maximum slumps and the Government reserves the right to require a lower water-cement ratio in any or all mixes than required to produce these slumps whenever, in the opinion of the contracting officer, such lower water-cement ratios are practicable and will produce concrete of better quality. 97. Cement.—Portland cement for the concrete, grout, gunite, and mortar will be furnished to the contractor by the Government as provided in paragraph 27, and the cement will be delivered to the contractor in carload lots in bulk: Provided, That, at the request of the contractor, cement for grout, gunite, and isolated minor items of concrete work, as determined by the contracting officer, will be furnished in sacks. The contractor shall return to the delivery yard, as the contracting officer may direct, all empty sacks cleaned of cement in a manner satisfactory to the contracting officer, securely bound in bundles in such manner and of such size as the contracting officer may direct. For all sacks not returned in serviceable condition the contractor will be charged the same amount that the sacks cost the Government. In order that cement will not become unduly aged after delivery, the contractor shall not use cement in the work direct from his freighting or other hauling or transporting operations whenever any cement is available that has been stored more than sixty (60) days after delivery to the contractor. Any cement in excess of five hundred (500) barrels that is kept by the contractor in excess of two hundred (200) days before it is used in the work will be condemned and its entire cost to the Government at the point of delivery to the contractor will be charged to the contractor. Storage bins for cement shall be so constructed that there will be no dead storage. Cement will be tested at the mills to see that it conforms to the requirements of the Government. The contractor shall give the contracting officer not less than thirty (30) days’ notice in writing of his cement requirements. The requirements shall be stated, in so far as practicable, in quantities of not less than carload lots. The contractor shall be responsible for the proper unloading of the bulk cement in such a manner that none of the cement will be damaged or wasted. Any cement damaged or wasted by the contractor due to carelessness in unloading and storing or otherwise will be charged to the contractor at its entire cost to the Government at the point of delivery to the contractor. It is now estimated that for all the items of concrete included in the schedule an average of between 1.1 and 1.2 barrels of cement per cubic yard of concrete will be used, this average including the cement used in grouting rock foundations and in horiZontal construction joints. However, as provided in paragraph 96, the concrete mixtures shall be under the directional control of the contracting officer, and no adjustment in compensation to the contractor will be made on account of any variation in the amount of cement used in the concrete and grout, should the actual average amount of cement per cubic yard be greater or less than the estimated limits. The unit prices bid in the schedule for the various items of concrete, grouting, gunite, and other work shall include the cost of the required hauling, handling and storing of the cement that is used in the concrete, gunite, grout, and other work. 98. Sand.-Sand for concrete, gunite, grout, and mortar shall be obtained from natural deposits, under the provisions of paragraph 37. The sand particles shall be hard, dense, durable, oncoated, nonorganic rock fragments that will pass a %-inch square or a 5%-inch round open*8. It must be free from injurious amounts of dust, lumps, soft or flaky particles, shale, alkali, *ganic matter, loam, mica, or other deleterious substances. The sand as it is used in the con*te must be so graded that concrete of the required workability, density, and strength can be made without the use of an exgeş of water or cement. The sand for concrete shall have a oneness modulus of not less than #3, nor more than 3.25, unless approval is given by the contracting officer to use sand not meeting this requirement. The fineness modulus will be deter*ed by dividing by 100 the sum of the percentages retained on Tyler standard sieves, numbers 4, 8, 14, 28, 48, and 100. The suitability of the sand will be determined by the contracting officer with the aid of tests made in accordance with the standard practice of the United States Bureau of Standards. The sand shall be washed unless specific written authority is given by the contracting officer to use unwashed sand. The contractor shall screen and, if required, wash all sand at the gravel deposits. The sand shall be such that tests of briquettes, made in proportion of three parts sand to one part cement, by weight, shall develop a tensile strength of not less than the strength developed by such tests with standard Ottawa sand. Any crushing, rolling, blending, screening, washing, or other operation on the sand required to meet these specifications shall be done by the contractor and the cost thereof shall be included in the unit prices bid in the schedule for the items of work in which the sand is used. 99. Broken rock or gravel.