Describe how water to cement ratio (W / C ratio) is the single most important factor governing the strength and durability of concrete?

insufficient water, there is not enough water for the cement to react, so that determines the strength. Also the concrete gives of more heat and the mixture can be to dry to pour

to much water, and there is not enough cement to react with, that also determines the strength, and the mixture will be pourable but the water evaporates and all that was water isn't concrete is there was no reaction with the cement. The concrete is also more open so bad fluids, salty fluids, salty air can penetrate more easily.

Water-cement ratio is the ratio of weight of water to the weight of cement used in a concrete mix. It 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.

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)

More typical Water-Cement ratios of0.4 to0.6 are used. For higher-strength concrete, lower water/cement ratios are used, along with a plasticizer.

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

Water to cement ratio (W/C ratio) is the single most important factor governing the strength and durability of concrete. Strength of concrete depends upon W/C ratio rather than the cement content. Abram’s law states that higher the water/cement ratio, lower is the strength of concrete. As a thumb ruleevery1% increase in quantity of water added, reduces the strength of concrete by5%. A water/cement ratio of only 0.38 is required for complete hydration of cement. (Although this is the theoretical limit, water cement ratio lower than0.38 will also increase the strength, since all the cement that is added, does not hydrate) Water added for workability over and above this water/cement ratio of0.38, evaporates leaving cavities in the concrete. These cavities are in the form of thin capillaries. They reduce the strength and durability of concrete. Hence, it is very important to control the water/cement ratio on site. Every extra liter of water will approx. reduce the strength of concrete by2 to3 N/mm2and increase the workability by25 mm. As stated earlier, the water/cement ratio strongly influences the permeability of concrete and durability of concrete. Revised IS456-2000 has restricted the maximum water/cement ratios for durability considerations by clause8.2.4.1, table5.

In addition to adequate answers you is duff Abrams in1919 he found relation between water /cement  ratio and strength of concrete where strength equal to K1/K2^w/c

w/c represents the W/C of the mix

K1,K2  are empirical constant Where to a certain extent from w / c concrete strength decreases .with increasing w / c

A. Water/cement ratio

Water to cement ratio (W/C ratio) is the single most important factor governing the strength and durability ofconcrete. Strength of concrete depends upon W/C ratio rather than the cement content. Abram’s law states that higher the water/cement ratio, lower is the strength of concrete. As a thumb rule every1% increase in quantity of water added, reduces the strength of concrete by5%. A water/cement ratio of only 0.38 is required for complete hydration of cement. (Although this is the theoretical limit, water cement ratio lower than0.38 will also increase the strength, since all the cement that is added, does not hydrate) Water added for workability over and above this water/cement ratio of0.38, evaporates leaving cavities in the concrete. These cavities are in the form of thin capillaries. They reduce the strength and durability of concrete. Hence, it is very important to control the water/cement ratio on site. Every extra liter of water will approx. reduce the strength of concrete by2 to3 N/mm2and increase the workability by25 mm. As stated earlier, the water/cement ratio strongly influences thepermeability of concrete and durability of concrete. Revised IS456-2000 has restricted the maximum water/cement ratios for durability considerations by clause8.2.4.1, table5.

B. Cement content

Cement is the core material in concrete, which acts as a binding agent and imparts strength to the concrete. From durability considerations cement content should not be reduced below300Kg/m3 for RCC. IS456 –2000 recommends higher cement contents for more severe conditions of exposure of weathering agents to the concrete. It is not necessary that higher cement content would result in higher strength. In fact latest findings show that for the same water/cement ratio, a leaner mix will give better strength. However, this does not mean that we can achieve higher grades of concrete by just lowering the water/cement ratio. This is because lower water/cement ratios will mean lower water contents and result in lower workability. In fact for achieving a given workability, a certain quantity of water will be required. If lower water/cement ratio is to be achieved without disturbing the workability,cement content will have to be increased. Higher cement content helps us in getting the desired workability at a lower water/cement ratio. In most of the mix design methods, the water contents to achieve different workability levels are given in form of empirical relations.

