This article is devoted to one of the most, if not the most important factor required to be specified, controlled, and insisted upon while manufacturing concrete. The significance of the Water to Cement ratio (W/C) has been understood by very few concrete producers. Many feel that controlling W/C means a reduction of water thereby producing stiff unworkable concrete mix. It is unfortunate that many in the field of concrete production have not realized that workability can be maintained at the desired level even while maintaining strict control over the low W/C. Simply put, if water is required to be increased, cement should also be increased such that specified W/C does not increase. Another simple way is to reduce the aggregate quantity or in other words to reduce the aggregate/cement ratio of the concrete mix.
All over the country on small, medium, and even on many large projects water addition and control are manually done (See figure No:1). if not strictly supervised the water addition is done without any care or technical consideration (See figure No:2).
As a result W/C most often exceeds the specified or stipulated limits. Generally, low strength concretes (15 MPa or 20 MPa) are specified in our country. As good quality cement is available, the strength of these low grade concretes is achieved easily with high W/C. this results in poor durability of reinforced concrete structures.
Before analyzing durability aspects and their dependence on W/C ratios, it is essential to understand the hydration process between cement and water.
HYDRATED CEMENT PASTE
When cement is mixed with water, then hydrated cement paste is formed. The hydrated paste consists of three parts. Hydration product, anhydrous cement, and capillary pore.
The percentage of these parts will vary with the degree of hydration of cement which will in turn depend on curing (duration of hydration, temperature, and humidity). After completion of hydration, the part containing anhydrous cement disappears and the cement paste only consists of capillary pores and hydration products. The volume of capillary pores reduces as the hydration process continues or progresses. It has been observed that cement water paste with a greater volume of water will also occupy a greater total volume of space and after completion of the hydration process will end up with a larger volume of capillary pores. Therefore it can be concluded that there will be a progressive reduction in the capillary porosity due to an increased degree of hydration and decreased W/C.
As the capillary pores in cement paste reduce the strength increases and permeability of the concrete or mortar prepared using the paste decreases.
For generally hydrated Portland cement mortars, powers developed an exponential concern of the type S= kx3, between the compressive strength (S) and the solids to space ratio (x), where k is a constant equal to 34,000 psi. The effect of increasing solid/space ratio (I-P) on compressive strength and permeability is given in Figure No. 3. If the solid/space ratio decreases from 0.7 to 0.6 or in other words the capillary porosity (P) increases from 0.3 to 0.4 the permeability drops from 110 x 10-12 to 20 x 10-12 cm/sec. Further increase in the solid/space ratio or decrease of W/C will not significantly reduce the permeability.
DEFINITION OF WATER/CEMENT RATIO
In 1918, extensive studies were conducted by Duff Abrams at the levis Institute, University of Illinois, and the relation between W/C and concrete strength was developed. This is known as Abram’s W/C rule and is expressed by the following equation.
Fc = k1/k2 w/c
Where fc is the compressive strength and k1 and k2 are empirical constant.
It was only thereafter that W/C started gaining in importance as many properties of hardened concrete are influenced by this ratio.
The W/C is difficult to define because the aggregates in the concrete absorb water within their mass, often to an unknown degree. If all the water absorbed by the aggregate particles is neglected and just water on the surface is considered along with water added to the mix then the W/C is called “free W/C”.
If water absorbed by the aggregate particles is also considered in addition to water on the surface of the particles as well as water added to the mix, then the W/C is called “total W/C”.
The free W/C has a direct bearing on the properties of cement pasted in concrete and therefore has a direct influence on concrete strength and durability. Hence W/C needs to be more accurately measured. The disadvantage, however, is that it is less easy to measure the amount of water only on the surface of the aggregate particles and without disturbing the absorbed water. Besides, this method involves getting the aggregate into a saturated but surface-dry condition. The total W/C is an as less accurate measure of the quality of cement paste in concrete. It is easier to measure by thoroughly drying the aggregate.