-The broken rock or gravel for concrete must be hard, dense, durable, uncoated rock fragments free from injurious amounts of soft, friable, thin elongated or laminated pieces, alkali, organic or other deleterious matter. It shall be so graded that concrete of the required workability, density, and strength can be made without the use of an excess of sand, water, or cement. The suitability of the broken rock or gravel will be determined by the contracting officer with the aid of tests made in accordance with the standard practices of the United States Bureau of Standards. Any crushing, blending, screening, washing, or other operation on the broken rock or gravel required to meet these specifications shall be done by the contractor and the cost thereof shall be included in the unit prices bid in the schedule for the items of work in which the broken rock or gravel is used. The broken rock or gravel shall be washed unless specific written authority is given by the contracting officer to use unwashed broken rock or gravel. The contractor shall screen and, if required, wash all gravel at the gravel deposits. The broken rock or gravel shall all pass through a screen having 2%-inch square or 3-inch round openings and shall be retained on a screen having 4-inch square or %g-inch round openings. It shall also be separated into three intermediate sizes by screens having %-inch square or %-inch round openings, and 1%-inch square or 1%-inch round openings. Screens having openings of other sizes or shapes may be used: Provided, That equivalent results, as determined by the contracting officer, are obtained. The relative amounts of each size of broken rock or gravel to be used in each mix of concrete and in all parts of the work will be determined by the contracting officer, and will be based on securing concrete having the required workability, density, impermeability, strength, and economy, without the use of an excess of sand, water, or cement and using in so far as practicable, the entire yield of suitable material from the natural deposits from which the broken rock or gravel is obtained. The contracting officer will determine the maximum size of broken rock or gravel to be used in each part of the work. For very thin or heavily reinforced parts, the maximum size will be that determined by the screen having %-inch square or %-inch round openings; for somewhat heavier portions of the work the maximum size will be that determined by the screen having 1%-inch square or 1%-inch round openings; and for the more massive portions the maximum size will be that determined by the screen having 2%-inch square or 3-inch round openings. 100. Cobble rock-Cobble rock shall in general be added to the mix. Such cobble rock shall be sound, clean gravel or broken rock of such size as will pass through a screen having 8-inch square or 9-inch round openings and be retained on a screen having 2%-inch square or 3-inch round openings. The amount of such cobble rock to be used shall be as determined by the contracting officer, based on producing the most economical concrete of the required strength and in so far as practicable, utilizing the entire yeild of the natural deposit or quarry from which the broken rock or gravel is obtained. The contractor shall screen and, if required, wash all cobble rock at the gravel deposits. The use of cobble rock will not be required or permitted where the concrete is reinforced, or in any concrete structure the least dimension of which is less than 30 inches. - 101. Samples of concrete aggregates.—Suitable samples of aggregates, as they are to be used in the concrete, gunite, grout, or mortar, shall be furnished and delivered to the contracting officer, either at the laboratory of the Bureau of Reclamation at Boulder City or at the laboratory at the dam site or at both, as required by the contracting officer, at least sixty (60) days in advance of the time when the pouring of the concrete is to begin, and at other times and in such amounts and manner as directed by the contracting officer. These samples shall be sufficient in quantity to permit the making of such test specimens as may be required for determining the suitability and proportions of the materials. The contractor shall give the contracting officer all necessary assistance in securing and handling the samples. The cost of all work required to carry out the provisions of this paragraph and the cost of hauling and handling cement used in testing concrete shall be included in the unit prices bid in the schedule for the items of work in which aggregates are used. 102. Water for mixing.—The water used in concrete, grout, gunite, and mortar must be reasonably clean and free from objectionable quantities of silt, organic matter, alkali, salts, and other impurities. It is contemplated that water equal in chemical analysis to the Colorado River water at the dam site prior to the beginning of construction work under these specifications will be satisfactory for mixing water. Water shall not be taken directly from the Colorado River and used in mixing but shall first be stored in settling basins so that the water used in mixing will be practically free from silt, or, at the option of the contractor, the silt may be removed by other methods as approved by the contracting officer. The entire cost of furnishing the mixing water of a quality as provided in this paragraph shall be included in the prices bid in the schedule for the items of work in which mixing water is used. 103. Mixing.—The cement, sand, broken rock or gravel, and cobbles shall be so mixed and the quantity of water added shall be such as to produce a homogenous mass of uniform consistency. Dirt and other undesirable substances shall be carefully excluded. All concrete shall be thoroughly mixed in a batch mixer of an approved type and size, and one so designed as to positively insure a uniform distribution of all the component materials throughout the mass during the mixing operation. Each mixer and its operation shall be subject to the approval of the contracting officer, and the use of any mixer that, in the opinion of the contracting officer, at any time produces unsatisfactory results shall be promptly discontinued until it is repaired or replaced. In general, only sufficient water shall be used in mixing to give a workable mix. The mixing of each batch shall continue not less than the number of minutes as stated in the tabulation at the end of this paragraph, after all materials, including water, are in the mixer, during which time the mixer shall rotate at the speed for which it has been designed or at such Speed as will produce a mass of uniform consistency at the end of the mixing period. The drum of the mixer shall have a peripheral speed of about 200 feet per minute. Overloading of mixers will not be permitted. A mechanically operated timing device, satisfactory to the contracting officer, shall be installed by the contractor as provided in paragraph 96.

