Can PPC be used for water retaining structures? What are the recommendations of BIS?
What is this question - PPC be used for water retaining structures?
Very wrong question .
Let me ask first why one should avoid PPC.
Any fly ash based cement is always better.
Portland cement without fly ash (or any other mineral admixture) should be used any where.
Please any fundamental books/literature on fly ash and its properties.
Please do not think old way on concrete. Read latest information and select material.
Portland cement without fly ash is hazardous ecologically and less durable, highly undesirable ecologically!
Do admixtures reduce the strength of concrete after 28 days?
They may or may not depending on many parameters.
However, it is possible to select admixture without affect on 28 day strength.
Do admixtures reduce the strength of concrete after 28 days?
Generally, their effect is neglegible; however, you can select admixture without affect on 28 day strength.
Usually, chemical admixtures with accelerate setting effect can reduce strength of concrete at higher ages.
Most of mineral admixtures, because of delayed pozzolanic effect may reduce strength of early age of concrete.
There are many types of admixtures and their performance varies widely from one to another depending on chemical composition, dosage used, and their function. Hundreds of products are being marketed nowadays. However, you can select the appropriate type so that you can modify the fresh properties as you need without affecting the strength and durability. This required some knowledge of the advantages and limitations of commonly used admixtures.
The admixtures I am familiar with are accelerators, water reducers, super plasticisers , and air entraining. Accelerators such as calcium chloride make the plastic concrete set fast and will reduce the ultimate strength. Water reducers and plasicisors will allow for a workable mix without affecting the water cement ratio and actually make the ultimate strength better than if the desired slump or workability had been achieved by adding water to the mix.
The question is raised - What are the recommendations of BIS?
Are BIS specifications not accessible?
Is is difficult to refer BIS codes and find out the answers?
Should such straight forward questions be put on public platform?
I feel we should use discussion sites more effectively so that the time spent on it is useful to most of the people who are in the discussion group.
I hope I have not hurt anybody and my intention is not restrict the questions to be raised.
The addition of High Range water reducers increase the work-ability of
concrete that has a low water content. Lower water content generally increases
strength of the concrete and reduces the permeability of the concrete.
Fly ash is used in concrete to mitigate alkali-silica re-activity of certain aggregates.
The amount of Fly Ash in concrete has been steadily increasing with time with
predicted benefits, and also intermittent problems such as low concrete strength
in low temperatures, low concrete strength when the source of the
flyash is changed and when the flyash has a high carbon content.
Concrete suppliers are now substituting up to 25 % of the cementacious
materials as flyash. We had better results with 16% to 20% flyash in
concrete made to achieve 4500 to 5,000 psi concrete ( 660 lbs of cementacious
material per cubic yard ). Amounts above 20% especially in the
22% to 25% range give strange strength curves depending upon
the temperature at placement ( flash sets in the summer, and low strengths in the winter, and tearing and fracturing of the surfaces during windy conditions. )
The most consistent results were in the 1990's when we were using
only 16% flyash, but 16% does not protect as well against alkali-silica re-activity.
Let me add my grain of salt - by admixture I understand a chemical added to improve performance and I will not discuss supplementary cementitious materials.(Call it professional interest)
Any admixture increasing the porosity of concrete WILL have a detrimental effect on late strength. Air entrainers will always do so since they work by creating a bubble network in concrete. Plasticizers and superplasticizers always help decrease batching water thus porosity - hence an increase in strength. Even retarders - which decrease early strength - have no or even an increasing effect on 28d strength and beyond.
As for water-retaining structures I do not know what you're referring to : for a dam you would probably want a reduced porosity but what matters much is the hydration HEAT. Given the very high volume of concrete involved the heat of hydration might be contained within the dam and that leads to considerable damage in the inner core. You would look for low-heat cements which very often are blended, so not pure OPC. This is how fly ash may come into play - mitigating hydration heat to avoid damage while decreasing porosity. But watch out for early strength !
For much smaller structures like basins, artificial ponds or basements some water-tightening admixtures exist under the names of 'water repellents' or 'water sealants'.
No,admixture does not reduces strength it adds more strength but be sure that concrete temperature must be maintain to below 32 degrees centigrade during hot weather concreting.A lot of factor that decrease concrete strength.
