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Slumps from 2 to 6 in. Interruptions of a duration greater than the initial setting time of the concrete should be treated as a construction joint.
Proper maintenance and calibration of equipment is essential to satisfactory performance and consistent production of concrete. Department of CommerceBozarth Batch timers with audible indicators used in combination with interlocks that prevent under- or over-mixing of the batch and discharge before completion of a wci mixing time are provided on automatic plants and are recommended on manual plants.
An agitator, if the truck body is equipped with one, aids in the discharge and ribbon-blends the concrete as it is unloaded. Give special attention to the amount and nature of material finer than the No.
304R-00: Guide for Measuring, Mixing, Transporting, and Placing Concrete (Reapproved 2009)
This average gradation can then be used for both quality control afi for proportioning purposes. If laitance is entrapped in the concrete, however, unit weight can be significantly below that of the fresh concrete. To portray more clearly certain principles involved in achieving maximum uniformity, homogeneity, and quality of concrete in place, figures that illustrate good and poor practices are also included in this guide.
The slope is advanced by pumping grout through successive rows of insert pipes until the entire slab has been grouted.
Since the Vietnam-Laos Wars, the direct in shrinkage by the Army is derived acj free. Liquid admixtures should be charged with the water or on damp sand, and powdered admixtures should be ribboned into the mixer with other dry ingredients. Another approach to accommodate long hauls is to use extended-set admixtures. With satisfactory plant maintenance, the batching accuracy should meet the tolerances given in Section 3. Setting of weights, starting the batch cycle, and discharging the batch are all manually controlled.
If a blockage occurs, the tremie should be quickly raised 6 in. Under no circumstances should any finishing tool be used in 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. Pay particular attention to the storage of special concrete aggregates, including lightweight, high-density, and architectural-finish aggregates.
Water should never be added to concrete in the truck body because no mixing is performed by the agitator. Manual buggies range in capacity from 6 to 8 ft3 0.
R Guide for Measuring, Mixing, Transporting, and Placing Concrete (Reapproved )
Stacking concrete by discharging the bucket too close to the lift surface or discharging buckets while traveling, commonly causes segregation. AEA effectiveness and the resulting dosage of AEA also depend on the cement fineness, cement factor, and water content, and the chemistry of cement and water, as well as that of other chemical and mineral admixtures used in the concrete.
Sufficient mixing, transporting, and placing capacity should be provided so that unfinished concrete lifts can be maintained plastic and free of cold joints. The use of HRWR for smaller volume placements in which flow distances are not as critical may be acceptable. Interlocks, however, interrupt the batching cycle when the scale does not return to 0.
It may be necessary to use a retarder to prolong the time the concrete will respond to vibration after it is placed. Water should be added under pressure, preferably at both the front and rear of the drum with it revolving at mixing speed, and then mixing is completed with the usual 70 to revolutions. Bureau of Reclamation ; U. When this occurs, the insert pipes should be pulled to just above the grout surface before the grout stiffens and rodded clear.
Use of pipe with thicker walls or weighted pipe can overcome buoyancy problems. The concluding three chapters deal with heavyweight, radiation-shielding concrete, lightweight concrete, and volumetric-measuring and continuous-mixing concrete equipment.
Admixtures are also available to prevent washout of cementitious materials and fines from concrete placed under water. The strength of grout alone, when determined in cubes or cylinders, may bear little relation to the strength of PA concrete made with the same grout because these units do not duplicate the weakening effect of excessive bleeding of the grout in place.
Initial lifting of the tremie should be done slowly to minimize disturbance of material surrounding the mouth of the tremie. Design and construction of concrete forms should comply with ACI R. In a truck mixer, there is little folding action compared with that in a stationary mixer. Use only as much water and fine aggregate as is required to achieve suitable workability for proper placement and consolidation by means of vibration.
Most of these pumps have an outlet port 5 in. Department of ReclamationU.