Lec 4 Flashcards
What are special concrete with use of admixture
High Strength Concrete (HSC)
High Performance Concrete (HPC)
Self Consolidation Concrete (SCC)
Fiber Reinforced Concrete (FRC)
Admixture
ASTM C125 & ACI SP-19
In concrete and cement mortar, besides water, aggregate,and hydraulic cement, one other material is added before or during mixing
Admixtures are used to
Improve the properties of fresh concrete or hardened concrete
Categories of concrete admixtures
Mineral admixtures : contain artificial materials (fly ash, slag) and natural materials (volcanic rocks)
Chemical admixtures: plasticizer, air entraining agent
What is Cement Replacement Materials (CRMs)
Mineral admixture
Used as a substitute for some Portland cement in a concrete
Some of them are by-products from other industrial processes, hence they are economical
Main reason of mineral admixture’s use
Give a variety of useful enhancements of or modifications to the concrete properties.
What are the two common features of CRMs
Particle size range is similar to or smaller than Portland cement;
Involved in the hydration reactions.
Types of Pozzolanic Materials
- pulverised fuel ash (pfa), called fly ash: the ash from pulverised coal used to fire power stations, collected from the exhaust gases before discharge to the atmosphere.
- ground granulated blast furnace slag (ggbs): slag from the ‘scum’ formed in iron smelting, which is rapidly cooled in water and ground to a similar fineness to Portland cement;
- condensed silica fume (csf), often called microsilica: extremely fine particles of silica condensed from the waste gases;
- calcined clay or shale: a clay or shale heated, rapidly cooled and
ground; - rice husk ash: ash from the controlled burning of rice husks after the rice grains have been separated;
- natural pozzolans; some volcanic ashes and diatomaceous earth.
What is fly ash
Generated by coal-fired power plants (thin,light,small)
Different from the bottom ash with large volume -> many pores and low strength
Can be used to replace part of cement, part of fine aggregate or cement
https://ibb.co/vPCmRKZ
Low calcium fly ash (class F)
High calcium fly ash (Class C)
In SLAG,
during the production of cast iron in a …….., when the mineral waste is …… in the air, an …….. is formed
What does slow cooling do?
Blast furnace, slowly cooled, air cooled slag (fine grinding)
Complete strength development, high hardness, can be used a pellets
In SLAG,
At high temperature, the liquid slag is ……… (quench) by spraying water and air, and it is in an amorphous glass state. Such particles are called “……… 水淬爐石.”
What does high activity do?
Rapidly cooled, water quenched slag (fine grinding)
Directly ground and added to cement
What is silica fume
It is very small (1micron long)
It replaces 15% of cement.
Each Portland cement particle, approximately has 2 million silica fume particles
Admixtures are chemicals that are added to the concrete immediately ………… mixing, significantly………, early age or hardened state to economic or physical advantage. Only ……….. are required, typically …….. of the cement.
Before or during mixing
Change its fresh
Small quantities
1-2% percent by weight
Types of chemical admixture
- Plasticizer: as water-reducers, since they can produce a concrete with the same workability at a lower water/cement ratio, hence increasing the strength or durability with the same cement content.
- Superplasticizers (SP): are more powerful than plasticisers and are used to achieve increases in fluidity and workability. Known as high-range water reducers.
- Accelerators: used to increase the rate of hardening of the cement paste (CaCl2).
- Retarders: delay the setting time of a mix (long distance, large pours, high temperature)
- Air entraining agents: are organic materials which, when added to the mix water, entrain a controlled quantity of air in the form of microscopic bubbles in t he cement paste component of the concrete.
