Hydration of cement

Transcription

Hydration of cement

“Materials, Durability And Deterioration”
Ir. Dr. Lim Char Ching
Bahagian Forensik (Struktur dan Jambatan)
Jambatan)
Cawangan Kej.
Kej. Awam,
Awam, Struktur dan Jambatan
Ibu Pejabat JKR Malaysia
email: cclim@jkr.gov.my
1
Contents
1. An overview of Portland cement: (production,
chemical composition, hydration reaction)
2. Types of cement replacement materials (pfa, ggbs,
csf, pozzolanic reaction)
3. Effects on concrete properties
4. Types of concrete
2
1
An Overview of Portland Cement
1824 :
Joseph Aspdin invented Portland Cement by
burning limestone and clay;
1400oC
Limestone
+ clay
Grind
Clinkers
+
Gypsum
(<5%)
Portland
cement
Copyright © 2011 Dr. Lim Char Ching
3
Chemical Compounds formed in the Cement Kiln
(Source: ACI Education Bulletin E3E3-01)
4
2
Chemical Compounds and Its Contribution
to Strength in Portland Cement
C3S
Tricalcium Silicate (54%)
Contribute to early strength
development
C2S
Dicalcium Silicate (17%)
Contribute to later age
strength development
C3A
Tricalcium Aluminate
(12%)
Insignificant to strength
development.
Can cause “flash“flash-set”
Tetracalcium
aluminoferrite (8%)
Insignificant to strength
development.
Reacts with gypsum and
helps to accelerate C3S and
C2S hydration
C4AF
5
(Source: ACI Education Bulletin E3E3-01)
6
3
Types of Portland Cement
1. Ordinary Portland Cement (OPC)
2. Rapid
Rapid--hardening Portland Cement (RHPC)
3. Sulphate
Sulphate--resisting Portland Cement (SRPC)
4. Low heat Portland Cement (LHPC)
Cement
Type
Compound Composition (%)
C3S
C2S
C3A
C4AF
OPC
54
17
12
8
RHPC
60
12
12
8
SRPC
43
36
4
12
LHPC
30
46
5
13
7
Hydration of cement
Cement + Water
(C3S, C2S)
CSH + Ca(OH)2
(gel)
Free water
Aggregate
in SSD
condition
Contributes
to strength
Contributes
to concrete
alkalinity
pH12.5 to
13.0
8
4
Hydration of Cement (1)
Unhydrated cement particles magnified 2000x
(ACI 308R-01)
9
Partially hydrated cement particles magnified
4000x (ACI 308R-01)
10
5
Close-up of a single hydrated cement particle magnified
11,000x (ACI 308R-01)
11
Concrete Composition
W
water
cement
1 m3
paste
sand
granite
Aggregates to
dilute cement
paste
Increasing
particle size
12
6
Concrete Mix Proportions
W
Water = 180 kg
Cement = 360 kg
1 m3
Sand = 700 kg
Granite = 1160 kg
13
Supplementary Cementing Materials (1)
TYPES : pulverised fuel ash (pfa), condensed silica
fume (csf), ground granulated blastfurnace slag (ggbs),
rice husk ash and sugarcane bagasse
PFA : A byby-product from the combustion of pulverised
coal in thermal power stations
CSF : A byby-product from the smelting process in the
production of silicon metal alloys
GGBS : A byby-product of iron manufacturing
Industrial wastes !!
14
7
Concrete Composition
1 m3
W
W
W
W
C
P
C
S
S
A
A
Binder
Increasing
particle size
15
Hydration of cement
Cement + Water
(C3S, C2S)
CSH + Ca(OH)2
(gel)
Pozzolanic reaction
Pozzolan + Ca(OH)2 + Water
(SiO2)
CSH
(gel)
16
8
Blended Cement (1)
1. Portland Pulverised
Pulverised--Fuel Ash Cement (MS 1227)
2. Portland Slag Cement (MS 1389)
LaFarge Malayan Cement Berhad
YTL Cement Berhad
17
Blended Cement (2)
(Courtesy LaFarge Malayan Cement Bhd)
18
9
Blended Cement (3)
Class
Concentration of
sulphate in
ground water (g/l)
Cement Type
1
< 0.4
OPC, PFA Cement, Slag Cement
2
0.4 – 1.4
OPC, PFA Cement, Slag Cement
3
1.5 – 3.0
PFA Cement, Slag Cement, SRC
4
3.1 – 6.0
PFA Cement, Slag Cement, SRC
5
> 6.0
As for Class 4 plus surface
protection
19
Effect of SCM on Concrete Properties (1)
Fresh Concrete:
1. Setting time – generally longer depending
on % replacement
2. Bleeding – reduced with the increase in %
replacement and fineness of pozzolan
3. Temperature rise – reduced with the
increase in % replacement
4. Workability – improves with the use pfa
20
10
Effect of SCM on Concrete Properties (2)
Hardened Concrete:
1. Strength development – generally lower
early age strength
