Water and its Impact on Cement Concrete

Water and its Impact on Cement Concrete

Water

Water is one of the most important ingredients for concrete. But people still ignore its significance in the aspect of its quality. The element is required for preparation of mortar, mixing of cement concrete and for curing work. The quality and quantity of waters has much influence on the strength of cement mortar and concrete in construction work (GharExpert.com, 2013).

Water occupies 6-8% of the composition of fresh concrete, it provides for cement hydration and for the workability of the fresh concrete mixture, but when in excess, it determinately affects concrete porosity and mechanical strengths (Neville, 1995). It is an important ingredient of concrete as it actively participates in the chemical reaction with cement and helps to form the strength giving cement gel (Shetty, 2006).

Adebakin, (2003) says that, of the world’s water bodies, only 2.5% is fresh water, the remaining constitutes seawater; and he defines the fresh water as that purified expanse of water, which is devoid of any form of impurities. While the seawater on the other hand, he considers as water containing high percentage of sodium chloride.

Quality of water

The water used for mixing and curing should be clean and free from injurious quantities of acids, alkalis, salts, oils, organic materials, sugar, vegetable growth and other substances that may be deleterious to blocks, stones, steel or concrete. Shetty, (2006) emphasizes that the quality of water affects its strength, so it is necessary to carefully look into the purity and quality of waters. Price, (2006) gives that the popular yardstick of suitability of waters for concrete mixing is that, ‘if waters is fit for drinking, it is fit for concrete’. This does not appear to be a true statement for all conditions, he adds.

Portable waters, whose P^H value should not be less than 6, is generally considered satisfactory for mixing and curing (GharExpert.com, 2013).

 Effects of bad quality water on cement concrete

It has been observed that certain common impurities in water affect the quality of mortar on concrete produced, and in spite of the best materials (i.e cement and aggregates e.tc) used in the cement concrete, required results are not obtainable (GharExpert.com, 2013). Some bad effects of water containing impurities are as follow (GharExpert.com, 2013):

• Presence of salts in water e.g calcium, iron salt, inorganic salt, and sodium e.tc are dangerous that they can reduce the initial strength of concrete and in a few cases, no strength is achieved. There will also be rusting problem in steel provided in the Reinforced Cement Concrete.
• Strength is also reduced when acids, alkalis, industrial waste, sanitary sewage and sugar are present in waters.
• Presence of silt or suspended particles in water has adverse effects on concrete strength.
• Presence of oil such as vegetable, mineral or linseed oil in water more than 2% reduces the strength of concrete up to 25%.
• The presence of vegetable/algae reduces the bond between cement paste and aggregate.

Opinions of researchers on the suitability of sea water for use in concrete

According to Mbadike and Elinwa, (2011), brack water is water that has more salinity than fresh water, but not as much as sea water. Brackish waters is also the primary waste product of the salinity gradient power. Salinity gradient or Osmotic power is the energy retrieved from the difference in salt concentration between sea water and river water. Water is said to be salty if it contains chlorides and sulphates (Tchobanoglous, 2003).

According to Chaneyenterprises.com, (2013), Salt does not damage concrete, but the effects of salt can. Salt does not chemically react with hardened concrete.  It does lower the freezing point of water, attracts moisture, and increase pressure of frozen water.  It can also increase the freeze-thaw cycles if the temperature fluctuates between 15°F and 25°F.  Concrete scaling can occur in the absence of salts too if there were problems at installation. The better quality the concrete and placement, the less likely that salt’s effects will have an adverse effect.

Salt waters contains magnesium chloride, sulphate ions and hydrogen carbonation ions that will essentially attack concrete to a certain degree, but what really starts to corrode in a concrete structure is any of the steel buried within. Concrete contains an alkaline environment that provides some protection against corrosion. The steel inside the concrete that is used for reinforcement will react with the concrete and form film that protects the steel. But however, the salt water works against this process.  The chloride and sulphate ions will weaken that film as the water soaks into the concrete.  Once the film is breached, then the corrosion process begins to work on the steel itself (Pondarmor.com, 2013).

Vicat, (1812) opines that the world seawater in most cases has salinity of the range 34-35%, though the properties of water to dissolved salts tend to vary within the ocean, the major component ions are evenly distributed in ocean water in relatively constant proportion that accounts for the defects and failures of buildings located in coastal areas.

According to Shelty, (2006), sea waters has salinity of about 3.5%. In that, sodium chloride has about 78% and 15% is chloride and sulphate of magnesium. He adds that sea water should also contain small quantities of sodium and potassium salts. These salts can react with reactive aggregates in the like manner as Alkalies in cement, therefore sea water should not be used even for Plain Cement Concrete (PCC) if aggregates are known to be potentially reactive (Shelty, 2006). He comments that sea water is not advisable for use in plastering a wall intended to receive painting finishing.

There are some reports that sea water does not appreciably reduce the strength of concrete although may lead to corrosion of reinforcement in certain cases (Fatoku, 2005). He discloses in his report that it only reduces the 28day strength of concrete by about 10-15% which can be made up for by re-designing the mix.

The overall picture of the quality of concrete is usually provided by its compressive strength (ccsenet.org, 2012). Obviously, water that is considered satisfactory for the mixing of concrete is as well good enough for curing (Emmanuel, Fatokun and Olabode, 2012).

Previous investigations and researches carried out regarding the effect of salt water on compressive strength of concrete either as mixing or curing water, or as both, have over the time produced undependable outcomes which present a lot of controversies in analysis of results and reaching a unanimous and unified conclusion (Emmanuel, Fatokun, and olabode, 2012). They referred to the reports of the past studies, which revealed that, the compressive strength of concrete increases if salt water is used for mixing, while recent reports have it that there is decrease of compressive strength of concrete if salt water is used for mixing.

According to Vicat, (1812) and Prascal et al., (2006), the chemical action of sea water on concrete is majorly due to the invasion by sulphate of magnesium (MgSO₄). This is made worse by the chlorides that are in the sea water which truncate the swelling that usually serves as the symptom of the attack by the sulphate in the water which invariably makes the concrete whitish in appearance. More severe attacks subject the hardened concrete to expansion which leads to spalls and cracks in the concrete. The final effect of this whole process, is that the concrete becomes liable and reduced to soft mud (Prascal et al., 2006). At first, according to these researchers, the concrete increases in strength during the early stage of this attack, but consequently followed by loss of strength which preceded the expansion, spalling and cracking. Bryant, (1964) adds that potassium and magnesium sulphates (KS, MgS) present in salt water can cause sulphate attack in concrete structures as they readily react with hydrochloride of calcium (Ca(OH)₂ that is in the cement by the hydration of C₃S and C₂S as given below:

S + CH + 2HCSH_{2 +} KH

S + CH + 2H – CSH₂+ MH.