501.03 Quality of Concrete; Proportioning. The proportioning of water, cement, and aggregates will be based on predetermined cement content, unless the special provisions require the substitution of proportions designed for minimum strength.

(a) Mix based on predetermined cement content.—The proportions shown in table 501-1 are designed to produce concrete containing 6.0 bags of cement plus or minus 2 percent per cubic yard, suitable for normal machine placement and having an air content within the limits specified and a slump of 12 to 3 inches for nonvibrated concrete. The weights are saturated surface-dry weights.

The percentage of air entrained in the mix shall be 7 percent plus or minus 2 percent where frost action is expected, and 5 percent plus or minus 2 percent where it is not.

Table 501-1.-Approximate proportions for air-entrained portland cement concrete of specified cement content

Table 501-1 is based on the assumption that the saturated surface-dry bulk specific gravity of sand, gravel, and stone is 2.65 and that of slag is 2.25; that the sand is well graded and has a fineness modulus between 2.6 and 2.9; and that the coarse aggregate is composed of about 60 percent of the larger separated component and about 40 percent of the smaller separated component, when separated at the 4-inch sieve for the 12-inch to No. 4 size, or at the 1-inch sieve for the 2-inch to No. 4 size.

Bulk specific gravity will be determined by using AASHO

T 84 or T 85, the slump using AASHO T 119, and the air content using AASHO T 152 for sand, gravel, and stone and ASTM C 173 for slag.

For batching, the quantities given in table 501-1 will be adjusted by the Engineer for differences in specific gravity in order to obtain the desired workability and slump, for the specified cement content, and for the free moisture content of the aggregates being used. The total weight of aggregates per bag of cement shall not be varied except to compensate for variations in specific gravity and/or moisture content.

(b) Mix based on minimum flexural or compressive strength.—If the special provisions stipulate that the proportions of cement, fine aggregate, coarse aggregate, and water to be used in the mix are to be based on design for minimum strength rather than on predetermined cement content, the Engineer will determine, from laboratory tests of the materials to be used, the nominal cement content and the proportions that will produce a workable concrete having a slump of between 12 and 3 inches if not vibrated (or between 2 and 11⁄2 inches if vibrated) and a flexural stength of not less than 550 pounds per square inch when tested by the thirdpoint method; or 650 pounds per square inch when tested by the mid-point method; or a compressive strength of 3,500 pounds per square inch when tested at 14 days in accordance with AASHO T 97, T 177, or T 22, respectively.

Laboratory test specimens will be made in accordance with AASHO T 126. The percentage of air entrained in the mix shall be 7 percent plus or minus 2 percent where frost action is expected and 5 percent plus or minus 2 percent where frost action is not expected. Slump will be determined using AASHO T 119, and air content using AASHO T 152 for sand, gravel, and stone, and ASTM C 173 for slag and other highly porous coarse aggregates. The cement content will be determined in accordance with AASHO T 121. The proportions will be such that the cement content is not less than 51⁄2 bags per cubic yard, and the mixing water not more than 52 gallons per bag of cement, including free surface moisture on the aggregates but not including moisture absorbed by the aggregates, in case the concrete is to be subjected to severe freezing and thawing or to salting; also, a cement content of not less than 5 bags per cubic yard and water not more than 61⁄2 gallons per bag elsewhere. In case the portions, as determined by the Engineer, or the nominal cement content, as determined by AASHO T 121 are found, in practice, to produce a mix unsatisfactory in plasticity, strength, or workability, using the aggregates on which the proportions were based, the Engineer may alter

the proportions, with adjustment of payment as stipulated in subsection 501.25 (c).

The special provisions may include tentative proportions for aggregates which the Contractor may choose to use and the grading of the aggregates used in determining the proportions. Such proportions will be considered only as tentative and for bidding purposes only. The Contractor will not be required to use such aggregates or gradings.

(c) General.-In the event the Contractor elects to use an air-entraining admixture in accordance with subsection 501. 02, the Engineer will determine by trial the amount of the particular admixture selected by the Contractor that will produce concrete having the specified air content and this amount shall not be varied except as directed by the Engi

neer.

When circumstances necessitate the use of admixtures other than those included in the specifications, the admixtures, such as accelerators and water-reducers will be permitted only with the written permission of the Engineer.

In the event the Contractor elects to use an air-entraining portland cement and it is found impossible to produce concrete having the required air content with the materials and mixing procedure employed by him, the Contractor shall make such changes in materials or methods of mixing, or both, as may be necessary in order to insure full compliance with the requirements of the specifications.

Batches involving fractional bags of cement will not be allowed unless cement in bulk is used.

In order that the concrete may attain strength more quickly, the Contractor may, at his option and without added compensation, use additional cement per batch but shall not exceed a total cement content of 8 bags per cubic yard of concrete.

