According to Goedjn :

I thought the point of the fiberglass was to limit spalling.
Does it really increase the tensile strength of concrete?

Air entrainment (and being careful with making sure you have
proper mixing/good quality materials) is the primary cure for
spalling. Fibermesh is supposed to prevent cracking like
rebar does. However, some suppliers suggest/say that fibermesh
helps limit spalling, early cracking, resistance to chipping
and improves surface durability too.

Obviously the suppliers have an incentive to tout the benefits,
but the existance of industry (eg: ASTM) tests to show the
actual improvement shows there must be some truth in it.

I have no doubt that steel mesh or full rebar (eg: stress concrete)
is better than fibermesh in terms of brute strength, but
fibermesh is very cheap compared to rebar, and is a good choice
in situations where you want a little more reassurance with
situation that doesn't really need steel.

I'll be doing something similar when I experiment with making
some "hypertufa". The recipe calls for 2 parts portland cement,
three parts sifted peat and three parts perlite. And a "handful"
of concrete fibermesh (a cup loosely packed for 5 gal of dry
ingrediants).

[Making some synthetic rocks to stick on top of cedar log
fence posts.]
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.