This comes from Dr. Rakesh Kumar "Designing Reinforced Concrete Structures for Long Life Span"

Quality management of material, methods, and testing.

Manage all design and construction aspects to ensure the structural integrity.

Designer should have adequate knowledge of material properties such as strength, creep, shrinkage, etc., of concrete and their affect on cracking of the concrete.

Design adequate depth of cover for the reinforcing steel.

Use of fly ash and/or other pozzolonic materials instead of ordinary portland cement only.

Use of high-quality aggregates free from deleterious compounds for preventing alkali-aggregate reactivity, and similar actions. Aggregates should also have proven reliability.

Concrete, from its proportions, mixing, methods of construction, (compacting and curing), should be given careful attention so that an adequately dense concrete, with full compaction and a desirable pore system may be ensured.

Adequate cover for the reinforcement ensuring highquality compaction and curing of the concrete. High-performance & self-compacting concrete may help in minimizing the potential of corrosion of reinforcement and deterioration of concrete due to poor quality of cover.

Corrosion resistant steel, steel coated with corrosion resistance layer such as cementitious material slurry, stainless steel, or other types of newer steel, may be used.

Concrete should be carefully tested and quality managed to meet long-term tests such as water and air permeability, shrinkage, creep, freezing and thawing, chloride-ion penetration by ponding and chloride diffusivity.

Prediction of life of structures based on corrosion rate of reinforcement.

The main issue to address here is that of the durability of the concrete to be designed. High strength, high performance concrete is the solution. I recommend that you consult literature on the design procedures for such a concrete.

Dear Alex. Thanks for your reply. It is true that I have to look for durable concrete. But how could I calculate at design that the life of our mix design is 300 years. Because we have to satisfy the regulator through design calculations.

Dear Shafi Ullah,

A 300 year old concrete would require a high performance concrete design. This is, indeed, a very broad topic. What you would have to do is to address all the long term problems a structure may suffer during the design stage. However, I would suggest you research about some admixtures and additives that would be helpful to achieve your goal.

1st : Use of pozzolans in your concrete mix: seek for literature regarding the use of fly ash, rice rusk, metakaolin and silica fume. These materials have the capacity of reacting with calcium hydroxide (byproduct of cement reaction that reacts with CO2 and has several negative effects on concrete) to form even more CSH, reducing concrete porosity and increasing resistance and durability. Zhang & Gjorv suggest that 16% of silica fume would react with all the calcium hydroxide in the mix (refer to: Effect of silica fume on pore structure and chloride diffusivity of low porosity cement pastes). 

2nd: this one is assuming you are working on reinforced concrete structure. If so, you may want to avoid two main deleterious agents. Chloride ions and concrete carbonatation. These two processes may reduce steel durability. Make sure you have enough concrete cover (depending on where you are building). Also, low porosity and low calcium hydroxide (when it reacts with CO2, it reduces concrete pH and affects steel protection) content help on this matter.

3rd: low water / cement ratio. Lower w/c ratios result in lower porosity and lower chloride difusivity in concrete. However it would reduce workability by a lot, so in this one I would suggest the use of superplasticizers. Also use good quality aggregates and study the effects of adding some filler to your mix to fill the pores.

4th: find and efficient curing process for concrete. You want to avoid cracks due to thermal expansion and retraction to increase durability.

5th: freeze-thaw process can also induce serious damage on concrete structures. Consider finding ways of dealing with it. A common option is using air entraining admixtures, however it may affect your concrete compressive resistance.

6th: run your mix designs and work on several testing regarding chloride diffusivity, workability (slump test, flow test, L-box, ...), carbonatation, long term shrinkage, and so on.

Please note that I am not taking cost into consideration. Once you addressed all the deleterious effects and found a suitable mix, you can run your mix through lifetime prediction studies based on corrosion rate of the reinforcement steel

 Hope it, somehow, helps

Dear Ramoel Serafini. 

Thank you for the detailed reply. As you suggested, the use of pozzolans, low w/c ratio, water proofing membrane /sealer, corrosion protection of steel, low permeability are the essential requirements for long life and durable concrete. In this regard, I used a freely available software of ACI, named Life-365, to design the concrete mix for > 300 years. Salient parameters of the mix (for calculated life of 304 years) come out to be as follows:

w/c = 0.40,

Fly ash = 7%,

Silica Fume = 15 % (more than 15% is not allowed by Life-365, I don't know why?),

Steel reinforcement: Black steel (1.2%),

Corrosion inhibitor (Ca-Nitrite) = 5 Litres/m3,

Barrier: Membrane (Sealer gives 297 years instead of 304 years),

Super-plasticizer: not yet determined.

Could you please review the above mix design and give me suggestions for further improvement?

