Building a pre-slope for a shower pan
I used sand mix (with acrylic fortifier) for the pre-slope I built for a new shower pan. After the mortar sets up(two days later) I checked the surface by knocking on it with a stick, I heard hollow ring instead of solid thud in some spots. What does this mean? The material in the spots where I heard the hollow ring was very low in strength and very easy to break. Should I remove the material from these spots? The fact that a PVC water proof membrane and more sand mix mortar plus stone tiles will go over this surface, does it matter that much that this pre-slope has some weak spots in it?
This pre-slope is built with wire mash on a poured concrete floor. The thickest part is 1.5" and the thinnest part next to the drain is about 1/4".