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Thewater–cement ratio (w/c ratio, orwater-to-cement ratio, sometimes also called theWater-Cement Factor,f) is the ratio of the mass of water (w) to the mass ofcement (c) used in aconcrete mix:
The typical values of this ratiof =w⁄c 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 higherstrength anddurability, but may make the mix more difficult to work with and form. Workability can be resolved with the use ofplasticizers orsuper-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 asground granulated blast-furnace slag (GGBFS),fly ash (FA),silica fume (SF),rice husk ash (RHA),metakaolin (MK), and naturalpozzolans. Most of supplementary cementitious materials (SCM) are byproducts of other industries presenting interesting hydraulic binding properties. After reaction withalkalis (GGBFS activation) andportlandite (Ca(OH)
2), they also formcalcium silicate hydrates (C-S-H), the "gluing phase" present in the hardened cement paste. These additional C-S-H are filling the concreteporosity and thus contribute to strengthen concrete. SCMs also help reducing theclinker content in concrete and therefore saving energy and minimizing costs, while recycling industrial wastes otherwise aimed tolandfill.
The effect of the water-to-cement (w/c) ratio onto themechanical strength of concrete was first studied by René Féret (1892) in France, and then byDuff A. Abrams (1918) (inventor of theconcrete slump test) in the USA, and by Jean Bolomey (1929) in Switzerland.
The 1997Uniform Building Code specifies a maximum of 0.5 w/c ratio when concrete is exposed tofreezing andthawing in moist conditions or tode-icing salts, and a maximum of 0.45 w/c ratio for concrete in severe, or very severe,sulfate conditions.
Concrete hardens as a result of the chemical reaction between cement and water (known ashydration and producingheat). For everymass (kilogram,pound, or anyunit of weight) of cement (c), about 0.35 mass of water (w) is needed to fully complete the hydration reactions.[1]
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 densesteel 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 aplasticizer to increase flowability.
A w/c ratio higher than 0.60 is not acceptable as fresh concrete becomes "soup"[2] and leads to a higher porosity and to very poor quality hardened concrete as publicly stated by Prof.Gustave Magnel (1889-1955,Ghent University, 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 firstprestressed concretegirder bridge in the USA: theWalnut Lane Memorial Bridge inPhiladelphia open to traffic in 1951.[3][4][5][6] 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 thegirders of this bridge remains in many memories:"American makes soup, not concrete".[7]
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 concreteporosity only filled byair. A higher porosity reduces the finalstrength of concrete because the air present in thepores iscompressible and concretemicrostructure can be more easily "crushed".
Moreover, a higher porosity also increases thehydraulic conductivity (K, m/s) of concrete and theeffective diffusion coefficients (De, m2/s) ofsolutes and dissolvedgases in the concrete matrix. This increases water ingress into concrete, accelerates itsdissolution (calciumleaching), favors harmful expansive chemical reactions (ASR, DEF), and facilitates the transport of aggressive chemical species such aschlorides (pitting corrosion ofreinforced bars) andsulfates (internal and external sulfate attacks, ISA and ESA, of concrete) inside the concrete porosity.
When cementitious materials are used to encapsulatetoxic heavy metals orradionuclides, 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 thediffusion of gases into the concretemicrostructure. A faster diffusion ofatmosphericCO
2 increases the concretecarbonationrate. When the carbonationfront reaches thesteel reinforcements (rebar), thepH of the concrete pore water at the steel surface decreases. At a pH value lower than 10.5, thecarbon steel is no longuerpassivated by analkaline pH and starts to corrode (general corrosion). A faster diffusion ofoxygen (O
2) into the concrete microstructure also accelerates therebar corrosion.
Moreover, on the long term, aconcrete mix with too much water will experience morecreep 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 thesegregation of fine and coarse aggregates (sand andgravels) 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 concreteslabs orrafts (with a dusty surface left after water evaporation).
For all the afore mentioned reasons, it is strictly forbidden to add extra water to aready-mix concrete 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 ofsuperplasticizer 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.
The water-to-cement ratio is the ratio between the weights of water and cement in a concrete mix. For proper hydration, this ratio (commonly called the w/c ratio) should be about 0.30, assuming no contribution to hydration from external water sources.
Prof. Gustave Magnel famous quote:Americans make soup, not concrete
See also this note by Prof. Eduardo C.S. Thomaz.
Jean Bolomey, 21-03-1879 (Saint-Légier-La Chiésaz) – 24-07-1952 (Pully)
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