Can we use both the sealer and membrane barriers for more life?

Dear Shafi Ullah

The design mix seems to be appropriate with some considerations to be made. First of all, let me answer your questions.

1) The program probably limits the amount of silica fume at 15% because you don’t need more than that to reach with Ca(OH)2, resulting in waste. Additionally, your mix has another pozzolan (fly ash) to compensate.

2) I am not sure about the usage of sealer and membrane barriers at the same time, I suggest you talk to the company that produces the product about your intentions and ask what their recommendations are. They should be able to give you a more direct answer than me.

For the mix design then, I would have some suggestions:

For superplasticizer I would say 0.3%-1% of the total cement content. I do not suggest you use more than 1%, once too much superplasticizer may result in problems later on. Thus, in this matter, it does not mean that 1% is better than 0.3% superplasticizer. The main idea is to give your mix workability to avoid problems, such as concrete honeycombs. If you can reach a desirable workability with only 0.3%, that’s great! Also, pay special attention for the amount of chloride in your admixtures, the less, the better.

The workability you need to have depends on what are you building. In my opinion, the best way to find out how much superplasticizer you need is to run a sample mix of your concrete with about 0.3% superplasticizer mixed with water. For the best results, when mixing the materials add the coarse aggregate first, then cement with a quarter of the total amount of water (only). Mix it for a while so you improve the transition zone properties of your concrete, resulting in a more resistant and durable concrete mix. Later on add the fine aggregate and the rest of water with more superplasticizer if needed. It’s a good practice to mix superplasticizer in water. You can run some workability tests while you are mixing to check the workability of your concrete.

a)     Slump test  - if you need regular concrete workability or;

b)     L-box and flow test if you need self-consolidating concrete.  

Pay special attention to the granulometric curve of your aggregates too they are decisive in workability.

Still about your mix design, you can go even further in w/c ratio. You can use w/c=0,30 if you plan on using superplasticizers. For a 0,40 w/c ratio you may not need superplasticizers (depending on your aggregate shape and distribution). However your concrete will be more porous than a 0,30 one with superplasticizer and your concrete will have less plasticity.   

Hope it somehow helps,

Cheers,

Dear Ramoel, its great.

1) The reason for max. 15% Silica Fume seems to be justified as according to Zhong and Gjrov, 16 % SF is sufficient to react with all the calcium hydroxide in the mix. 

2) For the combined usage of sealer and membrane, I shall consult the manufacturer.

3) For the mix design, I shall go for w/c = 0.35. This will need superplasticizer but give us a more impermeable mix than that with w/c=0.4 and at the same time more workable than that with w/c=0.3. This will reduce the requirement of Fly ash from 7% to 1%.

4) Now, I have to decide about the filler material. Could you suggest some suitable filler or else the mix is okay even without any filler. 

I am sure using filler will improve your compressive strength and reduce the specimen porosity. I would say use some filler if your construction is susceptible to ocean salt water drops, chemical attacks or near the ocean (aggressive environments). If you plan on building on a rural area I guess for durability purposes that may not be required because the environment is not that aggressive. Also, if you have good granulometric curves on your aggregates you may already have filler material on your fine aggregate composition. That may be enough for your application.

It has been observed that increased content of micro silica results in increase in water demand (& hence admixture). Moreover, such mixes becomes very sticky & difficult to place. 

Nowadays, micro silica is being replaced by ultra fine slag which contains not only SiO2 but also CaO & Al2O3. Ultrafine slag reacts with calcium hydroxide to convert in CSH gel. Moreover, ultrafine nature of materials increases its reactivity. It also improves the packing density of paste component. 

Thanks Mr Yatin.

It's right that micro silica can result in more demand of water. However, Ultrafine slag too has a very large surface area due to its fine particle size  and thus it will need much more water. Because the smaller the particle size, the larger thew surface area and the more the water demand.

I also thought so till I got hands on experience with ultrafine slag particles!!!

These particles (against the logic) reduce the water demand due to:

1, smooth surface texture of particles

2. surface negative charges acquired during grinding process &

3. particle size distribution (d95 < 10 microns) which fills the inter-particle voids of OPC

Compared to micro silica mix, water demand was reduced by 5 % for Ultra fine slag mix.

Regards,

Thank dear Yatin Joshi.

It is really very interesting and logical answer for reduction of water demand in case of using Ultra fine slag.

designing a concrete for 300 years of life span, main point which may be considered as

carbonation depth of concrete must be less than clear cover

permeability or depth of penetration of water must be very less even in the adverse environmental condition

Sorptivity must be minimum

and for high durability of concrete you can also try self healing concrete.

thank you

What tests you suggest for such concrete and what should be limiting values of the results?

Interesting and useful discussion!

Thanks for your interest, Boriana