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Many situations are faced wherein concrete is expected to be exposed to environments with more than one type of attack. Say, concrete exposed to sulphate attack and has aggregates which are reactive. Usually the solution to such problems would not have existed i we would have had choice of only one cement. but with the research and development works, with different additives in cement we have been able to figure out solutions for different problems exisiting in concrete.
but the most peculiar problem which was faced earlier was when, concrete was exposed to sulphate as well as chloride environent. Now for sulphate rich environment OPC is specified to have C3A content not higher than 5% (sulphate resisting cement). since sulphate reacts with C3A to form expansive ettringite and disrupt the concrete. but on the other hand C3A is very effective for resistance against chloride intrusion into concrete. thus concrete would get affected from chloride if not sulphate and vice versa. later on a very narrow boundary was drawn for uch cases, wherein, C3A content was specified to be between 5 to 8% in OPCs, ad it is said that this cement may prove effective against both threats.
About Me
- Avijit Chaubey
- Mumbai, Maharstra, India
- Concrete Technologist by Profession working with HCC Ltd., India. I have been awarded twice for my works, first time a Gold Medal from Union Ministry of Water Resources for a Paper titled "Design of Parabolic Channels", second time from M/S SPCL for suggestion in method for determination of silt in sand. Have also published papers in leading Journals and construction magazines of India.
Friday, February 27, 2009
Monday, February 23, 2009
Article published in "Civil Engg & Const. Review"
Much research has been carried out on properties of Concrete containing Fly Ash as replacement of cement. It has become a well known fact that Fly Ash does hold much positive advantages in Concrete in terms of resistance to Sulphate attack, Alkali Silica reaction, Carbonation, Chloride attack and in terms of economic benefits to users, or in terms of conservation of resources (since it replaces a part of Ordinary Portland Cement). In addition to these advantages, Fly Ash also reduces Heat of Hydration on account of its comparatively slow reactivity at early ages. These advantages/ facts need no mention and are very well known across the construction community. The main reason Fly Ash is able to perform this way is because of its pozzolanic property by virtue of which it reacts with by product of C3S/ C2S hydration i.e. CaOH2(CH). CH being an unstable material both chemically and physically creates nuisance in the Concrete leading to problems in durability. Chemical instability of CH relates to its tendency to react with:
1) Sulphates to form CaSO4 which further reacts with C3A (after Concrete has hardened) to form expansive Ettringite. This is Sulphate attack.
2) It produces a highly alkaline environment due to which Si-O-Si (Silicate bond present in Aggregates which leads to Alkali Silica Reaction) reacts with water to form expansive Silanol or Silica gel.
3) CH is a crystalline material which possesses some strength, but it has a tendency to react with atmospheric CO2 to form CaCO3, which by nature is an amorphous material possessing no strength.
4) On account of its physical instability, it is highly soluble in water, and leaches out of Concrete forming pores. These pores get interconnected to form a permeable Concrete. Chlorides, Carbon dioxide find their way into the Concrete through these pores, thereby accelerating process of corrosion in the reinforcement (Prakash Mehta, 2008).
It is clear that most of the problems relating to durability involves CH. Solution to this problem has been found through replacement of some percentage of Ordinary Portland Cement with a suitable pozzolanic material.
A pozzolanic material is characterized by its property of reactivity with CH in presence of moisture to form tri Calcium Silicate hydrate Gel, (the binding material in hardened Concrete). Fly Ash produced from thermal power plants, has proven to be a good pozzolanic material, and is widely used to replace certain percentage of OPC in Concrete.
Indian standards, which guide usage of Fly Ash in Concrete, have identified different ways to use Fly Ash in Concrete. IS: 3812 lays down requirement for different uses of Fly Ash in Concrete, they are, for use as admixture, as pozzolana and as fine aggregate in Concrete. It will be interesting to note that Fly Ash can be used in production of OPC in percentages not more than 5% (admixture) to improve the performance of OPC (IS 8112:1989, IS 12269:1987).
Prejudices:
Although most of the advantages relating to Fly Ash are well known among engineers, at least theoretically, it is unfortunate to note that most do not encourage Fly Ash for replacement of OPC in Concrete. Even some Government projects do not have provision for replacement of OPC with Fly Ash.
The main reason for the fear is inadequate understanding of the effect of Fly Ash on Concrete strength. Whenever Fly Ash is used for replacement of OPC, the practice is to equate it with OPC in terms of strength gain. From actual experience it is found that OPC with Fly Ash leads to slow strength gain compared to OPC. Moreover Concrete with Fly Ash is more sensitive towards temperature as compared to OPC. Meaning, decrease in temperature reduces the strength gain rate in Fly Ash Concrete more than Concretes with pure OPC. Probably this has led to so- called failures of Fly Ash Concretes in certain laboratories. The fear is not predominant only in construction industry, but even Cement companies which advocate usage of PPC over OPC; prefer OPC cement for production of Concrete in their RMC plants.
