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“BACK TO BASICS” 3A6 Concrete Technology

“BACK TO BASICS” 3A6 Concrete Technology. Dr. Roger P. West November 2006. Part A: Basic Concepts and Site Practice. 1. Introduction 2. Sampling Concrete Materials 3. Workability and Workability Measurement 4. Strength Development and Strength Measurement

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“BACK TO BASICS” 3A6 Concrete Technology

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  1. “BACK TO BASICS”3A6 Concrete Technology Dr. Roger P. West November 2006

  2. Part A:Basic Concepts and Site Practice • 1. Introduction • 2. Sampling Concrete Materials • 3. Workability and Workability Measurement • 4. Strength Development and Strength Measurement • 5. Sampling, Slump Testing and Cube Making

  3. 1. Introduction: • Testing to ensure: • Concrete has been specified, made and transported correctly. • Quality satisfactory for purpose intended.

  4. 2. Sampling Concrete Materials • Sampling: IS EN 12350-1. • Provide certificate. • Good equipment. • Take care / make representative. • Take from mixer or as close to discharge as possible, or both. • 1. Sample in middle section of load not ends (slump alternative). • 2. At irregular times not evident to mixer operator. • 3. Note appearance, stony, creamy, sandy etc..

  5. Sampling

  6. 3. Workability and Workability Testing • Objective is to produce a fully compacted concrete without a loss of homogeneity or workability • Includes mixing, transporting, discharging, placing, compacting and finishing • Problems: • Improper constituents (e.g. additional water) • Loss of workability prior to discharge (e.g. segregation / slump loss) • Lack of compaction • Finishing problems (e.g. excessive bleeding)

  7. Workability • Strongly associated with the slump test • Factors such as the fluidity, stability, pumpability, compactability and finishability all affect the workability • These properties are affected by: • Design of mix • Adjustment to the mix constituents • Environmental Conditions: • Ambient and concrete temperature • Relative humidity and the air speed • Degree of agitation and friability of aggregrates • Elapsed time since mixing • EN206-1 uses concept of consistence

  8. Plastic testing: Slump • Slump test: IS EN 12350-2.

  9. 4. Strength Development and Strength Measurement • Aggregates “glued” together by cement paste to form concrete • Cement hydration is a chemical reaction which requires water • Strength gain reflects degree of hydration • Strength gain depends on • Type of cement • Temperature history – temperature and time • Curing • Admixtures

  10. Factors Affecting Compressive Strength at 28 days • Aggregate content • Cement type and fineness • Water/cement ratio • Degree of compaction • Extent of curing • Temperature Between 65 and 80% of 28 day strength at 7 days

  11. Strength Measurement • 100mm or 150mm cubes at 7 and 28 days (note ratio 1:1 and square in plan) • 300mm x 150mm cylinders at 7 and 28 days (note ratio 2:1 and circular in plan) • Other tests – direct tension, bending and cores • Non-destructive testing

  12. Cube Making: • Cube making: IS EN 12390-2 • Prime objectives • to achieve full compaction • avoid loss of moisture • keep at proper temperature when in curing tank • Use proper tools. • Advantage of cube shape is ease of making accurate sides. • Effect of cube shape.

  13. The Slump Test

  14. Cube Making

  15. Part B:Specifications and Non-Compliance • 6. Cube Curing and Cube Testing • 7. Specification and Compliance – Slump Classes and New Concrete Grades • 8. Cube Reports and Cube Failures

  16. 6. Cube Curing and Cube Testing • Curing: IS EN 12390-3 • De-mould when stability of cube allows. • Prevent loss of moisture before placing in curing tank. • Loss in strength due to initial drying out is unrecoverable. • No provision for in-situ cubes. BS1881 gives method for temperature matched curing.

  17. Cube curing

  18. Cube testing: • Cube test: IS EN 12380-3 • Specification compliance. • Desirable properties enhanced by increasing strength. • Effect of load rate, dry cubes. • Effects of different machines. • General variations. • Validity of uniaxial compression.

  19. 7. Specification and Compliance • Consistence Classes • New Concrete Grades

  20. Consistence Classes • Quantitative tests include the slump test, Vebe, degree of compaction and flow table values according to ISEN 12350, Parts 2, 3, 4 and 5 respectively Table 3 from ISEN206-1: Table 11 ISEN 206-1: Slump classes Tolerances for target values of consistence ClassSlump in mm Target Value(mm) < 40 50 – 90 >100 Tolerance(mm) ± 10 ± 20 ± 30 S1 10 – 40 S2 50 - 90 S3 100 - 150 S4 160 - 210 S5> 220

  21. Identity Testing

  22. Meeting Concrete Specification - Compliance • The delivery docket doesn’t have to state the precise mix proportions of the concrete if a product data sheet is accepted • The docket must declare that it conforms to the specification in ISEN206-1. Other information such as time of batching may be relevant, in the case of non-compliance (if the concrete exceeds the consistence class tolerances) • The addition of water and admixtures is forbidden • In special circumstances, the producer can take responsibility for adding water providing that: … ‘ the limiting values in the specification are not exceeded’.

  23. Sampling for Assessing Conformity: Test Plan • Conformity assessed on site, in accordance with Table 13

  24. New Concrete Grades • Previously had 5 classes of exposure (mild, moderate, severe, very severe, extreme) in IS326 (or BS8110), with a reasonable choice of grades between 20 and 50 concrete. • Now in ISEN206, or Eurocode 2, have 18 exposure classes and only 5 grades in this range (cube strengths 20, 30, 35, 37, 45MPa) • Specify cylinder/cube strengths as , for example, C30/37. • ISEN206 NA Table X

  25. Criteria for cube failures • A strength (the characteristic 28-day strength) is specified based on design – the concrete Grade • In compression test, two tested cubes at 28 days = one result • Provided difference between individual results is within 15% of average • Running average of four cube results (for Grades < 50): • average of any 4 consecutive results must be greater than characteristic + 3MPa (or N/mm2) • Individual cube result: • every individual result must be greater than the characteristic -3MPa

  26. Concrete Cube Test Result Variability • Variability – 28 day cube results have a mean strength and a standard deviation • For an expected 5% defective level, the target mean strength is the specified characteristic strength plus 1.64 times the standard deviation

  27. Example • Grade 35 specified • Over 100 cube results available from site • Consider individual results and running group of 4 average • Consider percentage 7 vs 28 day results • Inspect histogram and calculate mean and standard deviation • Ensure actual mean is greater than target mean strength

  28. Actual Mean Compared to Target Mean Compressive Strength Characteristic =35MPa Actual Mean = 40.2MPa Standard Deviation = 4.65MPa Target Mean Strength = Characteristic + 1.64 Standard Deviation or TMS of 35 + 1.64 x 4.65 = 42.6MPa > Actual Mean => PROBLEM !

  29. Failure modes - Normal

  30. Failure modes - Abnormal

  31. Consequences of failure • In-situ testing, methods, validity, representative of what? • Cost of delays, loss in reputation.

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