Cement ... Concrete ... a basic guide

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Cement ... Concrete ... a basic guide

Postby MGV12 » Sun Oct 03, 2010 7:59 am

Whilst searching the Internet for something else I came across this informative article which may be useful to those building for the first time; and even others:

The Art and Science of Concrete

The art and science of concrete construction has a very long history, dating back to at least ancient Rome. Reinforced concrete construction has a much shorter history, only about one hundred years. Modern alternative concrete processes or hi-tech applications are only a few years old.

The Basics

In order to understand the basic principals of any technology it is important to understand the terminology. Cement and concrete are two entirely different things yet many times are incorrectly used interchangeably. Cement is the powdered raw form. Cement mixed with sand and aggregates (gravel) is concrete. Cement mixed with sand, clay and lime is referred to as stucco for exterior use, yet, is called plaster for interior or swimming pool use. A wet, high-pressure air applied mix is called shotcrete. However a dry, high-pressure air applied mix is called gunite. The basically same mixture is also referred to as mortar in masonry block and brick work. If you dilute this same mixture with more water into a flowable form it is called grout. Even though all of these terms are distinctly different, depending on the application, the actual chemical process that takes place, and the principles that apply, when cement is mixed with water and aggregates, are the same. Therefore, if you take the time to understand the basics of concrete, it will be invaluable in the future.

Water - Water comes in three different forms: A liquid (water), a gas (water vapor), and a solid (ice).

Waterproof - Waterproof concrete is concrete which is impervious or unaffected by liquid water or prevents the penetration of water. The term "waterproof" is frequently used inaccurately with regards to flooring materials. Waterproof concrete or waterproofing concrete does not stop water vapor movement. Concrete must be "vaporproof" before the application of flooring materials and surface coatings.

Vaporproof - Vaporproof concrete restricts or prevents the passage of water vapor.

Waterproof concrete is not necessarily vaporproof, but vaporproof concrete is always waterproof.

Water Vapor - Water vapor moves much faster and much more readily through concrete than water. Waterproof concrete does not stop water vapor. Water will not move through waterproof concrete, water vapor will. Additionally, when water vapor reaches the dew point under flooring materials it condenses becoming water. Therefore, to waterproof concrete is not the issue with regard to paints, coatings or flooring materials, the issue is to make concrete vapor proof and it will be inherently waterproof.

Moisture Vapor, Moisture Vapor Emissions - Moisture Vapor and Moisture Vapor Emissions are terms that the flooring industry uses to define water vapor. Moisture vapor emissions is the direct result of a number of factors. While irrelevant as to the cause or source moisture vapor emissions in concrete, which are higher than the flooring industry's maximum allowable levels, will cause flooring installations to fail.

Water Cement Ratio - This property is one of the most important factors in producing quality concrete. The water to cement ratio of concrete is simply the weight of the water divided by the weight of the cement. This ratio is presented as a decimal fraction. At placement of the slab the water-cement ratio directly affects permeability and the time it takes before flooring materials can be installed.

The water to the cement ratio is dependant in part on the local aggregates and sand available. Porous aggregates will require higher water to cement ratios to achieve a workable slump (consistency). Denser aggregates, being less absorptive, will require lower water to cement ratios to maintain a given slump. The resulting concrete, with porous aggregates and a higher water to cement ratio, will naturally have greater permeability due to a higher percentage of interconnecting capillary voids. These voids also make the concrete weaker.

Slump - Slump is measured with a slump cone. A slump cone is an inverted cone, 12" tall and open on both ends. The top is 4 inches wide and the bottom 8 inches wide. Fresh concrete is placed in the cone and "rodded" with a steel rod to compact the concrete. The cone is removed and placed next to this mass of fresh concrete. The difference in height between the top of the slump cone and the top of fresh concrete is the slump. More flowable concrete is said to have a higher slump. This means that the concrete has more water in it hence a higher water to cement ratio.

Hydrating Free Mix Water - Hydrating free mix water is the excess concrete mix water outflow from new concrete. In actual field conditions, new concrete construction may take five years or more, depending on the water to cement ratio of the concrete, for this free water to completely hydrate based on Portland Cement Association Research. Flooring cannot be installed when there is the presence of this water vapor outflow (hydrating free mix water).

Capillaries - Capillaries form within concrete in direct relationship to the water to cement ratio, the higher the water-cement ratio (more water) the greater volume of interconnecting capillaries, producing a more porous concrete making concrete inherently weaker.

Permeability - Concrete by its very nature is permeable. The interconnecting capillaries formed when concrete cures provides the perfect medium for water vapor transmission. The free mix water necessary for the ease of placement of concrete is much higher than is necessary for the complete hydration of concrete. It is the process of the hydrating cement and the curing of concrete that form these weakness planes and permeability.

Capillary Action - Capillary action (wicking action) attracts moisture from the soil through the interconnecting capillaries in the concrete slab. Moisture within the slab tries to evaporate into the room. Water vapor driven by vapor pressure, at room temperature, will condense at the dew point under impermeable flooring materials and will emulsify the adhesive. If the slab is in an expansive clay or high sulfate soil condition the moisture may also bring with it soluble alkalis, which will further attack the adhesive.

Hydrostatic - Hydrostatic water pressure is the pressure caused from elevated adjacent source of water such as landscaping. Higher landscaping saturates the soil at the foundation slab and provides a continuous source of water. Additionally, this water may take the form of perking free water moving up through any existing cracks in the slab causing flooring failures.

Curing - Curing is a process, which maintains the proper internal moisture level in concrete. The one significant variable that directly effects permeability, quality and the ultimate strength properties of concrete is curing. Prolonged and thorough moist curing is the most significant factor in producing waterproof and watertight, high quality, high strength concrete. One form of prolonged moist curing is to apply moist burlap to the concrete surface continually keeping it moist for a period of 28 days. Research shows that concrete continuously moist cured for a period of 28 days resulted in compressive strengths exceeding 4,500 psi. Yet, the same concrete air cured for the same period achieved only 2,550 psi. Furthermore, the compressive strengths at 180 days were actually lower for the air cured concrete, 2,500 psi, where the continuously moisture cured concrete achieved an ultimate compressive strength of over 5,750 psi.

Enhancing the Moist Curing Process

Although prolonged and thorough moist curing produces concrete far superior in all aspects over air cured concrete, prolonged moist curing cannot produce concrete which completely prevents water vapor transmission. Interconnecting capillaries are formed in the cement hydration process. Slow, moist curing is the most significant factor in producing high quality, high strength, waterproof and watertight concrete yet, without an enhancement to this curing process, it is impossible to produce concrete which is impermeable to water vapor transmission.

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Re: Cement ... Concrete ... a basic guide

Postby Mike Judd » Sat Mar 26, 2011 6:56 am

Hi , As a contractor in Sydney , I have had a great deal of work with, so called "Concrete Cancer" Apparently after intensive study it has been established that bare concrete exposed to rain, will be penetrated at the rate of 1.m.m per year creating an Alkaline action , when reaching the steel at what ever cover,( which can be some-times as little as 10m.m. ) it only takes 1 year to go around and start blowing apart. With that information it is wise to always give exposed concrete some protective covering.
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