The term W/C ratio is likely to disappear from concrete technology in the future. Concrete mixes besides comprising of cement, comprise of many types of cementitious material like fly ash, ground granulated blast furnace slag, silica fume, rice husk ash, metakaolin, and even fillers which are added to improve the durability of concrete, especially in severe environments. Hence, this ratio will now be known as a water-to-cementitious material ratio (W/CM) or a water-to-binder ratio (W/B).
The relation of the W/C ratio with strength will, therefore, be different when cementitious materials are used and the ratio is redefined as W/CM or W/B. it may be more relevant when very early age strength is to be established as at an early stage other ingredients of cement may not have undergone significant chemical reactions. In such cases, W/C only based on the mass of Portland cement will be more relevant. The W/C ratio’s influence on strength at various ages and durability will also depend on the type of cement used.
In the case of the use of liquid chemical admixture, the liquid content of the admixture should be included in the total mass of water in the mix while determining the W/C.
It s also important to note that cement and various cementitious materials have different specific gravities e.g. Portland cement has around 3.15 specific gravity as compared to 2.90 specific gravity of fly ash.
Some of those cementitious materials react with the calcium hydroxide produced by the hydration of Portland cement. The type and composition of cementitious material will affect the long term volume of solid products of hydration. It must be remembered that microstructures of concrete, which influence its strength and durability, depending on the volumetric proportions of various components of hardened concrete and not on proportion by mass.
Except when concrete is fully submerged in water, some cement will be left unhydrated. It is important to consider this when hardened concrete is being examined to determine the W/C ratio of the original mix.
ROLE OF W/C
The traditional role of the W/C ratio is a determinant of strength. It is also an indicator of durability.
D.A. Abrams stated “while there is agreement amongst authorities that the ratio of the amount of water to the amount of cement is a major influence on the strength of concrete, there is less agreement on the form of the relationship. The significance of the amount of water upon strength is the parameter”.
A more rational parameter is the relative density of the cement paste, which is also linearly related to strength.
As compressive strength is easily specified and tasted. It is mostly used as a measure to assess the quality of concrete. Within a normal range of strengths, the compressive strength is inversely related to W/C.
It must be noted that for fully compacted concrete produced, using sound and clean aggregates, strength and other desirable properties of concrete under a given job condition are governed by the quantity of water used per unit of cement.
Strength for a given W/C may vary due to the following reasons:
- Changes in aggregates such as maximum size, grading, surface texture, shape, strength, and stiffness.
- Different types of cement and their sources.
- Different types of cementitious admixtures used.
- Entrapped and entrained air content.
- Different types of chemical admixture and their dosages.
- Length of curing time and the presence of humidity around the structure.
In the past, the water content of the mix for given workability varied little with variation in strength. Earlier with the increase in the cement content and constant water content, the W/C ratio decreased while strength increased. With the advent of powerful water-reducing chemical admixtures, it is possible to significantly vary the water content, variation in strength can be significant without variation of cement content.
As W/C governs porosity of the hydrated cement paste, the value of W/C is relevant to many aspects of durability. Besides W/C, the following are another factor which determines the durability and permeability of concrete.
- Voids in concrete as a whole and not in cement paste alone (other voids being honeycombs, entrapped air, cracks, entrained air, etc.)
- The extent of connectivity between the pores that determine the penetrability of the aggressive agents.
- In case cementitious materials are used in cement, the influence of W/C on various properties is not the same as using neat Portland cement. The W/C is now redefined as a water/cementitious material ratio (W/CM) or water/binder ratio (W/B). the denominator CM or B = c + f k wherein c and f are the weight of cement and cementitious material or blinder added to the mix per unit volume and k accounts for the difference between the influence of Portland cement and binder. However, the value of k is different not only for different binders but also for the same type of binder. It is also different for different ages of concrete (hydration stages). After several months in the long term, there would be no difference expected between the cementitious material and Portland cement and therefore k would equal to one. The main reason for the difference between k value for different types of binders or the same type of binders from different sources is the volume occupied by the different binders in a mix per a given mass.
