Macro Synthetic Fibers are the new generation
of synthetic fibers. Currently all of the Macro Synthetic Fibers being marketed are classified as polyolefins. A Polyolefin
is an umbrella name for a number of ‘polys’ like polypropylene and polyethylene. The defining physical properties
of Macro Synthetic Fibers are length (1 ½”-2 ½”), configuration (thicker strands) and dosage (3-9 lbs/yd3).
It is important to realize
with the large dosage range (3-9 lbs/cy) that the selection of the quantity to use will be predicated on the degree of enhancement
required. In most cases this will be dictated by the results of the C1399 and C1609 tests. Attention directed to the mix proportions
is also critical. With the higher surface area of Micro Synthetic Fibers, at these dosage levels the amount of mortar
must be increased.
Macro Synthetic Fibers are
not to be considered as plastic shrinkage reinforcement or secondary reinforcement.
At the higher dosage levels the Macro Synthetic Fibers are designed to perform post-first crack relative to externally
applied loads. ASTM Test Methods C1399, C1609 and C1550 are the consensus test
methods intended to determine the load carrying capacity of the fiber reinforced concrete/shotcrete over a given deflection
range.....post first crack.
Either ASTM C1399
or C1609 are used to measure the post-first crack properties. In the case of acceptance for use in septic tanks and most applications,
C1399 seems to be preferred. A word of caution: it is important when selecting a laboratory to conduct this test, to make
sure this laboratory is qualified to do this test. This test method is not an
extension of ASTM C78.
Selecting the appropriate test to use is
tricky. In general, C1609 would be used for steel fibers and C1399 would be used
for the Macro Synthetic Fibers. The C1399 tests provide for the use of a steel
plate underneath the test specimen up through first crack. This reduces the shock
load to the Macro Synthetic Fibers, which could reduce the bond between the fibers and mortar.
