Three, Two, One

And there you have the basic recipe for concrete in The Third World.

Three shovels of gravel
Two shovels of sand
One shovel of cement and add water until it's 'wet enough'...

Hardly an exact science. And therein lies the problem.
You see, mixing concrete properly IS an exact science.

Without an exact ratio of sand, gravel, cement and water the resulting concrete can be a disaster waiting to happen.
Floors poured over earth can make due with poor quality concrete, but columns and beams that support second, third and even fourth
floors must have the structural integrity to withstand moderate eartquakes without pancaking.

The poor can't build cost effective, multi-story structures that can withstand a massive trembler, but it is heartbreaking to enter a village
that was utterly destroyed by a 5.0 earthquake - simply because the concrete was of an inferior quality.

As construction volunteers, you will teach concepts of rebar/reinforcement, proper preparation of sub-soils and you will
drive home the point that a three sack mix simply won't cut it for safe construction of a family home, a medical clinic or a school.

BASIC STUDY LINKS
Why is too much water bad?
How much concrete is needed?
Footing size?
Basic Rebar placement techniques