Water–cement Ratio
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The water–cement ratio (w/c ratio, or water-to-cement ratio, sometimes also called the Water-Cement Factor, ) is the ratio of the mass of water () to the mass of
cement A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mi ...
() used in a
concrete Concrete is a composite material composed of aggregate bound together with a fluid cement that cures to a solid over time. It is the second-most-used substance (after water), the most–widely used building material, and the most-manufactur ...
mix: :f = \frac = \frac The typical values of this ratio = are generally comprised in the interval 0.40 and 0.60. The water-cement ratio of the fresh concrete mix is one of the main, if not the most important, factors determining the quality and properties of hardened concrete, as it directly affects the concrete porosity, and a good concrete is always a concrete as compact and as dense as possible. A good concrete must be therefore prepared with as little water as possible, but with enough water to hydrate the cement minerals and to properly handle it. A lower ratio leads to higher
strength Strength may refer to: Personal trait *Physical strength, as in people or animals *Character strengths like those listed in the Values in Action Inventory *The exercise of willpower Physics * Mechanical strength, the ability to withstand ...
and
durability Durability is the ability of a physical product to remain functional, without requiring excessive maintenance or repair, when faced with the challenges of normal operation over its design lifetime. There are several measures of durability in us ...
, but may make the mix more difficult to work with and form. Workability can be resolved with the use of
plasticizer A plasticizer ( UK: plasticiser) is a substance that is added to a material to make it softer and more flexible, to increase its plasticity, to decrease its viscosity, and/or to decrease friction during its handling in manufacture. Plasticizer ...
s or super-plasticizers. A higher ratio gives a too fluid concrete mix resulting in a too porous hardened concrete of poor quality. Often, the concept also refers to the ratio of water to cementitious materials, w/cm. Cementitious materials include cement and supplementary cementitious materials such as
ground granulated blast-furnace slag Ground granulated blast-furnace slag (GGBS or GGBFS) is obtained by quenching molten iron slag (a by-product of iron and steel-making) from a blast furnace in water or steam, to produce a Glassy phase, glassy, granular product that is then dried a ...
(GGBFS),
fly ash Coal combustion products (CCPs), also called coal combustion wastes (CCWs) or coal combustion residuals (CCRs), are byproducts of burning coal. They are categorized in four groups, each based on physical and chemical forms derived from coal combust ...
(FA),
silica fume Silica fume, also known as microsilica, (CAS number 69012-64-2, EINECS number 273-761-1) is an amorphous (non-crystalline) polymorph of silicon dioxide, silica. It is an ultrafine powder collected as a by-product of the silicon and ferrosilicon a ...
(SF),
rice husk ash Rice hulls or husks are the hard protecting coverings of grains of rice. In addition to protecting rice during the growing season, rice hulls can be put to use as building material, fertilizer, insulation material, or fuel. Rice hulls are part of ...
(RHA),
metakaolin Metakaolin is the anhydrous calcined form of the clay mineral kaolinite. Rocks that are rich in kaolinite are known as china clay or kaolin, traditionally used in the manufacture of porcelain. The particle size of metakaolin is smaller than cement ...
(MK), and natural
pozzolan Pozzolans are a broad class of siliceous and aluminous materials which, in themselves, possess little or no cementitious value but which will, in finely divided form and in the presence of water, react chemically with calcium hydroxide (Ca(OH)2 ...
s. Most of supplementary cementitious materials (SCM) are byproducts of other industries presenting interesting hydraulic binding properties. After reaction with
alkali In chemistry, an alkali (; from the Arabic word , ) is a basic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a soluble base has a pH greater than 7.0. The a ...
s (GGBFS activation) and
portlandite Portlandite is a hydroxide-bearing mineral typically included in the oxide mineral class. It is the naturally occurring form of calcium hydroxide (Ca(OH)2) and the calcium analogue of brucite (Mg(OH)2). Occurrence Portlandite occurs in a variety ...
