DEVELOPMENT OF EQUIPMENT FOR COMPACTING SOIL-CEMENT INTO

PLASTIC MOLDS FOR DESIGN AND QUALITY CONTROL PURPOSES

W. Griffin Sullivan

Pavement Engineer In Training

Materials Division

Mississippi Department of Transportation

PO Box 1850, Jackson, MS 39215

601-359-1755 (ph) 601-359-1716 (fax) wsullivan@mdot.ms.gov

(Corresponding Author)

Isaac L. Howard

Associate Professor

Materials and Construction Industries Chair

Civil and Environmental Engineering

Mississippi State University

501 Hardy Road-Mail Stop 9546, Mississippi State, MS 39762

662-325-7193 (ph) 662-325-7189 (fax) ilhoward@cee.msstate.edu

Brennan K. Anderson

Former Graduate Research Assistant

Department of Civil and Environmental Engineering

Mississippi State University

501 Hardy Road-Mail Stop 9546, Mississippi State, MS 39762

662-325-3050 (ph) 662-325-7189 (fax) bka27@msstate.edu

Original Submission: August 1, 2014

4963 Words, 6 Figures (1500 words), 4 Tables (1000 words) = 7463 Total Equivalent Words

Revised Version 1: November 14, 2014

5243 Words, 5 Figures (1250 words), 4 Tables (1000 words) = 7493 Total Equivalent Words

Paper Prepared for Consideration for Presentation and Publication at the

94th

Annual Meeting of the Transportation Research Board

Sullivan et al. 1

ABSTRACT

In current soil-cement practice there exists a disconnection between laboratory mixture design,

pavement layer thickness design, and construction quality control. A device was developed to

integrate all three aspects by allowing compaction of soil-cement into single-use plastic cylinder

molds. The device is a metal split mold design that surrounds the slightly modified plastic mold

to prevent distortion of the specimen during compaction. Compaction was performed by two

methods: 1) a custom-built compaction frame (similar concept to ASTM D 1632 device), and 2)

a manual modified proctor hammer. This paper’s objective is to demonstrate the feasibility of

using this device to produce suitable specimens and provide discussion of possible applications.

Over 750 soil-cement specimens were compacted under laboratory conditions using the new

device. Specimens were analyzed for final dimensions (e.g. diameter, height), unconfined

compressive strength variability, and elastic modulus. Analysis showed that the device can

produce acceptable test specimens with unconfined compressive strength variability similar to

traditional Proctor specimen variability, and specimen elastic modulus values were observed to

be similar to those found in literature. Currently, the Mississippi Department of Transportation is

working towards incorporating this new compaction device into their soil-cement practices.

Sullivan et al. 2

INTRODUCTION AND BACKGROUND

Since the early 1930s, chemically stabilized soils have been utilized as an engineered material

for highway construction, particularly, in areas where native soils have marginal engineering

properties. The most common form of chemical stabilization is soil-cement, which is usually a

mixture of soil, water, and portland cement (1). Other chemical stabilization materials (or blends

of materials) can also be used, but portland cement is most common.

Soil-cement laboratory mix design is usually performed by state Departments of

Transportation (DOTs) using Proctor compacted specimens. Soil-cement pavement layers are

typically constructed using in-place roadway mixers and compaction equipment that is

monitored by method based approaches. A common method based approach to construction and

quality control (QC) is to monitor individual material quantities (i.e. soil, water, portland

cement) and final in place density absent any other verification that desired properties (e.g.

strength, elastic modulus) were achieved. Pavement layer thickness design is typically performed

using coefficients (e.g. AASHTO 1993 “a” and “m” terms) or with correlations to laboratory

properties (e.g. elastic modulus correlated to unconfined compressive strength) that were likely

measured on specimens prepared differently than they were during laboratory mix design.

Although soil-cement is used by several DOTs and has been for decades, techniques to

integrate laboratory mix design, pavement layer thickness design, and construction quality

control have been modest. Pavement performance is a function of all three components. With

implementation of the Mechanistic-Empirical Pavement Design Guide (MEPDG) on the horizon

for several DOTs, there arguably exists an even greater need to interface these three components

than there has been in previous years.

