05: Concrete

Question 1

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.

Answer 1

Fresh Concrete

Question 2

Used to describe the ease with which concrete mixes can be compacted (completely compacted using the lowest possible w/c)

Answer 2

Workability

Question 3

Factors of Workability

Answer 3

1. Water Content: adding water
   Increase workability, Decrease STRENGTH
2. Maximum size of aggregate:
   Increase surface area to be wetted, Decrease water in medium
3. Grading of aggregates: poor grading reduces consistency
4. Shape and Texture of aggregates: smooth surface gives better workability

Question 4

Measurements of Workability (Indirect)

Answer 4

1. Slump Test
2. Compacting Factor Test
3. Flow Table Test
4. VeBe Test
5. Kelly Ball Test

Question 5

Used in detecting variations in the uniformity of a mix of given nominal proportions.

1. ____= zero slump
2. ____ = slump varies but satisfactory
3. ____ = shear or collapse slump

Answer 5

Slump Test

1. Stiff Mix
2. Rich Mix
3. Lean Mix

Question 6

Measurement of the density ratio (actual recorded density to the density of the same concrete that is fully compacted).

Answer 6

Compacting Factor Test

Question 7

Measures the diameter of the spread after vibration.

Answer 7

Flow Table Test

(Self Consolidation Concrete/Self Compacting Concrete)

Question 8

Measures the depth of penetration.

Answer 8

Kelly Ball Test

Question 9

Separation of the constituents of a heterogeneous mixture so that the distribution is no longer uniform.

Answer 9

Segregation

Question 10

TWO FORMS OF CONCRETE SEGREGATION

Answer 10

1. Coarse particles tend to separate out since they settle more than the fine particles
2. Segregation on wet mixes (separation of water + cement) from the mix

Question 11

Water from the mix tends to rise to the surface of freshly placed concrete. This can be expressed as the total settlement per unit height of concrete.

Answer 11

Bleeding (Water Gain)

Question 12

A weak, friable layer on the surface of concrete and sand cement screeds which appears after curing.

Answer 12

Laitance

Question 13

Consists essentially of the elimination of entrapped air in concrete (ramming/vibration).

Answer 13

Compaction of Concrete

Question 14

Types of Vibration

Answer 14

1. Internal Vibration
2. External Vibration
3. Vibrating Tables

Question 15

The creation of an environment for a period of time (suitable temperature, prevention of loss of moisture or providing moisture).

Answer 15

Curing of Concrete (Sample and On-Site)

Question 16

Suitability of water for mixing and curing processes should be considered, quality of water is covered by saying water must be fit for drinking.

Answer 16

Quality of Mixing Water

Question 17

A type of concrete which is designed to be pump able with relatively high slump (6-8”).

Answer 17

Pumped Concrete

Question 18

A special technique used to make concreting in water (Tremie Method).

Answer 18

Underwater Concreting

Question 19

This is the product of binding and hardening of the concrete mixture which contains ingredients quantified in the appropriate proportions:

Answer 19

Hardened Concrete:

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).

Question 20

The ____ of concrete is a measure of the concrete’s ability to resist loads which tend to compress it. It is measured by crushing cylindrical concrete specimens in compression.

Answer 20

Compression Strength (or Compressive Strength)

Question 21

This is one measure of the tensile strength of concrete. It is a measure of an unreinforced concrete beam or slab to resist failure in bending.

Answer 21

Flexural Strength

Question 22

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 in compression on its side.

Answer 22

Tensile Strength

Question 23

____ is a loading in which a part of a member attempts to slide or shear along another part. ____, which is a twisting, is also complex and difficult to evaluate. When concrete fails, a ____ causes the failure.

Answer 23

Shear, Torsion and Combined Stresses

Question 24

Besides its ability to sustain loads, concrete is also required to be ____. The ____ of concrete can be defined as its resistance to deterioration resulting from external and internal causes.

Answer 24

Durable, Durability

Question 25

The external causes include the effects of environmental and service conditions to which concrete is subjected such as weathering, chemical actions and wear. The internal causes are the effects of interaction between the constituent material such as alkali-aggregate reaction, volume changes, absorption and permeability.

Answer 25

Durability

Question 26

External Causes

Answer 26

1. Leaching out of Cement 2. Actions of sulphates, seawater and natural slightly acidic water. 3. Occurrence of extreme temperatures, abrasion and electrostatic action.

Question 27

Internal Causes

Answer 27

1. Alkali-aggregate reactions 2. Volume change due to difference in thermal properties of the aggregate and cement paste. 3. Permeability of concrete.

Question 28

____ 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 concrete (____).

Answer 28

Shrinkage, Plastic Shinkrage, Autogenous Shinkrage, Drying Shinkrage

Question 29

Types of Concrete Defects

Answer 29

1. Cracks 2. Crazing 3. Blistering 4. Delamination 5. Dusting 6. Curling 7. Efflorescence 8. Scaling and Spalling

Question 30

These are formed in concrete due to many reasons but when these are very deep, it is unsafe to use that concrete structure. Various reasons for this and improper mix design, insufficient curing, omission of expansion and contraction joints, use of high slump concrete mix, unsuitable sub-grade etc.

Answer 30

Cracks

Question 31

This is also called as pattern cracking or map cracking is the formation of closely spaced shallow cracks in an uneven manner. This occurs due to rapid hardening of top surface of concrete due to high temperatures or if the mix contains excess water content or due to insufficient curing.

Answer 31

Crazing

Question 32

This is the formation of hollow bumps of different sizes on concrete surface due to entrapped air under the finished concrete surface. It may cause due to excessive vibration of concrete mix or presence of excess entrapped air in mix or due to improper finishing.

Answer 32

Blistering

Question 33

This is also similar to blistering. In this case also, top surface of concrete gets separated from underlying concrete. Hardening of top layer of concrete before the hardening of underlying concrete will lead to this. It is because the water and air bleeding from underlying concrete are struck between these two surfaces, hence space will be formed.

Answer 33

Delamination

Question 34

____, also called as chalking, is the formation of fine and loose powdered concrete on the hardened concrete by disintegration. This happens due to the presence of excess amount of water in concrete. It causes 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.

Answer 34

Dusting

Question 35

When a concrete slab is distorted into curved shape by upward or downward movement of edges or corners, it is called ___. It occurs mainly due to the differences in moisture content or temperature between slab surface (top) and slab base (bottom).

Answer 35

Curling

Question 36

This is the formation of deposits of salts on the concrete surface. Formed salts generally white in color. It is due to the presence of soluble salts in the water which is used in making concrete mix.

Answer 36

Efflorescence

Question 37

___, 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 this may occur.

Answer 37

Scaling and spalling