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

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

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

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

Sampling

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)

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

Plastic testing: Slump n Slump test: IS EN

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

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

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

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

The Slump Test

Cube Making

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

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

Cube curing

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

7. Specification and Compliance n Consistence Classes n New Concrete Grades

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) 100 Tolerance(mm) ± 10 ± 20 ± 30 S110 – 40 S S S S5> 220

Identity Testing

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 ISEN 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’.

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

New Concrete Grades n 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. n Now in ISEN206, or Eurocode 2, have 18 exposure classes and only 5 grades in this range (cube strengths 20, 30, 35, 37, 45MPa) n Specify cylinder/cube strengths as, for example, C30/37. n ISEN206 NA Table X

Exposure classes No risk of corro s-ion or attac k Carbonation-induced corrosion Chloride-induced corrosionFreeze/thaw attackAggressive chemical environments Sea waterChloride other than sea water X 0XC 1 XC 2 XC 3 XC 4 XS 1 XS 2 XS 3 XD 1 XD 2 XD 3 XF 1 XF 2 XF 3 XF 4 XA 1XA 2XA 3 Maxim um w/c ratio Minim um strengt h class C12/1 5 C25/ 30 C28/ 35 C30/ 37 C35/ 45 C40/ 50 C30/ 37 C35/ 45 C40/ 50 C28/ 35 C30/ 37 C40/ 50 C30/37C35/45C40/50 Minim um cement content (kg/m 3 ) ( 1) (2 ) 360 (1), (3) 320 (2) 400 (1) (3) 360 (2) Minim um air content (%) If less than C40/50 use 5.5% (10 agg.) 4.5% (14 agg.) 3.5% (20 agg.) 3.0% (40 agg.) (1) CEM I (2) Sulfate resisting cement (3) Use sulfate resisting cement if SO 4 2 > 1400 mg/l Other require - ments Note: These recommendations are for a CEM I cement.

Criteria for cube failures n A strength (the characteristic 28-day strength) is specified based on design – the concrete Grade n In compression test, two tested cubes at 28 days = one result – Provided difference between individual results is within 15% of average n Running average of four cube results (for Grades < 50): – average of any 4 consecutive results must be greater than characteristic + 3MPa (or N/mm 2 ) n Individual cube result: – every individual result must be greater than the characteristic -3MPa

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

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

Cube Ref. 7 day A 28 day B 28 day C 28 day Avge. % diff. Of 28d Ratio 7/28 % Runn- ing Avge Comment OK OK OMIT >15% OK OK OK Ind & Gp FAIL Ind & Gp FAIL Gp FAIL Gp FAIL OMIT 1no. 28d Gp FAIL OK OK OK

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

Failure modes - Normal

Failure modes - Abnormal

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