The durability and strength of concrete are highly dependent on the water-cement ratio. Sometimes fly ash which is a waste product of coal powered power plants are used to modify certain properties or as a replacement of some of the cement. Concrete is weak in tension. Its strength in compression depends on the proportions of the mix. The compressive strength is 2 to 5 times that of wood.
Most structural uses depend on concrete strength in compression and steel in tension.
These are air-entrained Portland cement formulated with a compound that releases many tiny bubbles of air during curing. They are recommended for agricultural applications even though they are slightly weaker than standard cement. An air-entrainment agent added to the cement produces millions of tiny bubbles in the concrete giving the concrete greater weathering resistance. Therefore, all concrete used for agricultural applications should be air entrained. The maximum size of coarse aggregate that can be used depends on the sizes and shapes of concrete members and the amounts and distribution of reinforcing steel.