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THE EFFECT OF PENNZSUPPRES®
D ON CALIFORNIA BEARING RATIO (CBR) VALUES
Products that improve the strength of locally abundant aggregates or
existing aggregates on unpaved roads can significantly reduce road
construction costs. The California
Bearing Ratio (CBR) test is one method that is used to measure the
strength of an aggregate or a roadbed. The independent Roads and Transport
Technology Division of the Council for Scientific and Industrial Research (CSIR)
of South Africa has measured the CBR of PennzSuppressâ
D treated aggregates and found that PennzSuppressâ
D can significantly improve the CBR value of an aggregate.
CSIR tested four locally available aggregates that consisted of granite,
andesite, ferricrete, and chert.
The physical properties of these aggregates are summarized in the following
table.
Physical Characteristics of Aggregates
| Sieve
Analysis |
SIEVE SIZE
|
AGGREGATE TYPE
|
|
(% Passing)
|
|
| |
(mm)1
|
Granite
|
Andesite
|
Ferricrete
|
Chert
|
|
37.5
|
100
|
|
|
100
|
| |
26
|
84
|
|
|
87
|
| |
19
|
77
|
100
|
100
|
85
|
| |
13.2
|
67
|
99
|
99
|
83
|
|
9.5
|
61
|
97
|
95
|
78
|
|
6.7
|
55
|
95
|
90
|
72
|
|
4.75
|
48
|
92
|
81
|
66
|
|
2
|
34
|
74
|
67
|
47
|
| |
0.85
|
22
|
48
|
61
|
35
|
| |
0.425 |
15
|
35
|
55
|
28
|
| |
0.25 |
12
|
29
|
49
|
24
|
| |
0.15 |
9
|
25
|
41
|
21
|
| |
0.075 |
7
|
20
|
31
|
16
|
| Atterberg
Limits and Bar Linear Shrinkage |
Liquid
Limit |
non-plastic
|
38
|
24
|
32
|
| |
Plastic
Limit |
non-plastic
|
28
|
18
|
24
|
| |
Placticity
Index |
non-plastic
|
10
|
6
|
8
|
|
Linear
Shrinkage |
non-plastic
|
7
|
7.1
|
5.5
|
| Density
& Moisture Content |
Max
Dry Density (kg/m3) |
2151
|
1980
|
2008
|
2030
|
|
Optimum
Moisture (%) |
7.4
|
12.7
|
10.5
|
13.3
|
| California
Bearing Ratio |
@
98% Mod AASHTO |
91
|
10
|
34
|
59
|
|
@
95% Mod AASHTO |
62
|
8
|
18
|
39
|
|
@
93% Mod AASHTO |
40
|
7
|
13
|
30
|
Each aggregate was treated with a 4:1 dilution (four parts water to one
part PennzSuppressâ
D) at three different application rates of 1.5 l/m2,
2.0 l/m2, and 2.5 l/m2 (equivalent to 0.33 gal/yd2,
0.44 gal/yd2, and 0.55 gal/yd2, respectively). Since
water is used to soak a sample for 96 hours prior to the CBR test, CSIR used
their modified CBR test that was developed for emulsified bitumen products. In
the CSIR modified CBR test, the sample was cured for 24 hours at 60o C
prior to testing.
With the exception of the granite, the best strength improvements were
obtained from the 2.0 l/m2 (0.44 gal/yd2) application
rate. Further improvements within the range of 59% to 103% above the control
sample were made to the CBR values of the granite aggregate using the 2.5 l/m2
(0.55 gal/yd2) application rate. A summary of the results of the
CBR values from the 2.0 l/m2 application is provided in the table
below.
CBR Values from Aggregates treated with 2.0 l/m2
of PennzSuppressâ D
| Aggregate |
Test
|
Control CBR
|
CBR @ 2.0 l/m2
|
Percent Improvement
|
| Granite |
98%
Mod AASHTO |
91
|
115
|
26%
|
|
95%
Mod AASHTO |
62
|
78
|
26%
|
|
93%
Mod AASHTO |
40
|
50
|
25%
|
| Andesite |
98%
Mod AASHTO |
10
|
34
|
240%
|
|
95%
Mod AASHTO |
8
|
30
|
275%
|
|
93%
Mod AASHTO |
7
|
24
|
243%
|
| Ferricrete |
98%
Mod AASHTO |
34
|
61
|
79%
|
|
95%
Mod AASHTO |
18
|
32
|
78%
|
|
93%
Mod AASHTO |
13
|
22
|
69%
|
| Chert |
98%
Mod AASHTO |
59
|
90
|
53%
|
|
95%
Mod AASHTO |
39
|
58
|
49%
|
|
93%
Mod AASHTO |
30
|
43
|
43%
|
As the data in the preceding table illustrate, significant improvements as
high as 275% above the control sample were recorded for the PennzSuppressâ D
samples. The biggest improvements were noted in materials that exhibited some
placticity. The greatest percentage increase in strength came from the weakest
aggregates. The results from the non-plastic granite samples indicate that
strength increase appears to increase in a linear fashion with increasing
application rates. For plastic materials, the 2.0 l/m2 application
rate appears to be the optimum application for strength increase for the
materials and application rates that were tested in this study.
CBR test data correlate with unconfined compressive strength data
previously generated for PennzSuppressâ D treated materials. Two separate
test methods confirm the strength improving properties of PennzSuppressâ D.
When used in road construction applications, aggregate strength improving
properties of PennzSuppressâ D will translate into:
- longer lasting roads;
- reduction of maintenance requirements; and,
- reduction of the thickness of required pavement overlays.
DEFINITIONS
California Bearing Ratio (CBR)
— A penetration test used to define the strength of a soil. The test uses a
circular piston of 1,935 mm2 to penetrate the surface of a
compacted material at a rate of 1.27 mm per minute to depths of 2.54 mm, 5.08
mm, and 7.62 mm. The penetration is expressed in Newtons and the CBR value is
calculated as follows:
With Standard penetration values of 13.344 kN, 20.016 kN, and 25.354 kN
for 2.54 mm, 5.08 mm, and 7.62 mm penetration depths, respectively.
Granite — Typically a light colored
crystalline igneous rock (formed from molten material) composed primarily of
aluminum silicate minerals containing abundant quartz.
Andesite — A dark colored finely
crystalline igneous rock (formed from molten material) composed primarily of
iron and magnesium silicate minerals that is relatively deficient in quartz.
Chert — A microcrystalline quartz formed in
sedimentary rocks. Chert is a hard dense rock composed of amorphous and
interlinking quartz crystals that are less than 30 microns in diameter.
Ferricrete — A conglomerate (naturally
cemented gravel) in which iron oxide is the primary cementing material.
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