Effect of Chemical Causes on Durability of Concrete | Concrete Technology

The durability of concrete is affected by physical, mechanical and chemical causes. The durability of concrete is most affected by chemical causes, which result in volume change, cracking of concrete and ultimately leading to deterioration of concrete. Now we shall discuss the effect of chemical causes on the durability of concrete.

The effects are: 1. Alkali-Aggregate Reaction 2. Attack by Sea Water 3. Acid Attack 4. Deicing Salts on Concrete 5. Frost Resistant Concrete.

Effect # 1. Alkali-Aggregate Reaction:

A chemical reaction between the active silica constituents of the aggregate and the alkalies of cement is known as alkali silica or alkali aggregate reaction. The reactive form of silica are opal (amorphous) chalcedony (cryptocrystalline fibrous) and tridymine (crystalline). These materials occur in several types of rocks as trap, andesites, rhyolites, siliceous lime stone, and certain types of sand stones. The reaction starts with the attack of (Na₂O and K₂O) on the reactive siliceous minerals in the aggregate by the alkaline hydroxide derived from the alkalies in the cement.

As a result of this reaction an alkali silica gel of unlimited swelling property is formed altering the borders of the aggregate. The gel is confined by the surrounding cement paste. The expansion of the gel produces internal pressure causing expansion of concrete. This expansion results in cracking and disruption of cement paste. The amount of swelling of gel depends upon the congenial condition of moisture and temperature in voids. The crack width may vary from 0.1 mm to 10 mm.

Till 1940, aggregate was considered as an inert material, but after the failure of concrete structures in 1940 due to alkali-aggregate reaction, nowadays great emphasis is laid on aggregate alkali reaction test on aggregates to be used in large^–and important concrete constructions.

It is observed that alkali-silica or aggregate reaction can take place only at high concentration of ‘OH’ ions, i.e., at high pH value in the pore water. The pH value of pore water depends on the alkali content of cement. High alkali cement may have a pH value of about 13.5 to 13.9 and the low alkali cement may have a pH value of 12.7 to 13.1. Thus an increase of 1.0% in pH value represents a ten time increase in hydrogen ions concentration. Thus low alkali cement which produces a low pH value in pore water is safe against reactive aggregates.

Alkalis not only come from cement, but also come from sand containing sodium chlorides, admixture mixing water, sea water penetration, blast furnance slag, fly ash and deicing salts etc., getting into the concrete. Alkalis from all these sources must be included while finding the total alkalis expressed as soda equivalent in 1 m³ of concrete if alkali reactive aggregate is used.

Soda Equivalent:

The actual content of Na₂O plus 0.658 times K₂O content in cement is called soda equivalent. The minimum alkali content of the cement at which the expansive reaction may take place is 0.6% of the soda equivalent. B-S 5328 part-1 specifies a maximum of 3 kg of alkalies per cubic metre of concrete.

Factors which Affect the Rate of Alkali-Aggregate Reaction:

The reactivity of aggregate depends upon the following factors:

1. Size of the aggregate particles

2. Porosity of the aggregate particles

3. Alkali content in cement

4. Fineness of cement particles

5. Availability of non-evaporable water in the paste

6. Alternate wetting and drying of concrete, and 

7. Temperature in the range of 10 to 40°C accelerates the reaction.

Preventive Measures:

Following measures can be taken to reduce the alkali aggregate reaction:

1. Addition of finely powdered silica to the concrete mix. The addition of reactive silica increases the surface area of the reactive aggregate. The increase in surface area of aggregates increases the calcium hydroxide-alkali ratio of the solution at the boundaries of the reactive aggregate. Under such conditions a non-expanding calcium alkali silicate gel or product is formed.

2. 20 gram of reactive silica should be added for each gram of alkali in excess of 0.5% of the mass of cement.

