Everything You Ever Wanted To Know About The Concrete In Your Swimming Pool

Everything You Ever Wanted To Know About The Concrete In Your Swimming Pool

What Is The Best Concrete Mix?

From Swimming Pool Steve

Concrete is the most ubiquitious man made substance in the world.  It might even be argued that the foundation of the world is built on concrete.  This versatile, strong material is singularly ideal for so many construction projects, not the least of which being the inground swimming pool.  Concrete is a mix of some basic and sometimes not so basic materials, which can cause confusion as to what really constitutes concrete.  This post from Swimming Pool Steve gives one of the easiest to understand treatises on concrete ever published.

Here is an excerpt from the post:

What Is The Best Concrete Mix?

“The best concrete mix” will depend on what you intend to do with the concrete – there are any 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 ration 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 or more often bonding glue. In areas where a faster set is needed it is not uncommon to add calcium to actuate the initial set more quickly. 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.

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