Concrete continues to be an important product for use in the construction of many different types of buildings, wall structures, floors and other items. However, with today's complicated high rise structures, the need to strengthen the product has become of utmost importance. According to the American Concrete Institute (ACI), high strength concrete must meet very specific requirements of at least 6,000 psi. To accomplish this, the concrete is generally manipulated within its basic cement and aggregate mixtures and mixed with compound additives like calcinated shale, fly ash, granulated blast furnace slag, metakaolin or silica fume. An explanation of these mixtures is noted below.
- Skill level:
Outline why high strength concrete is required. For example, it could be due to a reduced drying time allowed for the job. It could be to reduce the size of the support columns while still maintaining their strength. It might be to build extended structures like dams or bridges. Whatever the use, it will have an impact on how the concrete will be mixed. Additional considerations include, but are not necessarily limited to, the item's permeability, the estimated amount of material shrinkage and the needed workability within the product. Beyond that, one also has to consider ornamental considerations, such as the mixture's ability to be stamped and whether or not it will be stained.
Decide on the appropriate ratio between the dry and wet compounds of the mixture. In many instances, this ratio may change drastically, while in others it may alter only slightly. Much of this is dependent upon the way the concrete will be used as well as the additives that will be introduced into the basic mixture. If necessary, refer to the ACI website for guidance.
Calculate the type of aggregate product to be used and the appropriate ratio. Aggregate includes materials like gravel, limestone, granite and sand. The chosen aggregate will depend upon whether the product finish may be coarse or must be more fine in nature.
Figure out which additives, such as calcinated shale, fly ash, granulated blast furnace slag, metakaolin or silica fume, should be used. This decision will be somewhat dependent upon how the concrete is to be used. Considerations will include the product's need for strength and durability, its ultimate appearance, density requirements, and lastly where the concrete will be placed (like near standing water, in a damp, moist environment or in high humidity or other similar considerations). Fly ash tends to improve the concrete's finished look and allows for an extended drying time. Metakaolin and slag tend to lighten the concrete's final appearance. This can be important if the concrete will be stained.
Calculate the proportions of cement, water, aggregate and additives to be used. Depending upon how the concrete will be used and the qualities it requires, additives can replace as much as 40 per cent of the cement within the completed mixture. Again, you can refer to the ACI website to help determine the proper additives and to check for formula suggestions. Another option is to purchase a software program that can calculate the formula for you based upon the information that you provide concerning the concrete's requirements.
Mix the basic cement mixture reducing the amount of water as determined in Step 2 above. Continue mixing for approximately two to three minutes.
Cease mixing for a couple of minutes as you put the additives identified from Step 3 above into the mixture. Continue to mix for three to four minutes or until you get the desired mixture.
Check the concrete for proper consistency. The mixture is too dry if it crumbles and flakes as you try to mix it with a mixing tool. Add additional water in small quantities until the right texture is achieved. The mixture is too wet if it looks runny and doesn't appear to "set up." In this case, you may have to go back to Step 2 and begin all over again.
Pour the concrete for testing (if time allots). Decide how long to allow the mixture to sit before performing any actual tests. The standard waiting time is generally between 20 and 60 days.
Test the concrete for the desired results with regard to strength, durability, density, look and feel. If it meets the test guidelines, you are set to proceed with the final mixing of the product. If it does not, you will need to back up to Step 2 and recalculate the formulas and products chosen for the mixture.
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