45 Questions
What is the primary use of concrete blocks in construction?
Wall construction
What is another name for a concrete block?
Concrete masonry unit
What does the term 'precast' refer to in concrete blocks?
The blocks are formed and hardened before being brought to the job site
What is the typical feature of most concrete blocks?
They have one or more hollow cavities
Who designed the first hollow concrete block?
Harmon S. Palmer
In what year did Palmer patent his concrete block design?
1900
What is the typical color of concrete blocks?
Light gray
What determines the classification of concrete masonry units?
Their density
What is the main ingredient in the concrete mixture used to make concrete blocks?
Portland cement
What type of materials are used to manufacture normal-weight units?
Well-graded sand, gravel, and crushed stone
What is the purpose of the weigh batcher in the manufacturing process?
To measure the proper amounts of each material
How long are the dry materials blended together in the stationary mixer?
Several minutes
What is added to the mixer after the dry materials are blended?
Admixture chemicals and coloring pigments
What is the purpose of the rotating brush in some operations?
To remove loose material from the top of the blocks
How are the pallets of blocks arranged in the curing kiln?
On a set of rails
What type of curing kiln is most commonly used?
Low-pressure steam kiln
How long are the blocks held in the low-pressure steam kiln?
One to three hours
What is an advantage of lightweight units compared to normal-weight units?
Higher thermal and fire resistance
At what rate is steam introduced to raise the temperature of concrete blocks?
60°F per hour (16°C per hour)
What is the purpose of introducing steam to raise the temperature of concrete blocks?
To control the rate of temperature increase
What are the three basic forms of concrete block?
Solid, cellular, and hollow
What is the difference between the nominal dimension and the specified dimension of a concrete block?
The thickness of the mortar joint
What is the typical range for the dry density of concrete blocks?
500-2100 kg/m3
What is the main factor that affects the compressive strength of concrete blocks?
Mix composition
What is the classification of concrete masonry units based on compressive strength?
Load-bearing and non-load-bearing
What is the relationship between the strength and density of concrete blocks?
The strength increases with an increase in density
What is the purpose of specifying the nominal dimension of a concrete block?
To allow for the thickness of the mortar joint
What is the direction of the load applied to determine the compressive strength of individual concrete masonry units?
In the direction of the height of the unit
What is the purpose of controlling water absorption in concrete masonry units?
To reduce the effect of weathering and limit moisture loss after construction
How is the absorption of concrete masonry units determined?
By immersing the unit in water for 24 hours
What is the main factor that influences the magnitude of dimensional changes in concrete blocks?
The constituent materials and mix proportions
What is the most important type of shrinkage in normal applications of concrete blocks?
Drying shrinkage
What is the recommended strength of concrete blocks for use in extreme conditions of pollution and weather?
In excess of 7 Mpa
Why are open-texture blocks more susceptible to frost attack?
Because water can move freely within the block on freezing
What is the typical application of concrete blocks with adequate durability?
Most normal applications
What is the standard used to determine the strength requirements of load-bearing and non-load-bearing concrete masonry units?
ASTM C90
What is the primary cause of efflorescence in concrete blocks?
Reaction between atmospheric carbon dioxide and calcium hydroxides
What is the main factor that controls the fire-endurance of concrete blocks?
Numerous factors
What is the typical thickness of concrete blocks that can provide an adequate resistance to fire for up to 2 hours?
100 mm
What is the relationship between the thermal conductivity of a concrete block and its block density?
Thermal conductivity increases as block density increases
What is the effect of increasing moisture content on the thermal conductivity of a concrete block?
It increases the thermal conductivity
What type of concrete blocks have relatively low thermal conductivities?
Autoclaved aerated concrete blocks
What is the effect of increasing the block density of a concrete block on its thermal conductivity?
It increases the thermal conductivity
What is the typical duration of fire resistance for non-load-bearing concrete blocks of 100 mm thickness?
4 hours
What is the primary component of efflorescence in concrete blocks?
Sodium carbonates
Study Notes
Blocks
- A concrete block is a building material used in wall construction, also known as a concrete masonry unit.
- The first hollow concrete block was designed by Harmon S. Palmer in the United States in 1890.
- By 1905, around 1,500 companies were manufacturing concrete blocks in the United States.
Raw Materials
- Concrete blocks are made from a mixture of powdered portland cement, water, sand, and gravel.
- The mixture produces a light gray block with a fine surface texture and high compressive strength.
- Concrete masonry units are manufactured in three classes: lightweight, medium-weight, and normal-weight units, based on their density.
Manufacturing Process
- The process involves mixing, molding, and curing.
- Mixing involves blending dry materials (sand, gravel, and cement) with water and admixture chemicals.
- Molding involves compacting the mixed concrete into blocks, which are then pushed out of molds onto a flat steel pallet.
- Curing involves conveying the blocks to an automated stacker or loader, which places them in a curing rack.
Form and Size
- There are three basic forms of concrete blocks: solid, cellular, and hollow.
- Concrete masonry units are specified by their nominal dimensions, which are greater than their actual dimensions by the thickness of the mortar joint.
- Different types of concrete blocks include solid, cellular, hollow, common, facing, normal, insulating, lintel, quoin, cavity closure, and multipurpose blocks.
Properties
- The properties of concrete blocks depend on the type and proportions of constituent materials, manufacturing process, mode and duration of curing, and form and size of the block.
- Density ranges from 500-2100 kg/m3, depending on aggregate density, size, and grading, degree of compaction, and block form.
- Compressive strength is the basic requirement of concrete blocks, and is dependent on mix composition, degree of compaction, and aggregate type.
- Water absorption is controlled by ASTM standards to reduce weathering effects and limit shrinkage due to moisture loss.
- Dimensional changes occur due to variations in ambient moisture and temperature conditions, influenced by constituent materials, mix proportions, and manufacturing process.
- Durability is generally adequate for most normal applications, but may be affected by extreme conditions of pollution and weather.
- Efflorescence is rarely a problem with concrete blocks, but may occur as a result of a reaction between free hydroxides and atmospheric carbon dioxide.
- Fire resistance is good, with most concrete blocks providing adequate resistance to fire for up to 2 hours if load-bearing or up to 4 hours if non-load-bearing.
- Thermal conductivity is largely dependent on block density, with autoclaved aerated concrete and lightweight concrete blocks having relatively low thermal conductivities.
This quiz is for 1st year Civil Engineering students at the University of Halabja, covering topics related to building materials and testing.
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