Ready-mixed concrete will be moved from the mixer to the point of placing as quickly as dosable by methods that will maintain the needed workability and will prevent segregation.
Within 2 hours of the time of loading, the concrete shall be discharged from the truck-mixer. If reducing admixtures are used or in cool, humid weather or when chilled concrete is produced. The time of loading will start from adding the mixing water to the dry mix of cement and adding the cement to the wet aggregate whichever is applicable.
No water from the truck water system or elsewhere shall be added after the initial introduction of mixing water for the batch except when on arrival at the job site the slump of the concrete is less than specified. If additional water is added, the drum shall be turned an additional 30 revolutions, minimum.
The adding of water reducing agents shall be performed as required and with the control and record data as specified. Reduction in cement content shall not be allowed because of higher strengths attained due to admixtures.
After checking slump, "Steel Fibers" at equal temperature to mixed concrete or super-plasticizer must be placed into the mix at job site, with one additional adjustment to mix, before any other quality assurance tests are performed.
If these are not usable, the fibers can be added to the batching plant using conveyor belts or' blast ' machines similar to those used to attach the fibers on site. It is important to remember that the
fibers land freely in the concrete when using conveyor belts and are then mixed
in the concrete using appropriate mixing procedures. Blast Machines have the
advantage of spreading the fibers in the concrete mix at a consistently high
pace so that the steel fibers are evenly distributed in each concrete load. If
steel fibers are applied without the use of blast machines at the batching
plant or on the job site, fibers can be "balanced" in clumps. This is
the product of unequal fiber distribution in the concrete and requires extra
mixing time to correct it.
In the
concrete, there are two common types of fiber balls that may form.
·
Dry balls most often occur when the steel fibers are dumped
in clumps into the ready-mix truck resulting in balling due to the fibers not
being allowed to integrate into the concrete mix properly.
·
Wet balls or steel fibers clumps also occur when the
concrete is mixed too long or too quickly. Wet balls can also occur when high
dosage levels are used for a small diameter steel fiber.
·
Before the first truck mixer arrives, blast machines and
conveyor belts should always be mounted and set up for service.
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