Assembled by Danny Masci
With everyone ranting on how Concrete finishes are the most cost-effective way to create hardscapes there are a few things that people need to know to make sure that their hardscape is as beautiful and as long lasting as it can be.
Here at Concreation Canada Inc. our business is Concrete Colouring and Overlay solutions. So, our focus is in how to make sure your concrete hardscape is the best that it can be. With this in mind, we need to understand what additives are used and how they react in the curing.
Concrete sets in stages. These stages will vary in time based on ambient temperature and humidity as well as the concrete mix itself. In most cases you can walk on a poured concrete installation 24 to 48 hours. Heavy traffic needs to be avoided for 7 days when the concrete is about 70% cured, with the full strength of the concrete being realized after 28 days. Controlling the moisture in the concrete is the most critical part of the curing process. Not enough water and the concrete will never get to full strength. Too much water and the surface will remain weak and cause flaking.
Strength in the concrete is based on a combination of Air Void Clustering and the amount of unreacted Calcium Hydroxide in the cured concrete. Cement Manufacturers use Concrete Add Mixtures called Pozzolans to control the Chemical Reaction of transforming Calcium Hydroxide to Calcium Silicate Hydrates. These products control the water requirements in the mix and increase strength to the final finish.
Pozzolans are created as a by product of other industries or are man made. These products effect the concrete in different ways. They are also known as Admixtures.
Accelerating Admixtures are used to speed up the curing process. They do this buy increasing the heat within the concrete to speed up the chemical forming process. They are primarily used in colder weather. The most common of these products are Calcium Chlorides. As you can imagine, having Calcium Chlorides can also increase the corrosion of steel reinforcements in the concrete. There are other options available at a higher cost. Calcium Nitrate and Calcium Nitrite can both be used and will prevent corrosion. The issue is that they fall under Environmental Acts and are heavily regulated.
Retarding Admixtures and Plasticizers work to retain moisture and improve workability of the Concrete. These are generally used in Hot Conditions to increase the set time. Fly Ash, Slag, and VCAS are some examples of this type of Admixture. Fly Ash is a by product of coal burning plants. Slag is a by product of Metal Blast Furnace, and VCAS (Vitrified Calcium Aluminio-Silicate) is a man-made product. These products are used as a direct cement Replacement. They also create better chemical bonds in the concrete to improve Flexural Strength, Compressive Strength, and Abrasion Resistance. Be cautious when using Fly Ash when the ambient temperature is on the low side. Fly Ash based cement will not properly cure under colder conditions and will never realize full strength. If you are to use Fly Ash in colder weather make sure to use Warming blankets during the curing process.
The final Pozzolan to be discussed here will be Silica Fume. This product is considered a Superplasticizer and a Corrosion Inhibitor. This is a by-product created during the creation of silicon metals and ferrosilicon alloys. Consisting of mostly Silicon Dioxide, Silica Fume reacts with Calcium Hydrates in the Concrete Mix to create Calcium Silicate Hydrates. This chemical reaction increases the strength of the concrete while also closing the Air Voids to external corrosive materials while maintaining a high flowability of the wet concrete before setting allowing for long PUMP ranges.
This article is designed to be used as informational only. Remember to check with your Municipal, Provincial, and Federal guidelines when designing your concrete hardscapes. Concrete Mix and Admixture standards are controlled by:
ASTM C1679 and ASTM C1753 - Performance of Cementitious Materials and Admixture Combinations. CSA-A23.1-14 and CSA-A24.1-14 - Concrete Materials and Methods of Concrete Construction/Test methods and Standard Practices for Concrete. ACI 211.1-91 (2009) - Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete (Reapproved 2009).
Information is derived using: The Concrete Network (http://www.concretenetwork.com) Slag Cement Association (http://www.slagcement.org) The Concrete Countertop Institute (http://concretecountertopinstitute.com) Portland Cement Association (http://www.cement.org) Norchem (http://www.norchem.com)