lec 4 Flashcards
memorization
the concrete we describe as good, durable concrete, or quality concrete, is concrete that meets the structural and aesthetic requirements for the required life of the structure at maximum economy.
Good Durable Concrete
may be defined as a composite characteristics indicative of the ease with which the mass of plastic material may be deposited in its final place without segregation during placement, and its ability to conform to fine forming details.
Workability
is a measure of fluidity or wetness of the concrete and is measured by the slump test
Consistency
is the element of workability that indicates whether the concrete is harsh, sticky, or plastic. A good, plastic mixture is neither harsh nor sticky.
Cohesiveness and plasticity
delays in finishing, formation of a weak, non-durable surface if the concrete is finished when bleed water is present, formation of laitance, settlement of aggregate particles away from the underside of horizontal rebars or large aggregate particles, the voids thus formed become filled with water, causing a loss of bond with the steel on aggregate, and increased porosity of the concrete because of the water channels that were formed.
detrimental effects
The modulus of rupture is a measure of the flexural strength and is determined by testing a small beam, usually 6in x 6in cross section in bending. It may be loaded at center point or at third points. The ACI specifies a value of 7.5√(fc’) (psi)
flexural strength
28th day compressive strength of a 6” x 12” concrete cylinder
compressive strength
relatively low as compared to the compressive strength. Ranges from 0.10fc’ to 0.20fc’. Measured using the split cylinder test.
tensile strength
Difficult to determine experimentally than the other test because of the difficulty in isolating shear from other stresses.
Varies from 20% in normal load to a considerably higher, percentage of up to 85% of fc’ in case where direct shear exist in combination with compression
shear strength, torsion and combined strength
adopts the secant modulus of elasticity
modulus of elasticity
occurs during the first few hours after placing fresh concrete in the forms. Exposed surfaces such as floor slab are more easily affected by exposure to dry
plastic shrinkage
occurs after the concrete has already attained its final set and a good portion of the chemical hydration process in the cement gel has been accomplished.
drying shrinkage
elastic strain + creep strain + shrinkage strain
total strain
the separation of the coarse aggregate from the mortar. Results rock pocket (honey comb)
segregation
tendency of moisture to go up because of capillary action
bleeding
milk of concrete which develops at the top
laitance
during the period after concrete has been leveled off, surface water in the concrete rises to the top surface of the concrete. This movement of water is termed as bleeding.
bleeding
is a light gray or nearly white substance which appears on the top surface of a slab during or following consolidation and finishing. It consists of water, cement and fine sand or silt particles, it has no strength and is detrimental to the surface.
laitance
must have properties of concrete
workability
strength
durability
volume stability
freedom from cracks
freedom from blemishes
water tightness
economy
good appearance
factors affecting workability
- Size and gradation of aggregates
- Amount of mixing water
- Time of mixing
- Amount of cement
- The size and shape of forms
- Amount and characteristics of admixture (plasticizers)
elements of workability
consistency
cohesiveness and plasticity
concurrent property of workability
segregation
bleeding
latiance
parameters affecting concrete quality
- Quality of cement
- Proportion of cement in relation to water in the mix
- Strength and cleanliness of the aggregate
- Interaction or adhesion between cement paste and the aggregates.
- Adequate mixing of the ingredients
- Proper placing, finishing and compaction of the fresh concrete
- Curing at a temperature not below 50oF (23oC) while the placed concrete gains strength
- Chloride content not to exceed 0.15% in reinforced concrete exposed to chlorides in service and 1% for dry protected concrete.
economy
- Admixture and entrained air
- Quality assurance and control
- Large maximum aggregate size
- Efficient grading
- Minimum slump
- Minimum cement content
- Optimal automated plant operation