Of course usage of virgin Fly Ash for blending in Concrete at batching plant is much better than using inter-ground Fly Ash and OPC in the form of PPC. The sole reason being, that Fly Ash particles are spherical in shape, due to which they impart better workability to the Concrete in which they are introduced, Whereas when inter-ground with clinker to form PPC, the shapes get distorted, and these particles no more have their shape in spherical form. The result is higher water demand for desired workability. It won’t be wrong to say that water demand is a cumulative effect of particle shape, particle size distribution and fineness, implying that even after grinding of Fly Ash and OPC there may be possibility that PPC cement may have lower water demand up to certain time of grinding, as compared to OPC and un-ground Fly Ash. But the usual observation on site unfolds a different story, with water demand actually being higher for PPC than OPC in combination with virgin Fly Ash. This obviously calls for refining the process for production of PPC, with optimising the time of grinding so that there is minimum water demand. HCC has come across cases when a standard consistency of 26% with a blend of OPC and Fly Ash was achieved, i.e. a reduction by 2% when tested for pure OPC which gave a standard consistency of 28%.
What needs to be done:
Figure 1 gives a clear picture of effect on strength by replacing cement with Fly Ash. It can be seen that strength developed in Concrete with Fly Ash is always less than OPC Concrete, whereas most of cement companies show higher strength of Fly Ash based Concrete beyond 28 days in comparison to Concrete with equal quantity of OPC.
Fly Ash needs to be characterised by its Cementing Efficiency Index (Peter Hewlett, 2004) for different temperatures at different ages in combination to particular Cement.
W = W . -- - - - - - - - - - - - (i)
Cs (C+FK)
Here W, C & F are the weights of water, Ordinary Portland Cement and Fly Ash respectively for the given mix, and K is the cementing efficiency index of the Fly Ash. W/Cs is the equivalent water cement ratio, i.e. the required water cement ratio for the same strength but without Fly Ash. If we try to find out cementing efficiency indices of the Fly Ash used in a trial, reproduced in Table 1 (Amit Mittal, 2008), it comes out to be something between 0.45 to match strength for 28 days and 0.8 to match strength at 90 days (for 40% replacement with Fly Ash) and 0.63 to match strength for 90 days (for 50% replacement with Fly Ash) (figure 2). The steps to calculate cementing efficiency index is shown below:
From Table 1 we can find that for OPC (without Fly Ash), with 350 Kg Cement and 0.45 W/C ratio the 28 day strength is 37.8 MPa. The closest strength at 28 days is achieved with 450, 40% mix (total Cementitious, Percentage Fly Ash) using W/C ratio of 0.35.
Using Eqn. (i):
W = W .
Cs (C+FK)
Thus, 0.45= 158 .
(270+180*K)
Thus, K= 0.45 (This index is to match strength for 28 days of OPC Concrete).
This data can then be used to design Concretes with the desired percentage of Fly Ash for the required age of Concrete.
Another interesting property of Fly Ash should be incorporated in the mix design procedure, i.e. its ability to produce a better workability with lower water contents. A higher percentage of Fly Ash in cementitious material can yield better workability. M.L. Gambhir proposes multiplication factors both for water content and cementitious content for different percentages of Fly Ash (M. L. Gambhir, 2004).
Concrete made with OPC and Fly Ash when compared to Concrete made with Equal quantity of OPC alone, shows better durability in terms of Rapid Chloride Penetration tests, Sulphate resistance (Peter Hewlett, 2004), ASR, etc. whereas in the limits for Cement content in IS:456- 2000, minimum cement content holds same for all cements. It rather would be more appropriate to specify limits for test results on Concrete/ mortar for various aspects of durability viz. RCPT, Sulphate resistance, Mortar Bar Expansion (ASR) etc. rather than specifying minimum cement content per cubic metre of Concrete.
If PPC cement, available in the market, were to be compared with blend of same brand OPC and same Fly Ash, the cost for production of same grade of Concrete will be much less in case of Concrete made with blend of OPC and Fly Ash. The reason for comparing cost, is to just point out the inefficient usage of resources by cement companies. If we had to see this problem from the view point of sustainability, it might be clear that Energy consumption in producing equivalent grade of PPC Concrete will be much higher than the energy for OPC and PFA blend Concrete. Another reason for stating superiority of OPC and PFA blend is the situational advantage to increase or decrease the Fly Ash content to accelerate the production rate in construction. For example construction projects in Sub- Zero temperatures demand faster strength gain rate of Concrete to avoid damages due to freezing. In case of prestressed Concrete, prestressing is done only after achievement of certain strength, the faster the strength achievement, the more efficiently resources can be handled. In these conditions if one had to use PPC, the cost can work out to be much higher than OPC, since in these cases early age strengths holds more priority than 28 day strength.
Example:
An OPC Concrete gives 30 MPa strength at 28 days for W/Cs ratio of 0.5. The water content is 160 litres and cement content 320 Kg per cubic metre of Concrete. Now we desire to use 40% Fly Ash for replacing OPC, which has a Cementing Efficiency Index of 0.4 for 28 days, with the available OPC, so that the strength achieved is equivalent to OPC Concrete at 28 days.
Solution:
Fly Ash reduces water demand say by 12% as compared to OPC (M. L. Gambhir, 2004), so we reduce the water content to 141 litres.
