Chapter 4 : Durability problems in concrete
Freezing
and thawing:
Water could be absorbed in
the concrete finding its way into all of the pores. When the concrete is
subjected to low temperatures, the water will freeze although at lower
temperature than zero C° because the water in concrete usually contains salts.
The freezing point decrease as the size of the pore decreases. So there is no
single freezing temperature in the concrete and the water freezes in some pores
and stay liquefied in others.
As the gel pores are too
small to allow the water to freeze in it, and the air voids contain air not
water, the freezing of the water takes place in the larger capillary pores.
When the water in the pores
freezes, it undergoes an increase in volume about 9%. and because the water in
the concrete freezes gradually, the unfrozen water is subjected to a hydraulic
pressure from the frozen enlarged water.
Because the concrete is
saturated with no empty pores in which the pressured water can run in, the
pressure causes internal tensile stresses causing local failure of the concrete
as expansion (Thawing) occurs to contain the increased water volume .on
refreezing of the water in the pores when the temperature drops again after
rising, further expansion occurs. This repeated cycle of freezing and
defrosting (not a single freeze)
causes damage which varies from surface scaling to complete disintegration as
layers of ice are formed starting from the exposed surface and throw it’s
depth.
The important condition for
freezing and thawing to happen is full saturation of the concrete pores. As the
dry and below 80-90% saturation concrete is resistant to freezing and thawing
because the dry concrete contained no water to freeze and the partially
saturated concrete got enough empty pores that the pressured water can fill.
The presence of adjacent
air voids and empty capillary pores allows the relief of the hydraulic pressure
by the flow of the pressured water into these pores. So to prevent freezing and
thawing air must be entrained into the concrete by using air entraining agent
while the concrete is mixed which is effective only at low w/c ratios where the
volume of the capillary pores are small and segmented.
The hydraulic pressure
isn’t only a result of the expanding volume of the frozen water but other
factors increase this pressure. There is a thermodynamic imbalance between the
water in the gel pores and the frozen water in the capillary pores causing the
gel water to diffuse in the capillary increasing the pressure. Also at freezing
points only the pure water freezes increasing the concentration of salts in the
surrounding water, which causes the water pressure to increase by osmosis.
Sulphate
attack:
The cement composite
related to Sulphate attack is C3A, which hydrates as follows:
And when all the gypsum in
the cement is consumed:
The lime in the hardened cement (one of the hydration products of C3S And C2S) also reacts with sulphates to form gypsum which also is expansive but because of the high porosity, the lime in the hardened concrete is free, the lime leach out by dissolving in water and washed out to the other far side of the concrete where it reacts with CO2 forming calcium carbonate CaO which evaporates leaving a white deposit called Efflorescence which is harmless but extensive leaching of free lime increases the porosity and make the concrete weaker
Since it’s S3A that is attacked by sulphates type II or type V Portland cement which are low in C3A should be used but will not be useful unless the concrete is dense and low in permeability as in low w/c ratio mixes high in cement content.
Alkali-aggregate
reaction:
Aggregates produced from certain types of rocks contain a kind of silica called active silica (Opal, Chalcedony and Tridymite), which reacts with the alkali content in the cement (Na2O and K2O).
The alkali content of cement forms alkali hydroxides and attacks the active silica in the aggregates forming an alkali-silicate gel. This gel attracts water from the paste by absorption or osmosis and therefore increases in the volume.
Because The enlarged and swelled aggregates (part gel and part still solid) are surrounded by the hardened paste, internal pressure cumulates and leads to expansion, cracking and chipping of the paste. This is called the alkali-silica reactivity. The speed of this reaction is related to the size of the silica particles as fine particles lead to expansion in 4–8 weeks while larger ones do so after years, usually the damage occur after 5 years.
Other factors influencing the progress of the reaction are the porosity of the aggregates, the quantity of alkali in the cement, the availability of water in the paste and the permeability of the cement paste . The reaction mainly takes place in the exterior of the concrete under continuous wet condition or when there is cycles of wetting and drying and at high temperatures (10-38 C) and the danger of the reaction is present if the percent of the aggregate containing active silica is 0.5 % or more of the total aggregates.
To prevent the alkali-silica reactivity:
- Use non-reactive aggregate containing no active silica.
- Use cement with low alkali content not more than 0.6% usually expressed as a percentage of Na2O which cement makers use it to describe the total sum of actual Na2O and 0.658 time the K2O content).
- Use a fly ash or ground granulated blast furnace slag with type I Portland cement.
- Prevent contact between the concrete and external sources of moisture.
Another rare type of
Alkali-aggregate reaction is the alkali-carbonate reactivity between some
Dolomitic limestone aggregates and the alkali content in the cement but with
very little quantities of alkali-carbonate gel. The expansion of this gel is
related to the presence of clay.
Sea
Water Attack:
Although sweater contains sulphates it doesn’t have the same affect as the Sulphate attack in ground water because chlorides are present and because the Ettringite and the gypsum (products of Sulphate attack) are more soluble in chloride solutions than in water which means that they easily wash out from the concrete by the sea water without causing expansion but this increases porosity with time leading to a lower strength.
On the other hand, expansion takes place in the concrete when the water enters the concrete and rise up in the capillary pores above the water level where it evaporates and the salts crystallize in the dry pores generating pressure leading to cracking and chipping into fragments.
Concrete subjected to
repeated cycles of wetting and drying with sea water (between tides) is
severely attacked while concrete always submerged in water is less attacked
(expansion only above water level only) however with time the attack is slowed
down because the deposit in the pores of magnesium hydroxide which is formed
(along with gypsum) by the reaction of magnesium sulphates with the lime.
The sea water attack can be
prevented in the same measures of preventing Sulphate attack in ground water
but with no effect of the cement type only the low cement ratio and the low
permeability.
Carbonation:
The gas CO2 is present in the atmosphere especially in large cities. And in the presence of moisture it forms carbonic acid, which reacts with the CH hydrates [Ca(OH)2] to form CaCO3. It also decomposes other cement compounds. The continuous process of the carbonation will lead the concrete to contract causing carbonation shrinkage.
Carbonation proceeds from the surface inwards but does so extremely slowly. The rate of carbonation depends on the permeability of the concrete; it’s moisture content and the CO2 content and humidity in the air.
The carbonation can be easily determined but treating a freshly broken surface with phenolphthalein the free Ca(OH)2 is colored pink while the carbonated portion is uncolored.
The carbonation of concrete made using type I portland cement will slightly increase the strength and reduce permeability because the carbonation process produces water which will aid the further hydration of the remaining unhydrated cement grains and because CaCO3 deposits in the voids within the cement paste thus reducing the permeability.
Carbonation neutralizes the alkaline nature of the hydrated cement paste so the protection of the steel from corrosion is higher. But if the whole steel cover is carbonated, the moisture and oxygen reached the steel bars casing more corrosion.
The carbonation takes place when the internal humidity of the concrete is less than 100% but larger than 25%. If the concrete internal humidity is less than 25%, there is no sufficient moisture for the CO2 to form carbonic acid. And if the internal humidity is 100% (saturated concrete), the diffusion of CO2 into the paste is very slow.
Carbonation is greater in concrete that is exposed to moist air and protected from rainwater than in concrete frequently washed away by rain.