What is Concrete?
Concrete is an engineering material that simulates the properties of rock and is a combination of particles closely bound together.
Concrete is made up of three basic components: water, aggregate (rock, sand, or gravel) and Portland cement. Cement, usually in powder form, acts as a binding agent when mixed with water and aggregates. This combination, will be poured and harden into the durable material.
Components of Concrete Mix
There are three basic ingredients in the concrete mix:
- Portland Cement
- Aggregates (rock and sand)
Portland Cement – The cement and water form a paste that coats the aggregate and sand in the mix. The paste hardens and binds the aggregates and sand together.
Water– Water is needed to chemically react with the cement (hydration) and too provide workability with the concrete. The amount of water in the mix in pounds compared with the amount of cement is called the water/cement ratio. The lower the water to cement ratio, the stronger the concrete.
Aggregates– Sand is the fine aggregate. Gravel or crushed stone is the coarse aggregate in most mixes.
Desired Properties of Concrete
1. The concrete mix is workable. It can be placed and consolidated properly by yourself or your workmen.
2. Desired qualities of the hardened concrete are met: for example, resistance to freezing and thawing and deicing chemicals, watertightness (low permeability) , wear resistance, and strength. Know what you are trying to achieve with the concrete.
3. Economy. Since the quality depends mainly on the water to cement ratio, the water requirement should be minimized to reduce the cement requirement (and thus reduce the cost).
Common Concrete Admixtures
Admixtures are additions to the mix used to achieve certain goals.
Here are the main admixtures and what they aim to achieve.
Accelerating admixture-accelerators are added to concrete to reduce setting time of the concrete and to accelerate early strength. The amount of reduction in setting time varies depending on the amount of accelerator used (see your ready mix supplier and describe your application). Calcium chloride is a low cost accelerator, but specifications often call for a nonchloride accelerator to prevent corrosion of reinforcing steel.
Retarding admixtures-Are often used in hot weather conditions to delay setting time. They are also used to delay set of more difficult jobs or for special finishing operations like exposing aggregate. Many retarders also act as a water reducer.
Fly Ash– Is a by product of coal burning plants. Fly ash can replace 15%-30% of the cement in the mix. Cement and fly ash together in the same mix make up the total cementious material.
- Fly ash improves workability
- Fly ash is easier to finish
- Fly ash reduces the heat generated by the concrete
- Fly ash costs to the amount of the cement it replaces
Air Entraining Admixtures– must be used whenever concrete is exposed to freezing and thawing, and to deicing salts. Air entraining agents entrains microscopic air bubbles in the concrete: when the hardened concrete freezes, the frozen water inside the concrete expands into these air bubbles instead of damaging the concrete.
- Air entrainment improves concrete workability
- Air entrainment improves durability
- Air entrainment produces a more workable mix
Water reducing admixtures-reduces the amount of water needed in the concrete mix. The water cement ratio will be lower and the strength will be greater. Most low range water reducers reduce the water needed in the mix by 5%-10%. High range water reducers reduce the mix water needed by 12% to 30% but are very expensive and rarely used in residential work.
Concrete Reinforcement: Fibers vs. Welded Wire Mesh
Fibers can be added to the concrete mix in lieu of welded wire mesh.
The problem with welded wire mesh is that it often ends up on the ground from being stepped on as the concrete is being placed. (particularly if no support blocks are used). Another problem is that mesh does not prevent or minimize cracking-it simply holds cracks that have already occurred together.
If you could look into a section of concrete poured with fibers you would see millions of fibers distributed in all directions throughout the concrete mix. As micro cracks begin to appear due to shrinkage as water evaporates form the concrete (plastic shrinkage), the cracks intersect with the fibers which block their growth and provide higher tensile strength capacity at this crucial time.