Lesson 5 - Concrete Flashcards
The strength of concrete is affected by the degree of its compaction. It is vital, therefore, that the consistency (ability to flow) of the mix be such that, the concrete can be transported, placed, and finished sufficiently, easily and without segregation
Fresh Concrete
Used to describe the ease with which concrete
mixes can be compacted (completely compacted using the lowest possible w/c)
WORKABILITY
Factors of Workability
- Water Content
- Maximum size of aggregate
- Grading of aggregates
- Shape and Texture of aggregates
Water Content
adding water
↑ workability,
↓ STRENGTH
Maximum size of aggregate
↓ surface area to be wetted, ↑ water in medium
Grading of aggregates
poor grading reduces consistency
Shape and Texture of aggregates
smooth Surface gives better workability
Measurements of Workability (Indirect)
- Slump Test
- Compacting Factor Test
- Flow Table Test
- VeBe Test
- Kelly Ball Test
Used in detecting variations in the uniformity of a mix of given nominal proportions
SLUMP TEST
SLUMP TEST - Stiff Mix
zero slump
SLUMP TEST - Rich Mix
slump varies but is satisfactory
SLUMP TEST - Lean Mix
shear or collapse slump
Measurement of the density ratio
(actual recorded density compared to the density of the same concrete that is fully compacted)
Compacting Factor Test
Measures the diameter of the spread after
vibration (Self Consolidation Concrete/Self
Compacting Concrete)
Flow Table Test
Measures the depth of penetration
Kelly Ball Test
Separation of the constituents of a heterogeneous mixture so that the distribution is no longer uniform.
Segregation
TWO FORMS OF CONCRETE SEGREGATION
- Coarse particles tend to separate out since they settle more than the fine particles
- Segregation on wet mixes (separation of water + cement) from the mix
Water from the mix tends to rise to the surface of freshly placed concrete. It can be expressed as the total settlement per unit height of concrete.
Bleeding (Water Gain)
a weak, friable layer on the surface of concrete
and sand cement screeds which appears after
curing.
Laitance
Consists essentially of the elimination of
entrapped air in concrete (ramming/vibration)
Compaction of Concrete
Types of Vibration
- Internal Vibration
- External Vibration
- Vibrating Tables
The creation of an environment for a period of
time (suitable temperature, prevention of loss of
moisture or providing moisture)
Curing of Concrete (Sample and Onsite)
Suitability of water for mixing and curing
processes should be considered, the quality of water is covered by saying water must be fit for drinking.
Quality of Mixing Water
A type of concrete which is designed to be pump able with a relatively high slump (6-8”)
Pumped Concrete
A special technique used to make concreting in
water (Tremie Method)
Underwater Concreting
A product of the binding and hardening of the concrete mixture, which contains ingredients quantified in the appropriate proportions.
HARDENED CONCRETE
ingredients quantified in the appropriate proportions for hardened concretes
binder (cement)
coarse aggregate (gravel)
fine aggregate (sand, fraction <2mm)
water and optional additives (> 5% by weight of binder)
admixtures (<5% by weight of the binder).
a measure of the concrete’s ability to resist loads that tend to compress it. It is measured by crushing cylindrical concrete specimens in compression
Compressive Strength
measure of the tensile strength of concrete. It is a measure of the ability of an unreinforced concrete beam or slab to resist failure in bending
Flexural Strength
There is no field test for direct determination of
tension under axial loading. An indirect method
called the splitting tensile test, in which a standard test cylinder is loaded with compression on its side.
Tensile Strength
____ is a loading in which a part of a member attempts to slide or ___ along another part. ____, which is a twisting, is also complex and difficult to evaluate. When concrete fails, a __________ causes the failure.
Shear, Torsion and Combined
Stresses
is a loading in which a part of a member attempts to slide or move along another part.
Shear
which is a twisting, is also complex and difficult to evaluate.
Torsion
Besides its ability to sustain loads, concrete is also required to be resistant.
Durability
It can be defined as its resistance to deterioration resulting from external and internal causes.
Durability
It includes the effects of environmental and service conditions to which concrete is subjected such as weathering, chemical
actions and wear.
external causes
are the effects of interaction between the constituent material such as alkali-aggregate reaction, volume changes, absorption and
permeability.
internal causes
External Causes
- Leaching out of Cement
- Actions of sulphates, seawater and natural slightly
acidic water. - Occurrence of extreme temperatures, abrasion and
electrostatic action.
Internal Causes
- Alkali-aggregate reactions
- Volume change due to difference in thermal
properties of the aggregate and cement paste. - Permeability of concrete.
is caused by the setting of solids and the loss of free water from the plastic concrete, by the chemical combination of cement with water, and by the drying of the concrete.
Shrinkage;
- plastic
- autogenous
- drying
Types of Concrete Defects
- CRACKS
- CRAZING
- BLISTERING
- DELAMINATION
- DUSTING
- CURLING
- EFFLORESCENCE
- SCALING AND SPALLING
formed in concrete due to many reasons, but when these ruptures are very deep, it is unsafe to use that concrete structure.
Cracks
Various reasons for fissures and improper mix design, insufficient curing. omission of expansion and contraction joints, use of high slump concrete mix, unsuitable sub-grade etc.
Cracks
also called as pattern cracking or map cracking, is the formation of closely spaced shallow cracks
in an uneven manner.
CRAZING
occurs due to rapid hardening of the top surface of concrete due to high temperatures if the mix contains excess water content or due to insufficient curing.
CRAZING
is the formation of hollow bumps of different sizes on the concrete surface due to entrapped air under the finished concrete surface.
BLISTERING
It may be caused by excessive vibration of the concrete mix, or presence of excess entrapped air in a mix, or improper finishing.
BLISTERING
is also similar to blistering. In this
case also, the top surface of the concrete gets separated from the underlying concrete.
DELAMINATION
Hardening the top layer of concrete
before the hardening of the underlying concrete will lead to this defect.
DELAMINATION
It is because the water and air bleeding
from underlying concrete are struck between these two surfaces, hence space will be formed.
DELAMINATION
also called as chalking, is the formation of fine and loose powdered concrete on the hardened
concrete by disintegration.
DUSTING
This happens due to the presence of excess amount of water in concrete.
DUSTING
It causes the bleeding of water from concrete, with this, fine particles like cement or sand will rise to the top, and consequent wear causes dust at the top surface.
DUSTING
When a concrete slab is distorted into a curved shape by upward or downward movement of edges or corners,
CURLING
It occurs mainly due to the differences in
moisture content or temperature between slab surface (top) and slab base (bottom).
CURLING
is the formation of deposits of salts on
the concrete surface. Formed salts are generally white in color. It is due to the presence of soluble salts in the water which is used in making concrete mix.
EFFLORESCENCE
in both the cases, concrete surface gets deteriorated and flaking of concrete occurs. The main cause for this type of cases is penetration of water through concrete surface. This makes steel gets corroded and it may
occur.
SCALING AND SPALLING
