Presence of impurities in water for concrete mix leads to decrease in structural properties of concrete such as strength and durability to a large extent.
The effect of water impurities on properties of concrete can be expressed mainly in two terms:
- In terms of the difference in the setting times of the Portland cement mixes that consist of proposed mixing water when compared with the use of distilled water.
- In terms of the concrete strength that is compared with the control specimens that is prepared with the distilled water.
Effect of Water Impurities on Concrete Strength and Durability
A difference of compressive strength is measured on 28th day of curing. Up to 10% of the controlled test is adequate to measure the quality of the mixing water.
As per IS:456-2000, a difference in the initial setting time by a value +/-30 minutes, given that the initial setting time is not less than 30 minutes is prescribed.
Concrete is affected by the effluents that are expelled out from the sewerage works, sugar and the fertilizer industry, paint, gas works, and textile industries.
Various tests have shown that the usage of water or structure that are constructed near to a water body with the excessive amount of salts (dissolved salts) tend to decrease the compressive strength of the concrete by an amount of 10 to 30 percent. This decrease is the strength of concrete compared with that obtained by the concrete using distilled water.
The high content of chlorides in water tends to show surface efflorescence, dampness persistently and makes the reinforcement steel prone to corrosion. This problem in concrete structures due to water quality problems is more severe in the tropical regions, mostly in that mix that is lean.
The table-1 shows the reduction of compressive strength of concrete structures by the different salt content present in water.
Table-1: Effect of Different Salts in Water on Compressive Strength of Concrete
|Salt Content in Water (%)||Reduction in Compressive Strength (%)|
Water Impurities and their Effect on Concrete Properties
The effect of various properties on the concrete properties are explained below:
Effect of Suspended Particles in Water on Concrete Properties
If the mixing water contains suspended particles in an amount up to 0.02 percent by weight of total water used in concrete, it will not affect the concrete properties.
It is found that high content of suspended particles does not affect the strength of the concrete, but affect other properties of the same.
The Indian code IS: 456-2000 prescribes an allowable limit of suspended particles in water to be less than 2000mg/liter. Before the use of water in concrete, the muddy water should undergo settlement in the basin.
Effect of Miscellaneous Inorganic Salts in Water on Properties of Concrete
The salt content in water adversely affect the strength of the concrete. The major salts that can be present in water are salts of manganese, tin, lead, copper and zinc.
The presence of zinc chloride in water results in the retarding of concrete strength gain. This is determined as an observation of no strength gain at the second or the third day of concrete (when subjected to strength tests).
Another salt that brings destructive effect on concrete is the lead nitrate. Other salts like sodium phosphate, sodium arsenate, sodium iodate and sodium borate result in the reduction of the initial concrete strength to a very lower degree.
The sodium and potassium carbonates cause extremely large rapid setting in large concentrations. This results in the reduction of concrete strength.
The presence of calcium chlorides accelerates the setting and hardening of the concrete. But the presence of calcium chloride is restricted to 1.5 percent of the total weight of the cement used in the mix.
Effect of Salts in Seawater on Properties of Concrete
The seawater comprises of 3.5 percent of the dissolved salts. The seawater chemical composition is uniform throughout the world. Most of the chlorides are associated with the sodium, where some are with potassium, while sulfate is associated with magnesium.
When considering approximate values, different ion content due to the presence of salts can be specified as 51.3% of chlorides, magnesium in 3.6%, 7.2% of sulfates, 28.5% of sodium, 1.3% of calcium and 1% of potassium. But the total amount of salt may vary widely.
The ingress of any considered ion into concrete mass is directly proportional to the seawater’s salinity. This is with respect to a given mass of seawater.
When the chemical effects are of more priority it is mentioned that of all, sulfate is the most problematic. This is the reason for the development of sulfate resistant cement. The concrete of very lower water/cement ratio facilitates this requirement of sulfate resistance.
The salt content in sea water reduces the concrete strength by an amount of 10 to 20 percent. More than strength factor, the corrosion of the reinforcement is considered as the adverse effect. The chlorides are the main cause of corrosion.
The risk of reinforcement corrosion is higher when it is exposed to air than when it is submerged in water. Another effect of chlorides is the efflorescence.