W = W .
Cs (C+FK)
i.e. 0.5 = 141 . (Since Fly Ash is 40% of total cementitious)
(0.6Cm + 0.4Cm*0.4)
So, Cm= 372 Kg per cubic metre (total cementitious content).
Now the cementitious content is 372 Kg per cubic metre of Concrete out of which 150 Kg shall be Fly Ash and 222 Kg shall be OPC. The water cement ratio required now will be 0.38.
If the strength required was at 90 days instead of 28 days, and the cementing efficiency index found was 0.8, the total cementitious content then would have been 307 Kg per cubic metre of Concrete and water cement ratio required would be 0.46 (based on similar calculations shown above).
Economics:
320 kg of OPC costs much higher than combination of 222 kg of OPC and 150 Kg Fly Ash. The difference could be somewhere near Rs. 250 per cubic metre of Concrete (OPC cost- Rs. 5/Kg and Fly Ash Cost- Rs. 1.6/Kg). The heat of hydration from 320 kg of OPC at 3 days has been found out to be somewhere near 17.7 Mcal(Mega Calories), Whereas with the alternative combination, the heat of hydration comes down to 14.9 Mcal per cubic metre of Concrete(Based on actual test results as shown in table 2 and interpolation from SP 23: 1982 considering linear relationship between Heat of hydration and Fly Ash content), i.e. a decrease by 15% of heat in 3 days.
Each ton of cement produced releases 0.95 tons of CO2 in atmosphere (including energy consumption, if the heat is coal generated). It has been possible to reduce OPC by 100 Kg per cubic metre, or by 30%. Thus by replacing 40% cement, we are able to reduce CO2 emissions by 44 million tons per annum (Considering 155 million tonnes cement production per annum in India). Moreover the Fly Ash which otherwise creates an environmental nuisance will be used up in something productive.
Conclusion:
It becomes very necessary for the standards to look into this matter, and make necessary changes, in the mix design procedures for Concrete. It also is very necessary to include Cementing efficiency index and capacity to improve workability when used for replacement of OPC. Keeping in view that durability of Concrete increases when Fly Ash is used to replace OPC, same limits of cementitious content for durability does not seem justified for different types of cement. Rather limits on test results of durability for various tests of Concrete should be specified. Production of PPC is done by inter- grinding clinker of OPC and Fly Ash, which consumes up energy/ resources. If comparison of cost of Concrete made with PPC and Concrete made with blend of OPC and Fly Ash were to be done, the latter would mostly outperform the Concrete made with PPC. Cement companies should treat blend of OPC and virgin Fly Ash as benchmark, in terms of workability, cost, strength etc. when setting the performance targets for production of PPC. Although usage of PPC or blend of OPC and Fly Ash has become need of today to maintain sustainability in construction, it wont be beneficial to completely stop production of OPC, as it proves economical in comparison to Fly Ash based Concrete when high early age strengths are required from Concrete.
Adam Smith in his theory of ‘Invisible hand’ proposes that allocation of finite resources is done by an invisible hand. This invisible hand is referred to as price in terms of economics, if it were to be defined in a single word. The scarcer the resources are, the higher is the cost of the product made from these resources. So, if we have to choose an indicator for sustainable construction, the best indicator would be the cost. Thus two different Concretes made with different cost but same strength can easily indicate which is better in terms of sustainability. Standards can look into the problem of sustainability by also including cost of production of cement (since cost reflects the efficiency of usage of resources) per MPa strength of Cement. Although this might be a very crude step at this moment since not much data is available, but it surely will lead to better usage of resources in future. To start with, there could be data generated on effects of grinding of cementitious material on Workability, Strength etc. Then a suitable method can be devised to find optimum solution from the available data.
1) Sulphates to form CaSO4 which further reacts with C3A (after Concrete has hardened) to form expansive Ettringite. This is Sulphate attack.
2) It produces a highly alkaline environment due to which Si-O-Si (Silicate bond present in Aggregates which leads to Alkali Silica Reaction) reacts with water to form expansive Silanol or Silica gel.
3) CH is a crystalline material which possesses some strength, but it has a tendency to react with atmospheric CO2 to form CaCO3, which by nature is an amorphous material possessing no strength.
4) On account of its physical instability, it is highly soluble in water, and leaches out of Concrete forming pores. These pores get interconnected to form a permeable Concrete. Chlorides, Carbon dioxide find their way into the Concrete through these pores, thereby accelerating process of corrosion in the reinforcement (Prakash Mehta, 2008).
It is clear that most of the problems relating to durability involves CH. Solution to this problem has been found through replacement of some percentage of Ordinary Portland Cement with a suitable pozzolanic material.
A pozzolanic material is characterized by its property of reactivity with CH in presence of moisture to form tri Calcium Silicate hydrate Gel, (the binding material in hardened Concrete). Fly Ash produced from thermal power plants, has proven to be a good pozzolanic material, and is widely used to replace certain percentage of OPC in Concrete.
