If you are using the older matrix constructor, then you can use "evalm" to evaluate expressions.
For example:
> A := matrix([[1,2],[3,4]]);
> B := matrix([[10,20],[30,4]]);
> t := 0.25;
> evalm(A*t + B*(1-t));
[7.75 15.50]
[ ]
[23.25 4.00 ]
The newer Matrix form should just evaluate.