It is advised to use cement with high C3A content, as the chloride ion will be intercepted by the aluminate present. This is by the precipitation of calcium chloroaluminate which has no detrimental effects. This will hence increase the life period of steel and the durability of the structure.
The two main reasons for the presence of chloride ions in the concrete are
- Addition of calcium chloride as accelerating admixture
- Use of seawater as mixing water
For those cements that are not sulfate resistant, the use of CaCl2 will adversely affect the sulfate resistances of the concrete mix. This is not a problem if the cement has some amount of sulfate resisting measure in it.
Hence in cold weather conditions, the CaCl2 can be employed to accelerator add some content of sulfate resisting cement (in a measure equal to that added to the normal cement).
The codes do not support the use of calcium chloride when the sulfate resisting cement is employed. But in situations which are unavoidable, it is used in plain concrete that is submerged under water.
Effect of Acids and Alkalis in Water on Properties of Concrete
The water that consists of industrial waste are not suitable for concrete construction. The industrial water consists of detrimental acids or alkalies that depend on the waste product of the respective industry.
In terms of pH value, the water that has a pH value greater than 6 can be employed for the concrete construction. But the pH value will not give a proper and adequate measure about the acid content in the water.
The acid content in water can be gauged accurately based on total acidity, in the extend to satisfy the below requirement, i.e.:
The amount of 0.02 normal NaOH required to neutralize 100ml of water sample by using phenolphthalein as the indicator should not be greater than 5ml. Here, the acidity is equivalent to 49 ppm of H2So4 or 36 ppm of HCL.
Effect of Algae on Properties of Concrete
Algae are observed on the surface of the mixing water or on the surface of the aggregates. The algae (algae in aggregates) will combine with cement (cement + algae) reducing the bond between the cement paste and aggregates.
The algae entering the mix through water will result in the air entrainment in large quantities, which in turn result in the reduction of concrete strength.
Effect of Sugar on Properties of Concrete
The sugar content in water if is less than 0.05 percent by weight of water, then no adverse effect is observed in the concrete structure. The sugar content in 0.15 percent will result in retarding of setting time and the early strength of the concrete. But it is observed that the 28th day strength of concrete is improved.
The sugar content increased by an amount to 0.20 percent is said to improve the setting (time is accelerated). Further increase of sugar will cause rapid setting but the 28th day strength is affected.
Effect of Oil Contamination on Properties of Concrete
Mineral oil in water, that have no animal or vegetable oil content have no adverse effect on the concrete properties. The mineral oil content to a percentage of 2% is said to increase the strength of the concrete. But for more than 8% mineral oil, the strength is reduced.
The vegetable oil in water used for concrete manufacture shows its detrimental effects on the strength of concrete at its later stages.
Maximum Limit of Water Impurities for Concrete Construction
The table-2 specifies the limit of a number of solid impurities in water used for concrete production.
The pH value that is most suitable for construction of concrete is generally between 6 to 8. It is said that water equivalent to drinking water is best for construction. The solid contents in water are determined based on the procedures given in IS: 3025.
Table-2: Limits of Permissible Impurities in Water for Concrete Construction
|Types of Impurities in Water||Limit of Permissible Salt (Percentage weight of water)|
|Alkali Chlorides (as Cl2)|
|1. Plain Concrete||0.2|
|2. Reinforced Concrete||0.05|
Effect of Impurities in Curing Water on Concrete Construction
The main intention of curing is to let the water penetrate the concrete. No water is necessary for curing if proper steps were taken to prevent the loss of water from the concrete.
There is inevitably some loss of water from the surface of the structural members due to evaporation. The hydration process is carried out within the interior of the structure, but at the surface, the situation is not same as there is a lack of moisture or water content due to evaporation. Hence it is necessary to have curing.
If the water used is seawater, the chloride ions will enter the surface zone which will later move inwards by means of diffusion. It should be noted that most of the durability issues start from the surface or an attack from the surface to move inwards.
Marine structures that are intended to be submerged in the seawater have large risk due to the sea dissolved salts. But these problems are covered if proper curing is carried out by fresh water.
The iron content or organic matter in water that is used for curing will result in the staining or deposits in the concrete surface. As per IS: 456-2000, the presence of iron or tannic acid compounds are restricted in curing water.