Indian standards, which guide usage of Fly Ash in Concrete, have identified different ways to use Fly Ash in Concrete. IS: 3812 lays down requirement for different uses of Fly Ash in Concrete, they are, for use as admixture, as pozzolana and as fine aggregate in Concrete. It will be interesting to note that Fly Ash can be used in production of OPC in percentages not more than 5% (admixture) to improve the performance of OPC (IS 8112:1989, IS 12269:1987).
Prejudices:
Although most of the advantages relating to Fly Ash are well known among engineers, at least theoretically, it is unfortunate to note that most do not encourage Fly Ash for replacement of OPC in Concrete. Even some Government projects do not have provision for replacement of OPC with Fly Ash.
The main reason for the fear is inadequate understanding of the effect of Fly Ash on Concrete strength. Whenever Fly Ash is used for replacement of OPC, the practice is to equate it with OPC in terms of strength gain. From actual experience it is found that OPC with Fly Ash leads to slow strength gain compared to OPC. Moreover Concrete with Fly Ash is more sensitive towards temperature as compared to OPC. Meaning, decrease in temperature reduces the strength gain rate in Fly Ash Concrete more than Concretes with pure OPC. Probably this has led to so- called failures of Fly Ash Concretes in certain laboratories. The fear is not predominant only in construction industry, but even Cement companies which advocate usage of PPC over OPC; prefer OPC cement for production of Concrete in their RMC plants.
Of course usage of virgin Fly Ash for blending in Concrete at batching plant is much better than using inter-ground Fly Ash and OPC in the form of PPC. The sole reason being, that Fly Ash particles are spherical in shape, due to which they impart better workability to the Concrete in which they are introduced, Whereas when inter-ground with clinker to form PPC, the shapes get distorted, and these particles no more have their shape in spherical form. The result is higher water demand for desired workability. It won’t be wrong to say that water demand is a cumulative effect of particle shape, particle size distribution and fineness, implying that even after grinding of Fly Ash and OPC there may be possibility that PPC cement may have lower water demand up to certain time of grinding, as compared to OPC and un-ground Fly Ash. But the usual observation on site unfolds a different story, with water demand actually being higher for PPC than OPC in combination with virgin Fly Ash. This obviously calls for refining the process for production of PPC, with optimising the time of grinding so that there is minimum water demand. HCC has come across cases when a standard consistency of 26% with a blend of OPC and Fly Ash was achieved, i.e. a reduction by 2% when tested for pure OPC which gave a standard consistency of 28%.
What needs to be done:
Figure 1 gives a clear picture of effect on strength by replacing cement with Fly Ash. It can be seen that strength developed in Concrete with Fly Ash is always less than OPC Concrete, whereas most of cement companies show higher strength of Fly Ash based Concrete beyond 28 days in comparison to Concrete with equal quantity of OPC.
Fly Ash needs to be characterised by its Cementing Efficiency Index (Peter Hewlett, 2004) for different temperatures at different ages in combination to particular Cement.
W = W . -- - - - - - - - - - - - (i)
Cs (C+FK)
Here W, C & F are the weights of water, Ordinary Portland Cement and Fly Ash respectively for the given mix, and K is the cementing efficiency index of the Fly Ash. W/Cs is the equivalent water cement ratio, i.e. the required water cement ratio for the same strength but without Fly Ash. If we try to find out cementing efficiency indices of the Fly Ash used in a trial, reproduced in Table 1 (Amit Mittal, 2008), it comes out to be something between 0.45 to match strength for 28 days and 0.8 to match strength at 90 days (for 40% replacement with Fly Ash) and 0.63 to match strength for 90 days (for 50% replacement with Fly Ash) (figure 2). The steps to calculate cementing efficiency index is shown below:
From Table 1 we can find that for OPC (without Fly Ash), with 350 Kg Cement and 0.45 W/C ratio the 28 day strength is 37.8 MPa. The closest strength at 28 days is achieved with 450, 40% mix (total Cementitious, Percentage Fly Ash) using W/C ratio of 0.35.
Using Eqn. (i):
W = W .
Cs (C+FK)
Thus, 0.45= 158 .
(270+180*K)
Thus, K= 0.45 (This index is to match strength for 28 days of OPC Concrete).
This data can then be used to design Concretes with the desired percentage of Fly Ash for the required age of Concrete.
Another interesting property of Fly Ash should be incorporated in the mix design procedure, i.e. its ability to produce a better workability with lower water contents. A higher percentage of Fly Ash in cementitious material can yield better workability. M.L. Gambhir proposes multiplication factors both for water content and cementitious content for different percentages of Fly Ash (M. L. Gambhir, 2004).
Concrete made with OPC and Fly Ash when compared to Concrete made with Equal quantity of OPC alone, shows better durability in terms of Rapid Chloride Penetration tests, Sulphate resistance (Peter Hewlett, 2004), ASR, etc. whereas in the limits for Cement content in IS:456- 2000, minimum cement content holds same for all cements. It rather would be more appropriate to specify limits for test results on Concrete/ mortar for various aspects of durability viz. RCPT, Sulphate resistance, Mortar Bar Expansion (ASR) etc. rather than specifying minimum cement content per cubic metre of Concrete.