The Mississippi DOT (MDOT) relies heavily on soil-cement for pavement construction

due, at least in part, to an abundance of marginal native soils and lack of sufficient quantities of

native aggregates. In an effort to address the aforementioned need to integrate soil-cement

performance aspects, MDOT funded State Study 206 (SS206) at Mississippi State University, or

MSU (2). One of the SS206 outcomes was a new device, referred to as the PM device, which

was developed to allow compaction of soil-cement inside 76.2mm by 152.4mm plastic cylinder

molds.

The acronym PM refers to “plastic mold”. The PM device was designed to prevent

distortion of the plastic mold during compaction, and after removal from the mold, test

specimens have an approximate 2:1 height to diameter (h/d) aspect ratio. A 2:1 h/d ratio enables

elastic modulus testing (ASTM C469) for the MEPDG and is theoretically more suitable for

accurate unconfined compressive strength (UCS) test results. The device can be used in the

laboratory or in the field, and as a result, can serve as the medium to integrate laboratory mix

design (i.e. select cement content to achieve desired UCS), pavement layer thickness design (i.e.

measure the elastic modulus resulting from the design cement content for MEPDG input), and

construction quality control (i.e. produce specimens during construction in the same manner as

during design to verify UCS and/or elastic modulus).

The objective of this paper is to demonstrate the feasibility of using the PM device to

produce reasonable soil-cement specimens. A literature and practice review is presented that

supports the potential usefulness of the device. Thereafter, an experimental program was

conducted where hundreds of experiments were performed to validate the ability of the PM

device to produce reasonable soil-cement specimens, and also to measure their UCS and elastic

modulus. A discussion on potential implications of the PM device is then provided emphasizing

MDOT, and a series of concluding remarks are made.

Sullivan et al. 3

LITERATURE AND PRACTICE REVIEW

Soil-Cement Practices for USACE, PCA, and Southeastern DOTs

Methods for soil-cement design and construction quality control differ between the United States

Army Corps of Engineers (USACE), Portland Cement Association (PCA), and state DOTs. The

USACE and PCA design soil-cement mixtures based on durability and UCS. Durability is

determined using freeze-thaw (ASTM D560) or wet-dry (ASTM D559) testing. AASHTO T136

and AASHTO T135 are active test methods that can also be used for durability purposes.

TABLE 1 shows USACE and PCA design criteria. A mixture at the optimum cement content has

a weight loss less than the appropriate value in TABLE 1 and has an UCS that falls within the

given range or meets the noted minimum value (3,4).

TABLE 1 USACE and PCA Soil-Cement Design Criteria

Agency Soil Type

Max Weight Loss

after 12 Cycles1

(%)

7-day UCS

(kPa)

28-day UCS

(kPa)

PCA A-1, A-2, A-3 14 2069 - 4137 2758 - 6895

PCA A-4, A-5 10 1724 - 3447 2069 - 6205

PCA A-6, A-7 7 1379 - 2758 1724 - 4137

USACE Granular, PI < 10 11 3447 or 51712 Not available

USACE Granular, PI > 10 8 3447 or 51712 Not available

USACE Silt 8 3447 or 51712 Not available

USACE Clay 6 3447 or 51712 Not available

Unconfined Compressive Strength (UCS) tests performed according to ASTM D1633.

PI = plasticity index.

A-1 to A-7 are AASHTO M145 designations.

1: Cycles are either freeze-thaw (ASTM D560 or AASHTO T136) or wet-dry (ASTM D559 or AASHTO T135).

2: minimum requirements; 3447kPa for rigid pavements and 5171kPa for flexible pavements.

Presently, DOTs mostly rely on laboratory UCSs from Proctor specimens (h/d aspect

ratio of 1.15:1) to determine design cement contents. Designing based on UCS is appealing to

state DOTs because it takes less time (1-4 weeks) as compared to freeze-thaw and dry-wet

testing (4-6 weeks). Although most DOTs rely on UCS, each state DOT has generally developed

their own minimum strength criteria, curing protocols, and quality control programs. As an

example, TABLE 2 contains information concerning thirteen state DOT soil-cement practices in

the Southeast United States. There are several different protocols represented within these states.

