How Should Concrete Mixes with High Dosages of Macrosynthetic Fibers Be Adjusted?
December 10, 2014
In this article, we will review an acceptable approach to adjusting concrete mixes with high dosages of Macrosynthetic Fibers, such as ABC Polymer’s FiberForce 650™, Performance Plus DOT™ and Macro-Pro™, are used as reinforcement in the concrete at dosage levels from 3.0 to 11.0 pcy.
When we talk about a standard concrete mix design there are four principal ingredients: Cement, Coarse Aggregate, Fine Aggregate, and Water. The proportions of these materials are dictated by the project specifications as well as the physical properties of the ingredients. In addition to the standard ingredients, admixtures and additives can be included in the concrete mix to achieve specific properties. Admixture examples include air entrainment, water reducers, and accelerators. Examples of additives are silica fume, fly ash and color.
When and Why Should We Adjust the Concrete Mix?
We must review and potentially adjust the concrete mix design when macrosynthetic fibers are introduced into the matrix due totally to the surface area of the macrosynthetic fiber introduced at the higher dosage levels.
The standard dosage rate for microsynthetic fibers is 0.5 to 1.5 pcy. At this dosage level there is seldom insufficient mortar to coat the coarse aggregate and the microsynthetic fibers, thus no adjustments to mix proportions are required.
However, most of the defined engineering benefits attributable to the macrosynthetic fibers are achieved when the dosage level is 3.0 to 11.0 pcy. This higher dosage rate means an increase in the surface area of the fiber that the mortar matrix component of the concrete mix must be able to coat.
The fibers will not perform if they are not anchored in the concrete. To be anchored in the concrete the fibers must be encapsulated in the mortar; thus the mechanical bond is created. It becomes very apparent by visual inspection when an adjustment should be considered. If, when viewing the mix, only the coarse aggregate and the fibers seem to be visible, then there is a need to increase the mortar fraction.
A Typical Mix Adjustment Example
To demonstrate the mechanics of adjusting a concrete mix, we will go through the calculations that one would typically encounter in adjusting a mix design. This exercise is intended to illustrate what those schooled in the art would do to begin the process of adjusting the mix proportions.
For this example the initial mix design is as follows:
- Coarse aggregate is an ASTM #57 (1” to #4) crushed stone with a bulk dry specific gravity of 2.75. The dry compact unit weight of the stone is 102 pcf.
- The fine aggregate is a natural sand meeting the gradation of an ASTM concrete sand with a bulk dry specific gravity of 2.62.
- The cement is a Portland Type I with a specific gravity of 3.15.
- The desired 28-day compressive strength is 4,000 psi.
This will be a non-air mix, and we will use a high-range water reducer (HRWR) to enhance workability without adversely affecting the water-cement ratio. The macrosynthetic fiber will be either FiberForce 650 (Embossed Tape Macrosynthetic Fibers), Performance Plus DOT (Fibrillated Macrosynthetic Blend) or Macro-Pro (Highly Modified Fibrillated Macrosynthetic Fibers).
Ingredient STANDARD MIX – No Fibers STANDARD MIX ADJUSTED
FOR FIBERFORCE 650, PERFORMANCE PLUS DOT OR MACRO-PRO
Cement: 611 lbs 632 lbs2 #57 Stone: 1,866 lbs, dry 1,679 lbs, dry Sand: 1,326 lbs, dry 1,610 lbs, dry
(includes 124 lbs, dry, to offset
reduction in water)
Water: 325 lbs1 276 lbs FiberForce 650, Performance Plus DOT or Macro-Pro: — 3 lbs HRWR: — 3 oz/cwt of cement3 1 325 lbs = 39 gallons
2 We divided the amount of stone to be replaced between the cement
(10%) and the sand (90%). This is an arbitrary number and may be
adjusted to suit actual conditions.
3 When we add the HRWR we reduce the amount of water required by
15%. This volume reduction in water must be addressed. We did that
by increasing the quantity of sand.
Additional Adjustment Notes
As the macrosynthetic fiber dosage level increases from 3.0 pcy to 4.0 pcy and higher the amount of coarse aggregate replaced may need to be increased. This is in direct relationship with the increased surface area introduced by the fiber.
Our example does not show all of the actual calculations required to make these changes. A number of other factors might contribute to the adjustment in the weights, including the gradation of the coarse aggregate, the fineness modulus of the sand and, if used, whether the fly ash is Type ‘C’ or ‘F’.
What is important when we design a concrete mix is that the total volume of the ingredients will yield approximately 27.3 cubic feet per cubic yard (on paper). Granted, most ready mixers probably target 27.0 cubic feet per cubic yard, which is acceptable. What is not acceptable is the yield being less than 27 cubic feet per cubic yard.
In conclusion, whenever FiberForce 650, Performance Plus DOT or Macro-Pro is specified at a dosage rate of 3.0 pcy or more it is important that the concrete mix design is reviewed. Trial mixes prepared in the ready mixer’s laboratory are recommended so that the concrete contractor and ready mix professional can observe the consistency and workability of the mix.
Robert C. Zellers, PE, Director, Engineering Services (RZellers@ABCFibers.com) or Rob Yates, Application Engineer (Rob@ABCFibers.com), should be consulted during this process when ABC Polymer’s input is required, or whenever you have questions about adjusting a mix design for high dosages of macrosynthetic fibers.