Water-cement ratio
Encyclopedia
The water–cement ratio is the ratio of the weight of water to the weight of cement
used in a concrete
mix and has an important influence on the quality of concrete produced. A lower water-cement ratio leads to higher strength and durability, but may make the mix more difficult to place. Placement difficulties can be resolved by using plasticizer
s or super-plasticizers.
Often, the water–cement ratio is characterized as the water to cement plus pozzolan
ratio, w/(c+p). The pozzolan is typically a fly ash
, or blast furnace
slag
. It can include a number of other materials, such as silica fume, rice hull ash or natural pozzolans. The addition of pozzolans will influence the strength gain of the concrete.
The concept of water–cement ratio was developed by Duff A. Abrams
and first published in 1918, see concrete slump test
.
Concrete hardens as a result of the chemical reaction between cement and water (known as hydration
, this produces heat and is called the heat of hydration). For every pound (or kilogram or any unit of weight) of cement, about 0.25 pounds (or 0.25 kg or corresponding unit) of water is needed to fully complete the hydration reactions. This requires a water-cement ratio of 1:4 often given as a proportion: 0.25. However, a mix with a w/c ratio of 0.25 may not mix thoroughly, and may not flow well enough to be placed, so more water is used than is technically necessary to react with the cement. More typical water-cement ratios of 0.4 to 0.6 are used. For higher-strength concrete, lower water:cement ratios are used, along with a plasticizer to increase flowability.
Too much water will result in segregation of the sand and aggregate components from the cement paste. Also, water that is not consumed by the hydration reaction may leave the concrete as it hardens, resulting in microscopic pores that will reduce the final strength of the concrete. A mix with too much water will experience more shrinkage as the excess water leaves, resulting in internal cracks and visible fractures (particularly around inside corners) which again will reduce the final strength.
The 1997 Uniform Building Code
specifies a maximum 0.50 water-to-cement ratio (1:1) when concrete is exposed to freezing and thawing in a moist condition or to de-icing chemicals, and a maximum 0.45 water to cement ratio for concrete in severe or very severe sulfate conditions.
Cement
In the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed...
used in a concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
mix and has an important influence on the quality of concrete produced. A lower water-cement ratio leads to higher strength and durability, but may make the mix more difficult to place. Placement difficulties can be resolved by using plasticizer
Plasticizer
Plasticizers or dispersants are additives that increase the plasticity or fluidity of the material to which they are added; these include plastics, cement, concrete, wallboard, and clay. Although the same compounds are often used for both plastics and concretes the desired effects and results are...
s or super-plasticizers.
Often, the water–cement ratio is characterized as the water to cement plus pozzolan
Pozzolan
A pozzolan is a material which, when combined with calcium hydroxide, exhibits cementitious properties. Pozzolans are commonly used as an addition to Portland cement concrete mixtures to increase the long-term strength and other material properties of Portland cement concrete, and in some cases...
ratio, w/(c+p). The pozzolan is typically a fly ash
Fly ash
Fly ash is one of the residues generated in combustion, and comprises the fine particles that rise with the flue gases. Ash which does not rise is termed bottom ash. In an industrial context, fly ash usually refers to ash produced during combustion of coal...
, or blast furnace
Blast furnace
A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally iron.In a blast furnace, fuel and ore and flux are continuously supplied through the top of the furnace, while air is blown into the bottom of the chamber, so that the chemical reactions...
slag
Slag
Slag is a partially vitreous by-product of smelting ore to separate the metal fraction from the unwanted fraction. It can usually be considered to be a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides and metal atoms in the elemental form...
. It can include a number of other materials, such as silica fume, rice hull ash or natural pozzolans. The addition of pozzolans will influence the strength gain of the concrete.
The concept of water–cement ratio was developed by Duff A. Abrams
Duff Abrams
Duff A. Abrams was an American researcher in the field of composition and properties of concrete. He developed the basic methods for testing concrete characteristics still in use today. A professor with the Lewis Institute, he studied the component materials of concrete in the early 20th...
and first published in 1918, see concrete slump test
Concrete slump test
The concrete slump test is used for the measurement of a property of fresh concrete.The test is an emprical test that measures the workability of fresh concrete. More specifically, it measures the consistency of the concrete in that specific batch. It is also used to determine consistency between...
.
Concrete hardens as a result of the chemical reaction between cement and water (known as hydration
Hydration
Hydration may refer to:* Hydration reaction, a chemical addition reaction where a hydroxyl group and proton are added to a compound* Mineral hydration, an inorganic chemical reaction where water is added to the crystal structure of a mineral...
, this produces heat and is called the heat of hydration). For every pound (or kilogram or any unit of weight) of cement, about 0.25 pounds (or 0.25 kg or corresponding unit) of water is needed to fully complete the hydration reactions. This requires a water-cement ratio of 1:4 often given as a proportion: 0.25. However, a mix with a w/c ratio of 0.25 may not mix thoroughly, and may not flow well enough to be placed, so more water is used than is technically necessary to react with the cement. More typical water-cement ratios of 0.4 to 0.6 are used. For higher-strength concrete, lower water:cement ratios are used, along with a plasticizer to increase flowability.
Too much water will result in segregation of the sand and aggregate components from the cement paste. Also, water that is not consumed by the hydration reaction may leave the concrete as it hardens, resulting in microscopic pores that will reduce the final strength of the concrete. A mix with too much water will experience more shrinkage as the excess water leaves, resulting in internal cracks and visible fractures (particularly around inside corners) which again will reduce the final strength.
The 1997 Uniform Building Code
Uniform Building Code
The Uniform Building Code was a building code used primarily in the western United States.- History :The UBC was first published in 1927 by the International Council of Building Officials...
specifies a maximum 0.50 water-to-cement ratio (1:1) when concrete is exposed to freezing and thawing in a moist condition or to de-icing chemicals, and a maximum 0.45 water to cement ratio for concrete in severe or very severe sulfate conditions.