(), they also form
calcium silicate hydrate Calcium silicate hydrates (CSH or C-S-H) are the main products of the hydration of Portland cement and are primarily responsible for the strength of cement-based materials. They are the main binding phase (the "glue") in most concrete. Only well de ...
s (C-S-H), the "gluing phase" present in the hardened cement paste. These additional C-S-H are filling the concrete
porosity Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
and thus contribute to strengthen concrete. SCMs also help reducing the clinker content in concrete and therefore saving energy and minimizing costs, while recycling industrial wastes otherwise aimed to
landfill A landfill is a site for the disposal of waste materials. It is the oldest and most common form of waste disposal, although the systematic burial of waste with daily, intermediate and final covers only began in the 1940s. In the past, waste was ...
. The effect of the water-to-cement (w/c) ratio onto the
mechanical strength Mechanical may refer to: Machine * Machine (mechanical), a system of mechanisms that shape the actuator input to achieve a specific application of output forces and movement * Mechanical calculator, a device used to perform the basic operations of ...
of concrete was first studied by René Féret (1892) in France, and then by Duff A. Abrams (1918) (inventor of the concrete slump test) in the USA, and by Jean Bolomey (1929) in Switzerland. The 1997
Uniform Building Code The Uniform Building Code (UBC) was a building code used primarily in the western United States. History The UBC was first published in 1927 by the International Conference of Building Officials, which was based in Whittier, California. It was i ...
specifies a maximum of 0.5 w/c ratio when concrete is exposed to
freezing Freezing is a phase transition in which a liquid turns into a solid when its temperature is lowered below its freezing point. For most substances, the melting and freezing points are the same temperature; however, certain substances possess dif ...
and
thawing Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid. This occurs when the internal energy of the solid increases, typically by the application of heat or pressure, which incr ...
in moist conditions or to de-icing salts, and a maximum of 0.45 w/c ratio for concrete in severe, or very severe,
sulfate The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...
conditions. Concrete hardens as a result of the chemical reaction between cement and water (known as
hydration Hydration may refer to: * Hydrate, a substance that contains water * Hydration enthalpy, energy released through hydrating a substance * Hydration reaction, a chemical addition reaction where a hydroxyl group and proton are added to a compound * H ...
and producing
heat In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, ato ...
). For every
mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...
(
kilogram The kilogram (also spelled kilogramme) is the base unit of mass in the International System of Units (SI), equal to one thousand grams. It has the unit symbol kg. The word "kilogram" is formed from the combination of the metric prefix kilo- (m ...
,
pound Pound or Pounds may refer to: Units * Pound (currency), various units of currency * Pound sterling, the official currency of the United Kingdom * Pound (mass), a unit of mass * Pound (force), a unit of force * Rail pound, in rail profile * A bas ...
, or any
unit of weight A unit of measurement, or unit of measure, is a definite magnitude (mathematics), magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity. Any other qua ...
) of cement (c), about 0.35 mass of water (w) is needed to fully complete the hydration reactions. However, a fresh concrete with a w/c ratio of 0.35 may not mix thoroughly, and may not flow well enough to be correctly placed and to fill all the voids in the forms, especially in the case of a dense steel reinforcement. More water is therefore used than is chemically and physically necessary to react with cement. Water–cement ratios in the range of 0.40 to 0.60 are typically used. For higher-strength concrete, lower w/c ratios are necessary, along with a
plasticizer A plasticizer ( UK: plasticiser) is a substance that is added to a material to make it softer and more flexible, to increase its plasticity, to decrease its viscosity, and/or to decrease friction during its handling in manufacture. Plasticizer ...
to increase flowability. A w/c ratio higher than 0.60 is not acceptable as fresh concrete becomes "soup" and leads to a higher porosity and to very poor quality hardened concrete as publicly stated by Prof.
Gustave Magnel Gustaaf Paul Robert Magnel (born 15 September 1889 in Essen (Belgium); died 5 July 1955 in Ghent) was a Belgian engineer and professor at Ghent University, known for his expertise regarding reinforced concrete and prestressed concrete. Biograph ...
(1889-1955,
Ghent University Ghent University (, abbreviated as UGent) is a Public university, public research university located in Ghent, in the East Flanders province of Belgium. Located in Flanders, Ghent University is the second largest Belgian university, consisting o ...