Practice followed in U.K. to minimise the risk of alkali-aggregate or silica reaction is as follows:

1. The contact of concrete from the external source of moisture should be cut off or prevented.

2. Portland cement with an alkali content not more than 0.6% expressed as Na₂O (This is the sum of the actual Na₂O content plus 0.658 times the K₂O) content of cement should be used.

3. A blend (mixture) of ordinary type Portland cement and ground granulated blast furnance slag with a minimum of 50% slag should be used.

4. The mixture of ordinary Portland cement and fly ash cement, with a min of 25% fly ash should be used provided that the alkali content of the concrete supplied by the Portland cement component is less than 3.0 kg/m³ of concrete. The alkali content of concrete can be calculated by multiplying the alkali content of Portland cement (expressed as fraction) by the maximum expected Portland cement content.

5. British standard 5328 part-I 1991 specifies that the alkali content of the concrete should be limited to 3.0 kg/m³ which is now the alkali content of the composite cement (expressed as a fraction) times the maximum expected content of the cementitious materials.

Effect # 2. Attack by Sea Water:

In the present era, large numbers of concrete structures are constructed which are exposed to sea water either directly or indirectly. The coastal and off shore structures are exposed to simultaneous action of a number of physical as well as chemical deterioration processes. Concrete structures in sea water are sub­jected to freezing and thawing, abrasion by sand held in water and other floating bodies. The reinforcement of concrete is subjected to corrosion by the action of chlorine.

It has been observed that usually sea water contains 3.5% salts by weight of water. Abraham on the basis of his research work stated that sea water having 3.5% salinity can be used for plain concrete if the aggre­gates are non-alkali reactive. Sea water generally contains about 78% of sodium chloride and 15% magne­sium sulphate and chloride.

The ionic concentration of sodium and chloride are highest of the order of 11000 and 20,000 mg/litre respectively. The concentration of magnesium and sulphate is about 1400 and 2700 mg/litre respectively. The pH value of sea water is found to vary from 7.5 to 8.4. The average value is taken as 8.2. Sea water also contains some amount of carbon dioxide CO₂.

The magnesium sulphate reacts with free calcium hydro­xide Ca(OH)₂ in set Portland cement to form calcium sulphate, precipitating magnesium hydroxide, Mag­nesium sulphate, also reacts with the hydrated calcium aluminate to form calcium sulpho-aluminate. These reactions mainly are responsible for the chemical attack of concrete by sea water.

It has been observed that the deterioration of concrete in sea water is not caused by the expansion of concrete exposed to sulphate action, but due to the erosion or loss of constituents from the parent concrete mass without causing undue expansion. It may be due to the fact that the presence of chlorides in sea water may have restarted the swelling or expansion of concrete in sulphate solution. Concrete is also found to have lost some amount of lime content due to leaching.

Both calcium hydroxide and calcium sulphate are found more soluble in sea water, resulting in increased leaching out process. Thus in brief it can be said that concrete undergoes several reactions simultaneously when subjected to sea water. A less massive structure subjected to sea water shows the leaching effect more than expansion, whereas massive structures as dock wall show both effects leaching as well as expansion. The rate of chemical attack is found to increase in temperate zones.

Further it has been observed that most sever attack of sea water on concrete takes place just above the high water level. The parts between the high and low water levels are less affected and the parts below low water level which remain constantly immersed in water are least affected. The disruption of concrete is caused by the crystallization of salts in the portion of concrete above high water level. In cold climatic regions the freezing of water in pores at spray level of concrete is responsible for causing non durability of concrete. Freezing may also take place between tidal variation levels.

In shallow waters, the sea water contains certain amount of sand and silt. The wave velocity causes abrasion of concrete. The impact and mechanical force of wave also contributes to the non-durability of concrete.

Measures to Minimise the Sea Water Effects:

Following measures may be adopted to minimise the effects of sea water on concrete:

1. Use of right type of cement with low C₃A content.

2. Use of rich concrete i.e., use of more cement in concrete with low water-cement ratio. The rich concrete with low w/c ratio makes concrete dense and impervious to the attack of sea water. It has very little capillary pores, which held no water to cause expansion either due to crystallization or freezing.