2. Elastic modulus – similar to concrete
without pozzolan
3. Chemical resistance – improves concrete
resistance to chloride and sulphate
4. Carbonation – generally higher rate in
ggbs concrete
21
Chemical Admixtures
22
11
History of HRWR Development
Year
Chemical Base
Generation
Water
Reduction
1930
Ligno-sulphonates,
LignoGluconates
1st
10%
1970
Sulphonated
Melamine/Naphtalin
polymers
2nd
20%
1990
Vinyl--copolymers
Vinyl
3rd
30%
2000
Modified
Polycarboxylates
4th
40%
23
An Illustration of Superplasticiser Content
on SelfSelf-Compactibility
24
12
Fibres
25
Polypropylene Fibers
Developed in the early 1980’s
Polypropylene fibers
Dispersion of fibers
26
13
Benefits of PP Fibers in Concrete (1)
Inhibits cracking due to plastic and drying
shrinkage
A cost effective alternative to wire mesh for
crack control
Reduces concrete permeability and improves
durability
Improves impact and abrasion resistance
Reduces rebound and material loss in gunite
and shotcrete application
27
Plain Concrete
Fiber--RC
Fiber
PP fibers inhibit cracking
28
14
0.0 kg fibers
0.9 kg fibers
1.8 kg fibers
PP fibers improve durability
29
Typical Mechanical Properties of FiberFiber-RC
28
28--d compr.
strength
28
28--d flexural
strength
Splitting tensile
strength
Impact strength
(No. of drop)
Toughness
Index
Fiber--RC
Fiber
Plain Concrete
34.4 MPa
31.0 MPa
4.42 MPa
4.30 MPa
3.38 MPa
3.07 MPa
71
23
4.9
1.0
30
15
Contents
Deterioration of Reinforced
Concrete Structures
31
Deterioration of Reinforced Concrete
concrete
steel
Concrete
Deterioration
Corrosion of
Reinforcement
Chloride
Attack
Carbonation
Degradation of
Concrete Matrix
Sulphate
Attack
Acid Attack
Alkali-Aggregate
AlkaliReaction
32
16
Protection of Steel in Concrete
Concrete pH12.5 to
13.5
Passivating Layer
(Iron Oxide)
High Alkalinity Environment In Concrete
Protects Steel From Corrosion
33
Destruction of Passivating Layer
Two mechanisms that can destroy the
passivating layer :
(1) Chloride Attack
(2) Carbonation
34
17
Chloride Attack (1)
Sources of chlorides:
Sea water / sea water spray
Contaminated aggregates
Chloride-based admixture
De-icing salt
Transport mechanisms:
Diffusion
Permeation
Absorption
35
Chloride Attack (2)
Chloride content on
steel surface ≥ 0.4% by
wt. of cement
Destruction of
passivating layer
Steel corrosion and
volume expansion
Concrete cracking
and spalling
36
18
Corrosion of Steel: Volume Change
Steel bar
Level 1 corrosion
Fe
Fe3O4
Fe(OH)2
Level 2 corrosion
Level 3 corrosion
Fe(OH)3
Fe(OH)3. H2O
0
1
Level 4 corrosion
2
3
4
5
6
Volume (cm3)
37
Carbonation (1)
a) CO2 + H2O =
Carbonic acid
b) This carbonic acid
neutralises the
alkalinity of concrete
in the cover
c) When pH of concrete
falls below 10
10,,
passivating layer is
destroyed &
corrosion can take
place
38
19
Carbonation (2)
Acid
Alkali
1
7 8 9 10 11 12 13 14
Colourless
pH
Pink
Carbonated Concrete
(passivating layer is destroyed)
Uncarbonated
Concrete
(passivating
layer is intact)
Phenolphthalein Indicator for
Carbonation Test
Carbonated
concrete
39
Uncarbonated
concrete
40
20
Sulphate Attack (1)
Sulphate ions
C3A hydrates
Ca(OH)2
Sources of sulphates:
Sewerage
Groundwater and seawater
Rocks and soils
41
Sulphate Attack (2)
Sulphate ions
C3A hydrates
Ca(OH)2
1.
Sulphate ions + Ca(OH)2 = Gypsum
2.
Sulphate ions + C3A hydrates = Ettringite
42
21
Lecture 5a
1. What is “Concrete
Concrete Durability?”
2. BS 8110 Requirements on Durability
– Are They Adequate?
43
What is “Concrete Durability”?
ACI Manual – “The ability of a structure or its components
to maintain serviceability in a given environment over a
specified time”
Browne (1986) – “Ability of the concrete in preventing
steel corrosion. The attack process can be regarded as a
battle between the environment and the concrete cover
which acts as the defense system for steel. The enemies
range from chlorides, moisture, oxygen…”
44
22
The “Cretes” of Concrete Structural Element
