Can PPC used for concreting of heavy RCC structures? If “Yes”, for which grades? If “No”, Why?
ASTM C 618-78 describes Pozzolana as a siliceous or siliceous and aluminous material which in itself possesses little or no cementitious value but will, in finely divided form and in presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious Properties
A good question indeed!….there are a lot of misconceived notions about PPC…..lets start from basics:
a) How does action of Pozzolona help?:
Significant quantity of Calcium Hydroxide is produced during hydration of calcium silicates. Calcium Hydroxide is a useless material from strength/durability considerations.
Pozzolona converts this useless mass into cementitious produce; thereby improving the quality of concrete.
The pozzolonic action can be represented as:
Calcium Hydroxide + Pozzolona + Water———> C-S-H (gel)
b) Advantages of PPC:
- Concrete made with PPC is more impermeable
- Concrete made with PPC is more durable
- PPC produces less heat of hydration (and that too at a low rate) thereby controlling temperature differentials, which causes thermal strain and resultant cracking in mass concrete structures like dams.
- Its higher resistance to cracks means that it is less prone to corrosion of reinforcement.
- Better resistant to sulphate attacks and to some other destructive agents.
- Contributes to workability, reduced bleeding and controls destructive expansion from alkali-aggregate reaction.
- Reduces the leaching of calcium hydroxide liberated during the setting and hydration of cement.
c) If it has all the advantages as stated above….then why are there some concerns over use of PPC?
Inspite of its superior properties, there have been some concerns over its application. The reasons are:
- In the initial phase of introduction of PPC, there was a question mark (sometimes justified) over the type & quality of pozzolona being used. With increase in awareness & improvements in technology coupled with better quality control, this concern is no more valid for products of good manufacturers.
- For manufacture of good quality PPC, uniform blending of pozzolona material with cement is a must. Again, with increase in awareness & improvements in technology coupled with better quality control, this concern is no more valid for products of good manufacturers.
- One disadvantage of using PPC is that it causes reduction in alkalinity which reduces resistance to corrosion of reinforcement. This has been one of the prime reasons that some restrictions on its use in RCC were imposed by some standards. However, it is now established that this disadvantage is more than offset by increased impermeability & durability of concrete.
- Another disadvantage of use of PPC is its slower rate of development of strength. If PPC is manufactured by using the right type of reactive pozzolonic material, the PPC will not in any way be inferior (in fact it will be superior to PPC)to OPC except for the rate of development of strength of concrete upto 7 days. Its slower rate of development of strength does place some restriction on its application for constructions with rapid construction methodologies like Slip form techniques (where 4 hr to 6 hr strength is important).
- In view of slower rate of development of strength of concrete made with PPC, due care has to be taken to ensure that formwork is removed only after stipulated strength has been achieved (which is often correlated with a time period for removal of formwork).
d) Now lets come specifically to the question: Well the answer is:
PPC can be used for Concreting of Heavy RCC structures…there is no restriction on grades of concrete upto which it can be used.
IS 1489 Part 1 (Specifications for PPC-Fly Ash based) & IS 1489 Part 2 (Specifications for PPC-Calcined Clay based) state that it can “generally be used wherever 33 grade ordinary Portland Cement is usable under normal conditions”.
IS 456:2000 also allows use of PPC in CC/RCC. IS 456: 2000 is distinctly different than its earlier version of 1978 in that the revised code accepts and encourages the use of mineral admixtures in concrete. Clause 5.2 has been specifically added to mention the use of fly ash, metakaolin, rice husk ash, GGBS and silica fume.
PPC can be used & is being used for high grade concretes. In fact, presently most of the ongoing projects with high grade concretes use blended cement.
In this study, wide ranging investigations covering most aspects of mechanical behaviour and permeability were carried out for various mixes for compressive strengths of 60N/mm2, 80N/mm2 and 100N/mm2.
Compressive strengths of high strength SCC specimen with blended cements for 60N/mm2, 80N/mm2 and 100N/mm2 were observed to be higher by about 4 to 9% than the control specimen, for concrete with 50% Portland cement blended with 50% rice husk ash. Higher elastic moduli and reduced permeabilities were observed along with better sulphate and acid resistance.
The paper concluded that “Better strengths and improved durability of such high strength SCC make it a more acceptable material for major construction projects”.