What are the main factors impacting the hardened concrete stability and durability?

First of all take the composition of Concrete either fresh or hardened:

• Aggregates+Cement+Water+Admix+(Ice) ---------------------> Concrete

its mean stability and durability (i.e. quality of concrete) both are depending on above four ingredients. So, following parameter shall be considered to produce quality concrete.

1. Durable and strong enough aggregate must be used  because more than% volume of concrete is occupied by CA and FA.
2. Avoid to use rounded aggregate and try at your best to use crushed aggregate to make strong bond among aggregate with cement paste.
3. your aggregate must comply with BS: table4 and5 or BS EN0. (i.e. maintain your CA and FA percentage in a reasonable manner to make the concrete homogeneous)  
4. Check the compliance of all mechanical / physical and chemical properties of aggregate as per specification tolerance.
5.  Good quality of cement (Approved source) free from lumps and other contamination and shall stored in air tight silos.
6. Maintain adequate water to cement ratio and never go above as specified in specification and at least take twice MC of aggregate in a day and update your plant with MC value. This MC can control your W/C ratio as specified in mix otherwise your mix will be less strong and durable as well. 
7. Analysis your mix in terms of aggregate cement ration this ratio must come under4 to5%
8. Check the fresh concrete temperature at check point and site and monitor that the temperature of concrete shall not be more than C
9. To maintain concrete under these range use chilled water (3-6 C) and flack ICE
10. Avoid to cast your concrete when you have high ambient temperature and high wind velocity otherwise your concrete will get creak and that will make no seance for quality concrete.
11. Use good brand of admixture
12. Always use optimized mix design (with GGBS/Fly Ash and MS) to avoid high temperature of concrete and prevent crack to the concrete without bleeding and segregation.
13. These supplementary cement material shall make your concrete strong enough and durable because supplementary cement pack the tiny pore space (due to additional gel of C-S-H) which is not packed by the normal cement.   
14. Maintain at least2 % Air Voids in concrete to prevent to propagate micro crack / crack within the concrete) 
15. Most important proper curing is required.

For making durable concrete consider all above parameters along with expose / subsurface environmental condition.

please check ACI Code: for durability requirement of expose class of concrete or BS-1: for concrete guidance.  

 One of the main characteristics influcing durability of concrete is its permeability to the ingree of water, oxygen, carbon dioxide, chloride, sulphate and other potential deleterious substances. • Most of the durability problems in the concrete can be attributed to the volume change in the concrete. Volume change in concrete is caused by many factors. The entire hydration process is nothing but an internal volume change, the effect of hydration, the pozolanic action, the sulphate attack, the carbonation, the moisture movement, all type of shrinkages, the effect of chlorides, corrosion of steel, comes under the aspects of volume change in concrete. • The internal and external restraints to volume change in concrete results in cracks. It is the crack that promotes permeability and thus it becomes a part of cyclic action, till such time that concrete deteriorates, degrades, disrupts, and eventually fails.

 

 

the major parameters affecting concrete hardness are many, including: Cement content, Compaction, the curing, the thickness of concrete cover, and the permeability.

 

Below are the factors mostly affecting the properties of hardened concrete:-

• Water-cement ratio

• Type and amount of cement

• Type and amount of aggregate

• Weather condition

You can not tule one factor as the sololy responsible for concrete quality. Each element in concrete has properties that have an effect on final product which is the concrete, and to keep it short. Cement: the finer particle size the better. Aggregate (fine & caorce): must be clear of menerals that react nigativle with cement,therefore it might be washed to protect the final product, and best to have adquate size grading. And finaly the last but not the least, the water you use for the mix. It must meet only one condition: if it is potable then it is usable.