The presence of water in concrete has to be viewed in a proper perspective. Water is required for assisting mixing processes of all material, for lending workability to concrete and for cement hydration reaction.
Water is present in concrete from the start. Depending on ambient temperature, environmental conditions, and thickness of the concrete element, evaporable water in capillary pores and absorbed water is lost resulting in empty or unsaturated pores. Concrete will be vulnerable to water-related destructive phenomena when the exposure of the concrete to the environment leads to the resaturation of the pores and when it is insignificant or no water left after drying.
The resaturation of the pores will depend on the hydraulic (water) conductivity of the concrete element. This is also known as the coefficient of permeability (k), commonly known as “permeability”.
Permeability of concrete is of fundamental importance especially when there is a possibility of penetration of potentially aggressive chemicals (water, chlorides, sulfates, carbon-dioxide, etc.) either in liquid or gaseous form.
The study of the structure of hardened cement and water paste shown that the hydrated cement gel contains many very fine pores which diameters around 0.015 microns and occupy 28% by volume of the total cement paste. These pores are extremely fine and therefore fine and therefore virtually impermeable.
Capillary pores are considerably larger and have a diameter up to about 5 microns and they occupy up to 40% by volume of the total cement, paste depending on W/C used and the extent of chemical hydration which has taken place.
Voids larger than capillary pores would generally be entrapped air or voids resulting from inadequate placing and compaction of concrete. Generally, entrapped air to the extent of 1 – 2% of the total volume of concrete is expected to be present. To a much of entrapped air and voids must be avoided by using good mix design, god supervision, and quality control of all steps for concrete manufacture.
It is therefore extremely important to have low permeability by controlling the volume of the capillary pores by specifying and controlling the W/C at the site. Powers and others have shown the significance of W/C and the degree of hydration on the coefficient of permeability.
Chemical composition and fineness of cement do not have a significant influence on the coefficient of permeability. However, the coefficient of permeability significantly varies with W/C.
In figure A it is observed that the coefficient of permeability rapidly increases once W/C increases above 0.55.
The time necessary for cement paste which has been kept continuously damp to hydrate sufficiently for the capillary pores to be blocked has been investigated by powers and others and given in table 1 below.
Table 1 – Relation between age of concrete at which capillary pores become blocked and W/C
|Age of concrete at which capillary pores become blocked
In a hydrated cement paste, the control of the coefficient of permeability would depend on the size and continuity of the pores at any point during the hydration process. The coefficient of permeability of freshly mixed cement paste is of the order of 10-4 to 10-5 cm/sec. with the hydration process in progress, the capillary porosity reduces and so does the permeability (See table 2 given below) as observed by powers and others.
Table 2 – Reduction in permeability of cement paste (W/C = 0.70) with the progress of hydration
|Coefficient of Permeability (cm/sec x 10-11)
|2000 x 10-4
PERMEABILITY OF CONCRETE
It is logical to assume that by adding of aggregate particles of low permeability in the cement paste, permeability of the system will reduce because the aggregates would intercept the channels of flow within the cement paste matrix. In figure clearly indicates, on the contrary, that the addition of aggregates to cement paste or mortar increases the permeability considerably. It is also observed that the larger the aggregate size, the greater is the coefficient of permeability. The permeability coefficient for moderate strength concrete (containing 38mm, aggregate, 356 kg/m3 cement and 0.50 W/C) and low strength concrete used in dams (containing 75 to 150 mm, aggregate 148 kg/m3 cement and 0.75 W/C) are of the order of 1 x 10-10 and 30 x 10-10 cm/sec respectively.
The reason for increased permeability due to the presence of aggregates in the cement paste is micro-cracks which are present in the transition zone between the aggregate and the cement paste. Depending on the aggregate size and grading, bleeding occurs when aggregates are present in a concrete mixture, which weakens the strength of the transition zone. In the early days of hydration periods, the transition zone develops cracks due to differential strain between the cement paste and the aggregates. Thermal shrinkage and externally applied load. Cracks developed in the transition zone are too small to be seen with the naked eye but larger in width than most of the capillary pores present in the cement paste and therefore are responsible for establishing the interconnections between capillary pores which cause an increase in permeability.
