1. Learning Objectives Relevance of fresh concrete properties
Tests on freshly mixed concrete
Describe how fresh concrete properties are influenced by concrete constituents
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2. Relevance of fresh concrete properties
Why are we interested in the properties of fresh concrete?
Influence on the final quality of hardened concrete
Helps to detect variations in material and process
Uniform fresh concrete = Uniform hardened
Last chance to identify unsatisfactory quality
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3. Relevance of fresh concrete properties
Time
Distance 50 40 30 20 10
Temperature
º C
Job site
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4. Relevance of fresh concrete properties
Compaction ratio
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5. Tests on Freshly Mixed Concrete
Procedures used for testing freshly mixed concrete are standardized
Workability & consistency
Bleeding and settlement
Setting time
Air content
Temperature
Density
Used not only in the laboratory but also in the field
Designed to eliminate random variations that may occur in the test results and lead to unnecessary disputes on the quality of concrete
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6. Workability & Consistency
The ease of placing, consolidating and finishing freshly mixed concrete and the degree to which it resists segregation is called “workability”
Properties related to workability include:
Consistency
Segregation/cohesiveness
Mobility
Pumpability
Bleeding
Finishability
Degree of workability required is controlled by:
Placement method
Type of consolidation
Type of concrete
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7. Workability & Consistency
Factors influencing concrete workability are:
Method and duration of transport
Type and content of cement
Aggregate characteristics (grading, shape…)
Entrained air
Water content
Temperature (concrete & ambient)
Admixtures
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8. Measurement of Consistency
Although it is a different characteristic from workability, in practice the two terms are often confused and merged into one - the “slump” of concrete
Slump ASTM - 143
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9. Measurement of Consistency
Workability Terminology
Very stiff
Stiff
Plastic
Soft (wet)
Flowing
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10. Measurement of Consistency
Flow table - DIN 1048
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11. Influence of Water on Workability
Only very minor adjustments to workability should be made by the addition of water alone
Reduce strength and durability
Potential for drying shrinkage cracking
Therefore, adjustments to workability made by the addition of water should always be accompanied by an addition of cement to maintain a fixed W/C
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12. Influence of Water on Workability
Relation between water requirement of paste and concrete
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13. Influence of aggregates on Workability
Grading
Affects workability, because the water demand necessary to wet all the solids in the mix will depend on the surface area of the aggregates
The larger the particle size the greater the workability for a given cement content and W/C
Particle shape
Rounded or roughly cubically shaped aggregates will produce more workable concrete than that produced using flaky or elongated particles
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14. Sand
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15. Influence of temperature on Workability
ASTM C 1064
Glass or digital readouts thermometers are available
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16. Influence of temperature on Workability
Time Vs Slump 25 20 15
21 °C
Slump (mm) 10
32 °C 5
-20 10 40 70
100
Time(min)
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17. Workability & Consistency
Cohesiveness of concrete
Is a measure of its ability to resist the segregation of its constituents during handling, placing and compaction
Cohesive
No Cohesive
Segregation
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18. Cohesiveness
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19. Cohesiveness of concrete
Factors affecting the cohesiveness of concrete:
Water
The higher the water content the greater is the risk of segregation and bleeding
High water content retards the stiffening of concrete allowing sedimentation of heavier particles
Cold weather can also retard setting and promote bleeding
Dry mix can be friable and also prone to segregation
Grading
Mixes that are deficient in fine particles tend to segregate more readily during handling and bleed after finishing (entrained air can help)
Excessive fines make concrete “sticky” and very cohesive
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20. Bleeding and settlement
Bleeding is the development of a layer of water at the top or surface of freshly placed concrete
Bleeding is normal and should not diminish the quality of properly placed, finished and cured concrete
Some bleeding is helpful to control plastic cracking
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21. Excessive bleeding?
Cracks
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22. Measurement of Bleeding
Can be determined by the test method described in ASTM C 232 – rarely used in the field
Typical values range
from 0.1 to 2.5 % of mix water
Or 0.01 to 0.08 mL/cm2
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23. Setting time
Can be determined by the test method described in ASTM C 403 by means of penetration resistance measurements
Made on mortar sieved from the concrete mixture
Initial and final time of setting corresponds to a penetration resistance equal to 500 psi (3.4 MPa) and 4000 psi (27.6 MPa)
Typical values range
Initial set: 2 to 6 hours
Final set: 4 to 12 hours
Factors affecting setting time of concrete:
W/C ratio
Cement type and content
Temperature
Admixtures
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24. Measurement Setting time
Final set
Initial set
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25. Setting time Methods Vicat ASTM C 403 Limestone cement
Paste – w/c ~0.27
Concrete – w/c ~ 0.6
MSC
Competitor
Setting time
Initial
110 min
+ 30 min
280 min
- 50 min
Final
140 min
400 min
- 40 min
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26. Air content ASTM C231: pressure method ASTM C173: volumetric method
Applied pressure compresses the air within the concrete including in aggregates pores
Not suitable for lightweight aggregate concrete
ASTM C173: volumetric method
Removal of air from a known volume of concrete by agitating the concrete in an excess of water
Suitable for lightweight aggregate concrete
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27. Density
ASTM C138
Determine the volumetric quantity of concrete produced per batch
Can also give indications of air content provided the relative densities of the ingredients are known
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