O. Tavares

1. CON 124
Basic Concrete Mix Design Proportioning
Session 6
Concrete Durability

2. Session 6 Topics
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Alkali-Silica Reactivity (ASR)
Water/Cementitious Ratio
Shrinkage
Mechanisms
Testing procedures
Mitigation measures

3. Alkali-Silica Reactivity

4. Alkali-Silica Reactivity (ASR)… ACI 116


The reaction between the alkalies in portland
cement and certain siliceous rocks or minerals
present in some aggregates
The products of the reaction may cause
abnormal expansion and cracking of
concrete...

5. Alkali-Silica Reaction (ASR)
• Contributing factors:
– Reactive forms of silica
in the aggregate
– High-alkali (pH) pore
solution
– Sufficient moisture
Crack
Reaction Product

6. Mechanism
Reactive silica + alkalis Alkali-silica gel
OH-
OH-
Reactive
Silica
OH-
OH-

7. Mechanism
Silica gel + Water Expansion
H2O
H2O
Reactive
Silica
H2O
H2O

8. Mechanism
Expansion  Cracking
H2O
H2O
Reactive
Silica
H2O
H2O

9. Cracking and Appearance
of ASR Gel

10. Alkali-Silica Reaction (ASR)
Visual Symptoms
• Network of cracks
• Closed or spalled joints
• Relative displacements

11. Alkali-Silica Reaction (ASR)
Visual Symptoms
 Fragments breaking
out of the surface
(popouts)
Mechanism
 Alkali hydroxide +
reactive silica gel x
reaction product
(alkali-silica gel)
 Gel reaction product
+ moisture x
expansion

12. Alkali-Silica Reaction (ASR)
Visual Symptoms
 Fragments breaking
out of the surface
(popouts)
Mechanism
 Alkali hydroxide +
reactive silica gel x
reaction product
(alkali-silica gel)
 Gel reaction product
+ moisture x
expansion

13. Mitigation of ASR



Avoid reactive aggregates
Limit concrete alkali content, 5 lbs/cyd (low
alkali cement)
Supplementary cementitious materials


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Fly ash, slag, calcined clay (metakaolin)
Blended cement
Test for effectiveness of mitigation measures

14. Aggregates


The field-performance record of a particular
aggregate is best means for judging its
reactivity
If aggregates are shown by service records or
laboratory examination to be potentially
reactive, they should not be used when the
concrete is to be exposed to seawater or other
environments

15. ASR – The Key Components
Alkalis in Portland Cement


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Cement: Sodium and Potassium
Expressed as (Na2O + 0.658K2O)
ASTM C 150 - Table 2, Optional Chemical
Requirements


Low-Alkali Cement - Max 0.60%
Low-alkali cement to be used with reactive
aggregates (ASTM C 33)

16. ASR – The Key Components
& Other Sources of Alkalis in Concrete

Supplementary Cementitious Materials
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Fly Ash
Slag Cement
Silica Fume
High-Reactive Metakaolin
Sodium-Bearing Admixtures

17. Alkali-Silica Reactivity
The correlation at
14 weeks is not as
great as at later
ages.

18. Cement – Aggregate Test
ASTM C227
0.50
Na2Oeq=0.92%
6-months Expansion, %
0.40
Na2Oeq=0.57%
0.30
0.20
0.10
0.00
A
B
Aggregate
ASR Test Method alkali content and aggregate comparisons C227

19. Tests for ASR Potential
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ASTM C227—Mortar Bar Method
ASTM C441—Mortar Bar and SCMs
ASTM C289—Mortar Bar Method
ASTM C1260—Rapid Mortar Bar Test
ASTM C1293—Concrete Prism Test
ASTM C1567—Rapid evaluation of SCMs
…Others

20. Exposure Methods

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Warm and humid environment— “over water
at 38 °C”
Accelerated conditions—immersed in NaOH
solution at 80 °C

21. ASTM C227, C441 & C1293
“Over Water @ 38 °C”
Sealed
container in
38 C
room
Water
Wicking
material on
inner surface

22. ASTM C1260 & C1567
“Immerse in NaOH at 80 °C”
Polypropylene box with cover
Water
Bath
80 2 C
1N NaOH Solution

23. ASTM Test Methods Comparisons
ASTM
Test
Use
Specimen
Type
Test
Condition
Common
Duration
C 227
Cem-Agg
Combination
Mortar
Over H2O
@ 38 °C
90, 180 d
C 441
Effectiveness
of SCMs
Mortar
(Pyrex + HA
Cement)
Over H2O
@ 38 °C
14, 56 d
C 1260 and
[C 1567]
Aggregate
[Effectivenesof
SCMs]
Mortar
Immerse in
NaOH @ 80 °C
16 d
C 1293
Aggregate
[Effectiveness
SCMs]
Concrete
(Added
NaOH)
Over H2O
@ 38 °C
1 or 2 yr

24. ASTM C295

25. ASTM C295

26. Effect of Fly Ash on ASR
(Class F)
(ASTM C1293)
0.10
% Expansion
0.08
Control
15 % FA
25 % FA
0.06
1 yr limit
0.04
0.02
0.00
0
200
400
600
Time, days
800
1000
1200

27. Effect of Fly Ash –
Aggregate R and Cement NA
14-day Expansion, %
0.20
Fly Ash H
(CaO=18.6%)
0.15
Fly Ash L (CaO=2.4%)
0.10
0.05
0.00
0
20
Fly Ash, %
25
Effect of Fly Ash on aggregate, Fly Ash H is a Class C, whereas Fly
Ash L is a Class F

28. Effect of Slag on ASR
(ASTM C1293)
0.10
% Expansion
0.08
0.06
1 yr limit
0.04
Control
25 % Slag
40 % Slag
50 % Slag
0.02
0.00
0
200
400
600
800
Time, days
1000
1200

29. Effect of SCMs ASR
Influence of different
amounts of Class F fly
ash, slag, and silica
fume by mass of
cementing material on
mortar bar expansion
(ASTM 1260) after 14
days when using
reactive aggregate.

