The thought of concrete conjures images of a strong, durable steadfast material with a wide range of uses. But what does it take to get to that final product? Let’s take a quick look at how to achieve the right mix for strength and durability.
Concrete is essentially a mix of aggregate and paste [cement and water]. This sounds simple enough – however, to achieve a quality result you need quality ingredients, in the right quantity.
Aggregate can be described as any hard, inert material used in making concrete. This includes; natural gravel and sand, crushed quarry rock, boulders, large gravel or other naturally occurring stone. Concrete contains up to 75% volume (85% weight) of aggregate and so its properties greatly affect the outcome of the final product.
Industry leaders make sure that their aggregates undergo multiple tests to ensure that they lay the right foundation for the final product. Barbados-based concrete manufacturer, Ready Mix Ltd, conducts more than 15 separate tests in order analyze their raw materials. They even manufacture their very own aggregate to get as close as possible the perfect result: type M sand. The production of this sand allows the company to control the quality and consistency of their products. This type of sand is 30-40% calcium and of such high quality, it is considered fit for agricultural use as animal feed.
“Cement is to concrete as flour is to Fruit Cake” – Court of Pennsylvania on a decision concerning cement manufacturing.
Cement is the glue that holds the concrete together. The most popular type used worldwide today is Portland cement which was invented almost two hundred years ago as the result of an accident. Today’s manufacturing process, to produce Portland cement, mixes two main raw materials together – limestone and clay. Thus, producing calcium silicate. If not produced to standard, there are certain impurities present in cement that can affect durability. Reliable manufacturers will supply certificates that prove the impurities within the cement are within international standards.
Water is used in concrete to hydrate the mixture. While people may assume it is for “workability” it is a part of the chemical process that turns fresh concrete into the hard and strong product we know it can be. It is therefore incredibly important to use the correct water-cement ratio, as it directly impacts the strength and durability of the concrete. Too much water is also the main cause of concrete cracking and can also cause de-lamination.
With the right materials in place, it is then equally as important to implement good practices; in doing so you achieve the desired properties needed for the best results. These practices include; Consistency/Workability; Finishability; Uniformity; Bleeding Rate; Controlled Setting Time; Temperature; Durability; Watertightness; Strength; Abrasion Resistance; Appearance; Dimensional Stability; Design; Economy.
Sandy (Tan) – Concrete made with a combined aggregate in the sandy zone (usually WF < 40) tends to demand more water and result in a higher drying shrinkage than mixtures in the well-graded zone(s)
Rocky (Black) – Concrete made with a combined aggregate in the rocky zone (below the control line) rends to be harsh, difficult to finish and subject to poor consolidation and surface voids.
Course Gap-Graded (Yellow) – Concrete made with a combined aggregate in the coarse gap graded zone (75 < CF < 90) experiences variable results, including segregation, uncontrolled voids, and excessive laitance. Some projects using such a gradation can still result in defect-free pavement, bit the rick of defects is higher than if a well-graded mixture is used.
Well-Graded (Blue or Green) – Concrete made with a combined aggregate in the well-graded zones (29 < WF < 43 and CF < 75) places easily and typically is not prone to early distress. These concrete mixtures tend to slip and finish well, and conventionally have the least risk of uncontrolled cracking and/or other defects/problems.
The right mix for strong and durable concrete is a science as well as an art. It takes the right knowledge and skill, but the result is well worth it.
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Further reads: Know your Concrete Block: Quality and Utility