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Learning Objectives Relevance of fresh concrete properties

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Presentation on theme: "Learning Objectives Relevance of fresh concrete properties"— Presentation transcript:

1 Learning Objectives Relevance of fresh concrete properties
Tests on freshly mixed concrete Describe how fresh concrete properties are influenced by concrete constituents PECP

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 PECP

3 Relevance of fresh concrete properties
Time Distance 50 40 30 20 10 Temperature º C Job site PECP

4 Relevance of fresh concrete properties
Compaction ratio PECP

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 PECP

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 PECP

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 PECP

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 PECP

9 Measurement of Consistency
Workability Terminology Very stiff Stiff Plastic Soft (wet) Flowing PECP

10 Measurement of Consistency
Flow table - DIN 1048 PECP

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 PECP

12 Influence of Water on Workability
Relation between water requirement of paste and concrete PECP

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 PECP

14 Sand PECP

15 Influence of temperature on Workability
ASTM C 1064 Glass or digital readouts thermometers are available PECP

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) PECP

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 PECP

18 Cohesiveness PECP

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 PECP

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 PECP

21 Excessive bleeding? Cracks PECP

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 PECP

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 PECP

24 Measurement Setting time
Final set Initial set PECP

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 PECP

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 PECP

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 PECP


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