In this article we will discuss about:- 1. Definition and Significance of Durability 2. Effect of W/C Ratio on Durability 3. Causes of Non-Durability.
Definition and Significance of Durability:
The durability of cement concrete can be defined as its ability to resist weathering action, chemical attack, abrasion or any other process of deterioration. A durable concrete should maintain its original form and serviceability when exposed to adverse environment.
Significance:
It has been reported that in industrialized countries more than 40% of total resources of the building industry are spent on repairs and maintenance of concrete structures. In India also much money is spent on repair work. It is very unfortunate that not enough attention has been paid to durability aspect even in repair works.
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Secondly in the present age all good and favorable sites of construction have been already used and the construction of concrete work has to be taken up in more hostile environments. Thirdly all good concrete materials are in short supply. Thus the construction of modern concrete structures assumes much more importance for durability than in the past. Due to these reasons in revised IS 456-2000, the approximate values for cement content, maximum w/c ratio, and minimum grade of concrete has been suggested for different exposure conditions.
These recommendations have been given in Table 17.1 below:
Note:
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1. The minimum cement content and max. w/c ratio apply to 20 mm nominal maximum size of aggregate. For other sizes of aggregate suitable changes should be done as per Table 17.2 below.
2. Max. Cement Content. Cement content not including fly ash and ground granulated blast furnace slag in excess 450 kg/m3 should not be used unless specified.
For general information and also to carry out the mix design based on British method, department of Environment method (DOE method) Table 17.3 and 17.4 are given below.
Note:
1. Grade is the characteristic cube strength.
2. The values given in table 17.3 are applicable when the max. size of aggregate is 20 mm. When the size of aggregate is 10 and 14 mm, the content of cementitious material should be increased by 40 kg/m3 and 20 kg/m3 respectively. Conversely when the max. size of aggregate is 40 mm, the content of cementitious material should be reduced by 30 kg/m3. The least content of cementitious material in pre stressed concrete should be 300 kg/m3.
3. For freezing and thawing exposure air entrainment should be adopted.
Note:
1. Cementitious materials is inclusive of fly ash or slag.
2. Grade of concrete is the characteristic cube strength.
3. For very sever conditions only air entrained concrete should be used.
Effect of W/C Ratio on Durability:
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W/c ratio has a great impact on the durability of the concrete. Higher w/c ratio is the main cause of volume change and higher permeability of concrete. The use of higher w/c ratio causes permeability, volume change, cracks, disintegration and finally the failure is a cyclic process in concrete. Thus from durability consideration, the use of lowest possible w/c ratio is the fundamental requirement to produce dense and impermeable concrete.
There is a tremendous difference in the micro structure of concrete made with low w/c ratio and high w/c ratio. The permeability of concrete made with low w/c ratio decreases to such a level that the concrete is impervious to water. The lower permeability of concrete does not mean that there is no inter connecting network of capillaries in the concrete. These capillaries are so fine that water cannot flow through them.
When such concretes tested for chloride ions permeability test, it is observed that chloride ions diffuse in the concrete at a much lower rate 10 to 50 times slower than that of high w/c ratio concrete. Thus low w/c ratio concrete is less sensitive to carbonation, external chemical attack and other detrimental effects that make the concrete less durable. Further it has been observed that the corrosion of reinforcing steel can be checked by the use of low w/c ratio and provision of adequate cover, rather than using high w/c ratio and then protecting the steel by other means as by epoxy coating etc.
It is easy to propagate the virtues of low w/c ratio for all-round durability of concrete, but in actual practice it was not possible to use w/c ratio less than 0.4 till the invention of super plasticizers. Now, w/c as low as 0.25 or 0.2 can be used for producing flowing concrete.
In these low w/c ratio concretes there is not enough water available to fully hydrate all particles of cement. The water available can hydrate the surface particles only leaving plenty of particles un-hydrated, which can play an important role as they keep strength in reserve.
If due to any reason, environmental or structural, concrete gets cracked, the un-hydrated particles start hydrating as soon as moisture or water starts penetrating through these cracks. Thus the un-hydrated particles offer self-healing potential to improve the durability of concrete.
Causes of Non-Durability in Concrete:
The factors which cause non-durability in concrete may be classified as internal causes and external or environmental causes, these causes may also be categorised as physical, mechanical and chemical.
1. Physical Causes:
These causes arise due to frost action and from differences in thermal properties of cement paste and aggregates.
2. Mechanical Causes:
These causes are developed mainly due to abrasion.
3. Chemical Causes:
These causes are developed due to the chemical action between the seeping water through cracks or voids of concrete and cement paste. Thus these causes take place within the body of the concrete. Hence the permeability of concrete is of paramount importance.