104. Temperature of concrete.—Concrete when deposited shall have a temperature of not less than 40°F. nor more than 100°F. In freezing weather suitable means shall be provided for maintaining the concrete at a temperature of at least 40°F. for not less than 72 hours after placing or until the concrete has thoroughly hardened. The methods of heating the materials, if required, and protecting the concrete shall be subject to the approval of the contracting officer.

105. Forms.-Forms to confine the concrete and shape it to the required lines shall be used wherever necessary. Where the character of the material cut into to receive a concrete structure is such that it can be trimmed to the prescribed lines, the use of forms will not be required. The forms shall be of sufficient strength and rigidity to hold the concrete and to withstand the necessary pressure and ramming without deflection from the prescribed lines. The surfaces of all forms in contact with the concrete must be rigid, tight, and Smooth. In order to secure smooth surfaces on the main body of the dam and in the concrete linings in tunnels, adits, and shafts, the forms, if not entirely of metal construction, shall be faced with sheet steel or with other suit. able metal material. The sheets shall be carefully selected and shall be placed and maintained on the forms without wrinkles, bumps, or other imperfections. When such metal-faced forms have been used repeatedly and are, in the opinion of the contracting officer, unfit for further use they shall be discarded by the contractor and new forms shall be provided by the contractor, and the contractor shall be entitled to no additional compensation by reason of the necessity for discarding any forms which are unsuitable for use, as determined by the contracting officer. Suitable devices shall be used to hold adjacent ends and edges of panels or other forms together and in accurate alignment. Metal surfaces will not be required on forms which are to be used only once: Provided, That the lagging of forms for concrete surfaces that will be exposed to view and for all other concrete surfaces that are to be finished smooth, shall be tight, smooth-surfaced, and bevel-edged or matched. Forms without metal facing may be used more than once on such portions of the work as may be determined by the contracting officer, if, in the opinion of the contracting officer, they are maintained in serviceable condition and thoroughly cleaned before being reused. Metal rods or other similar devices to hold the forms will be permitted in the structures if proper means are used to take out or omit a portion of each of the rods from the nearest surface of the concrete to a depth of at least 2 inches. All holes left after removal of the rods shall be immediately and completely filled with cement mortar and the surface left smooth and in good condition. Wire ties will be permitted only where approved by the contracting officer and if used they shall be cut off flush with the concrete surface after the forms are removed. Before placing concrete the surfaces of all wooden, metal, and metal-faced forms shall be oiled with a suitable nonstaining oil of a quality satisfactory to the contracting officer, and just before placing concrete precaution shall be taken to see that all form anchors and ties are thoroughly secure and tight. Where forms are placed in successive units for continuous surfaces, care shall be exercised to fit the forms tightly over the completed surface so as to prevent leakage of mortal from the concrete. Where the thickness of tunnel, adit, or shaft lining is not sufficient to permit men to work behind the forms, inspection openings shall be provided in the forms as directed by the contracting officer. Forms shall be left in place until their removal is authorized by the contracting officer, and shall then be removed by the contractor with care so as to avoid injury to the concrete. Except as otherwise provided in paragraph 122, the contractor shal furnish all equipment, labor, and materials for forms, including lumber, metal forms or metal facing, bolts, spikes, nails, metal rods or wire ties, and oil, and the cost thereof shall be included in the unit prices bid in the schedule for concrete.