30. Differences in Specifications
for Effectiveness of SCMs for ASR
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ASTM C595 (blended cement)
ASTM C1157 (performance hydraulic cement)
ASTM C989 (slag cement)
ASTM C618 (fly ash and natural pozzolans)
ASTM C1240 (silica fume)

31. Test Methods Comparisons
ASTM Specification
Test Method
Specification Limit
Test Control
C595 and
C1157
C227 (Pyrex)
0.020 %
@ 14 d
N/A
C989
C441
0.020% @14 d or 75
% reduction
High alkali
cement
C618
C441 (as
modified in
C311)
Max. 100% of
Control @14 d
Low alkali
cement
< 0.60%
C441
At least 80%
reduction
@ 14 d
High alkali
cement
C1240

32. The Effect of
Water-To-Cementitious Ratio Law
“For given materials the strength of the concrete
(so long as we have a plastic mix) depends solely
on the relative quantity of water as compared
with cement and/or cementitious regardless of
mix or size and grading of aggregate.”
--Duff A. Abrams
May 1918

33. Water Cementitious Ratio
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Strength increases as the w/cm ratio
decreases
Concretes with the same w/cm ratio but
different ingredients are expected to have
different strengths
A lower w/cm ratio reduces set time

34. Relationship of
Water to cement
or cementitious
ratio, as W/cm
ratio decreases
strengths increase.

35. The strength
relationships
holds true at
various
ages, 7-day
and 28-day
respectively

36. As the w/cm ratio increase so
does the volume does the volume
of concrete

37. Regardless of w/cm ratio, the
weight of concrete is constant

38. Effect of w/cm
The type of
cement does not
impact concrete
durability as much
as water to
cementitious
ratio, irregardless
of the type of
concrete specified
5
4
Visual
Rating
3
w/cm = 0.38
w/cm = 0.47
w/cm = 0.68
2
1
0
2
4
6
8
10
12
14
16
Age, years
Types I, II, V, blended cements, pozzolans, slag

39. Effect of Cement Type
5
Type V (4 % C3A)
Type II (8 % C3A)
Type I (13 % C3A)
4
Visual
Rating
3
2
1
2
w/c = 0.38
4
6
8
10
Age, years
12
14
16

40. Effect of w/cm
Type V Cement
w/c = 0.65
Visual Rating = 5 @ 12 years
Source: PCA
Type V Cement
w/c = 0.39
Visual Rating = 2 @ 16 years
Effect of w/c comparing Type V cement at various w/cm
ratios, higher w/cm ratio, concrete less durable

41. Durable Concrete
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A low w/cm will produce less permeable
concrete and provide greater protection
against aggressive environmental conditions
A w/cm of 0.40 and adequate cover over the
steel performs significantly better than
concretes made with w/cm of 0.50 and 0.60
Frost-resistant normal weight concrete should
have a w/cm not exceeding 0.50

42. Schematic of Total Shrinkage

43. Shrinkage

Volume Reduction due to loss of moisture
from a concrete matrix as it hardens and dries.
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Plastic Shrinkage
Thermal Contraction
Drying Shrinkage
Autogenous Shrinkage
Settlement Shrinkage

44. Drying Shrinkage Mechanism
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Loss of moisture
Meniscus forms at air-water interface due to
surface tension

45. Drying Shrinkage Mechanism


Surface tension
forces exert inward
pulling force on the
walls of the pores
Most significant in
pore sizes ranging
from 2.5-50 nm
Capillary
Tension

46. Reducing Total Shrinkage Potential
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Keeping the water (or paste) content low
The higher the cement content of a mixture
the higher shrinkage
Increase Coarse Aggregate Content
Avoiding aggregates that contain excessive
amounts of clay in their fines

47. Additional Concerns on Shrinkage
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High concrete temperatures demand
increased water demand
Reduction in the effectiveness of the air-void
system due to higher temperatures
Winter effects the primary danger is that low
temperatures may hydrate slower

48. Rate of Hydration
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Early strength of a concrete mixture will be
higher with an elevated temperature
Chemical reactions are faster at higher
temperatures
With increasing temperature, the potential for
an imbalance in the cementitious paste
system will be exacerbated
Cement fineness affects the rate of heat
generation

49. Rate of Heat Loss
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Influenced by the thickness of the concrete
sections
Thinner concrete sections will not get as hot as
thicker sections
Thermal expansion and contraction of concrete
depends on concrete mix design
Coefficient of thermal expansion (CTE) changes in
length (or volume) for a given change in
temperature dependent on aggregate CTE

50. Please return to Blackboard and watch
the following videos:

Video 1: Durability

51. Questions?
Email cemtek@netzero.net