If you are a professional contractor you might notice the cracks and other irregularities present in exposed concrete, you could make the natural assumption that it must be very tricky and difficult to work with. While this may be somewhat true if poured under adverse conditions, for the most part, the overall quality of concrete used for construction purposes can be controlled from start to finish. The best way to ensure this is by employing an experienced concrete contractor familiar with every aspect of this material. Even though concrete is simply a mixture of water, portland cement, and aggregates, it is how these three things interact with each other and their environment that makes for a successful pour. This makes things a little more complex for any professional contractor. The following are some pertinent details about these essential elements of concrete.
WaterUsing pure, potable water that is fit to drink will ensure good binding of the dry materials. Any oils or particulates present in the water will significantly weaken the bond. Too much water in the mix will also weaken concrete, making a professional contractor look bad. The water/cement (W/C) mix ratio is known as Abrams’ Law, and can be affected by many variables, such as the moisture content of the aggregate and atmospheric humidity at the time of the pour. The strength of concrete decreases as the W/C ratio increases.
Cement:Before the 1940’s, cement quality was inconsistent, there were not professional contractors with any certifications, and many older foundations suffered accordingly. However, once the science of calcination was perfected, the processing kilns could turn out a more uniform product. There are five types of portland cement recognized by the American Society for Testing and Materials (ASTM). Each is distinct in their intended purpose, but all achieve the same strength rating after the curing period. Most residential concrete for construction calls for type 1, but many times type 3, with its fast curing properties, will be mixed in if it’s a rush job by a professional contractor. The strength of any concrete mix is heavily dependent on the cement content. The standard method of ordering ready-mix is to specify how much cement will be used for each cubic yard of concrete and is preferably expressed as a ratio of cement weight to the given volume, i.e 500 lb. per cubic yard.
Things all Professional Contractors should Know about Concrete
AggregatesThe main function of having aggregates in a mix is to keep the concrete from shrinking and cracking during hardening. The size of this material ranges from fine sand to 1.5″ diameter gravel. Most general construction calls for a certain proportion of fine to coarse particles in the mix to help fill in any voids that might develop. Aggregates play a huge role in the workability of concrete, and the adjustment of amounts and sizes of particles determine how well the mix fills the forms.
SlumpThis is a term used by concrete contractors to denote the moisture content of a concrete mix – the higher the slump, the wetter the mixture. The surface cracks and scaling you see on many residential foundations are a result of a high slump, and over time, can seriously compromise a structure.
Admixturesare agents that give concrete specific qualities. The four main types are:
- Air-Entraining is a method of adding air bubbles during the mixing process to create the shock absorbing voids that withstand the stresses of freezing and thawing temperatures.
- Set-Retarder can be added to ready-mix to increase the setting time by up to 60%. This works great in areas of the country like Dallas TX, that experience many hot days requiring extra working time on the site.
- A plasticizer is a water-reducing agent that helps to minimize any issues related to excess water, such as segregation and cracking. It also acts to strengthen the bond of concrete to steel reinforcing bar.
- Calcium Chloride is a chemical accelerator used during concrete pours in cold weather. It prevents the mix from freezing during setting and curing.