all material entering into the concrete shall be as deter- mined in the approved trial mixes. All necessary equipment shall be provided to determine and control the actual amount: of materials entering into each concrete mix. The proportions will be changed whenever, in the opinion of the Engineer, such changes become necessary to overcome deficiencies and obtain the specified strength, the desired durability, density, workability or consistency. Consistency in the field shall be measured by the slump test in accordance with the American Society for Testing Materials Standard C 143-39 The slump shall fall within the following tabulation: Type of Structures Slumps in Inches For Concrete with- For Concrete Conout an Admixture taining an Admixture Minimum Massive sections, pavements & slabs on the ground Thin walls & col-. umns, ordinary slabs or beams Maximum Minimum Maximum 1-1/2 h 1-1/2 3-1/2 3-1/2 6 3 5 5-11. BATCHING AND MIXING CONCRETE: Concrete shall be mixe by a mechanical batch-type mixing plant. Mixers shall be provided with adequate facilities for accurate measurement and control of each of the materials entering the mixer and for changing the proportions to conform to varying condition of the work. The mixing-plant assembly shall include adequate provisions for the inspection of operations at all times. The plant and its location shall be subject to approval by the Engineer, a. Batching Unit: Each batching unit shall be supplied with the following item: 1, Weighing Unit: <A weighing unit shall be pro- vided for each type of material to indicate the scale load at convenient stages of the weighing operation. The weighing units shall be checked at such times as directed by and in the presence of the Engineer and required adjustments shall be made before further use of the device. bob. Mixing Unit: 1. Operations: Mixers shall not be charged in 5-8