Norzainariah Binti Abu Hassan
Civil Engineering Department, Politeknik Melaka
Water- Cement
ratio
Amount and type of
Aggregate
Amount and type of
Cement
Weather conditions
Chemical Admixtures
Sand to Aggregate
ratio
1. Water-Cement ratio
• High w/c ratio increase in workability;
increase void in concrete results in segregation and bleeding and also shrinkage problem;
cement slurry will escape through joints of formwork.
• For proper hydration w/c should be about 0.35.
• Typical w/c ratio in practice 0.55 - 0.65.
• When water increased without increase in cement, the void content increases and the concrete strength drops.
• Low water to cement ratio leads to high strength but low workability.
• High water to cement ratio leads to low strength, but good workability.
2. Amount and type of Aggregate
• More the amount of aggregate less will be workability.
• Using smooth and round aggregate increases the workability.
• Workability reduces if angular and rough aggregate is used.
• Greater size of aggregate - less water is required to
lubricate it, the extra water is available for workability.
3. Amount and type of Cement
More ratio, less workability.
Since less cement mean less water, so the paste is stiff.
4. Weather conditions
Temperature - If temperature is high, evaporation increases, thus workability decreases.
Wind - If wind is moving with greater velocity, the rate of evaporation also increase reduces the amount of water and ultimately reducing workability.
5. Chemical Admixtures
Use of air entraining agent produces air bubbles which acts as a sort of ball
bearing between particles and increases mobility, workability and decreases
bleeding, segregation.
The use of fine pozzolanic materials also have better lubricating effect and more workability.
6. Sand to Aggregate ratio
If the amount of sand is more the
workability will reduce because sand
has more surface area and more contact area causing more resistance.
Slump Test
Compacting Factor Test
Cube Test
Measurement Of Workability
• Develop by Chapman in U.S. in 1913 and is very popular.
• The test is suitable for detecting changes in workability, e.g. increase in water
content or deficiency in the proportion of fine aggregate results increase in slump.
• Test is not suitable for very dry or wet mixes. Very dry mixes – zero or near zero slump and wet mixes –
completely collapse of the concrete produces undesirable values of slump.
APPARATUS
PROCEDURES
3 types of slump usua lly ob ser ved;
• True slump
• Usually observed with cohesive and rich mixes for which slump is generally sensitive to variation in workability.
• Shear slump
• Occurs after in leaser mixer and indicates lack of cohesion and generally associated with harsh mixes (loud mortar content).
• Collapse slump
• Usually associated with very wet mixes, generally indicative of poor quality concrete and frequently results from segregation of its constituent materials.
True slump
Shear slump
Collapse slump
• Developed in UK by Glenville et. al (1947) measures the degree of compaction for a standard amount of work.
• The test requires the weight of a partially and fully compacted concrete & the ratio of partially compacted of weight to the fully compacted weight (always <1) and is known as compacting factor.
• Normal concrete compactor lies between 0.80 to 0.92. The test is useful for drier mixes for which the slump test is not satisfactory.
• Not popular because some basic assumption is not correct and the procedure is not practical to be employed at site.
Introduction..
Why Does Concrete Harden??
It is the water in the concrete essential to the hardening process. The water, mixed with
cement, shrinks as it dries.
This causes the concrete to harden. The process through
which this happens is called
hydration.
• Strength of concrete
• Compressive strength
• Tensile strength
• Concrete Creep
• Durability
• Shrinkage
• Permeability and Absorption
• Volume Changes
• Chemical Attack
Following are the criteria of hardened
concrete:
1. Strength of concrete
Defined as the maximum load (stress) it can carry.
Compressive strength – depends on types of cement, mix proportions, methods of compaction and curing conditions.
Tensile strength – due to drying and temperature variation.
Water-Cement ratio:
• Lesser the water cement ratio, greater will be strength.
Type of cement:
• Type of cement affect the hydration process and therefore strength of concrete.
Amount of cementing material:
• It is the paste that holds or binds all the ingredients.
Thus greater amount of cementing material greater will be strength.
Type of Aggregate:
• Rough and angular aggregates is preferable as they provide greater bonding.
Admixtures:
• Chemical admixtures like plasticizers reduce the water cement ratio and increase the strength of concrete at same water cement ratio. Mineral admixtures affect the strength at later stage and increase the strength by increasing the amount of cementing material.
Compressive strength
• Most important property of hardened concrete.
• For ordinary construction
compressive strength in a range of 20 - 40 MPa.
• Low range (cast in-situ), High range (precast).
• Estimation of strength is by testing concrete prepared in standard cube 100 or 150mm (Cube Test) or by
Cylinder Test.
• Actual strength of concrete will not be the same as the capacity
measured from the test specimen.
• Concrete should be sample at 77- 115m3 for 7 and 28-day tests.
Tensile strength
• Provide mode of rupture of concrete.
• Beam Size
• 150 x 150 x 750mm, and 100 x 100 x 500mm
• If fracture occurs within the middle one-third of the beam
• fbl = Pl/bd3
• Provide indirect tensile strength of concrete
• fst = 2P/Ld
2. Concrete creep
• Concrete creep is defined as:
Deformation of structure under sustained load. Basically, long term pressure or stress on concrete can make it change in shape.
