Steel fibers added to mortar didn't improve the strength and may even have lowered it. What could be wrong with our procedure?
This effect has been reported by various research workers who have found that the strength of mortar (not concrete) is lower if steel fibers are incorporated in the mix. (See Fiber Reinforced Concrete, ACI Publication SP-44, pages 199-201.) It appears to be a natural property of the materials but is not completely understood.