- What are Nominal Mix and Design Mix –
Nominal mix is the mix proportion of aggregate where no trial mix or mix design is prepared. It’s based on previous work experience.
- What is SSD condition – Saturated Surface Dry is a condition of aggregate where the surface if the aggregate particles are dry. Surface absorption of water is not possible in this state.
- Capacity of Mixer Machine, Its RPM and Mixing Time – The Capacity of mixer machine varies manual machines has drum capacity of 260-500 lit (1/2 to 2 bag of cement), – 15-20 RPM or around 30 rotation.
- What is grading of aggregate and why it is necessary – Particle size distribution of aggregate in concrete is termed as grading. Aggregates are graded to minimize the void to obtain higher density, adequate workability and better durability.
- What happens if 12 mm stone chips are used in place of 20 mm size stone chips?
More water will be required to obtain the same degree of workability as the surface area will increase also more cement paste will require for the same strength of 20 mm aggregates.
- What is W/C Ratio?
Water cement ratio is the ratio of weight of water to weight of cement in concrete.
Minimum and maximum value of WC ratio. Min – 0.4 Max – 0.6 as per IS 456:2000
Slump Values –
|Placing Conditions||Degree of Workability||Slump (mm)|
|Blinding concrete: Shallow sections: Pavement Using pavers:||Very Low||In the ‘very low’ category of workability where strict control is necessary, for example pavement quality concrete, measurement of workability by determination of compaction factor will be more suitable than slump (IS 1199) and value of compacting factor of 0.75 to 0.80 is suggested.|
|Mass concrete; Lightly reinforced section in slabs, beam, walls, columns: Floors; Hand placed pavements ; Canal lining; Strip footings;||Low||25-75|
|Heavily reinforced sections in slabs, beams, walls columns||Medium||50-100|
|Slipform work; Pumped concrete||Medium||75-100|
|Trench fill; In-situ piling:||High||100-150|
|Tremie concrete;||Very High||In the ‘very high’ category of workability, measurement of workability by determination of flow will be appropriate. (IS 9103)|
- What is curing? How it is done and why? – Curing is the process of prevention of escaping water from the concrete. Ponding, Wet Gunny Bag, Water Sprinkling and Use of water proofing chemicals called curing compounds are methods of curing.
- What are segregation, Laitance and Bleeding?
Segregation is the separation of coarse aggregate from the concrete mix.
Laitance is the weak layer of concrete which is comprising thin layer of cement and sand particle, too much water causes bleeding and when the cement water set over the concrete laitance is formed. Also over working of concrete or improper finishing may lead to laitance.
Bleeding is the separation of cement water from the fresh concrete.
- Flakiness Index and Elongation Index.
Its index number representing how thin is the aggregate compared to its mean dimension. Its represented by the percentage by weight of particles whose least dimension (or thickness) is less than 3/5th of its mean dimension.
Elongation index is the percentage of the particle (by weight) whose largest dimension (length) exceeds the 9/5th of its mean dimension.