Learning Objectives

1. Learning Objectives • Relevance of fresh concrete properties • Tests on freshly mixed concrete • Describe how fresh concrete properties are influenced by concrete constituents PECP06 2005

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 PECP06 2005

3. 50 50 Temperature 40 40 30 30 20 20 10 10 0 0 º C Relevance of fresh concrete properties Time Distance Job site PECP06 2005

4. Relevance of fresh concrete properties Compaction ratio PECP06 2005

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 PECP06 2005

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 PECP06 2005

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 PECP06 2005

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 PECP06 2005

9. Measurement of Consistency • Workability Terminology • Very stiff • Stiff • Plastic • Soft (wet) • Flowing PECP06 2005

10. Flow table - DIN 1048 Measurement of Consistency PECP06 2005

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 PECP06 2005

12. Influence of Water on Workability • Relation between water requirement of paste and concrete PECP06 2005

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 PECP06 2005

14. Sand PECP06 2005

15. Influence of temperature on Workability • ASTM C 1064 • Glass or digital readouts thermometers are available PECP06 2005

16. Influence of temperature on Workability Time Vs Slump 25 20 15 21 °C Slump (mm) 32 °C 10 5 0 -20 10 40 70 100 Time(min) PECP06 2005

17. Cohesive No Cohesive Workability & Consistency • Cohesiveness of concrete • Is a measure of its ability to resist the segregation of its constituents during handling, placing and compaction Segregation PECP06 2005

18. Cohesiveness PECP06 2005

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 PECP06 2005

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 PECP06 2005

21. Cracks Excessive bleeding? PECP06 2005

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 PECP06 2005

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 PECP06 2005

24. Measurement Setting time Final set Initial set PECP06 2005

25. Setting time PECP06 2005

26. Air content • ASTM C231: pressure 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 PECP06 2005

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 PECP06 2005