Transcript Plastic Shrinkage Cracking

Plastic Shrinkage Cracking
How to make sure it happens!
Causing Plastic Shrinkage Cracking
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Specifications
Concrete Mix Design
Concrete Delivery
Placing the Concrete
Finishing the Concrete
Curing the Concrete
Specifications That Guarantee* Plastic
Shrinkage Cracking
Focus only on strength, and ignore all other
characteristics of the mix;
 Require a minimum cement content, and a
maximum water/cementitious ratio;
 Require or allow a “2 bin” mix to be used;
 Require only that an ASTM C33 sand and a
size 57 or 67 coarse aggregate be used;
(* = In real life you want to do the opposite!)
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Specifications That Guarantee* Plastic
Shrinkage Cracking
DO NOT EVEN MENTION EVAPORATION
POTENTIAL OR ITS LIMITS
 If “sufficiently foolish” to mention
evaporation potential, then make sure it is
measured only once sometime before the
actual placement.
(* = In real life you want to do the opposite!)
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Effects of Mix Design on Plastic
Shrinkage
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In order to insure* plastic shrinkage cracking,
make sure there is no bleed water;
Bleed water is a direct function of total
surface area in the mix:
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More surface area →Less bleed water
Less surface area → More bleed water
Gap graded mixes have more surface area.
(* = In real life you want to do the opposite!)
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Bleed water
visible as a
surface
shininess or
“sheen.”
Note
reflections
on watercovered
surface.
Plastic Shrinkage Cracking
1.) Bleed water appears on concrete surface
2.) Rate of evaporation exceeds bleed rate
3.) Concrete surface dries….
4.) Concrete surface tries to shrink….
5.) Moist concrete resists shrinkage….
6.) Stress develops in soft “plastic” concrete….
7.) “Plastic” shrinkage cracks form
Grading Influences:
 Total
Surface Area of Mix Components
Equivalent Volume
Equivalent Volume
Surface Area
290 in2
1800 in2
Surface Area of Cement
A single pound of typical Type I-II
cement has an approximate surface
area of:
 274,500 square inches
 1,900 square feet
Surface Area of Cement
The cement in a cubic yard of 6 ½ sack
concrete has a total surface area of:
 167,545,976 square inches
 1,163,513 square feet
 26.8 acres
Summary on Surface Area
More surface area requires more paste to coat each
particle;
 Larger particles have less surface area than smaller
particles;
 The best way to maximize aggregate volume and
minimize surface area is to use a uniform aggregate
gradation – Make sure you use gap graded
mixes*
 Too much cement is as harmful as too little!
(* = In real life you want to do just the opposite!)
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Impacts from Environmental
Factors
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Concrete Mix Temperature
Ambient Air Temperature
Wind Speed
Relative Humidity
All of this is tied together in the standard
Evaporation Potential Nomograph:
Evaporation Rate for Acoma Interchange I-40 Exit 102
June 6-7, 2005
0.35
0.3
Evaporation Rate
30 Minute Evaporation Rate
Evaporation Rate
0.25
0.2
0.15
0.1
0.05
0
12:00:00 AM
2:24:00 AM
4:48:00 AM
7:12:00 AM
9:36:00 AM
Time
12:00:00 PM
2:24:00 PM
4:48:00 PM
7:12:00 PM
12 :0
:0 7 P
9
12 :07 M
:4 A
7
M
1: :12