501.04 Equipment. Equipment and tools necessary for handling materials and performing all parts of the work shall be approved by the Engineer as to design, capacity, and mechanical condition. The equipment shall be at the job site sufficiently ahead of the start of construction operations to be examined thoroughly and approved.

(a) Batching plant and equipment:

(1) General.-The batching plant shall include bins, weighing hoppers, and scales for the fine aggregate and for each size of coarse aggregate. If cement is used in bulk, a bin, hopper, and separate scale for cement shall be included. The weighing hoppers shall be properly sealed and vented to preclude dusting during operation. Approved safety devices shall be provided and maintained for the protection of all personnel engaged in plant operation, inspection, and test

ing. The batch plant shall be equipped with a suitable nonresettable batch counter which will correctly indicate the number of batches proportioned.

(2) Bins and hoppers.-Bins with adequate separate compartments for fine aggregate and for each size of coarse aggregate shall be provided in the batching plant.

(3) Scales.-The scales for weighing aggregates and cement shall be of either the beam type or the springless-dial type. They shall be accurate within 0.5 percent throughout the range of use. When beam-type scales are used, provision, such as a "tell-tale" dial, shall be made for indicating to the operator that the required load in the weighing hopper is being approached. A device on weighing beams shall indicate critical position clearly. Poises shall be designed to be locked in any position and to prevent unauthorized change. The weigh beam and "tell-tale" device shall be in full view of the operator while charging the hopper, and he shall have convenient access to all controls.

Scales shall be inspected and sealed as often as the Engineer may deem necessary to assure their continued accuracy. The Contractor shall have on hand not less than 10 fifty-pound weights for frequent testing of all scales and shall provide means for hoisting the weights as needed.

(4) Automatic weighing devices.-When required by the contract, batching plants shall be equipped to proportion aggregates and bulk cement by means of automatic weighing devices of an approved type.

(b) Mixers:

(1) General.-Concrete may be mixed at the site of construction, or at a central point, or wholly or in part in truck mixers. Each mixer shall have attached in a prominent place a manufacturer's plate showing the capacity of the drum in terms of volume of mixed concrete and the speed of rotation of the mixing drum or blades.

(2) Mixers.-Mixing shall be in an approved mixer capable of combining the aggregates, cement, and water into a thoroughly mixed and uniform mass within the specific mixing period, and of discharging the mixture for placing, without segregation, on the prepared grade. Each paver or stationary mixer shall be equipped with an approved timing device which will automatically lock the discharge lever when the drum has been charged and release it at the end of the mixing period. The device shall be equipped with a bell or other suitable warning device adjusted to give a clearly audible signal each time the lock is released. In case of failure of the timing device, the mixer may be used for the balance of the day while it is being repaired, providing that each batch is mixed 90 seconds. The mixer shall be equipped with a

suitable nonresettable batch counter which shall correctly indicate the number of batches mixed.

The mixers shall be cleaned at suitable intervals. The pickup and throw-over blades in the drum or drums shall be repaired or replaced when they are worn down 4-inch or more. The Contractor shall (1) have available at the job site a copy of the manufacturer's design, showing dimensions and arrangements of blades in reference to original height and depth, or (2) provide permanent marks on blade to show points of 4-inch wear from new conditions. Drilled holes of 4-inch diameter near each end and at midpoint of each blade are recommended.

(3) Truck mixers and truck agitators.—Truck mixers used for mixing and hauling concrete, and truck agitators used for hauling central-mixed concrete, shall conform to the applicable requirements of AASHO M 157.

(4) Nonagitator trucks.-Bodies of nonagitating hauling equipment for concrete, shall be smooth, mortartight, metal containers and shall be capable of discharging the concrete at a satisfactory controlled rate without segregation. The concrete shall be discharged from the bottom of the container. If discharge of concrete is accomplished by tilting the body, the surface of the load shall be retarded by a suitable baffle. Covers shall be provided when needed for protection.

(c) Finishing equipment:

(1) Finishing machine. The finishing machine shall be equipped with at least two oscillating type transverse screeds or other comparable means for striking off the concrete as required by subsection 501.13.

(2) Vibrators.-Vibrators, for full width vibration of concrete paving slabs, may be either the surface pan type or the internal type with either immersed tube or multiple spuds. They may be attached to the spreader or the finishing machine, or may be mounted on a separate carriage. They shall not come in contact with the joint, load transfer devices, subgrade, or side forms. The frequency of the surface vibrators shall not be less than 3,500 impulses per minute and the frequency of the internal type shall not be less than 5,000 impulses per minute for tube vibrators and not less than 7,000 impulses per minute for spud vibrators.

When spud type internal vibrators, either hand operated or attached to spreaders or finishing machines, are used adjacent to forms, they shall have a frequency of not less than 3,500 impulses per minute.

(d) Concrete saw.-When sawing joints is elected or specified, the Contractor shall provide sawing equipment adequate in number of units and power to complete the sawing with a water-cooled diamond edge saw blade or an abrasive wheel