Surface
Skincrete
Covercrete
(Vital zone for
durability)
Heartcrete
(Vital zone for strength and
structural capacity)
45
What is “Concrete Durability”?
External Surface
Covercrete
(Vital zone for
durability. It
protects steel
against
corrosion)
Heartcrete
(Vital zone for strength and
contributes to overall
structural capacity)
46
23
Covercrete for
Durability
(cover thickness,
cement content,
cement type, w/c,
water content,
curing regime,
sorptivity,, Dc,
sorptivity
RCPT)
Heartcrete for
Structural Capacity
(conc. grade, fcu)
Typical CrossCross-Section of
Reinforced Concrete
47
Column A
Column B
Grade 30
concrete
Grade 20
concrete
Grade 30
concrete
Grade 20
concrete
Which is more durable ?
48
24
The 2Qs for Durability
1. QUANTITY of cover
* Is the Code’s provision adequate?
* What is the achievement of the specified
cover at site?
2. QUALITY of cover
* Is curing and compaction adequate?
* Is the concrete mix acceptable?
49
Factors Affecting Durability
1. Mix proportions – binder type, binder
content, w/c ratio, water content, sandsandaggregate ratio
2. Curing and compaction
3. Chemical admixture
4. Mineral admixture
50
25
Why Curing of Concrete?
“Curing is the process of controlling the rate and
extent of moisture loss from concrete during
cement hydration.”
Curing shall be carried out immediately after concrete has
been placed in position, thus, providing time for the
hydration of the cement to take place.
Since the hydration of cement does take time – days, and
even weeks rather than hours, curing shall be undertaken
for a reasonable period of time if the concrete is to
achieve its POTENTIAL STRENGTH and DURABILITY.
DURABILITY.
51
Methods of Curing Concrete
1. Wet or moist curing – preventing moisture loss by
continuously wetting the exposed concrete surface eg
eg.. Wet
burlaps, fogging, waterwater-ponding, plastic sheets, etc.
2. Curing compounds – are liquids usually sprayed directly
onto concrete surfaces which then dry to form an
impermeable membrane that retards the loss of moisture
from concrete.
3. Internal curing admixtures – are admixtures incorporated
into fresh concrete to inhibit moisture loss. These are
relatively new products and care should be taken to use
them.
52
26
Wet or Moist Curing of Concrete
Wetting by water
sprinkler
Using plastic sheet to
cover concrete
53
Wetting by water sprinkler
54
27
Fogging (in a chamber)
55
Water Ponding
56
28
Curing Compounds
Curing compounds are generally formulated from wax
emulsion, chlorinated rubber, synthetic and natural resins.
Comparative efficiency of curing compounds
(Cement Concrete & Aggregate Australia)
57
Effect of curing period on permeability of cement
paste (Cement Concrete & Aggregates Australia)
58
29
Why Compaction of Concrete?
“Compaction is the process which expels
entrapped air from freshly placed concrete and
packs the aggregate particles together so as to
increase the density of concrete.”
Proper compaction increases concrete strength,
improves impermeability and durability of
concrete.
59
Compaction of concrete is a 2-stage process. Firstly,
aggregates are set in motion and slump to fill the form.
Secondly, entrapped air is expelled (Cement Concrete &
Aggregates Australia)
60
30
Typical Loss of Strength in Concrete Through Incomplete
Compaction (Cement Concrete & Aggregates Australia)
61
Diameter of
Head (mm)
20 – 40
30 – 65
50 – 90
Radius of Action,
e (mm)
75 – 150
125 – 250
175 – 350
(Cement Concrete & Aggregates Australia)
62
31
Important Information Required from
Designer on Concrete Durability
1. Mix proportions – binder type? binder content?
w/c ratio? water content?
2. Curing method?
3. Slump value?
4. Nominal cover? (Incorrect to specify minimum
cover)
5. Concrete grade? (Incorrect to specify minimum
compressive strength at 28 days)
63
32

Hydration of cement

Transcription

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