Separate from the data presented in TABLE 2, a survey was made available to DOT’s at

the 98th

AASHTO Subcomittee Meeting on Materials (August 2012). Twenty DOTs responded

to the survey. Unconfined compression testing was, as expected, the most prevalent design

parameter with strength requirements of 689 to 5171kPa reported. At least ten of the responding

DOT’s used Proctor compacted specimens for design strength.

Field quality control is, in most cases, generally defined by six aspects as per USACE,

PCA, or the American Concrete Institute (ACI). They are: pulverization; cement content (spread

rate checks); moisture content; uniform mixing (visual check); compaction; and curing (4-6).

Most DOT practices are similar to those of USACE, PCA, and ACI, and most of the DOTs

investigated commonly check pulverization, cement content (spread rate), moisture content, and

Sullivan et al. 4

final density. Only Virginia DOT verified the cement content according to ASTM D806 after

mixing.

The aforementioned DOT survey found five of twenty respondents incorporating field

specimens into quality control operations. Three respondents indicated making field specimens,

while two respondents indicated cutting cores. All factors considered, consistent and widespread

field specimen preparation practices for strength and modulus verification does not appear

commonplace for field QC.

TABLE 2 Soil-Cement Practices for Southeastern United States DOT’s

State1 h/d ratio

2

Required Design UCS

(kPa) Lab Curing Protocol

AL 1.15 1720 to 4140 7-day moist cure, sealed in bag, 5hr soak

AR 1.15 or 1.00 2760 7-day moist cure, sealed in bag, 5hr soak

GA 1.15 2070 7-day moist cure, no soak

KY3

2.00 689 minimum 2-day moist cure at 49°C

LA 1.15 or 1.00 1034 to 3450 7-day moist cure, no soak

MS 1.15 or 1.00 1380 or 2070 7 & 14-day moist cure, sealed in bag, 5hr soak

NC 1.15 1380 7-day moist cure, 5hr soak

SC 1.00 or 0.76 Not available 7-day moist cure, overnight soak

TN4

1.15 8274 to 11721 28-day moist cure, 4hr soak

TX 1.33 1210 or 2070 7-day moist cure, no soak

VA5 1.15 Not available 7 & 28-day moist cure, 4hr soak

Table information was obtained from state DOT standard specifications and DOT officials as of May 2012.

1: FL and WV no longer utilize soil-cement in highway construction.

2: Specimen h/d ratio depended on material gradation in some cases.

3: KY utilizes soil-cement as a subgrade modifier to increase UCS by 300kPa with a minimum UCS of 689kPa.

4: Strength criteria shown in Table 2 are for subgrade treatments. For base courses, TDOT uses 5% cement by dry

mass for gravel materials and 4% for limestone materials. These dosages are applied with no design procedures.

5: Virginia requires durability testing for design.

MDOT Soil-Cement Practices From January 2009 through mid-July of 2014, MDOT has mixed and placed approximately 181

million square meters (218 million square yards) of soil-cement. Generally speaking, this would

equate to a soil-cement strip 4.6m wide by 38,600km long (or 15ft wide and 24,000miles long).

Shortages of local quality aggregates have led MDOT to heavily rely on soil-cement mixtures for

pavement subbase and base courses.

Materials used for soil-cement vary from A-6 and A-4 (usually for subbase courses) to A-

2-4 (used for base courses and most commonly used) based on AASHTO M145 classification. If

these soils are not located on site, then they are transported from nearby borrow pits. Type I

portland cement is most commonly used.

MDOT soil-cement mixture design is governed by Mississippi Test Method 25 (MT-25),

and is performed by MDOT Materials Division. MT-25 does not evaluate durability but relies on

UCS to determine the optimum cement content. Minimum 7 or 14-day UCSs for design are

1380kPa (200psi) for subbase layers and 2070kPa (300psi) for base layers. The design cement

content is the minimum cement content that achieves required UCS at 7 days for subbase designs

and 14 days for base designs. Mississippi Test Methods 8 and 9, which are similar to AASHTO

T99 and AASHTO T134, are performed to generate untreated and treated Proctor curves.

