Durability of Pre-Stressed Concrete

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 protection and the surface treatments (Burton and Pitt, 2001).

(Taken from USDT, 2010)

Resistance to alkali-silica reaction (asr)

The expansive reaction that takes place among the aggregate forms of silica, sodium alkalis and potassium is known as the ASR. 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 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 being given for the creep is that the water gets removed from the layers of calcium silicate crystallite as well as the possible arrangement of the bonds that takes place among the surfaces of the individual crystallites (Oleson et al., 2004).

(Taken from USDT, 2010)

Factors Influencing Creep

It has been noticed that a directly proportional relation exists between the shrinkage and creep, which means that the pre-stressed concrete which will shrink more will ultimately show a high creep. Experiments that have been performed suggest that there exists a strong relationship between the creep and shrinkage. It has also been noticed that little or no creeps would occur at all when the cement that has been hydrated, dries up. There exists an inverse relation between the humidity in pre-stressed concrete and the creep, which means that the lower the humidity in the pre-stressed concrete, the higher the creep is going to be (EPA, 2003).