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Forms should be built with sufficient strength and rigidity to carry the mass and fluid pressure of the actual concrete as well as all materials, equipment, or runways that are to be placed upon them. The most versatile concrete pumps use hydraulically operated concrete valves that have the ability to crush or displace aggregate that becomes trapped in the valve area.
304f In one method, the dry materials are batched into the ready-mixed truck and transported to the job site where all of the mixing water ac added. Moisture meters should be calibrated to oven-dried samples for optimum consistency of readings.
Variations in aggregate grading can occur due to degradation of the previously used aggregate during mixing or reclaiming. Concrete will then flow out and develop a mound around the mouth of the pipe.
Form faces should be treated with a releasing agent to prevent concrete from sticking to the forms and thereby aid in stripping.
Under no circumstances should any finishing tool be used aco an area before accumulated water has been removed, nor should neat cement or mixtures of sand and cement be worked into the surface to dry such areas. Give special attention to the amount and nature of material finer than the No.
Free Fall Of Concrete | ASCC Position Statements
Bureau of Reclamationthe U. In some cases, wetting the coarse aggregate in the stockpiles or on the delivery belts may be necessary to compensate for high absorption or to provide cooling. Cleaned joint surfaces should be continuously moist-cured until the next concrete placement or until the specified curing time has elapsed. The safety factor decreases as 304d pipeline wears due to the abrasiveness of the coarse and fine aggregate used in the concrete. This standard is available for individual purchase.
Various methods have been developed by concrete producers to handle and determine the volume of returned qci. Among the many tests used to check mixer performance, the following are the most common: In some cases, all returned concrete is transferred at the end of a day to a single mixer for treatment and holding. Plastic or rubber drop chutes or tremies can be used and shortened by cutting them rather than raising them as placement progresses.
Curing conditions are excellent and drying shrinkage is minimal. Together with the cement and water, the fine aggregate provides the mortar or fluid that conveys the coarse aggregates in suspension, thus rendering a mixture pumpable.
Where special circumstances dictate the use of coarser sand, Grading 3 is acceptable. Tighter controls, however, may be required for certain jobs. ACI R Guide for. Sieve analyses of coarse aggregate should be made with sufficient frequency to ensure that grading requirements are met. If a break in placement results in a planned or unplanned horizontal construction joint, the concrete surface should be green-cut xci it sets.
To help achieve maximum component life, safe and thorough cleaning of the pipeline is necessary at the end of each placement or at any time a lengthy delay in pumping operation occurs. 304f stabilized concrete is qci blended with freshly batched concrete before being sold. Commonly, pouring concrete of inclined surfaces is more challenging compare to horizontal surfaces and hence it needs more efforts to avoid segregation. Some of the more frequent problems that can detrimentally affect the quality of the concrete in paving are also common in other types of placement, namely, poor batch-to-batch mixing uniformity, variation in slump and air content, and nonuniform aic of the paste through the aggregates.
To prevent washing of concrete already in place, a go-devil should not be used to restart a tremie after loss of seal.
R Guide for Measuring, Mixing, Transporting, and Placing Concrete (Reapproved )
Once flow is controlled, either tremie or pump placement consists of the following three steps: Careful, consistent concrete aic with suitable mixture adjustments for changing ambient temperatures is required.
If the concrete is to be placed at night, the lighting system should be sufficient to illuminate the inside of the forms and to provide a safe work area. Additionally, a layer of aggregate is carefully dropped on top of the cloth to stabilize it and form a base for the bulk of the coarse aggregate to follow.
An inspection plan detailing locations and frequency of soundings should be developed. The worksheet makes provision for additional coarse and fine aggregate that can be adi to a mixture to improve the overall gradation and recognizes possible overlap of some coarse and fine aggregate components.
Ice-batching equipment should be insulated to avoid melting the ice. This planning should provide for the correct amount and type of concrete for the pump being used, provision for necessary pipeline, and agreement as to which personnel will provide aic labor necessary to the complete placement operation.
Because these walls are confined placements, the rate of concrete rise will be high, necessitating frequent removal of tremie sections to maintain flow.
If the seal is lost, or if the concrete is disturbed in any way, additional laitance will be formed when starting or restarting pipes. Curing measures should be ready for use at the proper time ACI This transporting equipment can be a ready-mixed truck operating aaci an agitator, or an open-top truck body with or without an agitator.
Where reinforcement is crowded or the placement is in relatively shallow patches, Grading 1 should be used. Stockpiles should be built up in horizontal or gently sloping layers, not by end-dumping.