Optimising Properties of Concrete to EC2

Optimising Concrete in EC2

Ian Gibb

www.urs-scottwilson.com

Learning Points

• Raw Materials

• Optimising Concrete Properties


Portland cement


Cement hydration

Portland cement (1000x)

Cement hydration

CH

CH
HEAT
CH
Calcium
Calcium – Silica - Hydrate
Silica
Calcium hydroxide Ca(OH)2
C-S-H Alumina

CH CH


Voids

Voids within C-S-H 0.001 m

CH
0.01 m

CH Capillary voids
0.1 m
CH
CH
1.0 m

10 m

CH
CH
100 m
CH
1mm
CH Entrapped voids

10mm

Cement hydration

CH

CH
CH

CH
C-S-H

C-S-H
CH CH


Cement Additions

• Portland cement (20 m)

• GGBS (<20 m) 2100X

• Fly ash (10 m)

• Silica fume (0.1 m)


Cement Additions


1000X

• GGBS (<20 m)

• Fly ash (10 m)



Cement Additions

2,000,000 SF grains : 1 PC grain


• GGBS (<20 m)

• Fly ash (10 m)



BS 8500

Portland cement
CEM Addition
replacement, %

I ~ 0–5

Silica fume 6 – 10
IIA
Fly ash 6 – 20

IIB-V 21 – 35
Fly ash
IVB-V 36 – 55

IIB-S 21 – 35

IIIA GGBS 36 – 65

IIIB 66 – 80


Cement Additions

CEM I CEM II/B-V

CH CH

CH CH
CH CH
CH CH

CH CH CH
CH
CH CH
CH CH


Cement Additions

CEM I CEM IIA-D

CH CH

CH CH
CH CH
CH CH

CH CH CH
CH
CH CH
CH CH


Aggregate


Aggregate

Recycled Blastfurnace China clay Lead
concrete slag stent shot


Aggregate

Rounded siliceous gravel Crushed limestone


Aggregate properties

• Strength

• Density (450-5000 kg/m3)

• Porosity

• Drying shrinkage

• Creep

• Thermal expansion


Admixtures


Fibres

ASTM C1018-97 (third point bending test)


Performance

• Compressive strength
• Tensile & flexural strength
• Modulus of elasticity
• Bond strength
• Creep & shrinkage
• Thermal expansion
• Thermal conductivity


Compressive Strength

Mix
Designation C12/16 C30/37 C40/50 C50/60 C60/75 C90/105

Target
mean
cylinder 20 38 48 58 68 98
strength
(fcm)
Target
mean cube
strength 26 47 60 70 85 115
(fcm, cube)


Nominal Limiting values for exposure class XS3 from table Cement
cover A.5 of BS8500-1 for different cement types and cover type
(mm) 45+ c 50+ c 55+ c 60+ c 65+ c

C45/55 C40/50 CEMI, IIA,
- - - IIB-S,
0.35 380 0.40 380 SRPC

C40/50 C35/45 C32/40 C28/35 C25/30
XS3 IIB-V, IIIA,
0.35 380 0.40 380 0.45 360 0.50 340 0.55 320

C32/40 C28/35 C25/30 C25/30 C25/30
IIIB, IVB-V
0.40 380 0.45 360 0.50 340 0.50 340 0.50 340


Design strengths are similar

Mix BS8110 EC2
• BS8110 Designation fcd fcd
fcd = 0.67 fcu/ γc
C20/25 11.2 11.3

• EC2 C30/37 16.5 17.0
fcd = αcc fck/γc

C40/50 22.3 22.7
αcc = 0.85 (from NA)

C50/60 26.8 28.3

C90/105 N/A 51.0


Other properties

• Generally linked to
compressive strength

• Can be optimised with
knowledge of materials


Tensile strength


Tensile strength

• Relevance:
• Cracking

• Shear / punching shear

• Bond and anchorage

• Deflection calculations

• Reinforcement design

• Fibre / un-reinforced sections


Tensile strength

• Higher not always advantageous!

• Consider calculation for steel required to control thermal
cracking


Shrinkage

• Why worry about shrinkage?


Calculating Shrinkage

• Combination of
• Autogenous shrinkage (typically <100µε) Important for
•Reinforcement
• Drying shrinkage
requirements
(plastic shrinkage ignored) •Deflections
•Wall / column
shortening
•Loss of pre-stress
•Stress relaxation


Autogenous Shrinkage

• Autogenous shrinkage
• Significant in high strength concrete
εca(∞) for C90/105 = 200µε
• BE EN 1992-1-1 recommends should be specifically considered
when new concrete cast against hardened concrete

Strength

Age

Drying Shrinkage

Cement class

Strength

Relative Humidity


Drying Shrinkage over time

Geometry

Age, curing period, geometry


Drying Shrinkage

300
C35/45
250 R
RH 80%
200
S
εcd(t)

150

100

50

0
1 10 100 1000 10000 100000
Days

450mm thick suspended slab cured for 9 days


Understanding cement class

Class Cement Type
The R & N in EN1992-1-1 are
different to R & N in EN197-1
R CEM42.5R,
CEM52.5N,
CEM52.5R

N CEM32.5R,
CEM42.5N,

S CEM32.5N


Understanding cement class

Class Cement Type
…. and BS8500-2 introduces
an L designation
R CEM42.5R,
CEM52.5N,
CEM52.5R

N CEM32.5R,
CEM42.5N,

S CEM32.5N


Effect of additions on autogenous
shrinkage
From CIRIA C660 Appendix A4


How to reduce shrinkage

• Aggregate content
(71 to 74% ~ 20%
reduction!)

• Aggregate size
(impacts paste volume)

• Aggregate type

• Admixtures
(water / shrinkage reducing)



(71 to 74% ~ 20%
reduction!)

• Aggregate size

• Aggregate type

• Admixtures
(water / shrinkage
reducing)



(71 to 74% ~ 20%
reduction!)

• Aggregate size 0.05

(impacts paste volume) 0.04

Length change (%)
0.03

• Aggregate type 0.02

0.01
• Admixtures
0

(water / shrinkage reducing) -0.01
0 10 20 30 40 50 60 70

Days
-0.02


Low-Shrink


Low-Shrink

• Typical dosage: 5-7 litres/m3

• Strength loss: 12-15% at 28-

days


Effects mitigated by creep


Drying shrinkage

• To reduce drying shrinkage
cracking:
• Adequate curing
(increases tensile strain
capacity)

• Reduce internal restraint
(movement joints)

• Crack control reinforcement


Conclusion


Thank you

Ian Gibb
Principal Materials Engineer

ian.gibb@scottwilson.com


Optimising Properties of Concrete to EC2

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