Properties of Hardened Concrete: Compressive Strength, Durability, and Testing, Lecture notes of Architecture

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“Concrete - Properties of Hardened

Concrete ”

September 2016

Introduction  Hardened Concrete Properties  Compressive Strength  Durability

Compressive Strength Test

 Compression test shows the compressive strength of hardened concrete  Testing is done in a laboratory off-site  Only work done on site is to make a concrete cylinder or cube for the compression test

Compressive Strength Test

 Concrete compressive strength depends on: -

 Quality and proportions of the ingredients

 The curing environment

Cube Making Procedures

4. When compacting by hand,

the standard compacting bar

(25mm x 25mm x 380mm

long x 1.8kg) should be

used

5. The number of strokes per

layer required shall not be

less than 35 strokes per

layer for 150mm cube

6. After the top layer has been

compacted, it shall be

levelled to the top of the

mould with a steel float

Cube Making Procedures

7. Immediately they are made,

the test specimens should

be covered with polythene

sheet to prevent evaporation

8. De-mould when stability of

cube allows

9. Place the cube in curing

tank

Durability

 Durability of concrete is its resistance to weathering,

chemical attack, abrasion, frost and fire

 Permeability should be as low as possible and is

improved by

 Full compaction  Proper curing  Low water content  High cement content

Durability

 Carbonation

 Reinforcement is protected by highly alkaline pore water in the hardened concrete  Carbon dioxide in the air neutralizes the free lime  If the reaction reaches the reinforcement, corrosion will occur  Very slow process depends on permeability  pH is lowered by : CO 2 +H 2 O+Ca(OH) 2 → CaCO 3 +H 2 O

Durability

 Frost attack

 Depends on permeability  Water is absorbed through pores in the concrete and expands on freezing, causing damage.  Surface of concrete will show signs of crack and spalling.  Improved by use of waterproofing agent or air-entraining agent to the concrete mix.

 Sulphate attack

 Tri-calcium aluminate in OPC is subject to attack by sulphates in the ground and ground water to form tri-calcium sulpho-aluminate.  The substance formed occupies a greater volume than the C 3 A.  The expansion cause cracking and spalling at the surface of the concrete.  Can be prevented by using sulphate-resisting cement in the concrete mix.

Durability

 Drying Shrinkage

 Defined as the reduction in volume of concrete caused by the chemical and physical loss of water during the hardening process.  If the shrinkage of concrete could take place without any restraint, the concrete would not crack.  It is the combination of shrinkage and restraint that causes tensile stresses to develop.  When the tensile strength of concrete is exceeded, it will crack.

Drying Shrinkage

 It depends on

 Humidity  Temperature of surrounding air  Rate of air flow over surface  Water and cement contents  curing

Lightweight Concrete

 Advantages

 Some lightweight aggregates are produced from waste materials, help to conserve materials resources  Foundation loads are reduced  Thermal insulation is improved  Better high-frequency sound absorption  Lightweight precast blocks are much easier to handle and cut than dense concrete blocks.