CE 416 – PRINCIPLES OF PRESTRESSED/REINFORCED CONCRETE
INTRODUCTION
- Concrete structural construction components are categorized as either,
I. Reinforced Concrete (RC)
II. Prestressed Concrete (PSC)
- The structural design of (I) and (II) are covered in the provisions in,
Chapter 4 (Structural Concrete) of
the
NSCP
2015,
Volume
1,
“Buildings,
Towers
and
other
Vertical Structures”, 7th Edition.
CE 416 – PRINCIPLES OF PRESTRESSED/REINFORCED CONCRETE
I. Reinforced Concrete (RC) is made up of two (2) materials:
1. Concrete which is strong in compression but very weak in tension,
and
2. Steel which is strong in both tension and compression.
- Concrete is composed of three materials,
1. Cement
2. Fine aggregates
3. Coarse aggregates
- Water is added to the mixture (cement, fine and coarse aggregates) and thoroughly mixed to
create a CONCRETE.
CE 416 – PRINCIPLES OF PRESTRESSED/REINFORCED CONCRETE
Cement
- Many types of cements are produced for different purposes. Some of them are,
1. Rapid Hardening Cement
- Attains high strength in 7 days, 3 days or even 1 day.
2. Quick Setting Concrete
- Used primarily in situations where the work needs to be completed in a very short period
of time (e.g. concreting under water).
3. Low Heat Cement
- Used for massive construction.
4. Blast Furnace Slag Cement
- Used as a cheaper alternative.
5. Pozzolanic Cement
- Ideal for concreting of marine structures and other concreting works under water.
CE 416 – PRINCIPLES OF PRESTRESSED/REINFORCED CONCRETE
Mixing Water
- Water is an important component for mortar or concrete.
- When the water is mixed in mortar, it reacts with cement and forms a binding paste which fills
small voids in the sand. This creates a close cohesion of sand particles and cement.
- In case of cement concrete, the voids formed between sand and coarse aggregate gets filled
with the paste forming a cohesive substance/concrete.
- The quantity and quality of water have vital effect on the strength of the concrete mix.
- Only clean water can be used in the concrete mix since impurities in dirty water generally has
an adverse effect on the concrete strength.
CE 416 – PRINCIPLES OF PRESTRESSED/REINFORCED CONCRETE
Aggregates
- For a good concrete mix, aggregates need to be clean, hard, strong particles free of
absorbed chemicals or coatings of clay and other fine materials that could cause the
deterioration of concrete.
- Aggregates, which account for 60 to 75 percent of the total volume of concrete, are divided
into two (2) distinct categories, fine and coarse aggregates.
- Fine aggregate is defined as material that will pass a No. 4 sieve and will, for the most part, be
retained on a No. 200 sieve. For increased workability and for economy as reflected by use of
less cement, the fine aggregate should have a rounded shape.
- The purpose of the fine aggregate is to fill the voids in the coarse aggregate and to act as a
workability agent.
- Coarse aggregate is a material that will pass the 3-inch screen and will be retained on the No.
4 sieve. As with fine aggregate, for increased workability and economy as reflected by the use
of less cement, the coarse aggregate should have a rounded shape.
CE 416 – PRINCIPLES OF PRESTRESSED/REINFORCED CONCRETE
2 Characteristics of Aggregates
1. Grading
- Particle size and distribution
2. Nature of particles
- Shape, porosity, surface texture.
Concrete Additives
- Admixtures provide enhanced concrete quality, improved durability, and deliver cost-benefits
to both the producer and the user.
- The sustainability and environmental profile of concrete is also enhanced by admixture use.
CE 416 – PRINCIPLES OF PRESTRESSED/REINFORCED CONCRETE
Commonly Used Concrete Admixtures:
1. Set Retarding
- These are used to delay the chemical reaction that takes place when the concrete starts
the setting process.
- This is commonly used to reduce the effect of high temperatures that could produce a
faster initial setting of concrete.
- This is a requisite for transporting ready-mixed concrete (RMC) to delay the setting while
the mix is in transit.
2. Air-Entrainment
- Air entrained concrete can increase the freeze-thaw durability of concrete.
- This type of admixture produces a more workable concrete than non-entrained concrete
while reducing bleeding and segregation of fresh concrete.
CE 416 – PRINCIPLES OF PRESTRESSED/REINFORCED CONCRETE
Commonly Used Concrete Admixtures:
3. Water Reducing
- Water reducing admixtures are chemical products that when added to concrete can
create a desired slump at a lower water-cement ratio than what is normally designed.
