Concrete QB solve

2022

a)what are the effects of impurities, in the mixing water, may have on concrete?
Impurities in mixing water can negatively impact concrete in the following ways:

1. Reduced Strength and Durability: Impurities like salts, acids, or organic

materials can weaken the concrete structure, reducing its long-term strength
and durability.

2. Corrosion of Reinforcement: Chlorides and sulfates in water can accelerate

the corrosion of steel reinforcement.

3. Delayed or Accelerated Setting: Some impurities can alter the setting time of
concrete, leading to difficulties in placing, finishing, and curing.

4. Surface Defects: Impurities can cause surface issues like discoloration and
efflorescence.

5. Volume Instability: Organic matter can lead to expansion, cracking, or

shrinkage, causing structural instability.

b)

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 c)

Alkali-Aggregate Reaction (AAR)

The Alkali-Aggregate Reaction (AAR) is a chemical reaction between the alkalis
(sodium and potassium) in cement and certain types of reactive minerals in
aggregates. This reaction forms a gel that absorbs water and expands, causing
cracking and damage to concrete.

Factors Affecting AAR

1. Alkali Content in Cement: Higher alkali levels increase the chances of the
reaction.

2. Reactive Aggregates: Aggregates containing silica or carbonate are prone to

reacting with alkalis.

3. Moisture Availability: Water is needed for the reaction to take place

4. Temperature: Higher temperatures speed up the reaction process.

Controlling AAR
1. Use Low-Alkali Cement: Reduces the alkali levels available for the reaction.

2. Select Non-Reactive Aggregates: Use aggregates that are not prone to react
with alkalis.

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 3. Limit Moisture Exposure: Minimize the concrete's exposure to water to
reduce the reaction.

4. Add Pozzolanic Materials: Materials like fly ash or silica fume can reduce the
alkalis and prevent the reaction.

a)

b)
Sulfate Attack on concrete occurs when sulfate ions, typically found in soil or
water, react with the components of hardened concrete, leading to its

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 deterioration.

Process:
1. Chemical Reaction: Sulfates react with calcium hydroxide and other
compounds in the cement, forming expansive compounds like gypsum and
ettringite.

2. Expansion and Cracking: These expansive compounds cause the cracking,

and weakening of the structure.

3. Loss of Strength: As cracks develop, concrete strength reduces, making it

more vulnerable to further damage from moisture and freeze-thaw cycles.

Effects:
Surface scaling and spalling

Cracking and disintegration

Reduction in strength and durability

Prevention:
Use sulfate-resistant cement.

Ensure a low water-cement ratio.

Provide proper curing

c)

Factors influencing shrinkage in concrete:

1. Water-Cement Ratio: More water leads to more shrinkage as it evaporates.

2. Cement Content: Higher cement content increases shrinkage.

3. Aggregate Size: Larger aggregates reduce shrinkage by holding the concrete

together.

4. Curing: Poor curing causes more shrinkage due to fast drying.

5. Temperature: Hot and dry conditions make concrete shrink faster.

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 6. Mix Proportions: Mixes with more cement paste shrink more.

These factors control how much concrete shrinks as it dries.

2021

a)

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Concrete QB solve 6
 5 (A) Define the terms:
1. Water-Cement Ratio: The ratio of the amount of water to the amount of
cement in a concrete mix. A lower ratio results in stronger concrete.

2. Durability of Concrete: The ability of concrete to withstand weathering action,

chemical attack, and abrasion while maintaining its strength and integrity over
time.

3. Tensile Strength of Concrete: The ability of concrete to resist tension or

pulling forces.

4. Joint in Concrete: A break or interruption in a concrete structure, usually

provided to accommodate movement or reduce stresses. Types of joints
include expansion joints, contraction joints, and construction joints.

5 (B) Cube specimen shows much higher compressive strength

than cylinder specimen, Why?
Cube specimens tend to show higher compressive strength than cylindrical
specimens due to the shape effect. Cubes have a more uniform distribution of
stress during testing, while cylinders experience stress concentration at the ends.
Additionally, the length-to-diameter ratio of cylinders affects their performance
under compression.

5 (C) Main objectives of concrete mix design:

The main objectives of concrete mix design are:

1. Achieve the desired strength (in this case, 40 MPa).

2. Ensure durability.

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 3. Provide a workable mix that is easy to place and compact.

4. Economical use of materials, minimizing cost while maintaining quality.

6 (A) Summarize causes of bleeding of concrete and the effects

on concrete performance due to bleeding:
Causes of Bleeding:

1. High water content in the mix leads to water separation.

2. Poorly graded aggregates result in insufficient particle packing.

3. Low cement content or use of cement with low fineness.

4. Improper mixing or inadequate compaction of concrete.

5. Overuse of admixtures, which can increase water separation.

Here are the disadvantages of bleeding in concrete:

1. Weak Surface Layer:

Bleeding water rises to the surface, making it weak and reducing

durability.

2. Reduced Bond Strength:

Bleeding creates a water layer between aggregates and cement paste, so

it weakens the bond and overall strength.

3. Surface Cracking:

Evaporating bleed water can cause surface cracks and reduced strength.

4. Increased Porosity:

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 Excessive bleeding increases concrete porosity, making it less dense and
increased chemical damage.

5. Finishing Delays:

Bleeding keeps the surface too wet, delaying finishing work and affecting
schedules.

6. Segregation:

It can cause heavier particles to settle and lighter ones to rise, reducing
uniformity and strength.

6 (C) Briefly explain expansion joint and construction joint of

concrete:
1. Expansion Joint:

Purpose: Allows for expansion and contraction of concrete due to

temperature changes or other movements.

Location: Typically placed at intervals in large concrete structures to

prevent cracking.

Material: Filled with flexible materials like rubber or bitumen to absorb

movement.

2. Construction Joint:

Purpose: Used when concrete placement is done in sections or at

different times.

Location: Usually at the end of one day's work

Function: Provides a location for future connection while maintaining the

structural integrity.

4o

Concrete QB solve 9