Sooner, rather than later, researchers will develop an easy-to-use measurement device that monitors fiber dispersion in hardened concrete. And that time is near, according to a technical paper published in the Sept./Oct. 2006 issue of the ACI Materials Journal.
In the report, a team of researchers lead by Dr. Nilufer Ozyurt, a research assistant at the Istanbul Technical University in Turkey and others at Northwestern University in Evanston, Ill., reported on a new nondestructive monitoring technique called AC-impedance spectroscopy (AC-IS). Researchers found the method to be effective in determining fiber clumping and fiber orientation in steel fiber-reinforced cement pastes.
This may prove to be an important step for the advancement of fiber concrete. Producers and contractors have traditionally struggled with determining proper dosage rates and feed rates when introducing fibers into their mixes. Until now, the only accepted method of quality control in this area has been image analysis, which is destructive and time-consuming.
Fortunately for concrete producers, fiber suppliers have recognized the problem of proper dosage rates. For large steel-fiber jobs, producers should refer the two methods of dispensing suggested in ACI 544, “Guide for Specifying, Proportioning, Mixing, Placing, and Finishing Steel Reinforced Concrete.”
For nonmetal fibers, producers at first exclusively relied on pre-weighed packages, whose bags would dissolve in the drum's mixing action. But in the last few years, pushed by an ever-increasing demand for all types of these lighter fibers, manufacturers have invested heavily in upgrading fiber feeding equipment.
These delivery methods vary. A manufacturer's delivery method is based on its product's best handling feature.
Along with tighter dosage rate control, these feed systems offer producers less waste and increased productivity. Drivers no longer need to climb ladders to toss in bags, only to climb down to rev up the mixer, and then return to see if the dispersion is correct. Many units also allow producers to order in larger shipping quantities and save on in-plant handling and storage costs.
Another feature many of these units offer is the ability to link dispensing rates to the batch controller. For example, engineers at Buckeye Building Fibers, a subsidiary of Buckeye Technology, have released an updated model of their automated dispensing equipment that provides an I/O port for data transmission.
In many locations, the batch operator needs to only monitor the auger's activity. The controller automatically sets dosage rate, determined by the specific mix design. The operator is even notified of bin inventory, so that the hopper can be refilled long before it runs empty.
To learn more about the following fiber dispensing systems, circle the reader service number on the reader service card.