(loop repeat 3 concat "foo") works as expected.
However, help for loop does not mention it:
loop is an alias for `cl-loop' in `cl.el'.
The Common Lisp `loop' macro.
Valid clauses are:
for VAR from/upfrom/downfrom NUM to/upto/downto/above/below NUM by NUM,
for VAR in LIST by FUNC, for VAR on LIST by FUNC, for VAR = INIT then EXPR,
for VAR across ARRAY, repeat NUM, with VAR = INIT, while COND, until COND,
always COND, never COND, thereis COND, collect EXPR into VAR,
append EXPR into VAR, nconc EXPR into VAR, sum EXPR into VAR,
count EXPR into VAR, maximize EXPR into VAR, minimize EXPR into VAR,
if COND CLAUSE [and CLAUSE]... else CLAUSE [and CLAUSE...],
unless COND CLAUSE [and CLAUSE]... else CLAUSE [and CLAUSE...],
do EXPRS..., initially EXPRS..., finally EXPRS..., return EXPR,
finally return EXPR, named NAME.