SELECTING WATER/CEMENT RATIO
The W/C is the ratio by weight between water and cement. The selected W/C must be the lowest value required to meet the following design considerations.
It is extremely important to first select W/C ratio based on the environmental conditions or exposure conditions within which the concrete structure is to be constructed.
The requirements of durability as per IS 456, for various environmental or exposure conditions, are given below in the table.
Requirement of durability as per IS : 456
Minimum Cement Contents and Maximum water-cement ratio required in cement concrete to ensure durability under specified conditions of exposure.
Note 1: When the maximum water-cement ratio can be strictly controlled, the cement content in the above table may be reduced by 10 percent.
Note 2: The minimum cement content is based on 20 mm aggregates. For 40 mm aggregates, it should be reduced there about 10 percent; for 12.5 mm aggregate, it should be increased there about 10 percent.
DETERMINATION OF W/C
Accuracy of batching allowed, both for water and cement, is generally between + 1% to 3%. Therefore the likelihood of variation of W/C can be
to 1/1.03 W/C to 1.03 W/C
i.e. (0.97 to 1.03) W/C. if W/C=0.5, it can vary at the time of concrete production between 0.485 to 0.515 i.e. ± 0.03 of the specified value.
While investigating the quality of concrete in an existing structure, the issue of determination of W/C comes into dispute. This gives rise to two main problems.
- Although there are some petrographic and chemical methods for measuring the W/C, their precision is low, because they require a number of assumptions to be made for interpretation. A report of concrete society, “Analysis of Hardened Concrete” stated. “In favorable circumstances, with reliable analysis, the result is likely to be within 0.1 of the actual W/C.” Even with a precision of ± 0.1 or at best ± 0.05 of the true value, the variation is quite large for assessing the quality of concrete and W/C accurately.
- The problem with determining the W/C is that this term has a proper meaning only at the time when concrete has set and begun to harden. The W/C at such time will depend on a number of factors some of which are known while others are unknown.
- Water added to the mix at the mixer.
- Water content on the aggregate surface.
- Water content in chemical admixtures.
- Water added subsequently prior to placing concrete.
- Water lost during transportation due to evaporation.
- Water lost during transportation due to absorption by aggregates.
- Water lost after placing if bleeding occurs.
- Water lost during placing itself.
- Water lost during compaction.
It is not possible to establish the original W/C of the concrete mix during its production. Thus it is always safer to justify quality by specifying and determining the compressive strength of concrete and establishing criteria for evaluation and acceptance based on the same.
It is always prudent to specify high-strength concrete such that W/C required to satisfy the strength requirement is much lower than the W/C required to satisfy the durability requirement.
Durability aspects are practically well taken care of when high strength concrete is specified. Durability aspects cannot be practically taken care of when low-strength concrete is specified even with a restriction on W/C. this is because acceptance criteria of low strength concrete cannot be established by compressive strength when the durability requirement of W/C is lower than W/C required to attain the compressive strength.
Many sites in one country do not give importance to W/C. most often 1:2:4 concrete is still used where W/C > 0.8. This is one out of the many reasons, for poor durability of concrete structures needing costly repairs and rehabilitation.
When concrete is made using very high Water/Cement, as stated above, large capillary pores in cement paste allow high permeability of water along with other chemicals like chlorides and sulfates (if present) which cause determination of concrete. To constituent durable concrete structures, it is essential to specify low W/C but since W/C is difficult to measure or control, it is sensible to specify high strength (over 30 MPa) of concrete so that durability requirement for maximum W/C and minimum cement contents are fully satisfied.
Simple and accurate tests to determine W/C of fresh concrete have still not been developed. At present, such tests are cumbersome and not accurate. In any case, adequate care has to be exercised in batching of water, cement, and cementitious materials to control the W/ or W/CM or W/B. these ratios control permeability and hence durability of concrete. They are thus responsible for the strength of the concrete.