a- Briefly explain how various environmental factors affect the durability of concrete? b- What is the mechanism of sulphate attack on concrete; how it can be controlled?

 

Q#  a-      Briefly explain how various environmental factors affect the durability of concrete?                    

b-      What is the mechanism of sulphate attack on concrete; how it can be controlled?            

 

Answer A

Durability

       A DURABLE CONCRETE IS ONE THAT PERFORM satisfactorily in workig enviromet during its anticipated exposure conditions service. 

Durability is the ability of a physical product to remain functional, without requiring excessive maintenance or repair, when faced with the challenges of normal operation over its design lifetime. There are several measures of durability in use, including years of life, hours of use, and number of operational cycles. In economics, goods with a long usable life are referred to as durable goods.

A DURABLE CONCRETE IS ONE THAT PERFORM satisfactorily in workig enviromet during its anticipated exposure conditions service.

dURABILITY Of concrete is its ability to resist weathering action, chemical attack, abrasion or any other process deterioration 

Environmental factors affect the durability of concrete;

Physical Durability

Physical durability is against the following actions

1.   Freezing and thawing action

2.   Percolation / Permeability of water

3.   Temperature stresses i.e. high heat of hydration

Chemical Durability

Chemical durability is against the following actions

1.   Alkali Aggregate Reaction

2.   Sulphate Attack

3.   Chloride Ingress

4.   Delay Ettringite Formation

5.   Corrosion of reinforcement

 

TEMPERATURE

Durability of concrete may be defined as the ability of concrete to resist weathering action, chemical attack, and abrasion while maintaining its desired engineering properties. ... When the temperature increases the volume of the concrete increases and when the temperature falls the concrete contracts

MOISTURE

Higher water-to-cement ratios result in greater spacing between the aggregates in cement, which affects compaction. Similarly, increased moisture levels reduce the concrete’s compressive strength and durability. As concrete’s surface area increases, particularly with the addition of fine aggregates, so does the demand for water. The increased water leads to a higher water-to-cement ratio.

When excess water creates greater spaces between aggregate materials, the voids fill with air after the moisture evaporates. The resulting inadequate compaction reduces the concrete’s strength. Concrete with trapped air levels as little as 10 percent experience reductions in strength of up to 40 percent.

Quality of water

if the water has pollutant i it, it will result in decrease in strength of concrete and the workability is goig to decrease

Freezing and Thawing:

Deterioration of concrete from freeze thaw actions may occur when the concrete is critically saturated, which is when approximately 91% of its pores are filled with water. When water freezes to ice it occupies 9% more volume than that of water. As the seasons pass, concrete goes through the process of freezing and thawing resulting in repeated loss of concrete surface With the addition of an air entrainment admixture, concrete is highly resistant to freezing and thawing.

Alkali aggregate reaction:

 Use of non-reactive aggregate from alternate sources . use of low alkali ordinary Portland cement having total alkali content not more than 0.6 percent (as Na20 equivalent).Measures to reduce the degree of saturation of the concrete during service such as use of impermeable membranes.

Answer B

sulfate attack

    sulfate attack of concrete is a complex process, which includes physical salt attack due to salt crystallization and chemical sulfate attack by sulfates from soil, groundwater, or seawater. Sulfate attack can lead to expansion, cracking, strength loss, and disintegration of the concrete

Sulfate attack can be 'external' or “internal”. “External”         

EXTERNAL:

Due to penetration of sulphates into the concrete from outside For example : high-sulphate soils and ground waters, or atmospheric or industrial water pollution.        

INTERNAL:

Due to a soluble source being incorporated into the concrete at the time of mixing, gypsum in the aggregate.       

MECHANISM:

        Sulfate attack has occurred at various locations throughout the world. Some common sulfate environments are soils, groundwater, transport fluids, contained soils or fluids, and seawater. Many concrete structures are exposed to these environments and accumulation of sulfates at an exposed face increases the potential for deterioration.

The mechanism of distress for sulfate attack is an expansive pressure caused by the transformation of monosulfoaluminate to ettringite. This mechanism can be described as a sequence of processes. First, the external sulfate reacts with calcium hydroxide to saturate the pore solution and precipitate gypsum (CaSO4 • 2H2O). The increased concentration of SO4 promotes the transformation of monosulfoaluminate to ettringite (3CaO• Al2O3• 3CaSO4 • 32H2O). This transformation causes an increase in solid volume, which results in the deterioration of concrete by inducing cracking, softening, and spalling. The expansion mechanism is caused by pressure from ettringite crystal growth or swelling due to absorption of water. Another measurable concrete property identified with the sulfate attack mechanism is a decline in compressive strength.

Different sulfates can effect the concrete differently. Magnesium sulfate is the most severe because of the presence of magnesium ions. These ions can cause additional corrosive reactions through the formation of Mg(OH)2 and ettringite. This also decomposes the C-S-H.

 

PROTECTION:         

The methods given below can be adopted to protect concrete from sulphate attack.

USE OF SULPHATE RESISTING CEMENT:

1.QUALITY CONCRETE:-

A well designed, placed and compacted concrete which is dense and impermeable exhibits a higher resistance to sulphate attack. Similarly, a concrete with low water/cement ratio also demonstrates a higher resistance to sulphate attack.

2.USE OF AIR-ENTRAINED ADMIXTURE:

Use of air-entrainment to the extent of about 6% has beneficial effect on the sulphate resisting qualities of concrete. The beneficial effect is possibly due to reduction of segregation, improvement in workability, reduction in bleeding and in general better impermeability of concrete.

3.USE OF POZZOLANA:

Incorporation of or replacing a part of cement by a pozzolanic material reduces the sulphate attack. Admixing of Pozzolana converts the leachable calcium hydroxide into non-leachable cementitious product. This pozzolanic action is responsible for impermeability of concrete. Secondly the removal of calcium hydroxide reduces the susceptibility of concrete to attack by magnesium sulphate.

4.HIGH PRESSURE STEAM CURING:

High pressure steam curing improves the resistance of concrete to sulphate attack. This improvement is due to the change of C3AH6 into a less reactive phase and also to the removal or reduction of calcium hydroxide by the reaction of silica which is invariably mixed when high pressure steam curing method is adopted.