If PPC cement, available in the market, were to be compared with blend of same brand OPC and same Fly Ash, the cost for production of same grade of Concrete will be much less in case of Concrete made with blend of OPC and Fly Ash. The reason for comparing cost, is to just point out the inefficient usage of resources by cement companies. If we had to see this problem from the view point of sustainability, it might be clear that Energy consumption in producing equivalent grade of PPC Concrete will be much higher than the energy for OPC and PFA blend Concrete. Another reason for stating superiority of OPC and PFA blend is the situational advantage to increase or decrease the Fly Ash content to accelerate the production rate in construction. For example construction projects in Sub- Zero temperatures demand faster strength gain rate of Concrete to avoid damages due to freezing. In case of prestressed Concrete, prestressing is done only after achievement of certain strength, the faster the strength achievement, the more efficiently resources can be handled. In these conditions if one had to use PPC, the cost can work out to be much higher than OPC, since in these cases early age strengths holds more priority than 28 day strength.
Example:
An OPC Concrete gives 30 MPa strength at 28 days for W/Cs ratio of 0.5. The water content is 160 litres and cement content 320 Kg per cubic metre of Concrete. Now we desire to use 40% Fly Ash for replacing OPC, which has a Cementing Efficiency Index of 0.4 for 28 days, with the available OPC, so that the strength achieved is equivalent to OPC Concrete at 28 days.
Solution:
Fly Ash reduces water demand say by 12% as compared to OPC (M. L. Gambhir, 2004), so we reduce the water content to 141 litres.
W = W .
Cs (C+FK)
i.e. 0.5 = 141 . (Since Fly Ash is 40% of total cementitious)
(0.6Cm + 0.4Cm*0.4)
So, Cm= 372 Kg per cubic metre (total cementitious content).
Now the cementitious content is 372 Kg per cubic metre of Concrete out of which 150 Kg shall be Fly Ash and 222 Kg shall be OPC. The water cement ratio required now will be 0.38.
If the strength required was at 90 days instead of 28 days, and the cementing efficiency index found was 0.8, the total cementitious content then would have been 307 Kg per cubic metre of Concrete and water cement ratio required would be 0.46 (based on similar calculations shown above).
Economics:
320 kg of OPC costs much higher than combination of 222 kg of OPC and 150 Kg Fly Ash. The difference could be somewhere near Rs. 250 per cubic metre of Concrete (OPC cost- Rs. 5/Kg and Fly Ash Cost- Rs. 1.6/Kg). The heat of hydration from 320 kg of OPC at 3 days has been found out to be somewhere near 17.7 Mcal(Mega Calories), Whereas with the alternative combination, the heat of hydration comes down to 14.9 Mcal per cubic metre of Concrete(Based on actual test results as shown in table 2 and interpolation from SP 23: 1982 considering linear relationship between Heat of hydration and Fly Ash content), i.e. a decrease by 15% of heat in 3 days.
Each ton of cement produced releases 0.95 tons of CO2 in atmosphere (including energy consumption, if the heat is coal generated). It has been possible to reduce OPC by 100 Kg per cubic metre, or by 30%. Thus by replacing 40% cement, we are able to reduce CO2 emissions by 44 million tons per annum (Considering 155 million tonnes cement production per annum in India). Moreover the Fly Ash which otherwise creates an environmental nuisance will be used up in something productive.
Conclusion:
It becomes very necessary for the standards to look into this matter, and make necessary changes, in the mix design procedures for Concrete. It also is very necessary to include Cementing efficiency index and capacity to improve workability when used for replacement of OPC. Keeping in view that durability of Concrete increases when Fly Ash is used to replace OPC, same limits of cementitious content for durability does not seem justified for different types of cement. Rather limits on test results of durability for various tests of Concrete should be specified. Production of PPC is done by inter- grinding clinker of OPC and Fly Ash, which consumes up energy/ resources. If comparison of cost of Concrete made with PPC and Concrete made with blend of OPC and Fly Ash were to be done, the latter would mostly outperform the Concrete made with PPC. Cement companies should treat blend of OPC and virgin Fly Ash as benchmark, in terms of workability, cost, strength etc. when setting the performance targets for production of PPC. Although usage of PPC or blend of OPC and Fly Ash has become need of today to maintain sustainability in construction, it wont be beneficial to completely stop production of OPC, as it proves economical in comparison to Fly Ash based Concrete when high early age strengths are required from Concrete.
Adam Smith in his theory of ‘Invisible hand’ proposes that allocation of finite resources is done by an invisible hand. This invisible hand is referred to as price in terms of economics, if it were to be defined in a single word. The scarcer the resources are, the higher is the cost of the product made from these resources. So, if we have to choose an indicator for sustainable construction, the best indicator would be the cost. Thus two different Concretes made with different cost but same strength can easily indicate which is better in terms of sustainability. Standards can look into the problem of sustainability by also including cost of production of cement (since cost reflects the efficiency of usage of resources) per MPa strength of Cement. Although this might be a very crude step at this moment since not much data is available, but it surely will lead to better usage of resources in future. To start with, there could be data generated on effects of grinding of cementitious material on Workability, Strength etc. Then a suitable method can be devised to find optimum solution from the available data.
