How To Make Concrete Do What You Want
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Learning how to work with concrete is one of the most important trade skills to know. Whether you are a contractor or just a home owner learning how to work with concrete is going to pay dividends time and again for you. Learning solid concrete fundamentals will allow you to take on projects for pennies on the dollar that you would normally need to hire a mason or concrete contractor to do. Furthermore if you take the time to learn a little about the construction medium you are working with beforehand this will allow you to do jobs only once...as opposed to doing all that work only to have it fail early again!
Fortunately concrete working is not black magic and for the most part it will be a simple and straight forward process for you...but be warned that the subject of concrete is anything but simple, and truly concrete is one of the deepest technical subjects that you can dive into, should you desire. But for the most part, the average home owner with a pothole in their concrete driveway does not need to know about rare chemical reactions in certain structural concrete applications. By the time that you get to the bottom of this page you might not be able to determine an engineered mix design for a site specific application...but you will know how to make strong concrete, how to form and place it, how to modify certain properties like how strong it is, how heavy it is, how fast or slow that it sets up, as well as most of the finishing tricks and techniques you will need.
Please note that no matter how much you study up on concrete, and how easy concrete workers make finishing large flat expanses of concrete look, doing this is actually very hard. Flat work finishing takes practice, muscle memory, experience working with set times, and a dozen other advanced concrete techniques. Be warned that while it is certainly possible to do concrete work on your own, it is important to manage your expectations since it would would be very easy to overestimate your skill finishing concrete.
What Is The Best Concrete Mix?
"The best concrete mix" will depend on what you intend to do with the concrete - there are many different mix variations for application specific uses. This page will teach you how to choose the correct concrete mix for your needs as well as how to mix, place and finish it correctly.
Concrete is the most versatile building material on the planet so learning a little about how to work with it would be a great idea. "Concrete", "mortar", "cement", "grout" and "plaster" are all unique concrete mixes - but are essentially all the same. An aggregate, such as sand, pebbles or rocks mixed with portland cement powder and water. The water is the catalyst in a chemical reaction that causes the mix to harden during a month long curing process. No matter which type of concrete mix you are dealing with, the properties are all the same.
In order to gain a working understanding of concrete processes we need to define some terms
Portland cement - The powder additive that you mix with aggregates such as sand which results in concrete after being actuated by water. Portland is essentially the glue that holds concrete together.
Concrete - The generic catch-all term that describes cement powder and aggregate that has been solidified. The most technically accurate form of "concrete" would be portland cement, sand and gravel mixed together in a ratio of 1:2:3
Mortar - A form of concrete that uses only cement powder and sand as an aggregate. No gravel or larger aggregate would be used in mortar. Sometimes referred to as grout, especially when speaking in terms of swimming pool floors in vinyl liner pools.
Grout - Grout is a mortar product (no large aggregate) that is most commonly modified with bonding agents for use as a filler in between joints in tile installations. Tile grout is available in sanded and unsanded versions where the sanded grout is better for larger grout joints and unsanded is preferable for smaller, detailed work. A "grout bottom" in a swimming pool would be a mortar product of sand and cement and not at all the same as tile grout in terms of the mix design.
Plaster - Mortar based plaster is a product made from cement powder and very fine aggregates. The result is a very dense mortar concrete that is water resistant and suitable for fine detailed work, stamps and finishes. Strong, but not load bearing strong, plaster is typically used as a decorative surface treatment. Other terms for different kinds of pool plaster are marcite, marblite, quartzite and many proprietary names as well. Mortar plaster is different from gypsum or lime based plasters, though they function in a similar fashion.
Exposed aggregate - Exposed aggregate, or pebble finishes are commonly found in swimming pool interiors as well as decorative finished flat concrete work. Essentially this is concrete that includes (and is seeded with) a specifically decorative aggregate stone that is small and has multiple colors. As it begins to set the top layer of portland cement is washed off revealing the "exposed aggregate" inside the concrete.
