Ilin. 600 11 27 272 2in. 580 101 27 27 3in. 560 10 26 27 For No. 2 concrete gin. 530 17 26 31 .Jin. 520 161 27 31 Jin. 480 151 28 31 lin. 460 15 29 31 lain. 430 14 28 30 2in. 420 131 28 291 3in. 410 13 27 291

TABLE 6-QUANTITIES OF MATERIALS TO MAKE 1 CUB. YD. OF CONCRETE

Quantities to mix with f bag of cement Sand and gravel If aggregate used Sand, moist Gravel or metal or if already mixed Approx, amount is graded up to Quantities to mix with 1 bag of cement Approx, amount (loose measure) (loose of concrete measure) (loose measure) of concrete cub. ft. cub. ft. cub. ft. Sand, moist Gravel or metal or (loose measure) (loose measure) cub. ft. cub. ft. cub. ft. Sand and gravel if already mixed (loose measure) cub. ft. cub. ft. For No. 1 concrete (using 6 gals, of water) (using 6 gals, of water) fin. 21 4 44 41 ■ Jin. 21 41 42 42 2in. 4 42 21 51 42 5 5 5 5 tin. 2! 51 54 51 flin. 21 51 52 5s 2in. 21 52 51 52 3in. 21 52 6 6 For No. 2 concrete (using 8 gals, of water) (using 8 gals, of water) gin. 4 6 71 \ •u 2*0. 4 61 71 6 2 2in. 4 64 7i 6 2 4 71 8 7 1 in. 4 72 82 71 ' lain. 4 81 81 72 2in. 4 4 si 81 81 82 72 8 3in. 4 4 81 81 82 9 8 81 x

TABLE 7—QUANTITIES OF SAND AND GRAVEL TO MIX WITH 1 BAG OF CEMENT TO MAKE CONCRETE