Concrete is strong in compression because the granite aggregates are very strong in compression . If the concrete is properly mixed in the correct ratio of materials , when the concrete is vibrated there is virtually no “reasonable space “ between the aggregate . They are virtually touching . The extra smaller materials fills in the voids caused by irregular shapes of the aggregates .
There is extra water an cement in the mix to make the concrete workable and that rises to the surface when finishing to give an aggregate free surface to trowel . The larger aggregates are much stronger in concrete than the smaller . The smaller gravel is intended for thin applications like 1.5 “ thick stepping stones . Concrete is not that strong in tension because the aggregate only has the bonding effect of the cement to prevent the aggregate from “sliding “ apart .
To give the concrete tensile strength , steel rods are added usually near the bottom of a beam or slab . That is a lot different than pressing the aggregate together as in compression . For those that are saying the concrete will eventually fail , nothing last forever . Concrete takes 50 years to completely cure to full strength before it even starts downhill