Friday, February 20, 2009
Casey Concrete will pay $25000 for releasing cement water
A company based in Truro, U.S., has been fined for $25000 for polluting by releasing Wash water from Transit mixers into water body.
As per Indian standards of CPCB, the limit on pH of release water is between 5.5-9. the pH of Water released from transit mixers after washing usually goes up to 12 and above. The water, though can be used for manufacturing of concrete again, it seems baffling as to what prevents responsible companies to include a system of recycling of waste water. Also, the monitoring of environmental agencies over the effluents is limited, since it becomes difficult to track each industrial unit effluents for various parameters pertaining to hazards concerned with environment.
The usual system of checking on pollution, on account of carbon dioxide release, is to tax industries for each unit of production. But since taxing reduces the efficiency of markets, alternate option was found, viz. carbon credits. In short, as per this system, if a company can reduce it's caron dioxide emissions, it is eligible for some points (carbon credit) based on the quantity of reduction of carbon dioxide emission. These credits can be sold to another company, whose production can fluctuate heavily with change in carbon dioxide release.
It is a High time for environmental agencies to develop a similar system, pertaining to all parameters of environmentally harmful elements. In case of concrete producing industries, the water released is highly alkaline, whereas there can be other inductries who may be releasing acidic effluents, and finding difficult to treat the same. Fining and taxing has become a sort of cheap threat, which is being used by govt. for each and every problem. A better solution would be to combine, effluents of opposite nature, release the water after neutralisation, and use the salts for further production. This will not only solve the problem of pollution, but will also, help in generating employment.
Tax should always be not the first option for a government to curtail or control certain things, but they should be used only after other options do not exist. Taxes increase the deadweight losses of economy, and may also create hindrance in restoration of global economy.
As per Indian standards of CPCB, the limit on pH of release water is between 5.5-9. the pH of Water released from transit mixers after washing usually goes up to 12 and above. The water, though can be used for manufacturing of concrete again, it seems baffling as to what prevents responsible companies to include a system of recycling of waste water. Also, the monitoring of environmental agencies over the effluents is limited, since it becomes difficult to track each industrial unit effluents for various parameters pertaining to hazards concerned with environment.
The usual system of checking on pollution, on account of carbon dioxide release, is to tax industries for each unit of production. But since taxing reduces the efficiency of markets, alternate option was found, viz. carbon credits. In short, as per this system, if a company can reduce it's caron dioxide emissions, it is eligible for some points (carbon credit) based on the quantity of reduction of carbon dioxide emission. These credits can be sold to another company, whose production can fluctuate heavily with change in carbon dioxide release.
It is a High time for environmental agencies to develop a similar system, pertaining to all parameters of environmentally harmful elements. In case of concrete producing industries, the water released is highly alkaline, whereas there can be other inductries who may be releasing acidic effluents, and finding difficult to treat the same. Fining and taxing has become a sort of cheap threat, which is being used by govt. for each and every problem. A better solution would be to combine, effluents of opposite nature, release the water after neutralisation, and use the salts for further production. This will not only solve the problem of pollution, but will also, help in generating employment.
Tax should always be not the first option for a government to curtail or control certain things, but they should be used only after other options do not exist. Taxes increase the deadweight losses of economy, and may also create hindrance in restoration of global economy.
Sunday, February 15, 2009
Why so much resistance?
It just makes me wonder as to why, Shree Ram Sena is being Maligned by media. Shree Ram Sena came up with a threat to get couples married if seen toghether on Valentine's Day. Have these couples not decided to get married to each other? is it not because they love each other and will like to get married to each other? if not, will they just enjoy with this dirty trend of "live in relation ship" and get married to someone else? Is a threat to get a couple married more severe than violence? probably it is, if the couples do not understand their responsibility.
Advertisement of I-Pill shows a girl who has conceived and scared to death to get the child aborted, why? advertisements for contraceptives are targeted towards unmarried females more than married couples. Well there can be N number of people who will laugh on this comment of mine, and make me feel like a fool, saying the relation between these both issues does not exist.
But it should seriously be given a thought, once, at least. Would you accept your mother or father, who has had affairs before marriage? parents would also, never want to get their childs involved into such acts ever! we Indians are very easily accepting western culture but deep within us we still feel ashamed of the western culture.
Media when first reported about the Mangalore Incident, had clearly mentioned, that the families which stayed close to the pub had complained of drug abuse etc. in the pub. Reacting to which Shree Ram Sena had carried out this operation. Ofcourse this turned out to be a rumour later on. It seems very baffling that later on these neighbours were not questioned, as to why have they created such rumours? it would surely have un earthed the incidents, or behaviour of these boys and girls, which would have made them think this way.
Many of my friends thank their wives, since their wives have forced them to give up vices and lead a more healthy and peaceful life. I wonder what these girls intend to do to their children(forget their spouse)? probably once their children are born, the children would be fed with bottles of Beer instead of milk, isn't it?
