44 Questions
What is the standard size of the cube specimen according to BS 1881, Part 108: 1993?
150 × 150 × 150 mm
How many times should each layer be stroked by a hemispherical-tipped steel rod when preparing a cube specimen?
35 times
What is the required temperature for curing a cube specimen?
20 ± 1°C
What is the standard size of the cylinder specimen according to ASTM C470-81?
150 × 300 mm
What is the length/diameter ratio of the cylinder specimen?
2
What is the primary reason for grinding or capping the upper surface of the cylinder specimen?
To level and smooth the compression surface
How many categories can the factors affecting the compressive strength of concrete be discussed under?
3
Which of the following is NOT a category of factors affecting the compressive strength of concrete?
Environmental conditions
Which region primarily uses the cylinder specimen method?
North America
What is the primary property of concrete considered in structural design?
Compressive strength
What is the relationship between compressive strength and tensile strength?
Tensile strength increases at a decreasing rate with compressive strength
What is the empirical formula suggested by Raphael to relate tensile and compressive strengths?
ƒ𝑡=0.3(ƒ𝑐)2/3
What is the purpose of performing a four-point bending test?
To determine the tensile strength of concrete
Why is the use of cylinder becoming more popular, particularly in research laboratories?
Because it gives more uniform results than cube
What is the effect of the platens of the testing machine on the strength of cube?
It decreases the strength of the cube
What is the modification by Oluokun to Raphael's formula?
ƒ𝑡=0.2(ƒ𝑐)0.7
What is the relationship between the strength of cube and cylinder?
The strength of cylinder is 0.8-0.85 times the strength of cube
What is the expression used in the British Code of Practice BS 8007: 1987 (2006 Eurocode 2) to relate tensile and compressive strengths?
ƒ𝑡=0.12(ƒ𝑐)0.7
What is the primary function of sheltering the surface from wind and sunshine during construction?
To reduce the rate of evaporation
Why is nearly twice as much water added to the concrete mix?
To increase the workability of the concrete
What happens to the excessive water in the concrete after curing?
It migrates from the interior to the surface
What is the primary factor affecting the magnitude of the ultimate shrinkage?
All of the above
What percentage of the ultimate shrinkage occurs during the first year?
90%
Why does the shrinkage increase as the ratio of the surface to volume increases?
Because the surface area increases the rate of evaporation
What is the range of shrinkage strain values for plain concrete members?
0.0004 to 0.0007
What is the effect of reinforcement on shrinkage in concrete members?
It reduces the shrinkage
What is the range of shrinkage strain values for reinforced concrete members?
0.0002 to 0.0003
What is the primary goal of measuring the strength of concrete test specimens?
To determine the strength of the concrete in the actual structure
Why are cores cut from in situ concrete often of different proportions?
Because the diameter of the core depends on the size of the core-cutting tool
What is the purpose of trimming cores to the h/d ratio of 2 before testing?
To estimate the strength of the same concrete as if it had been determined on a specimen with h/d = 2
What is the consideration of ACI 318-08 for concrete in the part represented by a core test?
The average strength of 3 cores is equal to at least 85 per cent of the specified strength
What is the purpose of using cores in concrete testing?
All of the above
What is the value of D for cores drilled horizontally according to the British standard?
2.5
Why are standard correction factors for strength of cylinders with different ratios of height to diameter used?
To estimate the strength of the same concrete as if it had been determined on a specimen with h/d = 2
What is the main reason for testing cores for strength?
To estimate the strength of the concrete in the actual structure
What is the primary cause of concrete deterioration?
Coupling effect of environmental factors and loading
What is the common result of major durability problems in concrete structures?
Cracking and spalling
What is the primary factor affecting the durability of concrete?
Permeability and diffusivity
What is permeability defined as?
The property that governs the rate of flow of a fluid into a porous material under pressure
What affects the permeability of concrete?
Porosity in concrete
What is the difference between permeability and diffusivity?
Permeability is when pores are filled with water and diffusivity is before saturation
What is diffusivity defined as?
The rate of migration of ions or moisture in concrete under the action of a higher concentration side to lower concentration side
What is the result of the corrosion of the reinforcing steel?
Cracking and spalling
What are the five major durability problems of concrete structures?
Corrosion, freeze/thaw damage, salt scaling, alkali-aggregate reactions, and sulfate attack
Study Notes
Specimen Methods
- The cube specimen method is used in Europe and China, while the cylinder specimen method is used in North America.
- The standard size of the cube specimen is 150 × 150 × 150 mm, prepared by pouring concrete into a cubic mold in three equal layers, with each layer being stroked 35 times by a hemispherical-tipped steel rod.
- The standard cylinder size for compression testing is 150 × 300 mm, prepared by pouring concrete into the cylindrical mold in three equal layers, with each layer being stroked 25 times by a hemispherical-tipped steel rod.
Factors Affecting Compressive Strength
- Characteristics and proportions of materials
- Curing conditions
- Testing parameters
Relation between Compressive and Tensile Strengths
- As compressive strength (fc) increases, tensile strength (ft) also increases but at a decreasing rate.
- Empirical formulas relate tensile and compressive strengths, such as ft = 0.3(fc)2/3 and ft = 0.2(fc)0.7.
Comparison between Cube and Cylinder Strength
- The strength of the cube is different from the strength of the cylinder due to the restraining effect of the platens of the testing machine.
- Normally, the strength of the cylinder is taken as 0.8-0.85 times the strength of the cube.
Effect of Height/Diameter Ratio on Strength of Cylinders
- Standard cylinders have a height/diameter ratio of 2, but sometimes specimens of other proportions are encountered.
- Correction factors are used to estimate the strength of cylinders with different ratios of height to diameter.
Test Cores
- Cores can be used to detect segregation or honeycombing, check the bond at construction joints, or verify the thickness of pavement.
- The average strength of 3 cores should be at least 85% of the specified strength, and no single core should have a strength lower than 75% of the specified value.
Drying Shrinkage and Durability
- Drying shrinkage occurs as the concrete volume shrinks due to moisture loss, and the shrinkage strain is time-dependent.
- The magnitude of the ultimate shrinkage is primarily a function of the initial water content of the concrete and the relative humidity of the surrounding environment.
- Factors influencing durability include environmental factors, mechanical loading, and the coupling effect of both.
Permeability and Diffusivity
- Permeability is the property that governs the rate of flow of a fluid into a porous material under pressure.
- Diffusivity is the rate of migration of ions or moisture in concrete under the action of a higher concentration side to lower concentration side.
- Both permeability and diffusivity affect the durability of concrete.
This quiz covers the different methods used to prepare concrete specimens, including the cube specimen method used in Europe and China, and the cylinder specimen method used in North America.
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