25 A
M
2: :12
01 A
:5 M
2: 2
39 A
M
3: :52
17 A
: M
3: 52
55 A
M
4: :52
33 A
: M
5: 52
11 A
M
5: :52
49 A
: M
6: 52
27 A
M
7: :52
05 A
: M
7: 52
43 A
M
8: :52
21 A
:5 M
8: 2
59 A
M
9: :52
37 A
10 :52 M
:1 A
10 5:52 M
:5 A
3
11 :52 M
:3 A
1
12 :52 M
:0 A
12 9:52 M
:4 P
7 M
1: :52
25 PM
2: :52
03 PM
:
2: 52
41 PM
3: :52
19 PM
:
3: 52
57 PM
4: :52
35 PM
:5
2
PM
Windspeed (mph)
Evaporation Potential
Time of Day
0.35
Time of Day
0.3
0.25
0.2
0.15
0.1
0.05
0
4:53:53 PM
4:17:52 PM
3:41:52 PM
3:05:52 PM
2:29:52 PM
1:53:52 PM
1:17:52 PM
12:41:52 PM
12:05:52 PM
11:29:52 AM
10:53:52 AM
10:17:52 AM
9:41:52 AM
9:05:52 AM
8:29:52 AM
7:53:52 AM
7:17:52 AM
6:41:52 AM
6:05:52 AM
5:29:52 AM
4:53:52 AM
4:17:52 AM
3:41:52 AM
3:05:52 AM
2:29:52 AM
1:53:52 AM
1:19:12 AM
12:43:12 AM
Windspeed (mph)
Air Temperature
11:47:22 PM
9:37:07 PM
The relationship of air
temperature (Ta) to
Evaporation Potential
is:
E ~ (Tc2.5-rTa2.5) x 106
37
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9:
Effects of Air Temp on Evaporation
Potential
90
80
70
60
50
40
30
20
10
0
12 :0
:0 7 P
9
12 :07 M
:4 A
7: M
1: 12
25 A
: M
2: 12
01 A
: M
2: 52
39 A
: M
3: 52
17 A
: M
3: 52
55 A
: M
4: 52
33 A
: M
5: 52
11 A
: M
5: 52
49 A
: M
6: 52
27 A
: M
7: 52
05 A
: M
7: 52
43 A
: M
8: 52
21 A
: M
8: 52
59 A
: M
9: 52
37 A
10 :52 M
:1 A
5
10 :52 M
:5 A
3
11 :52 M
:3 A
1
12 :52 M
:0 A
9
12 :52 M
:4 P
7: M
1: 52
25 PM
:
2: 52
03 PM
:
2: 52
41 PM
:
3: 52
19 PM
:
3: 52
57 PM
:
4: 52
35 PM
:5
2
PM
Windspeed (mph)
Time of Day
0.35
Time of Day
0.3
0.25
0.2
0.15
0.1
0.05
Evaporation Potential
0
4:53:53 PM
4:17:52 PM
3:41:52 PM
3:05:52 PM
2:29:52 PM
1:53:52 PM
1:17:52 PM
12:41:52 PM
12:05:52 PM
11:29:52 AM
10:53:52 AM
10:17:52 AM
9:41:52 AM
9:05:52 AM
8:29:52 AM
7:53:52 AM
7:17:52 AM
6:41:52 AM
6:05:52 AM
5:29:52 AM
4:53:52 AM
4:17:52 AM
3:41:52 AM
3:05:52 AM
2:29:52 AM
1:53:52 AM
1:19:12 AM
12:43:12 AM
Windspeed (mph)
Relative Humidity
11:47:22 PM
9:37:07 PM
The relationship of
relative humidity to
Evaporation Potential
is:
E ~ (Tc2.5-rTa2.5) x 106
37
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9:
Effects of Relative Humidity on
Evaporation Potential
60
50
40
30
20
10
0
12 :0
:0 7 P
9
12 :07 M
:4 A
7
M
1: :12
25 A
M
2: :12
01 A
:5 M
2: 2
39 A
M
3: :52
17 A
: M
3: 52
55 A
M
4: :52
33 A
: M
5: 52
11 A
M
5: :52
49 A
: M
6: 52
27 A
M
7: :52
05 A
: M
7: 52
43 A
M
8: :52
21 A
:5 M
8: 2
59 A
M
9: :52
37 A
10 :52 M
:1 A
10 5:52 M
:5 A
3
11 :52 M
:3 A
1
12 :52 M
:0 A
12 9:52 M
:4 P
7 M
1: :52
25 PM
2: :52
03 PM
:
2: 52
41 PM
3: :52
19 PM
:
3: 52
57 PM
4: :52
35 PM
:5
2
PM
Windspeed (mph)
Evaporation Potential
Time of Day
0.35
Time of Day
0.3
0.25
0.2
0.15
0.1
0.05
0
4:53:53 PM
4:17:52 PM
3:41:52 PM
3:05:52 PM
2:29:52 PM
1:53:52 PM
1:17:52 PM
12:41:52 PM
12:05:52 PM
11:29:52 AM
10:53:52 AM
10:17:52 AM
9:41:52 AM
9:05:52 AM
8:29:52 AM
7:53:52 AM
7:17:52 AM
6:41:52 AM
6:05:52 AM
5:29:52 AM
4:53:52 AM
4:17:52 AM
3:41:52 AM
3:05:52 AM
2:29:52 AM
1:53:52 AM
1:19:12 AM
12:43:12 AM
Windspeed (mph)
Wind speed
11:47:22 PM
9:37:07 PM
The relationship of
wind speed to
Evaporation Potential
is:
E ~ (1+0.4V) x 106
37
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9:
Effects of Windspeed on Evaporation
Potential
20
18
16
14
12
10
8
6
4
2
0
Evaporation Retarders
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EVAPORATION RETARDERS ARE NOT
CURING COMPOUNDS
Evaporation retardiers are applied to the
surface of the freshly “struck off” concrete to