- With this type of admixture, concrete properties are improved and help place concrete
under difficult conditions.
- Water reducers have been used primarily in bridge decks, low slump concrete overlays,
and patching concrete.
4. Accelerating
- Accelerator concrete admixtures are used to increase the rate of concrete strength
development, or to reduce concrete setting time.
- Calcium chloride is the most common accelerator component, however, it could
promote corrosion activity of steel reinforcement.
CE 416 – PRINCIPLES OF PRESTRESSED/REINFORCED CONCRETE
Commonly Used Concrete Admixtures:
5. Shrinkage Reducing
- Shrinkage reducing concrete admixture is added to concrete during initial mixing. This
type of admixture could reduce early and long-term drying shrinkage.
- Shrinkage reducing concrete admixture can be used in situations where shrinkage
cracking could lead to durability problems or where large numbers of shrinkage joints are
undesirable for economic or technical reasons.
6. Super Plasticizers
- The main purpose of using superplasticizer is to produce flowing concrete with very high
slump in the range of 7-9 inches to be used in heavily reinforced structures and in
placements where adequate consolidation by vibration cannot be readily achieved.
- The other major application is the production of high strength concrete.
CE 416 – PRINCIPLES OF PRESTRESSED/REINFORCED CONCRETE
Commonly Used Concrete Admixtures:
7. Corrosion Inhibiting
- Corrosion inhibiting admixtures fall into the specialty admixture category and are used to
slow corrosion of reinforcing steel in concrete.
- Corrosion inhibitors can significantly reduce maintenance costs of reinforced concrete
structures throughout a typical service life of 30 to 40 years.
CE 416 – PRINCIPLES OF PRESTRESSED/REINFORCED CONCRETE
Structural Properties of Concrete:
- Concrete has relatively high compressive strength but significantly low tensile strength,
and as such, is usually reinforced with materials that are strong in tension.
- The elasticity of concrete is relatively constant at low stress levels but starts decreasing at
higher stress levels as cracking develops.
- Concrete has a very low coefficient of thermal expansion and as it matures, concrete
shrinks.
- All concrete structures will crack to some extent, due to shrinkage and tension, and
concrete which is subjected to long-duration compressive forces is prone to creep.
- The unit weight of reinforced concrete used for load calculations can be conservatively
taken as 24 kN per cubic meter.
- For normal weight concrete, the modulus of elasticity is,
𝐸𝑐 = 4,700 𝑓𝑐′
𝑓𝑐′ = 𝑐𝑜𝑛𝑐𝑟𝑒𝑡𝑒 𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑖𝑣𝑒 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ (𝑀𝑃𝑎)
CE 416 – PRINCIPLES OF PRESTRESSED/REINFORCED CONCRETE
Important Properties of Concrete:
1. It possesses a high compressive strength
2. It is a corrosion resistant material and atmospheric agents have no appreciable effect on
it.
3. It hardens with age and the process of hardening continues for a long time after the
concrete has attained sufficient strength.
4. It is more economical than the steel.
5. It binds rapidly with steel and as it is weak in tension, the steel reinforcement is placed in
cement concrete at suitable places to take up the tensile stresses. This is termed as
“Reinforced Concrete or simply RC”.
6. Under the following two (2) conditions, it has a tendency to shrink; (a) There is initial
shrinkage of cement concrete which is mainly due to the loss of water through forms,
absorption by surfaces of forms, etc., (b) The shrinkage of cement concrete occurs as it
hardens, this tendency of cement concrete can be minimized by proper curing of concrete.
CE 416 – PRINCIPLES OF PRESTRESSED/REINFORCED CONCRETE
Important Properties of Concrete:
7. It has a tendency to be porous. This is due to the presence of voids which are formed
during and after its placement.
8. It forms a hard surface, capable of resisting abrasion.
9. It should be remembered that apart from other materials, the concrete comes to the site in
the form of raw materials only. Its final strength and quality depend entirely on local conditions
and persons handling it.
CE 416 – PRINCIPLES OF PRESTRESSED/REINFORCED CONCRETE
Important Properties of Concrete:
7. It has a tendency to be porous. This is due to the presence of voids which are formed
during and after its placement.
8. It forms a hard surface, capable of resisting abrasion.
9. It should be remembered that apart from other materials, the concrete comes to the site in
the form of raw materials only. Its final strength and quality depend entirely on local conditions
and persons handling it.
END