“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.