CHAPTER 2

CONCRETE

LEARNING OUTCOME:
By the end of this session, the students SHOULD BE able to:

Explain the basic science and engineering fundamentals pertaining to classification,properties, advantages and disadvantages of concrete.

INTRODUCTION
 Concrete is most widely used as construction material.
 Obtained by mixing – cement, water + aggregates
(sometimes admixtures in required proportions)
 These mixture then placed in forms and allowed to cure and becomes hard like stone.
 The hardening is caused by chemical reaction between water and cement and its continues for a long time, and consequently the concrete grows stronger with age.

INTRODUCTION (cont.)
• The strength, durability and other characteristics of concrete depend: o Properties of its ingredients o Properties of the mix o Method of compaction o Other controls during placing, compaction and curing CLASSIFICATION OF CONCRETE
 It is usual to specify a particular concrete by the proportion (by weight) of the constituents and their characteristics.
 Example, 1:2:4 concrete refers to particular concrete mixed of cement, sand and coarse aggregate in a ratio 1:2:4 (with a specified type of cement, water cement ratio & max. size of coarse aggregate).
 Concrete can be classified either: o Prescribed mix concrete or o Designed mix concrete
-Depending on the specifying requirements of the desirable properties of concrete (strength, workability, durability and permeability. CLASSIFICATION OF CONCRETE (cont.)
Prescribed mix concrete:

o Suitable for small jobs or use initially before designed mixes have been established o Cube strength test is not required by specification but may needed for other purposes

Designed mix concrete

o Used for economy of material content o Must be sufficient similar concrete batches to set up testing regime based on statistical interpretation of results. o Cubes will normally be tested at 28 days

PROPERTIES OF CONCRETE
• Concrete has to satisfy performance requirements in the plastic state and also hardened state.
• Plastic state – concrete should be workable and free from segregation and bleeding.

Segregation
 Is the separation of coarse aggregate

Bleeding
 Is the separation of cement paste from the main mass
● Hardened state – concrete should be strong, durable and impermeable, & have min. dimensional changes.

GRADES OF CONCRETE
• Concrete generally graded according to its compressive strength at 28 days.
• Various grades of concrete are grouped into 9 categories as stipulated in codes of practice BS 8110 – best known based on their characteristics strength in N/mm2
• Table below shows the tabulation of concrete grade based on BS
8110.

GRADES OF CONCRETE (cont.)
Grade

Characteristic
Strength
(N/mm2 )

7
10

7.0
10.0

Mass concrete

15

15.0

Reinforced concrete using Light weight aggregate

20
25

20.0
25.0

Reinforced concrete using Heavy weight aggregate

30

30.0

Prestressed Post-tensioned concrete

40
50
60

40.0
50.0
60.0

Lowest grade suitable for specific purposes

Prestressed Pre-tensioned concrete

GRADES OF CONCRETE (cont.)
• Concrete of grades 7 & 10 – suitable for lean concrete bases and for mass concrete and these need not be designed.
• Concrete of grade lower than grade 15 - not suitable for reinforced concrete works.
• Concrete of grade lower than grade 30 – not to be used in the prestressed concrete works.

ADVANTAGES OF CONCRETE
• Economical – in long run as compared to other engineering materials. Except cement, it can be made from locally available coarse and fine aggregates.
• Concrete possesses a high compressive strength and minimal of corrosive and weathering effects. When properly prepared, its strength = a hard natural stone.
• The fresh concrete can be easily handled and moulded into any shape or size according to specifications. The formwork can be re-used a number of times for similar jobs resulting in economy.

ADVANTAGES OF CONCRETE (cont.)
• Strong in compression and has unlimited structural applications in combination with steel reinforcement. The concrete and steel have approximately equal coefficients of thermal expansion. The concrete is extensively used in the construction of foundations, walls, roads, airfields, buildings, water retaining structures, docks and harbours, dams, bridges, bunkers and silo etc.
• Concrete can be sprayed on and filled into fine cracks for repairs by the guniting process.
• Concrete is durable and fire resistant and requires very little maintenance.
• Concrete can be pumped and hence it can be laid in the difficult positions also. DISADVANTAGES OF CONCRETE
• Concrete has low tensile strength and hence cracks easily.
Therefore, concrete is to be reinforced with steel bars or meshes. • Fresh concrete shrinks on drying and hardened concrete expands on wetting. Provision for contraction joints has to be made to avoid the development of cracks due to drying shrinkage and moisture movement.
• Concrete under sustained loading undergoes creep resulting in the reduction of prestress in the perestressed concrete construction. DISADVANTAGES OF CONCRETE (cont.)
• Concrete expands and contracts with changes in temperature. Hence, expansion joints have to provided to avoid the formation of cracks due to thermal movement.
• Concrete is not entirely impervious to moisture and contains soluble salts – cause efflorescence.
• Concrete is liable to disintegrate by alkali and sulphate attack.
• The lack of ductility inherent in concrete as a material is disadvantageous with respect to earthquake resistant d