This deformation usually occurs in the direction the force is being applied. Like a concrete column getting more compressed, or a beam bending.
• Creep does not necessarily cause
concrete to fail or break apart. Creep is factored in when concrete
structures are designed.
Factors affecting creeping
Influence of Aggregate
• The aggregate influences the creep of concrete through a restraining effect on the magnitude of creep. The paste which is creeping under load is restrained by aggregate which do not creep. The stronger the aggregate the more is the restraining effect and hence the less is the magnitude of creep. The modulus of elasticity of aggregate is one of the important factors influencing creep.
• It can be easily imagined that the higher the modulus of elasticity the less is the creep. Light weight aggregate shows substantially higher creep than normal weight aggregate.
• Hence, the stiffer the aggregate, the lower the creep.
• The higher the volume of aggregate, the lower the creep.
Influence of Mix Proportions
• Referring to the amount of paste content and its quality.
• A poorer paste structure undergoes higher creep.
• Creep increases with increase in water/cement ratio, where creep is inversely proportional to the strength of concrete.
Influence of Age
• Age at which a concrete member is loaded will have a predominant effect on the magnitude of creep.
• Quality of gel improves with time, where older gel creeps less.
• Young gel under load being not so stronger creeps more.
• Nevertheless, it should be noted that the moisture content of the concrete being different at different age can also influences the magnitude of creep.
Effects of creep
• In reinforced concrete beams, creep increases the deflection with time and may be a critical consideration in design.
• In mass concrete structures such as dams, on account of differential
temperature conditions at the interior and surface, creep is harmful and by itself may be a cause of cracking in the interior of dams. Therefore, all
precautions and steps must be taken to see that increase in temperature does not take place in the interior of mass concrete structure.
• Loss of prestress due to creep of concrete in prestressed concrete structure.
3. Durability
• Durability of concrete may be defined as the ability of concrete to resist weathering action, chemical attack, and abrasion while maintaining its desired engineering properties.
• Normally refers to the duration or life span of trouble free performance.
• Concrete ingredients, their proportioning, interactions between them, placing and curing practices, and the service environment determine the ultimate durability and life of concrete.
• For example, concrete exposed to tidal seawater will have different requirements than an indoor concrete floor.
• A durable material helps the environment by conserving resources and reducing wastes and the environmental impacts of repair and replacement.
• Durability is also the ability of concrete to:
1. Resist freezing and thawing
2. Resist chemical - Resistance to Sulfate Attack, Seawater Exposure, Chloride Resistance and Steel Corrosion, Resistance to Alkali-Silica Reaction (ASR), Abrasion Resistance
1. Resistance to Freezing and Thawing
➢Water gains 9% in volume upon freezing
➢Nighttime freezing followed by daytime thawing.
➢Approximately 40 cycles/year,
in average and max of 200 cycles/year.
➢Air Entraining Agent or air-entrained concrete will helps the concrete to
withstand a great number of cycles of freezing and thawing without distress.
2. Resist Chemicals
➢ Concrete in chemical manufacturing and storage facilities is specially prone to chemical attack.
a) Resistance to Sulfate Attack: Severe at locations where the concrete is exposed to wetting and drying cycles.
b) Seawater Exposure: Most vulnerable in the tidal or splash zone where there are repeated cycles of wetting and drying and/or freezing and thawing.
Chemical resistance Abrasion resistance
Corrosion resistance
4. Shrinkage (Pengecutan)
• One of the important factors that contribute to the cracks in floors and pavements is that due to shrinkage.
• The term shrinkage is described as the various aspects of volume changes in concrete due to loss of moisture at different stages due to different reasons.
• Three types of shrinkage:
1. Plastic shrinkage 2. Drying shrinkage
3. Carbonation shrinkage
Factors affecting shrinkage
1. Drying condition or in other words, the relative humidity of the atmosphere at which the concrete specimen is kept - magnitude of shrinkage increases with time and also with the reduction of relative humidity.
2. The rate of shrinkage decreases rapidly with time. It is observed that 14 to 34 per cent of the 20 year shrinkage occurs in 2 weeks, 40 to 80 per cent of the 20 year shrinkage occurs in 3 months and 66 to 85 per cent of the 20 year shrinkage occurs in one year.
3. Water/cement ratio of the concrete also affects the rate of concrete shrinkage. The richness of the concrete also has a significant influence on shrinkage.
4. Harder aggregate with higher modulus of elasticity like quartz shrinks much less than softer aggregates such as sandstone.
5. Permeability and absorption
• Permeability (Kebolehtelapan) refers to the ease with which water can pass through the concrete.
• Absorption (Penyerapan) is the ability of concrete to draw water into its voids.
• Low permeability is a very important requirement for hydraulic structures and usually associated with more stronger and durable concrete.
6. Volume changes
• Due to subsequent drying of concrete, variations in temperature and alternate wetting and drying.
• When changes in volume is resisted by internal and external force, this can produce cracking.
• Cracks in concrete reduce its resistance to the action of leaching, corrosion of reinforcement, attack by sulphates etc.
7. Chemical attacks
• 2 forms of most common chemical attacks on concrete:
- leaching – it lowers concrete durability
- sulphate attack – contributes to breakdown of structures.
• How to improves?
= increase impermeability
