Performance-based versus Prescription-based FRC Specifications
May 28, 2015
Which method is best when writing specifications for Fiber-reinforced Concrete? This article provides the historical, testing and physical property related data — plus more — that leads to the reasoning behind ABC Polymer’s preferred method of specification writing.
FRC History — Micro Beginnings
Synthetic Fibers as reinforcement for concrete were introduced in the mid-1970s and have positively evolved through the years. Initially, we offered what we now call Microsynthetic Fibers. Microsynthetic fibers were limited to monofilament nylon fibers and monofilament and fibrillated polypropylene fibers. The configuration and physical properties were basically the same for like materials.
In essence, one product of a given chemical base and configuration looked like and had the same basic physical properties of all of the others in the same category. The lengths and dosage rates were very restricted, as were the applications. The length was typically 3/4″ for most applications, and the dosage rate ranged from 0.5 pcy to 1.5 pcy for most applications.
FRC Evolution — Micro to Macro
Along came Macrosynthetic Fibers in the early years of the 21st Century. The focus of the development of macrosynthetic fibers was to generate maximum mechanical bond between the fibers and the concrete matrix so as to increase the post-first crack properties of Macrosynthetic Fiber-reinforced Concrete (FRC).
These products exhibited some common properties like length (1½” standard and 2″) and chemical base, polyolefin (a blend of polypropylene and polyethylene resins). Furthermore, the cross-sectional area (denier) of macrosynthetic fibers was much greater than that of microsynthetic fibers.
There were other major differences, starting with the configuration of the fibers (there was no standard), the physical properties of the fibers and the most important parameter, the increased dosage rate (minimum of 3.0 pcy) of the fibers. The dosage rate reflected the mechanical bonding properties of macrosynthetic fibers as determined by either ASTM Test Method C1609 or ASTM Test Method C1399 coupled with the application. The dosage rate is determined based on these parameters plus the general properties of the concrete and the Welded Wire Fabric or rebar specified.
Since the real focus was on the contribution of the macrosynthetic fibers to the engineering properties of the concrete comparing the actual physical properties of the individual macrosynthetic fibers became secondary to the performance of the fibers in the concrete composite as measured by one of the ASTM consensus test methods noted above (C1399 or C1609).
The original intent of these tests was to compare the performance of one macrosynthetic fiber with another or with the family of macrosynthetic fibers. In making this performance comparison one could then make a judgment as to the dosage level of one macrosynthetic fiber versus another macrosynthetic fiber. Initially, the C1399 test method was the test method of choice to measure the post-first crack benefits of macrosynthetic fibers.
Using the Average Residual Strength data from C1399, the engineer could theoretically determine an adequate dosage level of macrosynthetic fiber required for a given application, be it a slab-on-ground, elevated slab-on-composite steel deck or in precast products. The performance of macrosynthetic fibers in the concrete matrix superseded the need to quantify the physical properties of the fibers themselves.
For example: the aspect ratio only has value if the fiber is a straight cylindrical fiber, such as ABC Polymer’s Mono-Tuf™. If the fiber in question is rectangular and/or has multiple branches (fibrils) extending out from the main fibril there is no way to determine the aspect ratio. This is the case with a fibrillated type fiber like our Fibril-Tuf™. Each of those branches is important, with each branch providing an additional anchor site that contributes to the bond between the fiber and the mortar. See photos below.
Aspect ratio is the relationship of the fiber length to its diameter, which is valid for FiberForce 150™ (Mono-Tuf™) — a straight, cylindrical fiber.
Aspect ratio is not valid for Fibril-Tuf™ — a fibrillated fiber with multiple branches.
Re-evaluating FRC Test Methods
ASTM Test Method C1609 is a reinvention of ASTM Test Method C1018 (abandoned when C1609 was adopted). This test method, throughout its advancement into multiple iterations, has always been used for the testing of steel fiber-reinforced beams. Over the last eight-plus years the use of this test method for all FRC beam specimens has become the norm.
Although a number of inherent issues have been identified with this test method that affects the accuracy of the test, it is now the recommended or preferred test method. Data generated from this test method provides a better picture of the energy to resist deflection and crack-opening widths than the C1399 test method. Being a continuous test that is closed-loop versus open-loop, as in the C1399 test method, it provides a more accurate means of measuring all higher-dosed FRC.
FRC Test Method Issues — Some Resolved/Some Remain
Issues revolving around these test methods are being addressed by ASTM Subcommittee C09.42. For example, we now know that conducting the test with less than 3.0 pcy of macrosynthetic fibers in the mix will produce results with increased variability.
We have spent considerable time enhancing the precision of the yoke that encircles the test beam. This has now been accomplished.
We have also identified a concern with the method used to fabricate the Fiber-reinforced Concrete beams. It was determined that using the standard third layer approach caused a disruption in the three-dimensional fiber network. Placing the FRC in three segments side-by-side in the beam mold produced the same network separation. It was then determined that a single dump of the fresh FRC was the appropriate approach.
Two identified issues that remain to be resolved are: 1) the appropriate design for the rollers that support the beams, and 2) the effect of the energy transfer from the test frame to the beam at first crack for a range of test frames.
The Key Macrosynthetic Fiber Property
If there was a single physical property of macrosynthetic fibers to be considered, it would be percent elongation. The lower the percent elongation the better the fibers resist deformation, producing greater resistance to the separation of the two faces of a crack. Also when synthetic fibers elongate the cross-sectional area decreases, which can result in the fibers debonding from the concrete matrix. With the complexity of the configurations of macrosynthetic fibers available and the manner by which the fibers bond in the concrete matrix, debonding due to elongation of the fiber is mitigated.
Write a Performance-based FRC Specification
In general terms, there are two ways to write a specification: prescription and performance. With a prescription specification everything is written into the specification, from the material properties to the composition of the mixture. The prescription approach does not consider the optimization of materials when united in the concrete composite. Conversely, a performance-based specification utilizes consensus plain concrete and FRC test methods to establish target composite property as determined by the engineer using standard equations, which then requires the parties to the contract to produce the optimum composite.
ABC Polymer engineering considers optimizing the composite for the application performance the preferred choice. Our position is to use the concrete composite test data, which directly correlates with performance, versus requiring specific physical properties of the fibers that may or may not directly (or indirectly) correlate with the performance of the macrosynthetic fibers in the concrete composite. Performance-based specifications will ensure a better picture of the effectiveness of the three-dimensional reinforcement system and should be the preferred specification method.
Prepared by R.C. Zellers, P.E., Chief Engineer