Water/cement ratios required to achieve target mean strengths are interpolated from graphs given in IS10262 Clause3.1 and3.2 fig2. The cement content is found as follows: –

Thus, we see that higher the workability of concrete, greater is cement content required and vice versa. Also, greater the water/cement ratio, lower is the cement content required and vice versa.

C. Relative proportion of fine, coarse aggregates gradation of aggregates

Aggregates are of two types as below:

a. Coarse aggregate (Metal): These are particles retained on standard IS4.75mm sieve.

b. Fine aggregate(Sand): These are particles passing standard IS4.75mm sieve.

Proportion of fine aggregates to coarse aggregate depends on following:

i. Fineness of sand: Generally, when the sand is fine, smaller proportion of it is enough to get a cohesive mix; while coarser the sand, greater has to be its proportion with respect to coarse aggregate.

ii. Size& shape of coarse aggregates: Greater the size of coarse aggregate lesser is the surface area and lesser is the proportion of fine aggregate required and vice versa. Flaky aggregates have more surface area and require greater proportion of fine aggregates to get cohesive mix. Similarly, rounded aggregate have lesser surface area and require lesser proportion of fine aggregate to get a cohesive mix.

iii. Cement content: Leaner mixes require more proportion of fine aggregates than richer mixes. This is because cement particles also contribute to the fines in concrete.

Durable concrete is somewhat porous concrete  The W / C ratio has a great influence on the porosity of the doughhydrated cement because it directly governs the spacingInitial suspended between cement grains in waterof mixing. Over the W / C, the lower the porosityaccessible to water is reduced and the resistance tocompression increases.

I agree with the answer given by Mr. Yaqoub يعقوبعبداللهعبدالواحدالعمر.

According to the strength,in structural concretes, the project resistance (fck ), that is to say, the minimum value of resistance is25 N / mm2 in reinforced concrete or prestressed.

According to the durability, the following important requirements about concrete dosage and behaviour must be obeyed:

1) maximum water/cement ratio

2) minimum concrete contents

And another requirements are : Minimal entrained air content, Using a sulphate resisting cement, Using a resistant cement to seawater, Resistance against erosion or Resistance against the alkali-aggregate reactions ; each one where it was appropriate.

The typical W/C ratios are used in this range: between0.60 and0.45 depending on the kind of the environmental exposure. You can check it in the data table37.3.2.a on page153 in the Spanish Concrete Instruction EHE-08

If you respect the concrete rules in this way, and use high quality sand and gravel you will get the strength and durability desired in the concrete produced.

A lower water-cement ratio leads to higher strength and durability.

With increase in water in concrete the strength of concrete decrease as voids are filled with water. For this purpose a slump test is carried out at site. However w/c ration can not be considered the single factor governing the strength. It also depends on fine and coarse aggregate. 

Concrete is a structural composite material made up of aggregates (fine and coarse), cement and water. The cement acts as a binder which determines the cohesive strength of the concrete. However, cement is usually procured dry and water is added during batching to initiate the chemical reaction between the cement and water. Upon addition of water, cement hardens up or solidify - a reaction which is exothermic in nature. 

This is however a sensitive factor for the targeted concrete strenth - if the amount of water added is too low, then the cement does not complete its chemical reaction; likewise if the amount of water added is too much, the cement will be diluted and cannot harden up. The water/cement ratio is therefore the optimum amount of water that must be added to a unit amount of cement to achieve the targeted strength. 

Also, water/cement affects workability - a term used to refer to the relative ease of use of concrete on a construction site. The higher the water/cement ratio, the higher the workability, the lower the strength. The converse is also true!

The most common values of water/cement ratio in use ranges from 0.3 to 0.55 depending on a lot of factors such as:

- type of cement use (ordinary portland cement, rapid hardening etc)

- type of fine aggregrates 

-the targeted concrete strength

- the site working conditions (available working space, type of form-work, working at height, etc)

Thank you my dear friend about your useful question and also useful answers , thanks for all