ITEM 522 PORTLAND CEMENT CONCRETE PLANTS

Transcription

1 ITEM 522 PORTLAND CEMENT CONCRETE PLANTS Description. This Item shall govern for the plant facilities, equipment and operation of portland cement concrete plants Class of Plants. The class of plant shall be in accordance with one of the classes specified herein. Unless the class and type of plant is specifically indicated on the plans, either a central or transit mix batch plant may be furnished. (1) A Class 1 plant shall be for the exclusive use of the Department for a single project and when required, will be designated on the plans as one of the following: (a) Class 1A: Central mix plant (b) Class 1B: Transit mix or central mix batch plant (2) A Class 2 plant shall be for the exclusive use of the Department which may be used for more than one project as specified by the Engineer and when required, will be designated on the plans as one of the following: (a) Class 2A: Central mix plant (b) Class 2B: Transit mix or central mix batch plant (3) A Class 3 plant shall be a plant designated for the exclusive use of the Department during continuous placements only, as specified by the Engineer, but available for commercial work during other periods and when required, will be designated on the plans as one of the following: (a) Class 3A: Central mix plant (b) Class 3B: Transit mix or central mix batch plant (4) A Class 4 plant shall be a commercial plant, either central mix or transit mix, which will be required to dedicate sufficient trucks. All classes of plants shall be located to meet the time requirements for hauling and placing concrete Approval of Plant. Before mixing and hauling concrete to Departmental projects, the plant will be thoroughly inspected and must be approved by the Engineer. All scales, water measuring devices, mixers, agitators, hoppers and storage bins shall be in satisfactory operating condition and together with the method of operation, storage facilities, materials and any other necessary item required in the manufacture and delivery of concrete will be inspected for approval by the Engineer. Weighing and measuring equipment shall be in good working condition, properly calibrated and shall meet all requirements of Item 520, "Weighing and Measuring Equipment". The mixer shall have a plate affixed showing the manufacturer's recommended operating data. The data shall include the drum paddle speed of rotation for mixing and agitating and the capacity of the unit for complete mixing and/or agitating only. A copy of the manufacturer's design, showing dimensions and arrangement of blades, shall be available.

2 When the equipment and/or facilities fail to meet the specified requirements, approval of the plant will be withdrawn until the defects are corrected and the plant reapproved Equipment. (1) General Requirements for Batch Plants. The batch plant shall be provided with adequate bins for batching all aggregates and materials required by the specifications. A separate scale and hopper entirely free and independent of that used for weighing aggregate shall be provided for weighing bulk cement and fly ash. The scale provided may be used for the accumulated weighing of the bulk cement and fly ash. Admixture dispensing equipment shall be as specified in Item 437, "Concrete Admixtures". (2) Types of Mixing Equipment. Portland cement concrete shall be mixed and delivered by means of one of the following approved methods: (a) Mixed completely in a stationary mixer and transported to the point of delivery in a truck agitator or a truck mixer operating at agitation speed (Central mix concrete). (b) Mixed completely in a stationary mixer and transported to the job site in approved non-agitating trucks. This method of transporting will be permitted for concrete pavement only (Central mix concrete). (c) Mixed completely in a transit mix truck and transported to the point of delivery at mixing and/or agitating speed (Transit mix concrete). (d) Mixed completely in a truck mounted paving mixer and transported to the point of delivery in the truck mounted paving mixer or in an approved non-agitating truck. (e) Mixed completely in a mobile, continuous volumetric mixer at the job site in accordance with Item 421, "Portland Cement Concrete". (f) Mixed completely in a paving mixer, operated at the construction site or at a central location and used for mixing concrete for concrete pavement only. If a paving mixer is furnished and operated at the site of construction, it shall be equipped with a power controlled boom and bucket, so designed as to permit uniform distribution of the concrete for the full width of the pavement. (3) General Requirements for Mixers and Agitators. (a) General. Mixers shall be an approved stationary or truck-type mixer capable of combining the ingredients into a thoroughly mixed and uniform mass conforming to the requirements of Item 421, "Portland Cement Concrete". Facilities shall be provided to permit ready access to the inside of the drum for inspection, cleaning and repair of blades. Mixers and agitators may be subject to daily examination for changes in condition due to accumulation of hardened concrete and/or blade wear. Any hardened concrete shall be removed to the satisfaction of the Engineer before the mixer or agitator is approved for use. Worn blades and/or fins, shall be repaired or replaced with new blades or fins when worn as much as 10 percent from the original height of the manufacturer's design.

3 In some instances blade or fin extensions may have been added similar to those shown in Figure 1. When these extensions have been made to the original blade or fins by welding or bolting, repair or replacement of the extensions as specified herein will not be required. Any original blade or fin with holes therein must be repaired or replaced. FIGURE 1 (b) Stationary Mixers. These mixers shall be either a rotating drum type or twin paddle with stationary drum type. Multiple drum mixers will be permitted provided that their operation is properly synchronized. The mixing time for multiple drum mixers shall be determined exclusive of the time required to transfer concrete from one drum to the next drum. A transit mix truck or a truck mounted paving mixer mounted on a stationary base will not be considered a stationary mixer. (c) Truck Mounted Paving Mixers and Transit Mix Trucks. Truck mounted paving mixers and transit mix trucks shall comply with the following requirements: Each truck mounted paving mixer shall be approved by the Engineer prior to use on the project. It shall be so designed that uniform and low slump concrete (approximately 1-1/2 inch slump) can be mixed without segregation. The mixer shall be capable of discharging low slump concrete at a speed of one (1) cubic yard per 10 seconds or faster. Truck mounted paving mixers and transit mix trucks shall be equipped with actuated counters by which the proper number of revolutions of the drum as specified in Article may be readily verified. The counters shall be read and recorded at the start of mixing at mixing speeds. Each unit shall have adequate water supply and accurate metering or gauging devices for measuring the amount of water used Operation of Plant and Equipment. (1) General. The production and delivery of concrete shall equal or exceed the rate specified on the plans or approved by the Engineer for continuous placements. In all cases, the delivery of concrete to the placement site shall assure compliance with the time limits in the applicable specifications for depositing successive batches in any monolithic unit. The Contractor shall satisfy the Engineer that adequate standby trucks are available. For all uses of concrete, except miscellaneous, either a printout or batch ticket record shall be

