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Chapter 3 - Obtaining Silica-Fume Concrete
Producing SFC

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In this case, a 25 lb (11 kg) repulpable bag is being added to the truck mixer. These bags are intended to be added directly to a central or truck mixer without opening. The bags are designed to disintegrate through a combination of wetting and grinding the paper during concrete mixing.

The Silica Fume Association is aware of several instances in which the bags have failed to disintegrate as intended. The result is the appearance of fragments of paper in the surface of the concrete. This problem seems to be particularly persistent during construction of flatwork such as bridge decks. We believe that the problem is caused by inadequate wetting and grinding of the paper during concrete mixing. The problem is particularly evident in concrete mixtures that have a very low water-cementitious materials ratio, that contain a small maximum sized coarse aggregate such as inch (13 mm), or that contain rounded aggregates. Pan-type concrete mixers are also very prone to problems with these bags.

The remedy for this situation is really very straight forward: if you have any doubts about the performance of the bags, conduct testing to determine whether the bags will deteriorate under the conditions and materials that will be used on your project. Testing should follow these steps:

  • Make concrete using project materials and project mixers (for truck-mixed concrete, test all trucks to be used)
  • Simulate haul time that will be expected
  • Discharge the concrete and look for paper fragments

If fragments are seen or if there is any question of performance, DO NOT add the bags directly. Instead, simply empty the bags into your mixer.