Research Paper: Durability of Pre-Stressed Concrete Seawater

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Chemical resistance

Pre-stressed concrete is one of those few materials that can not only withstand very harsh temperatures but it also has the ability to resist harsh chemicals to a great extent. This is the reason that pre-stressed concrete is one material that is very frequently used to build the transportation and treatment facilities for highly aggressive chemical (Ashley and Lemay, 2008).

However, at times it is possible that the pre-stressed concrete might come in contact with some really harsh chemicals that will result in its deterioration; this happens very often in the chemical manufacturing and storage facilities. The sulfates and the chlorides affect the pre-stressed concrete. The calcareous aggregates and the concrete paste get dissolved when it gets attacked by the aggressive acids (Ashley and Lemay, 2008).

In order to avoid this, the pre-stressed concrete can be made less permeable and the surface treatment can be done on the surface of the pre-stressed concrete as well to save it from the acids (Ashley and Lemay, 2008).

(as cited in USDT, 2010)

Resistance to sulfate attack

If the cement paste has not been made properly and there are large quantities of sulfate present in the water or the soil then these sulfates can destroy the pre-stressed concrete. In the hardened cement paste there are hydrated compounds which are attacked by the sulfates and in this way the pre-stressed concrete starts deteriorating (Ashley and Lemay, 2008).

This kind of deterioration usually takes places in the environment where the pre-stressed concrete is exposed to wet and then dry cycles rather than only the wet cycles. The best way to deal with the deterioration of this kind is to make the cement paste with low water to cementitious ratio (about 0.40) and to use the cement that is specifically designed for the sulfates (Ashley and Lemay, 2008).

Sulfate attack in concrete and mortar

There are two kinds of sulfate attacks:

1. External attack where the sulfate penetrates the pre-stressed concrete from the outside

2. Internal attack where the sulfate gets mixed into the pre-stressed concrete paste.

External sulfate attack

External sulfate attack is the most common type of the external attack on the concrete. In this type of attack the sulfate that dissolved into the water penetrates the pre-stressed concrete structure and starts deteriorating it. On the surface, the pre-stressed concrete might look just fine but inside the composition of the pre-stressed concrete starts to change. However, the severity of these changes mainly depends on the severity of the attack and the durability of the concrete. Some of these changes are: the bond between the aggregate and the cement is lost; there is extensive cracking; expansion etc. (Jing, 2009).

The main impact of all these changes on the pre-stressed concrete is that it loses its strength. However, these above mentioned impacts usually take place when the potassium sulfate or the sodium sulfate attacks the concrete (Jing, 2009).

Some of the other sources from which the sulfate can come which attacks the concrete are:

Seawater.

The bacteria present in the sewers can be responsible for the sulfate attack as sulfur dioxide is produced by the bacteria; this sulfur dioxide gets dissolved in the water and oxides hence, producing the sulfuric acid.

Sulfuric acid can also be produced from the oxidation of the sulfate that is present in the clay which might be lying next to the concrete paste.

The sulfate attack of the mortar can be produced by the sulfate that is present in the bricks in the masonry. This can happen in such a way that the sulfate in the bricks keeps getting released over a long period of time. This attack is especially dangerous in the places where moisture has penetrated as well (Jing, 2009).

(Taken from USDT, 2010)

Internal sulfate attack

Usually the internal surface attack takes place when there is contamination in the pre-stressed concrete paste, some sulfate gets mixed into the paste, when sulfate-rich aggregates are used or when there is an excess of the gypsum added into the cement paste. With the help of proper screening processes the internal sulfate attacks can be avoided (Jing, 2009).

Delayed ettringite formation

Delayed ettringite formation which is also known as the DEF is a special type of internal sulfate attack. DEF is a type of sulfate attack that is being faced by many countries. The main reason for this attack is the concrete that is cured at the elevated temperatures such as in the steam curing (Giovannardi, 2006).

The concrete structures can get seriously damaged by the expansion that is caused by the DEF the reason behind this expansion is the ettringite formation in the paste. The excess of sulfate within the cement is not usually the reason behind the DEF however; the presence of sulfate can increase the percentage of expansion. The thing to remember here is that when the ettringite is heated over 70c it is destroyed. The main reason behind the occurrence of the DEF is the re-emergence of the ettringite which had earlier been dissolved during the hydration but now once the pre-stressed concrete has hardened it emerges once again. It is when the crystals of ettringite try to exert more pressure on the hardened concrete that the pre-stressed concrete starts deteriorating (Lancaster, 2005).

The amount of ettringite is relatively small in the normal pre-stressed concrete as it gets limited by the sulfate that has been contributed by the cement initially. Conditions necessary for DEF to occur are: alkali-silica reaction (ASR) is also something that the DEF can commonly be associated with; mostly, however not necessarily always, the high temperatures (>65-70 c approx.) can lead to DEF too; and, the permanent or intermittent saturation of water after the curing (Oleson et al., 2004).

The affects of the various compositions of cement on the DEF are still not properly understood. There are some factors that have shown very strong behavior with the DEF however, the causes of these behaviors aren't clear. Following are some of the cement-related factors with which the DEF showed a positive co-relation when tests were performed in the laboratory:

1. high C3A

2. high MgO

3. high sulfate

4. high C3S

5. cement fineness

6. high alkali (Oleson et al., 2004)

Although the resistance of pre-stressed concrete to deformation is seen as one of the most ideal characteristics of the concrete however, due to this resistance even small amounts of changes in the shrinkage or resistance can alter the structure or the usefulness of pre-stressed concrete (Oleson et al., 2004).

Settlement and Bleeding

It is said that before the settling pre-stressed concrete is in the plastic state, the particles that are present in the paste get dispersed in the water whereas, the aggregate gets dispersed by the cement paste. Once the pre-stressed concrete has been placed, the particles then start coming closer to each other in order to get settled. This process usually takes about an hour. The change in the volume of the pre-stressed cement is usually very little and in extreme cases it can be as much as 1%, but this change in volume doesn't affect the strength of the pre-stressed concrete as it is in plastic or semi-plastic state. During this process of the settlement of concrete the water appears at the surface of the pre-stressed concrete. This phenomenon is known as bleeding (Oleson et al., 2004). The graph below shows the bleeding rate per unit thickness of concrete (cm) versus the water-cement ratio:

(Taken from USDT, 2010)

It is very important to ensure that the finishing process of the concrete is started after the bleeding process is completely over and the water has dried up; because otherwise, when the finishing process is started, the water and the moisture will be trapped underneath the surface of concrete and this will make the concrete weak (Oleson et al., 2004).

Structural flaws can also arise due to settlement. It is possible that the contact between the concrete and the steel gets weak because of the layer of water that is present between the horizontal bars that are used for the reinforcement. This problem can be taken care of by making use of the vibration to get rid of all the water when the cement is in the plastic state. However, it needs to be remembered that the reinforcing should not be touched (Oleson et al., 2004).

Creep of Concrete

Creep of concrete is a phenomenon that results when the strain on the pre-stressed concrete increases gradually with time. The creep that occurs in the pre-stressed concrete that is drying under load is going to be once or twice larger due to the continuous moisture that it will be in.

Presently, the reason that is… [END OF PREVIEW]

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