PROPERTIES OF CONCRETE AS A CONSTRUCTION MATERIAL Flashcards
is a dominant structural material used in construction industry. Concrete is a mixture of cement, water and aggregates. As with most rocklike mass, concrete has a very high compressive strength but have low tensile strength. As a structural, concrete can be made to carry tensile stresses. In this regard, provided reinforcing steel bars are used to give additional strength to resist tensile
stresses.
Reinforced Concrete
Concrete as a material
- Roadways or Transportation System
- Airstrips
- Infrastructures
- Water Distribution
- Underground Structures
Prestressed Concrete
is those in which cracking and tensile forces are greatly reduced or eliminated by the imposition of internal stress that are of opposite character to those that will be caused by the service or working loads.
The materials used in prestressed concrete are concrete and high strength steels also known
tendons
tendons were tensioned before the concrete was placed. After the concrete had hardened sufficiently, the tendons are cut, and the
prestress force is transmitted to concrete by bond. These methods are well suited for mass production where the tendons can run to several meters long across several beams in the casting bed,
pre-tensioning
the tendons are tensioned after the concrete is placed and has gained the required strength. The tendons are placed inside hallow ducts or tubes located in the form. When the concrete has hardened, the tendons are stretched and mechanically attached to end anchorage. In this method, the prestress force is transferred to the concrete by end bearing.
post-tensioning
Uses of Prestressed Concrete
- Long span bridges
- High buildings
- Hollow slabs
- simple span box girders
- cable bridges
- Flat slab floor
Factors Affecting Strength of Concrete
- Water-Cement Ratio
- Properties and Proportions of Concrete Constituents
- Method of Mixing and Curing
- Age of Concrete
- Loading Conditions
- Shape and Dimensions of Tested Specimen
the higher the ratio, the lower the strength. In fact, the relationship is approximately linear when
expressed in terms of C/W, the ratio of cement to water by weight.
Water-Cement Ratio
An increase in cement content and the use of well-graded aggregates increase the strength of concrete. Special admixtures are used to produce desired quality and strength of concrete.
Properties and Proportions of Concrete Constituents
The use of mechanical concrete mixers and proper time mixing have both favorable effects on strength of concrete. Also using a vibrator may lessen the percentage of air voids in concrete. A void ratio of 5% may reduce the concrete strength up to 30%. The curing time influences also the strength of concrete. The longer the period of moist storage, the greater the strength.
Method of Mixing and Curing
In practice, the strength of concrete is determined from cylinders or cubes
tested at the age of 7 days and 28 days. As practical assumptions, concrete at 28 days is 1.5 times as strong as at 7 days. The strength of concrete increases with
age and hydration of cement continues for months.
Age of Concrete
The compressive strength of concrete is estimated by testing a cylinder or
cube to failure in a few minutes. Under sustained loads, the compressive strength of concrete reduces by 30%.
Loading Conditions
The common sizes of concrete specimens used to predict the compressive strength, as discussed in earlier course, are either 6in x 12in (150mm x 300mm) or 4in x 8in (100mm x 200mm) cylinders. The greater the ratio of specimen height
to diameter, the lower the strength indicated by compression strength.
Shape and Dimensions of Tested Specimen
- weighs 2400 kg/m^3 (145 lbs/ft^3)(3915 lbs/yard^3)
- development of strength starts at 7 days
- common strength values is 10 MPa (1450 psi) to 40 MPa (5800 psi)
- about 28 days 75-80% of the total strength is attained
- not durable against severe condition e.g freezing and thawing
Normal-Weight Concrete
- weighs 1800 kg/m^3
- primarily used to reduce dead load or for thermal insulation, mailability, or fill
- higher cost, need for mare care in placing, greater porosity, and more drying shrinkage
Light-Weight Concrete
- weighs 3200 kg/m^3
- concrete of 60.5 kN/m^3 can be produced using heavier than ordinary aggregates
- upper limit can be achieved with steel shot as fine aggregates and steel punching as course aggregates
- used principally in radiation shields and counterweight
Heavyweight Concrete
- criterion of quality concrete
- specimens used to determine compressive strength may be cylindrical, cubical, or prismatic
- specimens are cured and then tested at the age of 28 days by the use of static load until rapture occurs
Compressive Strength
FAILURE OF CONCRETE UNDER THREE MODES
- under axial compression
- splitting fracture
- combination of shear splitting
specimen may fail in shear and resistance due to both cohesion and internal friction
under axial compression
result in separation into columnar pieces
splitting fracture
failure occurs
combination of shear splitting
- compressive strength of concrete shall not be less than 17 MPa
- determined by testing to failure of 28 day old 150x300mm concrete cylinder
- max. stress at a strain of 0.002
- rapture occurs at a strain of 0.003
- 21-42 MPa compressive strength of concrete may be adopted
- high strength concrete with strength greater than 42 MPa is an important material for design concrete structure
Stress-Strain Curves of Concrete
- concrete is a brittle material
- low tensile capacity can be due to the high concentrations of stress in the concrete under load
- direct tension test, due to slight misalignment and stress concentration in the grasping instruments, are not accurate in predicting the tensile strength of concrete
Tensile Strength
concrete cylinders is place in axis horizontal in a compression system machine
splitting test
ratio or normal stress to corresponding strain for tensile and compressive stresses below limit of material
Modulus of Elasticity, Ec
when the load is removed the material returns to its original length
elastic limit
loaded beyond the elastic limit, thus permanent deformation
plastic strain
the decrease in volume of concrete during hardening and drying under constant temperature
Shrinkage
concrete expands with increasing temperature and contracts
Expansion Due to Rise in Temperature
- is a slow plastic deformation under constant stress
- concrete is elastoplastic material
- under sustained loads plastic deformation continues to develop over a period of time
- high rate of deformation during the first 4 months after application of load
creep
the ability of the section to deform beyond its yield point without significant stress loss
Ductility
- reinforcing steel bar is placed in concrete element, mainly tension zone to resist tensile forces
- also used to increase the member’s compression resistance
Steel Reinforcement
Types of Steel Reinforcement
Round bars
Welded fabrics and mats
Pre-stressed Concrete wires and strands
widely used for reinforced concrete available in 6mm to 3mm - either plain or deformed bars
Round bars
consist of series of longitudinal and traverse cold-drawn steel wires - at right angle and welded together at all points of intersection
Welded Fabrics and Mats
used for high strength steel - high tensile steel wires of diameter 5mm-7mm are used
Pre-stressed Concrete Wires and Strands
the most important factor affecting the mechanical properties is and stress-strain curve of the steel is the
chemical composition
