Measurement and batching of cement, fine and coarse aggregates shall be by weight. Scales in use shall be accurate when static load tested to + 0.4 percent of the total capacity of the scale. One sack of cement shall be considered to weigh 94 pounds net. Bulk and cement from fractional sacks shall be weighed.
3.1 STRENGTH OF CONCRETE: The concrete shall attain a minimum expected strength of concrete at 28 days of 3,500 pounds per square inch compressive strength and/or 550 pounds per square inch flexural strength "modulus of rupture".
Concrete specimens to check the adequacy of the delivered concrete for strength shall be made and cured in accordance with ASTM C-31, making and curing concrete compressive and flexural test specimens in the field. Compressive strength specimens shall be tested in accordance with ASTM C-39, and flexural strength determination shall be in accordance with ASTM C-78. If concrete test(s) is ordered by the Planning Commission, or its duly authorized representative, testing shall be performed by a commercial laboratory.
3.2 PROPORTIONING CONCRETE: It shall contain not less than six sacks of cement (94 lbs. per sack) per cubic yard of concrete and a maximum of 5-1/2 gallons of water per sack of cement, including surface water contained in the aggregates. The proper proportions of cement, water and aggregates shall be determined in accordance with ACI Standard 613, "Recommended Practice for Selecting Proportions for Concrete", or the Portland Cement Association booklet "Design and Control of Concrete Mixers".
The entrained air shall be obtained by using air-entraining agent. All concrete shall be air- entrained in accordance with the following:
Maximum size of Aggregate (inches) | Air Content Percent by Volume |
1-1/2, 2, 2-1/2 | 5 " 1% |
3/4, 1 | 6 " 1% |
3/8, 1/2 | 7-1/2 " 1% |
The air content of freshly mixed air-entrained concrete shall be checked by the Planning Commission, or its duly authorized representative. Concrete with air contents above or below the amount specified shall be corrected by adjustments in the mix design or quantities of air-entraining admixture being used.
3.3 CONSISTENCY: The slump of the concrete shall be from 1 to 4 inches when hand finishing techniques are employed and 1 to 3 inches when a mechanical finishing machine is used. The consistency shall be measured as described in the current ASTM Standard Method of Slump Test for Consistency of Portland Cement Concrete (Designation C-143) or the Method of Test for Ball Penetration for Portland Cement Concrete, ASTM (Designation C-360).
3.4 READY-MIXED CONCRETE: All ready-mixed concrete shall be furnished in accordance with the current ASTM specifications for ready-mixed concrete (Designation C-94).
When construction conditions are such that it is absolutely necessary for trucks hauling concrete to operate on the grade between forms, they shall not back over previously deposited fresh concrete.
3.4.1 TIME OF DELIVERY: Concrete shall be delivered and discharged from a truck mixer or agitator truck within a period of 1-1/2 hours at air temperatures up to 85°F. and one hour at air temperatures higher than 85°F. after introduction of the water to the cement and aggregates or of the cement to the aggregates. Delivery tickets shall have this time clearly shown thereon, and the contractor shall check the same to be certain that delivery is made within the period specified.
3.4.2 TYPE OF DELIVERY EQUIPMENT: Concrete shall be delivered in truck mixers or agitator truck (trucks providing mechanical agitation by revolving drums or revolving blades in a stationary drum) operated after time required for thorough mixing of the concrete at the speed designated by the manufacturer as agitating speed.
3.5 JOB-MIXED CONCRETE: Job-mixed concrete shall be mixed in a drum mixer which shall conform to the concrete paving mixer standards of the Mixer Manufacturers Bureau of the Association General Contractors of America. The mixer shall be capable of combining the aggregates, cement, and water into a thoroughly mixed and uniform mass within the specified time and of discharging the material without segregation. The entire contents of the drum shall be discharged before recharging. The volume of the mixed materials per batch shall not exceed the manufacturer's guaranteed capacity of the mixer.
3.5.1 TIME OF MIXING: The mixing of each batch shall continue for not less than one minute after all materials, except water are in the mixer. The mixer shall rotate at the rate recommended by its manufacturer. The mixer shall be provided with a batch timing device which shall be subject to inspection and adjustment by the Planning Commission, or its duly authorized representative, at any time.
3.6 RETEMPERING OF CONCRETE: Retempering of concrete which has partially hardened, by remixing, with or without water, shall not be permitted.
(Ord. 1971-2, passed 4-6-71)