prevent the excessive loss of bleed water
while the concrete is initially setting;
Evaporation Retarders
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Create a mono-molecular film essentially the
same as a large water-proof soap bubble
laying on the surface of the concrete;
The film is extremely fragile and will be
completely destroyed if touched lightly by
any finishing tools
It should remain untouched until the surface is
ready for the final floating and finishing.
Factors to insure* Plastic Shrinkage
Cracking
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Mix Design
Use smaller top sized aggregate to make it easier to finish
the mix;
 Use gap graded “2 bin” mix;
 DO NOT use uniform gradation;
 Use extra cement to make sure compressive strength is
achieved far earlier than specified;
 Oversand the mix to make the finishers happy.
 Make sure sand is extra dirty.
(* = In real life do you want to do the opposite!)
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Factors to insure* Plastic Shrinkage
Cracking
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Environmental Factors
Make sure that concrete is placed under windy,
dry conditions;
 Do not consider evaporation potential;
 Do not erect or consider use of wind breaks;
 Place concrete at convenient time of day,
regardless of weather or wind;
(* = In real life you want to do the opposite!)
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Factors to insure* Plastic Shrinkage
Cracking
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Environmental Factors (continued)
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Do not use evaporation retarder. If evaporation
retarder is used, make sure finishers get onto slab
immediately after placement so film is destroyed;
(* = In real life you want to do the opposite!)
CURING
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A good curing system makes sure that the
evaporation potential at the surface of the
concrete is 0.0;
In normal climates, a single curing system is
adequate;
In New Mexico, multiple curing systems
should be considered mandatory.
Different Curing Systems
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In order of most effective to least effective;
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Flooding with water;
Covering with saturated burlap or burleen (and
making sure burlap does not dry out);
Covering with visqueen;
Double coat of curing compound;
Single Coat of curing compound.
To Insure* Plastic Shrinkage Cracking
from Lack of Curing
Do not use any curing system;
 If curing system is used, use only one method,
and apply that as late as possible, just before
leaving project for the evening;
 If burlap is used, make sure it is dry or
becomes dry soon, so moisture is actually
wicked from the concrete;
(* =In real life you want to do the opposite!)
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To Insure* Plastic Shrinkage Cracking
from Lack of Curing
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Do not consider a multiple system, such as:
Curing compound applied immediately after final
finish;
 Applying a second coat of curing compound in a
direction perpendicular to that of first coat;
 Covering curing compound with saturated burlap;
 Covering saturated burlap with visqueen,
weighted down around edges.
* = In real life you want to do the opposite!
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