Durability of Pre-Stressed Concrete Seawater exposure

Chloride Resistance and Steel Corrosion

Resistance to alkali-silica reaction (asr)

Abrasion Resistance

Lietrature Review

Concrete crack

Concrete surfaces spall

High Humidity and Wind-Driven Rain

Ultraviolet resistance

Inedible

Resistance to freezing and thawing

Chemical resistance

Resistance to sulfate attack

Sulfate attack in concrete and mortar

External sulfate attack

Internal sulfate attack

Delayed ettringite formation

Settlement and Bleeding

Creep of Concrete

Factors Influencing Creep

The extent to which a product can withstand deterioration and how long it can last is known as the durability of that product. In today's world the durable products have a lot of advantages such as; they don't need to be repaired or renovated soon which saves the earth's resources, building and re-creating buildings and doing construction again and again that adds up to the solid waste materials that fill the landfills.

Usually the buildings that have a design life of 30-50 years can stay standing for 80-100 more years after that. However, this doesn't usually happen as, these buildings are demolished not because they had deteriorated but due to obsolescence. Even during the renovation of a building it is advisable and very environment friendly to keep the outer structure standing and renovate the building from the inside because the concrete shells are very strong and can withstand harsh weathers and abrasions.

Introduction

The durability of various structures and building differ from each other based on the geographical locations of the buildings as well as the environmental conditions that they will have to face. For example; a building built on the sea shore will have different durability levels as it will be exposed to the sea waves as compared to a building that is built in desert. The life and durability of concrete mainly depends upon the portions of ingredients in them along with the techniques through those ingredients are mixed and applied (Baek, 2005).

(Taken from USDT, 2010)

Background of the study

Seawater exposure

The buildings or the pre-stressed concrete that are exposed to seawater have to be built with very carefully selected materials and with the use of different techniques. Especially the materials for the buildings that are in the tidal zone have to be selected very carefully because these buildings have to withstand very harsh temperatures such as the storms and floods as well as thawing and/or freezing. The reason why concrete is being used in the buildings near the sea is because it has given excellent results. The factors that needs to be kept in mind while doing construction in the seawater exposed areas are that the pre-stressed concrete should have minimum level of permeability in order to protect it from the sulfates and chlorides in the water. Also, the steel used in building the structure or the frame of the building should be kept completely and properly covered with concrete along with the water-cementation ratio not being more than 0.40 (Baek, 2005).

(Taken from USDT, 2010)

Chloride Resistance and Steel Corrosion

The durability of the pre-stressed concrete does not get affected by the chloride that is present in it. The pre-stressed concrete helps in protecting the steel that is embedded in it from corrosion. This happens in such a way that the high pH level of the pre-stressed concrete helps in building a non-corroding passive oxide coating on the steel which protect it from the corrosion. Usually the pH level within the pre-stressed concrete is as high as 12.5. However, if the seawater does manage to penetrate the pre-stressed concrete, the steel can corrode as the chloride ions present in it can destroy the protective coating around the steel. An electric cell is created along the steel or between its bars once the threshold of chloride corrosion is reached, which results in the beginning of the corrosion process (Burton and Pitt, 2001).

Although the tendency of concrete to resist the chloride is very good but in scenarios such as the building of bridge decks this resistance can further be improved. This can be achieved by including the supplementary cementations materials like silica fumes, by keeping the water-cementations ratio low (0.40), by keeping the moist curing for at least seven days. All this helps in decreasing the permeability of the pre-stressed concrete. Another method through which the penetration of chloride into the steel can be reduced is by thickening the layers of pre-stressed concrete on the top of the steel. Some of the other methods through which the steel corrosion can be reduced are by epoxy-coated reinforcing steel, pre-stressed concrete overlays, by using corrosion inhibiting admixtures, cathodic...

Only in the scenarios when the expansion has become huge does the reactivity create problems. When there are networks of cracks, or movements of some part of the structure or spalling or closed joint observed, it simply means that an alkali-aggregate reaction is taking place. With the help of the supplementary cementitious materials or the proper selection of aggregates ASR can be controlled (Burton and Pitt, 2001; Li et al., 2003).
Abrasion Resistance

Although concrete is resistant to the affects of abrasive weather to a great extent however, when it comes to the situation where it has to face extreme abrasion then its durability varies. The examples of severe erosion and abrasion are when the steel studs are allowed to be put on the tires and fast moving particles in the water or floating ice etc. It has been observed through various studies that concrete that has the compressive strength of around 12,000 to 19,000 psi works very well in the situations where the abrasion is very severe (Burton and Pitt, 2001; Li et al., 2003).

Literature Review

Concrete crack

Like many other materials concrete shrinks and contracts as well when it dries out. Upon shrinking the pre-stressed concrete usually cracks. This is the reason why the joints are put between the concrete structures by the construction workers as it helps the concrete to crack in a neat and straight line at the joint. Other kinds of joints that are also placed in between the concrete walls are the constructions or the control joints (Lin, 2007; Li et al., 2003).

(Taken from USDT, 2010)

Concrete surfaces spall

There are many reasons behind the spalling or flaking of pre-stressed concrete takes place but this can be prevented. Given below are some of the reasons because of which the concrete flaking occurs:

1. Before beginning the pre-stressed concrete finishing operations it is very important to let the water sheen on the surface as well as let the excess water dry. The reason behind this is the fact that in case the water hasn't dried up properly and we start the pre-stressed concrete finishing operations the concentration of water near the top of the concrete will be very high which will make the pre-stressed concrete weaker, increasing the chances of spalling.

2. If the pre-stressed concrete is not air-entrained then in the colder climates where there is a chance of freezing the surface of the pre-stressed concrete resulting in spall.

3. The water-cemetitious ratio should be kept at approximately 0.45 or less (Lin, 2007; Li et al., 2003).

(Taken from USDT, 2010)

High Humidity and Wind-Driven Rain:

Humid climate, moist air or the wind-driven rain doesn't affect the concrete as it has resistance against all these conditions because concrete is impermeable to all these things. Usually the moisture that does enter a building enters it through the joints in between the pre-stressed concrete and the moisture that enters through the joints generally doesn't affect the pre-stressed concrete. However, it is said that if the concrete has some permeable material it allows the concrete to breathe which helps in drying out the moisture (Lin, 2007; Li et al., 2003).

The moisture that gets trapped within the pre-stressed concrete not only corrodes the steel but it also rots the wooden framing, sheathing and insulation. The exterior insulation finish systems (EIFS) haven't faced as many problems regarding moisture and corrosion as these materials do not really have the tendency to rot or corrode (Lin, 2007; Li et al., 2003).

Ultraviolet resistance

The pre-stressed concrete doesn't get affected by the ultraviolet ray from the solar radiation. In order to keep the color of paint more visible for a longer period of time colored pigments are added into the cement which helps in maintaining the color even after the paint has all faded and dried out (Lin, 2007; Li et al., 2003).

Inedible

The insects can't really enter or penetrate the pre-stressed cement as it is not edible. However, some relatively softer materials might still provide the insects with some pathways to enter. But due to the hardness of concrete insects can't enter it (Lin, 2007; Li et al., 2003).

Resistance to freezing and thawing

One of the most dangerous weather factors that can deteriorate the concrete is if freezing or thawing takes places when the pre-stressed concrete is still wet, it can result in deterioration in such a manner that when the water freezes the paste or the aggregate particles will expand (Lin, 2007; Li et al., 2003).

Chemical resistance

Pre-stressed concrete is one of those few…