Sometimes it makes me feel that Talibanisation is the most correct thing, if it can stop the community from going on wrong path, but sadly it goes to the other extreme.
Human race has evolved from apes, because humans understand difference between good and bad, humans know the value of service to others, we really care about how the society will be affected by our deeds, contrary to animals, who just know to gratify their own needs, not realising the after effects.
Advertisement of I-Pill shows a girl who has conceived and scared to death to get the child aborted, why? advertisements for contraceptives are targeted towards unmarried females more than married couples. Well there can be N number of people who will laugh on this comment of mine, and make me feel like a fool, saying the relation between these both issues does not exist.
But it should seriously be given a thought, once, at least. Would you accept your mother or father, who has had affairs before marriage? parents would also, never want to get their childs involved into such acts ever! we Indians are very easily accepting western culture but deep within us we still feel ashamed of the western culture.
Media when first reported about the Mangalore Incident, had clearly mentioned, that the families which stayed close to the pub had complained of drug abuse etc. in the pub. Reacting to which Shree Ram Sena had carried out this operation. Ofcourse this turned out to be a rumour later on. It seems very baffling that later on these neighbours were not questioned, as to why have they created such rumours? it would surely have un earthed the incidents, or behaviour of these boys and girls, which would have made them think this way.
Many of my friends thank their wives, since their wives have forced them to give up vices and lead a more healthy and peaceful life. I wonder what these girls intend to do to their children(forget their spouse)? probably once their children are born, the children would be fed with bottles of Beer instead of milk, isn't it?
Sometimes it makes me feel that Talibanisation is the most correct thing, if it can stop the community from going on wrong path, but sadly it goes to the other extreme.
Human race has evolved from apes, because humans understand difference between good and bad, humans know the value of service to others, we really care about how the society will be affected by our deeds, contrary to animals, who just know to gratify their own needs, not realising the after effects.
Monday, February 9, 2009
Air Entrained Concrete
In concrete, formation of air bubbles is inevitable due to agitation during mixing, kneading, toppling, turning, etc. Normally these air bubbles escape, or coalesce to form bigger bubbles which partly dissipate during placement and consolidation. The bubbles or pockets that do not escape are called entrapped air.
Air entrainment is the intentional creation of tiny air bubbles in concrete. The bubbles are introduced into the concrete by the addition to the mix of an air entraining agent, a surfactant (surface-active substance, a chemical that includes detergents). As we all know detergent used for washing of clothes, does not produce air bubbles in the water all by itself but is formed after agitating the water for sometime. Similarly, the mixing operation is essential to let bubbles form in concrete after addition of air entraining agent. Air entraining agents help to stabilise smaller diameter air voids by modifying the property of cement paste, i.e. reducing the surface tension of water. These air bubbles are created during mixing of the plastic (flowable, not hardened) concrete, and most of them survive to be part of the hardened concrete. These are called entrained air.
The main purpose of air entrainment is to protect the concrete from damage due to freezing and thawing cycles. A hardened concrete always has minute cracks and capillary pores. Whenever it comes in contact with water, the fissures/ pores absorb and retain water due to forces of capillary action. It is an inherent property of water to expand during transition from liquid stage to ice at freezing temperature. So, during sub-zero temperature the water present in the fissure & pores within concrete changes into ice which is much larger in volume and consequently exerts disruptive internal pressure within the concrete. The solution to this problem has been found in artificially creating some quantity of air voids with closer spacing between them (usually referred as air entrainment), which acts as chamber for the extra volume of frozen water.
Obviously the potential of damage is dependent on how much of the voids are filled with absorbed water. If there is enough space left for accommodating the expanded volume of ice, no internal pressure would be generated during freezing. It is therefore necessary to understand the degree of saturation that is safe, which is expressed by the term ‘Critical Saturation’. The ratio of volume of water present in these voids of concrete to the total volume of voids expressed as percentage is known as saturation in concrete. The term Critical saturation has been coined to explain the role of percentage saturation, in concrete, on aspects related to service life and durability of structure. It is defined as that percentage of saturation beyond which volume of water on freezing expands beyond the capacity of voids present in concrete and consequently damages the concrete. It has been studied, that water can expand up to 9% on freezing. Critical saturation thus, is considered as 91% saturation, since beyond this limit, chances of damage due to freezing are increased.
It is normally recommended to have air content of 3 to 7 % to prevent damage of concrete due to freezing and thawing cycles. But it is not only the volume of entrained air that influences the durability against freezing and thawing, but also the pattern of distribution of these voids. When there is expansion of water due to freezing, the air voids should be
1) Close enough so that water expands and travels to reach another void without damaging the concrete,
2) Enough in volume so that the expanded volume of frozen water does not exceed the total volume of voids.
3) Not forming clusters around aggregates or in the paste,
4) Not more than the volume specified in the specifications.
