“BACK TO BASICS”3A6 Concrete Technology

Dr. Roger P. WestNovember 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 5. Sampling, Slump Testing and Cube

Making

1. Introduction:

Testing to ensure:– Concrete has been

specified, made and transported correctly.

– Quality satisfactory for purpose intended.

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..

Sampling

3. Workability and Workability Testing

Objective is to produce a fully compacted concrete without a loss of homogeneity or workabilityIncludes 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 compactionFinishing 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

Slump test: IS EN 12350-2.

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

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

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

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.

The Slump Test

Cube Making

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

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.

Cube curing

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.

7. Specification and Compliance

Consistence Classes 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

Class Slump in mm Target Value(mm) < 40 50 – 90 >100  Tolerance(mm) ± 10 ± 20 ± 30 S1 10 – 40S2 50 - 90S3 100 - 150S4 160 - 210S5 > 220

Identity Testing

Extract from Table B-1, EN206 –1: Identity criteria for slump For samples taken in accordance

with I.S. EN 12350-1 For spot samples taken from initial discharge

Specified slump class Not less than mm

Not more than mm

Not less than mm

Not more than mm

S1 0 60 0 70 S2 40 110 30 120 S3 90 170 80 180 S4 150 230 140 240 S5 210 No requirement 200 No requirement

Specified target slump, mm

Not more than the following from the specified target value mm

< 40 -20 +30 -30 +40 50 to 90 -30 +40 -40 +50 > 100 -40 +50 -50 +60

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’.

Sampling for Assessing Conformity: Test Plan

Conformity assessed on site, in accordance with Table 13

Production Minimum rate of sampling Subsequent to first 50 m3 of productiona First 50 m3 of

production

Concrete with production control certification

Concrete without production control certification

Initial (until at least 35 test results are obtained)

3 samples 1/200 m3 or 2/production week

1/150 m3 or 1/production day

Continuousb (when at least 35 test results are available)

1/400 m3 or 1/production week

a Sampling shall be distributed throughout the production and should not be more than 1 sample within each 25 m3. b Where the standard deviation of the last 15 test results exceeds 1,37 , the sampling rate shall be increased to that required for initial production for the next 35 tests results.

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

Exposure classes 

No risk of

corros-ion

or attac

k

Carbonation-induced corrosion

Chloride-induced corrosion Freeze/thaw attack Aggressive chemical environments

 

Sea water Chloride other than sea water  

X 0 XC 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 1 XA 2 XA 3 

Maxi

mum w/c ratio

--- 0.65 0.60 0.55 0.55 0.55 0.50 0.45 0.55 0.50 0.45 0.60 0.55 0.55 0.45 0.55 0.50 0.45

Minimum

strengt

h class

C12/15

C25/30

C28/35

C30/37

C30/37

C30/37

C35/45

C40/50

C30/37

C35/45

C40/50

C28/35

C30/37

C30/37

C40/50

C30/37 C35/45 C40/50

Minim

um cemen

t conten

t (kg/m3

)

--- 280 300 320 320 320 360 400 320 360 400 300 320 320 400 320(1)

280(2)

360(1), (3)

320(2)

400(1)(3)

360(2)

 

Minim

um air

content (%)

If less than C40/50 use5.5% (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 SO4

2 > 1400 mg/l  

Other require

-ments

 

Note: These recommendations are for a CEM I cement .  

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

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

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

Cube Ref.

7 dayA

28 dayB

28 dayC

28 day Avge.

% diff.Of 28d

Ratio 7/28 %

Runn-ingAvge

Comment

21 35.0 45.0 48.0 46.5 6.5 75 45.1 OK

22 32.0 43.0 42.5 42.8 1.1 75 44.7 OK

23 31.5 39.5 32.5 36.0 19.4 88 OMIT >15%

24 28.0 39.0 40.0 39.5 2.5 71 43.1 OK

25 32.0 37.5 42.5 39.8 12.7 80 42.2 OK

26 35.0 42.0 41.0 41.5 2.4 84 40.9 OK

27 23.0 28.5 31.0 29.8 8.4 77 37.7 Ind & Gp FAIL

28 20.5 25.5 25.5 25.5 0.0 78 34.2 Ind & Gp FAIL

29 17.5 33.0 31.5 32.3 4.6 54 32.3 Gp FAIL

30 25.5 34.5 30.5 32.5 10.7 78 30.0 Gp FAIL

31 37.8 44.5 - OMIT 1no. 28d

32 35.5 41.5 45.5 43.5 9.2 82 33.5 Gp FAIL

33 38.0 47.0 42.0 44.5 11.2 85 38.2 OK

34 37.5 44.5 41.5 43.0 7.0 87 40.9 OK

35 32.5 36.5 39.0 37.8 6.6 86 42.2 OK

1 1

5

9

1820

26

16

12

4

0

5

10

15

20

25

30

1 2 3 4 5 6 7 8 9 10

Cube Result Range (MPa)

Fre

qu

en

cy

Series1

Target Mean Strength = Characteristic + 1.64 Standard Deviation

or

TMS of 35 + 1.64 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

In-situ testing, methods, validity, representative of what?

Cost of delays, loss in reputation.