, Belgium) during an official address to American building contractors at the occasion of one of his visits in the United States in the 1950s to build the first
prestressed concrete Prestressed concrete is a form of concrete used in construction. It is substantially prestressed (Compression (physics), compressed) during production, in a manner that strengthens it against tensile forces which will exist when in service. Post-t ...
girder bridge A girder bridge is a bridge that uses girders as the means of supporting its deck. The two most common types of modern steel girder bridges are plate and box. The term "girder" is often used interchangeably with "beam" in reference to bridge d ...
in the USA: the
Walnut Lane Memorial Bridge The original Walnut Lane Memorial Bridge was a prestressed concrete girder bridge in Philadelphia, Pennsylvania, designed by Belgian Engineer Gustave Magnel and built by the City of Philadelphia. Completed and fully opened to traffic in 1951 ...
in
Philadelphia Philadelphia ( ), colloquially referred to as Philly, is the List of municipalities in Pennsylvania, most populous city in the U.S. state of Pennsylvania and the List of United States cities by population, sixth-most populous city in the Unit ...
open to traffic in 1951. The famous sentence of Gustave Magnel, facing reluctance from a contractor, when he was requiring a very low w/c ratio, zero-slump, concrete for casting the
girder A girder () is a Beam (structure), beam used in construction. It is the main horizontal support of a structure which supports smaller beams. Girders often have an I-beam cross section composed of two load-bearing ''flanges'' separated by a sta ...
s of this bridge remains in many memories: ''"American makes soup, not concrete"''.Engineering News Record, 1954, February 25 When the excess water added to improve the workability of fresh concrete, and not consumed by the hydration reactions, leaves concrete as it hardens and dries, it results in an increased concrete
porosity Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
only filled by
air An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...
. A higher porosity reduces the final
strength Strength may refer to: Personal trait *Physical strength, as in people or animals *Character strengths like those listed in the Values in Action Inventory *The exercise of willpower Physics * Mechanical strength, the ability to withstand ...
of concrete because the air present in the pores is
compressible In thermodynamics and fluid mechanics, the compressibility (also known as the coefficient of compressibility or, if the temperature is held constant, the isothermal compressibility) is a measure of the instantaneous relative volume change of a f ...
and concrete
microstructure Microstructure is the very small scale structure of a material, defined as the structure of a prepared surface of material as revealed by an optical microscope above 25× magnification. The microstructure of a material (such as metals, polymer ...
can be more easily "
crushed Crushed may refer to: Music * "Crushed" (Roland Lee Gift song) a 2009 single by Roland Lee Gift * "Crushed", a song by Cocteau Twins from the 1991 compilation album '' The Box Set'' * "Crushed", a song by Dala from the 2009 album ''Everyone Is So ...
". Moreover, a higher porosity also increases the
hydraulic conductivity In science and engineering, hydraulic conductivity (, in SI units of meters per second), is a property of porous materials, soils and Rock (geology), rocks, that describes the ease with which a fluid (usually water) can move through the porosity, ...
(''K'', m/s) of concrete and the
effective diffusion coefficient The effective diffusion coefficient of a in atomic diffusion of solid polycrystalline materials like metal alloys is often represented as a weighted average of the grain boundary diffusion coefficient and the lattice diffusion coefficient.P. Hei ...
s (''De'', m2/s) of
solutes In chemistry, a solution is defined by IUPAC as "A liquid or solid phase containing more than one substance, when for convenience one (or more) substance, which is called the solvent, is treated differently from the other substances, which are ...
and dissolved
gas Gas is a state of matter that has neither a fixed volume nor a fixed shape and is a compressible fluid. A ''pure gas'' is made up of individual atoms (e.g. a noble gas like neon) or molecules of either a single type of atom ( elements such as ...
es in the concrete matrix. This increases water ingress into concrete, accelerates its dissolution (
calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...
leaching), favors harmful expansive chemical reactions ( ASR, DEF), and facilitates the transport of aggressive chemical species such as
chloride The term chloride refers to a compound or molecule that contains either a chlorine anion (), which is a negatively charged chlorine atom, or a non-charged chlorine atom covalently bonded to the rest of the molecule by a single bond (). The pr ...
s (
pitting corrosion Pitting corrosion, or pitting, is a form of extremely localized corrosion that leads to the random creation of small holes in metal. The driving power for pitting corrosion is the depassivation of a small area, which becomes anodic (oxidation re ...