3. Provision of adequate cover to reinforcement.

4. Use of pozzolanic material.

5. Full compaction and good construction joints are necessary for the good durability of concrete in sea water.

6. For better durability, where-ever possible high pressure steam cured prefabricated concrete elements should be used.

Effect # 3. Acid Attack:

No Portland cement is fully resistant to acid attack. In damp conditions sulphur dioxide (SO₂) and carbon dioxide (CO₂) as well as some other fumes present in the atmospheric air form acids. These acids diffuse or penetrate into the concrete through porous structure of the concrete mass. These acids attack the calcium silicate hydrate (C-S-H) gel and calcium hydroxide Ca(OH)₂.

Ca(OH)₂ is the most vulnerable part of the cement for the acid attack. This reaction makes the concrete very weak. Certain acids, such as oxalic and phosphoric acids are harmless, whereas hydrochloric acid (HCl) is most reactive. Sulphuric acid (H₂SO₄) is less reactive than (HCl). Silicious aggregates are more resistant to acid attack than calcarious aggregates. The acidic attack takes place in industries and some agricultural conditions such as floors of dairies.

In practice, the degree of acidic attack increases with the increase in acidity. Usually the acidic attack takes place when the pH value is below about 6.5. At pH value less than 4.5 the acidic attack is very severe. For pH values between 3 and 6, the rate of acidic attack is proportional to the square root of time. This indicates that the rate of attack depends on the capacity of hydrogen ions to diffuse through the calcium silicate hydrates known as (C-S-H) gel after the calcium hydroxide Ca(OH)₂ has been dissolved and leached out.

If the concentration of free carbon dioxide in water is between 15 to 60 ppm, the concrete is attacked by this water. Pure water formed by melting of ice or by condensation and water with carbon dioxide with in excess of 60 ppm is found aggressive. The pH value of such waters may be as low as 4.4.

The domestic sewage though alkaline in nature, causes deterioration of sewers, especially at higher temperatures, when sulphur compounds in the sewage are reduced by anaerobic bacteria to hydrogen sulphide H₂S. Though this gas is not destructive agent in nature by itself but when it is dissolved in moisture film on the exposed surface of the concrete, it undergoes oxidation forming sulphuric acid. The acidic attack takes place above the flow level of the sewage. The cement is gradually dissolved and progressive deterioration takes place. In this reaction of sulphuric acid, the calcium sulphate formed, reacts with the calcium aluminate in cement and forms calcium sulpho-aluminate, which on crystallization causes expansion and disruption of concrete.

The attack of calcium hydroxide Ca(OH)₂ can be reduced by the following measures:

i. By treating the surface with sodium silicate known as water glass. The sodium silicate when reacts with Ca(OH)₂, it forms calcium silicate in the pores.

ii. Surface treatment with coal tar, rubber or bituminous paints, epoxy resins etc. has also been found useful.

Effect # 4. Deicing Salts on Concrete:

The chemical salts used for removing or clearing the ice and snow from concrete roads in winter season of cold regions are known as deicing agents or salts. The use of these salts has been found to have adverse effect on concrete, probably by increasing the severity of the freezing and thawing cycles.

Salts used for Deicing:

Normally following salts can be used for deicing snow or ice:

(a) Sodium chloride (NaCl) and calcium chloride (CaCl₂)

(b) Urea

(c) Ammonium salts

(a) Sodium and Calcium Chloride:

Normally sodium chloride or calcium chloride is used for deicing snow clearance on cement concrete roads. The repeated use of these salts with intervening periods of freezing or drying cause surface scaling of concrete roads.

(b) Urea:

Sometimes urea is used as deicing salt. Though it has been found less deleterious (harmful), but it has been found less effective in removing ice.

(c) Ammonium Salts:

The uses of these salts even in small concentration have been found very harmful. Thus their use as deicing salts is not recommended. In other words they should not be used as deicing salts.