Stamped concrete - Stamped concrete is flat finished concrete that has been troweled smooth and impressed with a texture imprint from a rubber mat. Integral mix colors or dry release powders can add color to the concrete, or color can be added after the concrete is cured already using paints or acid etching.
Specialty Concrete Mix Recipes
Since there are so many different applications for concrete and mortar mixes, this section will discuss some of the commonly used mix designs and what they would typically be used for. If you intend to patch an existing concrete of any kind it is important to patch with a product similar to the base you are patching. Concrete of different mix designs can result in patchwork delaminating.
Pool plaster - This is a very dense plaster mortar used for interior finishes of concrete swimming pools. You would typically mix 2 parts fine white silica sand with one part federal white cement powder (or 1 part fine sand and one part plaster / extra fine white sand). This mix is then typically modified with a latex acrylic. In areas where a faster set is needed it is not uncommon to add calcium to actuate the initial set more quickly however this can potentially compromise the quality and integrity of the mix. Asbestos was also used sometimes in pool plaster, which certainly increased the lifespan, though not something that you would typically do anymore. Beware of 20-50 year old pool plaster that is in uncommonly good shape for its age as it could potentially contain asbestos fibers.
Vermiculite concrete - Vermiculite is a lightweight mineral that can be added to concrete (in place of some/all sand) which results in a concrete with unique properties. The vermiculite, being so lightweight as opposed to dense like sand, results in a mix substantially weaker than sand based mortar - however the concrete has a slightly "soft" or spongy finished strength that is great for protecting vinyl liners as well as being 10x as thermally resistant as regular concrete! Vermiculite concrete has a somewhat shimmery appearance as though there are gold flecks all through the concrete.
General use concrete - The most common concrete mix for general construction use is a "3 - 2 - 1 mix". This mix would have 3 parts gravel (usually 1/2" or 3/4" clear gravel), plus 2 parts masonry sand, plus 1 part portland cement. This is the cheapest concrete mix by volume, while still being strong enough to meet the demands of modern construction.
Mortar - Mortar, being concrete with no large stone aggregate, can have a ratio from 1-1 all the way up to 10 or 12 parts sand to 1 part of cement powder. The texture, finished strength , finishing ability, cure time and application for each mix is unique - for example you might build a natural stone wall with a mortar that is 8-1, or you may do a fine detail repair to some decorative concrete with a 1-1 mix. The strongest ratio of sand to cement is 3-1 and as such this should be the most common mix to use. If a finished detail is needed like with stamped concrete, a mix of 2-1 should produce very strong results that are slightly better suited to accept detailed impressions.
Thin set - Thin set is the standard for tile adhesion. Essentially thin set is a mortar concrete that has been heavily modified with bonding agents, such that thin set is very well known for bonding to just about anything. If you have existing tile work that is falling off the pool you will be inclined to stick the tiles back on however the new thin set will have trouble bonding to the old thin set since very limited water can transfer between the old and new layer. If water can not permeate the substrate layer then you can not get a strong mechanical bond with the new layer.
Bonding slurry - Bonding slurry is a thick, modified concrete paste that you use as a transitional layer in between old, existing concrete and a new concrete mix that you want to put over the old. This mix is water and portland cement mixed in equal parts (no sand) with a minimum of 10% bonding glue added. The result should be thick enough that you could paste it on with a brush (like half-melted butter). This is a deep technical subject and something that I cover in much greater detail in this article about how to bond new concrete to old concrete.
Modified Concrete Admixtures
Modified concrete, or fortified concrete, is any concrete mix that has additional ingredients added to it to change something about the properties. Concrete can be modified with bonding agents, accelerants, retarders, water reducers, air entrainers, colors and specialty aggregates. This would allow you to have limited control over the set time, strength, color, finishing properties, freeze resistance, viscosity and ability to bond to other concrete.