of reinforced bars) and
sulfate The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...
s (internal and external sulfate attacks, ISA and ESA, of concrete) inside the concrete porosity. When cementitious materials are used to encapsulate
toxic heavy metal A toxic heavy metal is a common but misleading term for a metal-like element noted for its potential toxicity. Not all heavy metals are toxic and some toxic metals are not heavy. Elements often discussed as toxic include cadmium, mercury and ...
s or
radionuclide A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess numbers of either neutrons or protons, giving it excess nuclear energy, and making it unstable. This excess energy can be used in one of three ...
s, a lower w/c ratio is required to decrease the matrix porosity and the effective diffusion coefficients of the immobilized elements in the cementitious matrix. A lower w/c ratio also contributes to minimize the leaching of the toxic elements out of the immobilization material. A higher porosity also facilitates the
diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
of gases into the concrete
microstructure Microstructure is the very small scale structure of a material, defined as the structure of a prepared surface of material as revealed by an optical microscope above 25× magnification. The microstructure of a material (such as metals, polymer ...
. A faster diffusion of
atmospheric An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosphere ...
increases the concrete
carbonation Carbonation is the chemical reaction of carbon dioxide to give carbonates, bicarbonates, and carbonic acid. In chemistry, the term is sometimes used in place of carboxylation, which refers to the formation of carboxylic acids. In inorganic che ...
rate. When the carbonation
front Front may refer to: Arts, entertainment, and media Films * ''The Front'' (1943 film), a 1943 Soviet drama film * '' The Front'', 1976 film Music * The Front (band), an American rock band signed to Columbia Records and active in the 1980s and ...
reaches the steel reinforcements (rebar), the pH of the concrete pore water at the steel surface decreases. At a pH value lower than 10.5, the
carbon steel Carbon steel is a steel with carbon content from about 0.05 up to 2.1 percent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states: * no minimum content is specified or required for chromium, cobalt ...
is no longuer passivated by an alkaline pH and starts to corrode ( general corrosion). A faster diffusion of
oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
() into the concrete microstructure also accelerates the
rebar Rebar (short for reinforcement bar or reinforcing bar), known when massed as reinforcing steel or steel reinforcement, is a tension device added to concrete to form ''reinforced concrete'' and reinforced masonry structures to strengthen and aid ...
corrosion. Moreover, on the long term, a concrete mix with too much water will experience more creep and drying shrinkage as excess water leaves the concrete porosity, resulting in internal cracks and visible fractures (particularly around inside corners), which again will reduce the concrete mechanical strength. Finally, water added in excess also facilitates the segregation of fine and coarse aggregates (
sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural ...
and
gravel Gravel () is a loose aggregation of rock fragments. Gravel occurs naturally on Earth as a result of sedimentation, sedimentary and erosion, erosive geological processes; it is also produced in large quantities commercially as crushed stone. Gr ...
s) from the fresh cement paste and causes the formation of honeycombs (pockets of gravels without hardened cement paste) in concrete walls and around rebar. It also causes water bleeding at the surface of concrete slabs or
raft A raft is any flat structure for support or transportation over water. It is usually of basic design, characterized by the absence of a hull. Rafts are usually kept afloat by using any combination of buoyant materials such as wood, sealed barre ...
s (with a dusty surface left after water evaporation). For all the afore mentioned reasons, it is strictly forbidden to add extra water to a
ready-mix concrete Ready-mix concrete (RMC) is concrete that is manufactured in a batch plant, according to each specific job requirement, then delivered to the job site "ready to use". There are two types with the first being the barrel truck or in–trans ...
truck when the delivery time is exceeded, and the concrete becomes difficult to pour because it starts to set. Such diluted concrete immediately loses any official certification and the responsibility of the contractor accepting such a deleterious practice is also engaged. In the worst case, an addition of
superplasticizer Superplasticizers (SPs), also known as high range water reducers, are Concrete additive, additives used for making high strength concrete or to place Self-consolidating concrete, self-compacting concrete. Plasticizers are chemical compounds enabli ...
can be made to increase again the concrete workability and to salvage the content of a ready-mix concrete truck when the maximum concrete delivery time is not exceeded.


References


Further reading

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