Sullivan et al. 5

Treated Proctors are performed at an estimated optimum cement content. Six specimens are

prepared (two at estimated cement content, two at ±1%) according to MT-9. Proctor specimens

are removed from molds, placed in plastic bags and cured in a moisture room. After 7 and 14

days, one specimen at each cement content level is tested for UCS according to MT-26 (similar

to ASTM D1633).

MDOT maintains statewide records of soil-cement mixture designs, and this database

was analyzed from January 2005 through December of 2010 for use herein. Ninety four base

course mix designs using A-2-4 soil were investigated and their average design cement content

by mass was 4% (range of 2.7-6.3%). Design cement contents for base courses in this general

range has been working well for MDOT, and MDOT is not necessarily interested in modifying

these values in an overall sense. This is noteworthy because the PM approach presented herein

can be implemented in a manner that does not result in noticeable shifts in cement content within

the auspices of an agency. It could also be incorporated in a matter to refine design cement

contents should that be desired for another agency.

MDOT’s construction practices and quality control measures are method based. Soil-

cement layers are commonly constructed using in-place mixers. Quality control measures include

cement spread rates, material pulverization, moisture content, mixing uniformity (visual

inspection), and final density. After construction, the soil-cement layer is kept moist and covered

with a bituminous curing seal within 24 hours. No traffic, construction or public, is allowed on

the soil-cement layer for 7 or 14 days depending on recommendations from the mix design.

Within MDOT there exists a disconnection between laboratory mix design, pavement

layer thickness design, and construction quality control. Soil-cement mixtures are designed to

have a desired UCS, but the strength of material after field mixing is not checked.

Strength and Modulus Literature Review

According to Griffin and Tingle (7), there is no standard method (besides field cores) for

determining the strength capacity of cement stabilized soils after construction. Conceptually,

three general approaches can be taken to this problem: 1) strength indices such as the dynamic

cone penetrometer or Clegg Hammer; 2) maturity or thermal profile approaches; and 3)

compacting specimens on site. Griffin and Tingle (7) is a good reference for strength indices, and

Howard et al. (2) provides laboratory and field data related to thermal profile approaches that

interface with the PM device described in this paper. This paper indirectly addresses compacting

strength specimens on site for quality control purposes.

Several research efforts have been made to correlate UCS and elastic modulus values of

chemically stabilized materials. Howard et al. (2) provides a more detailed literature review with

studies of direct pertinence to this paper presented herein.

Kolias and Williams (8) derived a relationship between gradation modulus, uniaxial

compressive strength (i.e. unconfined compressive strength), and modulus of elasticity for

chemically stabilized soils. The gradation modulus is defined as the sum of the percentages

passing the 37.5mm, 19.0mm, 9.5mm, 4.75mm, 2.36mm, 1.18mm, 600μm, 300μm, 150μm and

75μm sieves and dividing by 100. The approach taken by (8) gives a rapid approximation of the

modulus of elasticity without direct laboratory testing. Soils tested ranged from a flint gravel

aggregate to a fine grained silty material, and specimen dimensions included prismatic (101.6mm

by 101.6mm by 254mm) and cylindrical (101.6mm diameter by 254mm tall). The observed

relationship between gradation modulus, UCS, and modulus of elasticity is expressed in

Equation 1. This relationship was verified with multiple other studies (9-11).

Sullivan et al. 6

Efp = (15.5 – 1.3*G)(fp)1/2

[1]

Where:

Efp = modulus of elasticity at a strength level of fp (GPa)

fp = uniaxial compressive strength (MPa)

G = gradation modulus

James et al. (12) conducted a study on seven typical Mississippi soils used for chemically

stabilized pavement layers. One objective of this study was to observe density and moisture

effects on the measured UCS and elastic modulus of soil-cement mixtures. Equation 2 was used

to calculate elastic modulus using measured UCS results. Note that the MEPDG also uses

Equation 2 as a level 2 input.