4 furnished by the Contractor for each batch of concrete. The information to be shown on the printout or batch ticket shall be the same as shown on Department Form 596, "Concrete Batch Ticket", for that portion of the ticket to be recorded at the batch plant. When documented as part of the mix design data, the identity of materials need not be shown on the printout or the batch ticket. Revolution counter readings may be handwritten on automatic printout tickets. Additional mortar, consisting of one (1) sack of cement, three (3) parts of sand and sufficient water to coat the inside of the mixer drum, shall be added to the initial batch of concrete mixed and after each wash out. Spilling of materials from the mixer drum during charging or mixing shall be corrected by additional aprons and baffles or by reducing the size of the batch. The entire contents of the drum shall be discharged before any materials are placed therein for the succeeding batch. The maximum size batch shall not exceed that of the rated mixing capacity of the mixer. Except as provided below for transit mix trucks or truck mounted paving mixers, retempering or remixing of concrete will not be permitted. Admixtures shall be measured and dispensed as specified in Item 437, "Concrete Admixtures". An approved work plan for the use of high-range water reducers will be required. The work plan will govern when a conflict exists with these specifications. (2) Central Mix Plants. When a stationary mixer is used for the entire mixing operation, the concrete shall be mixed not less than 50 seconds nor more than 90 seconds, or as recommended by the manufacturer of the mixer, measured from the time the last material enters the mixer to the time discharge of the concrete into hauling equipment begins. When necessary to achieve suitable mixing, the Engineer may require adjustment of the mixing time. (3) Truck Mounted Paving Mixers and Transit Mix Trucks. The mixing speed shall be attained as soon as all ingredients are in the mixer. Each complete batch shall be mixed not less than 70 nor more than 100 revolutions of the drum at mixing speed, except as provided below. Trucks will be permitted to transport concrete to the job site at mixing speed if equipped with double actuated counters which will separate revolutions at mixing speed from total revolutions. Trucks equipped with a single actuated counter counting total revolutions of the drum shall mix the concrete at the plant not less than 50 nor more than 70 revolutions at mixing speed, transport the mix to the job site at agitating speed and complete the required mixing before the concrete is placed. Mixing and agitating speed shall be as designated by the mixer manufacturer. All revolutions after prescribed mixing shall be at agitating speed. Except for short periods of time during discharge, the drum shall be kept in continuous motion from the time mixing is started until the discharge is completed. For concrete mixed in a transit mix truck a portion of the mixing water, required by the batch design to produce the desired slump, may be withheld and added at the job site, but only with the permission and under the supervision of the Engineer. When water is added at the job site, 25 revolutions (minimum) at mixing speed, will be required to uniformly disperse the additional water throughout the mix. Water used to flush down the blades after charging shall be accurately measured and included in the quantity of mixing water. The introduction of the initial mixing water, except blade washdown water and that permitted in this article shall be prior to or simultaneous with the charing of the aggregates and cement.

5 When used as an agitator only, the loading of transit mix trucks shall not exceed 80 percent of the drum volume. (4) Storage of Materials. All material shall be stored in accordance with Article (5) Measurement of Materials. All material shall be measured in accordance with Article Mixer Performance Test. Mixers producing concrete which is acceptably uniform to the satisfaction of the Engineer need not be tested. All other mixers will be tested as specified herein. Mixers failing any of these tests will not be permitted on further work until corrective measures are taken. Corrective measures may include mixing smaller loads, requiring a more efficient charging sequence, blade repair, drum cleanout or other measures necessary to comply with the performance requirements. Mixers shall be capable of mixing a batch using normal job mixing requirements to a uniformity conforming to Table 1. Tests will be performed at the job site after completion of mixing requirements. In making a performance test, the variation within a batch will be determined for each concrete property listed in Table 1 as the difference between the highest and the lowest value obtained from different portions of the batch. The comparison will be between two (2) representative samples, taken after discharge of approximately 15 percent and 85 percent respectively of the maximum load recommended by the manufacturer. Both samples shall be obtained within a period of 15 minutes or less. TABLE 1 REQUIREMENTS FOR UNIFORMITY OF CONCRETE Requirement, expressed as maximum permissible difference in results Concrete Property of tests of samples taken from two (2) locations in the concrete batch. Weight per cubic foot calculated to an air-free basis (Test Method Tex-417-A) 1.0 lb./cf. Air content, percent by volume of concrete (Test Method Tex-416-A) 1.0 % Slump If average slump is 4 inches or less If average slump is 4 to 6 inches (Test Method Tex 415-A) 1.5 in. 1.0 in. Coarse aggregate content, portion by weight of each sample retained on No. 4 sieve, percent 6.0 % Measurement and Payment. The work performed, materials furnished and all labor, tools, equipment and incidentals necessary to complete the work under this Item will not be measured or paid for directly, but will be considered subsidiary to the various bid Items of the contract.