Overview of Reinforcement Categories
October 17, 2009
Microsynthetic Fibers are the original generation of synthetic fiber and are comprised of polypropylene monofilament and fibrillated fibers and nylon monofilament fibers. Typically these fibers are ¾” (19mm) long and are used at a dosage rate of 1 to 1.5 pounds per cubic yard as a means to address plastic shrinkage reinforcement and/or secondary/temperature-shrinkage reinforcement. The length range for microsynthetic fibers is ¼” to 2 ½” and the dosage range is 0.5 to 3.0 pcy.
The standard specification references for Microsynthetic Fibers are ASTM C1116, Section 4.1.3 and Note 2 and ICC ES AC32, Sections 3.1.1 and 3.1.2. These products are key to the replacement of conventional/older generation secondary reinforcement (i.e. wire mesh), providing a new marketing tool and profit center for the ready-mix producer while providing the contractor with a ‘Value Engineered’ product and the owner and with long-term durable concrete.
Macrosynthetic Fibers are the new generation of synthetic fibers. Currently, all of the Macrosynthetic Fibers being marketed are classified as polyolefins. Polyolefin is an umbrella term that includes a number of ‘polys’ like polypropylene and polyethylene. The defining physical properties of Macrosynthetic Fibers are length (1 ½”-2 ½”), configuration (thicker strands) and dosage (3-9 lbs/cy). The length is important in establishing a greater mechanical bond with the mortar portion of the concrete. In general terms, the configuration provides for a greater cross-section thus a higher net tensile strength. The dosage level picks up where the Micro’s peak and carry performance to a higher level. Macrosynthetic Fibers are designed to perform post-first crack. When we talk about post-first crack we are talking about cracks imposed by an externally applied load. Specifically, the macrosynthetic fibers are utilized to transfer the externally applied load across the crack. ASTM Test Methods C1399, C1609, and C1550 are used to measure the load carrying capacity of the macro synthetic fiber reinforced concrete/shotcrete over a given deflection range post-first crack.
Steel Fibers have morphed through a number of changes both in configuration and aspect ratio. The standard products either have a circular cross-section (ASTM A820 Type I) or a rectangular cross-section (ASTM A820 Type II) or a shaved, modified circular cross-section (ASTM A820 Type V). Other than the cross-section, all of the other requirements found in A820 are precisely the same for all three. The aspect ratio is important to the ability of the steel fiber to disperse in the concrete. We typically use an aspect ratio of 60 as the upper limit. Glued hooked end Type I steel fibers are an exception.
Currently, there are three basic configurations of steel fiber available: Continuously deformed (Types I, II and V), end deformed (Types I and II) and the hooked end, both glued and loose (Type I). In general hooked end, steel fibers produce higher post-first crack/toughness numbers and the continuously deformed steel fibers produce higher flexural strength numbers, particularly Type II. The end-deformed product will provide good results for both post-first crack and flexural strength.
The steel fiber length will range from ¾” (19mm) to 2” (50mm).
Steel fibers at dosage levels of 40 lbs/cy or above, more realistically 50 lbs/cy or above will increase the flexural strength of the concrete. With an increasing flexural strength, an engineer is provided an opportunity to either decrease the thickness of the slab-on-ground or increase the load capacity when compared to the non-steel fiber designed slab. Finally, an engineer can also use this increased load capacity as a Safety Factor. This benefit of the steel fibers would be relative to a Plain Structural Concrete slab-on-ground design.
ASTM Test Methods C1609, C1399 and C1550 can be used to measure the post-first crack properties of steel fiber reinforced concrete/shotcrete. The standard 3-point flexural strength test, ASTM C78, is used for steel fiber reinforced concrete.
When steel fibers are used at a dosage level less than 40 lbs/cy the use would be as a secondary reinforcement. One approach is to replace the weight of the wire mesh used as secondary reinforcement with an equivalent weight of steel fibers. For example: If a 6×6 W2.9 x W2.9 wire mesh is to be replaced by an equivalent weight of steel fiber in a 6” thick slab, the dosage would be 20 lbs/cy.
Typical blends combine steel fibers with Microsynthetic Fibers. We also find blends of steel fibers with Macrosynthetic Fibers as well as Macrosynthetic Fibers and Microsynthetic Fibers. These blends are intended to perform at a higher level than Microsynthetic Fibers by themselves as Secondary/Temperature-Shrinkage Reinforcement. This statement is based on the fact that most blends contain either 1.0 or 1.5 pounds of Microsynthetic Fibers, which would be considered a standard dosage level for secondary reinforcement. In the case of steel fiber/synthetic fiber blends the combined materials may also increase the flexural strength of the concrete/shotcrete. The dosage level of steel fiber must exceed 40 lbs/cy and more realistically 50 lbs/cy to achieve a gain in the flexural strength.
The ‘standard’ lengths of steel fiber used in blends are 1” (25mm) and 1 ½” (38mm), the standard length of Microsynthetic Fiber is ¾” (19mm) 1 ½” is the standard length for Macrosynthetic Fibers.
Polypropylene monofilament and fibrillated fibers and nylon monofilament fibers are all considered Microsynthetic Fiber candidates for use in blends. Type I or Type II continuously deformed steel fibers are commonly used. The typical Steel/Microsynthetic fiber blend totals 24 pounds/cy. This Blend will have either 1.0 or 1.5 pounds of Microsynthetic Fibers (1.0 lbs if polypropylene or nylon monofilament fibers are used) and either 22.5 or 23.0 pounds of steel fibers depending on the Microsynthetic Fiber selected.
This blend provides the benefits of the Microsynthetic Fibers in reducing plastic shrinkage cracks and drying shrinkage cracks in concert with the toughness benefits of the Steel Fibers.
When the Blend totals 24 pounds per cubic yard with either 22.5 or 23.0 pounds of steel fiber, there is insufficient steel fiber to contribution to the flexural strength.
More and more, Macro/Microsynthetic fiber blends are being specified. A common blend is 4 lbs/cy of Macro synthetic fibers and 1 lb/cy of Microsynthetic fibers. This blend provides a high fiber count and therefore provides excellent crack control.
R.C. Zellers, PE/PLS
Director, Engineering Services