The primary determinate of strength in concrete is the water to cement ratio.
High range water reducers reduce the W/C so the 7 day and 28 day strength
increases dramatically. A mix we are now using has measured W/C between
0.33 and 0.36 with air content between 5.0 % and 7.5 %. we expect 7 day strengths
in excess of 5,000 psi and 28 day in excess of 7,500 psi.
The mix has 660 lbs of cement per cubic yard of which 22 % is fly ash.
The mix has 20 oz of ADX 2 and 40 oz of ADX 6 per cubic yard ( High Range water
Reducers ).
Weight per cubic foot is running between 138.5 and 141.4 lbs per cubic foot
depending upon how the delivered temperature activates the 0.9 oz of
air entraining agent per cubic yard.
Like Dr Bapat I think we are talking MINERAL ADDITIONS rather than ADMIXTURES.
I agree with most comments about admixtures. Additions have an interesting effect on mitigating heat of hydration and bringing a bit more of durability.
Do admixtures reduce the strength of concrete after 28 days?
Any admixture would affect the properties adversely or beneficially depending upon the compatibility between Portland cement and chemical admixture and in case of mineral admixture, it depends upon its content and type.
No mix should be used without reliable specific laboratory data generated on various combinations of cement and admixture. If this is done, there is no question of reduction of strength. Nobody should say strength would reduce and this can be taken care in mix design using lower w/c ratio and modifying mix proportions at the laboratory stage itself
Can PPC be used for water retaining structures? What are the recommendations of BIS?
Why do you think technically PPC can not be used? PPC can be used anywhere.
Dear colleagues,
To be simple, if the strength is reduced by something that was added to concrete, this stuff is not an admixture but a bad stuff. We assume that an admixture to be used in concrete technology is a material that has been previously tested and all inconveniences are already known. Best Wishes.
Deal all, admixtures- such as superplastisizers- are mainly used for the increase of the workability of the concrete mixtures, without affecting strength. However, when an admixture is used, we usually reduce water content, so strength could be increased. I totally agree with Dr S. El. Allem that "There are different types of admixtures and their performance varies widely from one to another depending on chemical composition, dosage used, and their function".
I quite agree with Dr Luiz Pereira de Oliveira"s contribution.
There are many factors contributing to the reduction of strength after 28 days. It's depend on the performance of the concrete itself and subjected to intrinsic properties and also the environment of the concrete to be exposed as well. However, design is normally based on the strength of the concrete the age of 28 days.
It depends on what is admixture is intended to be used for. Admixtures are generally used to enhance one or more properties of the cementitious systems
Depend on kind, admixtures can reduce or increase hydration reaction. For example pozolans can increase hardened concrete strength in long term. But sulphate calcium that opposes the fast initial set, may cause decrease strength in long period. Or bubble producing admixture cause the decrease in strength.
In our study, we used Cr(VI) reducing agent in cement up to 1% which retards the strength as per control except some chloride additives. Generally, sulfate additives such as iron sulfate and tin sulfate retards the hydration and strength due to the formation of calcium sulfate and decreases the Calcium silicate formation. it means sulfate additives is attracted towards calcium hydroxide to make calcium sulfate. Besides this cations such as Fe, Mn, and Sn are also influence the strength because in the presence of Fe, the very well developed, long ettringite crystals are produced, in the presence of Mn, the manganese hydroxide is formed, the Sn is present in C–S–H, and the plate-like forms of Sn–Ca compounds are detected.
This problem has been solved by our research by avoiding above participation of anions and cations during hydration. we will provide full text soon.
Thanking you
Look at articles related to Fast-Track Concrete. Various addatives and the use of Type III Cement seem to raise the initial strength fairly high, in a short period of time, followed by a gradual reduction in strength.
Look at 6 hour, 12 Hour, and 24 Hour Fast-Track Concrete mixes. Stay away from clorides as they attack the reinforcing steel, especially the black rebar.
It depends on the type of admixture and what you want to achieve. As pointed out pozzolans will increase strength on long term retarders depending on the type may have varying degrees of effect on strength and other properties of concrete.
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