What Is Concrete

aggregate or crushed rock for concrete
Concrete basics – Aggregate or crushed rock for concrete

Above is a hand full of concrete aggregate.  It is clean, sharp, and hard crushed rock, that is perfect for making good concrete.  The course texture and sharp edges allow it to bond well to the other ingredients giving a stronger mix than rounded pebbles or softer rock would do.

Modern concrete started with the introduction and manufacture of Portland Cement in England around 1842.  There are many variations of concrete, but the basic concrete that will be delivered from a local batching plant by a transit mixer (the truck with a rotating bowl on the back), or that you may mix yourself on site, consists of the following ingredients.

  • Standard grade, general purpose, grey Portland Cement.  Sold in bags of around 20 kilograms.
  • Crushed rock or aggregate, of various sizes, graded to give a selection from the largest to the smallest.
  • The largest size of aggregate depends on the work being done. The concrete has to act like a fluid packing tightly around rebar and fitting neatly into corners.
  • On large civil engineering structures the size may be quite large but in general the transit mixers that we see in our cities and towns will be using a maximum size of 20mm or 3/4″.
  • Certain jobs like paving work may use a maximum size of 10mm or 1/2″
  • There may be one or two intermediate sizes of aggregate and the the smallest aggregate in the mix is sand.
  • The finest particles in the mix are provided by the cement.
  • There has to be enough cement in the mix so that when it is combined with the water there will be enough cement paste to coat every piece of aggregate to make a homogeneous mass.
  • Clean freshwater.   It has been said that the water in concrete should be good enough to drink.