It is clear that a clay floor is not capable of withstanding the loads which occur in the modern-day industrial environment, although its steady moisture balance also has advantages⁸⁾. The question is what can be used to hold together industrial floors so that sharp-edged metal objects can be dropped without digging large holes and small wheels with a diameter of 25 mm, a width of 100 mm and wheel loads of 500 kg ⁹⁾ can be moved repeatedly along the same path without tearing the sand grains out of the composite material.

Building a sandcastle
In actual fact our floors consist principally of sand, just like the sandcastles which you can no doubt still remember clearly from your last holiday at the seaside. Here I must add the following remark: sand can also be bonded together strongly using water. If the water is then allowed to freeze, you get an excellent load-carrying surface, rather like the frozen tundra. But to return to the sandcastles which young children build, they use "Iiquid" (water) as the binder, which is a very tricky job! A little too much water and the sand flows away, whilst drying out in the sun or overnight also causes the sandcastle to crumble ¹⁰⁾. This phenomenon is also of significance in connection with the water-cement -ratio.

Grains of sand in our floors are held together by a variety of binders in different quantities and with various methods of functioning. These binders include:

• tar
• bitumen
• lime
• plaster
• anhydrite
• magnesite
• cement
• polymers.

By way of example, a proportion of 1 part by volume cement to 8 parts by volume aggregate is adequate to ensure that lean concrete for the substructure of concrete floors holds together. This "holding together" is what is meant by cohesion as opposed to adhesion, which is used to describe the grip between layers of different composition. These two terms become confused when describing the boundary between aggregate (sand) and the particular binder used. Here it is again a question of "adhesion". Thus the vocabulary we use depends on which aspect is being considered or, in other words, it depends on the "power of our microscope".

In general the cement content per cubic metre of ready concrete is at least 300 kg. The concrete then weighs 2,300 kg. Thus 13% by weight of cement is adequate to produce a B 25 grade concrete.