Concrete

Question 1

What is a material

Answer 1

A substance from which something else can be made

Question 2

What properties is material selection based on

Answer 2

-Mechanical properties, eg avoid failure/deformation
-Chemical Properties
Constructability
Cost
Safety

Question 3

What is cement and what is it made from

Answer 3

A powder substance used for bonding together separate parts to form a single whole. Contains limestone, sand, aluminium and iron ore

Question 4

How is cement manufactured

Answer 4

• Each element is reduced in size by grinding
• Heated to 1400C to remove water allowing chemical reaction to take place, fusion into clinker
• As clinker leaves kiln it is quickly cooled
• Finished by adding gypsum which delays setting time

Question 5

Types of cement

Answer 5

• Portland-most commonly used
• Masonry-brick laying
• Rapid Hardening- particle size smaller therefore larger surface area allowing a faster reaction to take place
• Sulphate Resisting
• Low Heat Blast Furnace
• Blended Cement

Question 6

How is cement stored and why is this

Answer 6

It is stored in paper bags which include an inner lining of plastic to prevent water from reach the powder. This is because cement is hygroscopic so absorbs water, if it comes into contact with water hydration occurs

Question 7

What is concrete made from

Answer 7

A mixture of cement, water, aggregate/sand in controlled proportions

Question 8

What does heterogenous mean

Answer 8

It is a material that is non uniform. Concrete is heterogenous as it consists of randomly distributed sizes of aggregate particles bonded together by a cent paste matrix

Question 9

What does grading aggregate mean, and why is it important

Answer 9

Grading is making sure a variety of different sized particles are distributed. IF all particles were the same size then there would be large voids -> decreased strength and an uneconomical mix

Question 10

Desirable properties of aggregate

Answer 10

• Clean
• Hard
• Durable
• Capable of good adhesion
• Good shape

Question 11

What is the difference between setting and hardening concrete

Answer 11

Setting is when the concrete changes from liquid to solid and becomes rigid to withstand a certain amount of pressure but still contains moisture. During setting concrete doesn’t gain much strength. Whereas hardening is when the concrete gains strength

Question 12

What is hydration

Answer 12

The chemical reaction between water and cement that causes the cement to set and harden. An exothermic process

Question 13

What is curing

Answer 13

The process of hardening and refers to the maintenance of optimal conditions of humidity and temperature

Question 14

Desirable properties of concrete

Answer 14

• Strength
• Water-Tightness
• Abrasion Resistance
• Durability
• Resistance to cracking

Question 15

What are the 2 tests for workability of concrete

Answer 15

1. Slump Test

2. Cube Test

Question 16

What factors affect the strength of concrete

Answer 16

1. Age, max strength is reach after about 28 days
2. Water:Cement Ratio, too much water and it never hardens, too little and hydration can’t take place, optimum is about 0.4
3. Duration of Curing, strength greater if cured in moist environment compared to in air
4. Curing temperature, optimum curing temperature is 22c
5. Type of Cement,
6. Compaction

Question 17

How much does concrete initially shrink in a year and why is this a problem

Answer 17

over a year shrinkage can be 0.05%, this is due to the loss of moisture from the hydrated cement paste. The issue is it can lead to cracking in the concrete if it hasn’t been considered. If the concrete is reinforced the shrinkage will help the concrete bond to the bars