106. Placing.—Concrete shall be placed in the work before the cement takes its initial set The contractor will be charged for any cement wasted at its cost to the Government at the poin'

of delivery to the contractor. All foundation surfaces upon or against which concrete is to be placed shall be free from mud and debris. After cleaning, and immediately before placing concrete, all approximately horizontal foundation surfaces for the dam and other parts of the work, as directed by the contracting officer, shall be covered with a layer of mortar 1 inch thick, consisting of the regular concrete mixture without the coarse aggregates, or as directed by the contracting officer. The concrete mortar shall be thoroughly worked with brooms or otherwise into all irregularities of the surface. Concrete shall then be placed immediately upon the fresh mortar. When the placing of concrete is to be interrupted long enough for the concrete to take its final set the working face shall be given a shape, as determined by the contracting officer, by the use of forms or other means, that will secure proper union with subsequent work. All concrete surfaces, such as the horizontal construction joints of the dam, upon or against which concrete is to be placed and to which the new concrete is to adhere, shall be roughened, and all laitance and loose or defective surface concrete shall be removed by thorough scrubbing, brush.ing, chipping, sand-blasting, washing, and blowing with compressed air. The cleaned surfaces shall be thoroughly moist when concrete or mortar is placed upon or against them, but shall be free from pools of water. The cleaning of concrete surfaces as described above shall be done while the concrete is still sufficiently soft to permit thorough cleaning. Any surface which is allowed to become thoroughly hardened before cleaning shall be completely chipped to a depth as directed by the contracting officer. All horizontal construction joints in the dam shall be provided with keys as shown on the drawings, or as directed by the contracting officer. All horizontal concrete surfaces for the dam, and other parts of the work as directed by the contracting officer, which have set for 48 hours or more, and upon which concrete is to be placed, and to which the new concrete is to adhere, shall be covered with a layer of mortar approximately 9% inch thick, consisting of the regular concrete mixture without the coarse aggregates, or in proportions determined by the contracting officer. Concrete shall then be placed immediately upon the fresh mortar. Methods of placing which will deliver concrete of the required consistency into the forms and into final position without segregation shall be used. Placing of concrete in the dam shall, in general, be done by means of bottom-dump buckets or other methods whereby each complete mixer batch or combination of mixer batches is conveyed in one mass to its location in the dam. Methods of conveying concrete to any of the structures, by which the mixed batch or combination of batches is progressively loaded into chutes, belts, conveyors, or other similar equipment and carried in a thin continuous flow to the forms, will not be permitted: Provided, That such methods of conveyance may be used, if approved in writing by the contracting officer, for Very limited, isolated sections of the work. The continuous flow methods of conveyance are excluded under these specifications: First, because of the hazard to workmen from falling rock; and second, because of the difficulty of controlling the consistency of the concrete mix. Concrete shall be deposited in all cases as nearly as practicable directly in its final position and shall not be caused to flow in the mass in a manner to permit or cause segregation. Dropping the concrete vertically a distance of more than 5 feet or depositing a large quantity at any point and running or working it along the forms will not be permitted. Except as intercepted by Contraction joints, all concrete shall be placed in continuous horizontal layers, the thickness of which generally shall not exceed 12 inches. All lifts in placing concrete shall be rammed, tamped, or worked with suitable appliances, tamping bars, shovels, or forked tools until it completely fills the forms, closes snugly against all surfaces, and is in perfect and complete contact with any steel used for reinforcement. Where smooth surfaces are required, and for all surfaces which Will be permanently exposed to the weather, and for all surfaces next to metal conduits or other embedded metal around which it is desirable to prevent leakage, the adjacent concrete shall be