Spacing of air voids in practical applications is not constant as can be seen in Fig.1, and generalization of spacing is impossible, though spacing between air voids is said to be the most important factor in deciding durability of a structure. Consequently a measurement method has been used widely to check if spacing of air voids in the air void system is acceptable which is often referred to as ‘Spacing Factor’. Spacing factor is calculated based on certain assumptions. The calculation is done by first assuming that all air voids are of same size, and spaced equally as corners of a cubic lattice. To do so, the volume of cube lattice is calculated by setting ratio of unit cube to unit air void, equal to, ratio of total air voids volume to total paste volume (paste volume=
Cement volume + water volume + admixture volume). Spacing factor is not the distance between centre of one air void to another but is the distance of periphery of any air void from centre of the cube (ΔΉ).
In other words Spacing factor is the thickness of imaginary shell which makes up an air bubble in the cement paste. This is the distance that frozen water has to travel from one void to enter into another. ASTM/ ACI specifies a maximum spacing factor of 300 microns, and average 200 microns as requirement for durable concrete in addition to the Air content for different categories of exposure and Maximum Aggregate Size. ASTM 457 has various methods to calculate the spacing factor of Air Voids.
Since shearing action of aggregates and mixer cause air bubbles to be formed and divided into smaller sizes, it becomes critical to give ample time for mixing of concrete at the batching plant so that enough air voids are formed and stabilised. Air entrainment is affected by following factors:
1) Higher carbon content in Fly Ash reduces the efficiency of Air entraining admixtures, thereby creating hindrance in consistency of air content, when carbon content in fly ash keeps on varying,
2) 0.1% dosage of admixture w.r.t. cement weight increases the air content by about 2-3%. Though this should be confirmed during trial mix, with proper tests for air content.
3) Pumping of Air entrained concrete, usually leads to decrease in air content, since pressure of pumping leads to expulsion of air voids from the system.
4) Excessive Vibration also leads to loss of air voids in concrete. Care should be taken to impart not more than few seconds of vibration to concrete, so that not much of entrained air is lost.
Usually it is perceived that air content and size of air voids dictate the spacing factor in concrete, which holds true in many cases. But there are cases of clustering of air voids at one location, which not only disturbs the spacing consistency of air voids, but also leads to remarkable loss in strength of concrete (Fig. 5). This usually is seen around coarse aggregates. As a thumb rule each 1% of void is responsible for 5% loss of strength, whereas a clustering similar to rating 2 (fig. 5) can reduce the strength by 20%.
Clustering of air voids is a consequence of re-tampering of concrete (addition of water to concrete at site), and must always be avoided especially in Air-entrained concrete.
Following table shows the recommended Air content (Entrapped + Entrained) for various conditions of exposure with different sizes of aggregates.
Air entrained concrete has some additional advantages over Non Air- entrained concrete like 1) Improved workability, 2) Better Cohesiveness.
Admixtures for air entrainment
Admixtures used for Air entrainment into concrete as discussed previously are surfactants, like anionic Sodium Vinsol resin. When other admixtures are also used in concrete in addition to air entraining agent, their interaction and their combined interaction with cement become important for compatibility reasons. Calcium Chloride added to concrete in cold weather to overcome freezing damages should not be mixed with air entraining agent rather should be added separately. Some Air entraining Agents are also known to retard the strength gain rate.
There is a new Air entraining agent developed, with a commercial name of Micro Air, which has capacity to produce ultra stable Air voids of small size and are closely spaced, it can be used in concretes containing high carbon fly ash. The Air entraining admixtures are also available in Powder form, which find their use in production of dry mix air entrained shotcrete, since liquid air entraining admixtures pose lot of problems for addition into Dry mix shotcrete.
Friday, February 6, 2009
Concrete Mix Design
Many naive Engineers, assume Mix Design of Concrete to be a very difficult task, but on the contrary I feel it is nothing more than a combination of Basics of Maths that we have learnt in our Schools, Knowledge of effect of material properties to some extent, Lot of imagination and playfulness. the most basic and important thing is to recognise the fact that 1 cubic metre of concrete will be equal to sum of individual volumes of its ingredients. The reason that Engineers like to design mix of concrete for a specific volume is because it gives 2 advantages 1) It gives you idea on economics of your concrete (2 concretes of same grade but different costs per cubic metre of concrete, will easily denote the better option). 2) The proportions remain undistorted, plus in addition you also can detect exact correlation between concrete property and content of a particular ingredient. if you just had to design concrete on the basis of inter proportions, as was done earlier, it would have been very difficult to economise the mixes, or to improve the properties of concrete. for example let us say you made a concrete with 10 kg cement, 20 kg fine aggregate, 40 kg coarse aggregate and you got less strength. you decide to increase the cement (thus indirectly reduce w/c ratio) and the new mix proportion you think is 12 kg cement, 20 kg fine aggregate and 40 kg coarse aggregate. if you had to see the economics you would say that the cost of concrete has increased by an equivalent rate of 2 kg of cement. But if you really had to check the economics, the new cost of your concrete would be higher by 2 kg of cement and lesser by 1.7 kg of total aggregates. the Formula 980= C/Sc + W + Fa/Sfa * 1/p is nothing but an equation which says that 1 cubic metre of concrete is equal to volume of cement per cubic metre of concrete plus volume of water per cubic metre of concrete plus volume of total aggregates per cubic metre of concrete.
Avijit Chaubey +919920131802.
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