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 PennzSuppress^â D treated aggregates and found that PennzSuppress^â 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 PennzSuppress^â 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 60^o 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 PennzSuppress^â 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 PennzSuppress^â 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 PennzSuppress^â D treated materials. Two separate test methods confirm the strength improving properties of PennzSuppress^â D. When used in road construction applications, aggregate strength improving properties of PennzSuppress^â 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.