The deicing salts produce osmotic pressure and cause movement of water towards the top layer of the slab where freezing takes place. Greatest damage has been observed to occur when concrete is exposed to relatively low concentration of salts (2 to 4% solution).

According to Verbeck and Klieger this action has been attributed as physical in nature and not as chemical. The repeated use of these salts cause surface scaling of the concrete roads resulting in severe damage to the surface Fig.17.8 shows the effect of calcium chloride concentration on scaling of a non-air entrained concrete after 50 cycles of freezing and thawing without removal of solution. The extent of surface scaling from 0 to 5.0 values shows no scaling, while 5 values indicates bad scaling.

Action of Deicing Salts:

Mather has suggested the actin of deicers as follows:

According to Mather deicers increase the corrosion of steel of the concrete. The sequence of action is as follows. The deicer melts the ice or snow, which is usually ponded by the adjacent ice. The resulting liquid is absorbed. Due to the lower freezing point of this liquid, it remains in liquid state.

As more and more ice melts, the melt water becomes diluted till its freezing point reaches near to the freezing point of water. This water then freezes. Thus freezing and thawing takes place as often as without the use of deicers or even more often as possibly an insulating layer of ice has been destroyed. Thus it can be said that deicers increase the saturation and possibly increase the number of cycles of freezing and thawing.

This action promotes the corrosion of steel. This argument supports the physical nature of the damage action of the deicers, which is independent of whether the deicer is organic or not or a salt or not. However there is a possibility of leaching of calcium hydroxide Ca(OH)₂, which has a greater solubility in the chloride solution than water. There is also possibility of formation of chloro-aluminates under wetting and drying conditions.

Measures to make Concrete Surface Resistant to Scaling:

The concrete surface may be made more resistant to scaling by the following treatments:

i. By air entrainment.

ii. By using rich mixes.

Portland pozzolana cement has been found less resistant to deicing salts than ordinary port-land cement.

The extent of damage of scaling by deicing salts has been found to be influenced by the procedure adopted. For example, the air drying of the concrete after wet curing but before to the exposure cycles (freezing and thawing) has been found to increase the resistance to surface scaling. However the drying out must be done before moist curing for sufficient duration to allow the cement paste to hydrate properly and extensively.

In case of air entrained concrete, the period of curing required for the development of high resistance to salt scaling is about the same as that necessary for the development of adequate strength of concrete to resist the applied loads.

The damage to the concrete would be severe if the deicing salts are applied within a few weeks of placing of the concrete. However some protection can be obtained by sealing the concrete with linseed oil.

On the dry surface of the concrete a mixture of boiled linseed oil diluted in equal parts both kerosene oil or mineral spirits can be applied in two coats on the surface of the concrete. Though the oil slows down the ingress of the deicer solution, but it does not seal the surface of the concrete. Hence the evaporation from the surface is not prevented.

The most severe damage to the concrete takes place when it is subjected to alternate freezing and thawing while the deicing solution remains on top of the concrete surface rather than being replaced by fresh water before each refreezing. On the other hand if the deicing solution is removed from the top surface of the concrete before the refreezing takes place, no scaling of the top surface of the concrete would take place even with non-air entrained concrete.

Determination of resistance to Concrete Scaling:

The resistance of concrete to the scaling can be determined as follows:

The concrete specimen covered with deicing fluid is subjected to repeated cycles of freezing and thawing. The resistance to the scaling of the concrete is determined from the loss of weight, depth of spalling and the size of the spalled area. However no reliable data about the success of this test is available.

Effect # 5. Frost Resistant Concrete:

1. Frost resistance concrete can be produced by the use of air entrainment in concrete on admixtures.

2. Secondly the frost resistance of concrete can be increased by adopted mixes with low w/c ratios. The amount of water should be such that the paste can have only small capillaries and only little freezable water. However it is essential that substantial hydration of cement should take place before the exposure to the frost. Such concrete has less permeability and does not absorb water during wet weather.