E = 1200*qu [2]

Where:

E = elastic modulus (psi)

qu = unconfined compressive strength (psi)

EXPERIMENTAL PROGRAM

Two types of compaction were performed with the PM device: 1) custom-built large compaction

frame not conforming to any current test method designation (referred to as PM-CF) and 2)

compaction with a modified proctor hammer (referred to as PM-P). Over 750 laboratory

specimens were compacted using the PM-CF and PM-P approaches. To evaluate dimension

variability, all specimens were analyzed for dimensionality (e.g. diameter, height) after

compaction. A subset of specimens (180) was analyzed for UCS variability as compared to

traditional Proctor specimens. Another subset of specimens (54) was evaluated to determine

elastic modulus according to ASTM C469. Reference (2) provides several additional

experimental details.

Materials Tested

Five soils were sampled from project borrow pits (referred to as Pit soils) and evaluated. These

soils are typical for soil-cement base construction in Mississippi. TABLE 3 shows pertinent soil

properties, while cement properties can be found in (2). For the UCS variability and elastic

modulus analysis, Pit soils A, B, and C were evaluated at their respective design cement contents

using Type I portland cement from one source. Pit soils D and E were only evaluated for

feasibility of producing specimens in plastic molds.

Sullivan et al. 7

TABLE 3 Properties of Pit Soils Tested

Soil Property Pit A Pit B Pit C Pit D Pit E

% Passing 2 mm (No. 10) 100 100 100 100 100

% Passing 420 μm (No. 40) 79 95 90 98 100

% Passing 250 μm (No. 60) 60 62 54 71 80

% Passing 150 μm (No. 100) 25 27 30 ― ―

% Passing 105 μm (No. 140) 21 25 27 ― ―

% Passing 75 μm (No. 200) 20 24 26 19 16

Plasticity Index NP NP NP NP NP

USCS Classification SM SM SM SM SM

AASHTO M145 Classification A-2-4 A-2-4 A-2-4 A-2-4 A-2-4

MDOT Classification1

9C 9C 9C 9C 9C

Design Cw (%)2 4.2 4.3 3.2 6.2 6.4

Treated Optimum γd (kg/m3)

3 1920 1812 1935 1796 1737

Treated OMC (%)2 11.8 14.0 11.4 14.7 13.6

― = no data available; NP = non plastic; USCS = unified soil classification system.

1: Criteria for class 9C material can be found in the 2004 Mississippi Standard for Road and Bridge Construction

under section 703.07 for granular material.

2: Design cement content by dry mass of soil (Cw) is based on a minimum 2070kPa (300psi) unconfined compressive

strength design criteria as determined by Mississippi Test Method 25.

3: Treated max dry density (γd) and optimum moisture content (OMC) were determined by MT-9 (similar to

AASHTO T134).

Compaction Equipment

FIGURE 1 shows photos of the compaction equipment designed by MSU and fabricated by a

local machine shop. Detailed drawings for all components are located in (13).

Plastic molds used measured 76.2 by 152.4mm and met the requirements of AASHTO

M205 for single-use concrete molds. Molds were modified slightly by sanding a ridge off the

bottom and cutting a 35mm diameter hole in the bottom to facilitate specimen extraction from

the mold after compaction. A 76.2mm diameter and 1.6mm thick aluminum plate was inserted

into the mold to provide a solid surface for specimen extraction, and the plastic cut-out was taped

to the mold bottom to fill in the gap between the plate and exterior mold bottom.

The PM device is a metal split-mold designed to closely surround the outside diameter of

a plastic mold to prevent distortions during compaction. A top collar helps with alignment to

prevent impact with edges of the plastic mold and aids with material transfer. The PM-CF device

was designed to compact a known amount of material to a prescribed height, thus achieving a

target density. For PM-CF, the PM device was attached to the frame base, and the compaction

head (connected to a guide rod) was lowered into the PM device to rest on top of the soil.

Compaction was performed by dropping a 6.8kg weight from a height of 305mm and hitting a

striker plate which transfers the energy to the soil. This design is very similar conceptually to the

dropping-weight compacting machine described in ASTM D1632. The PM-P device was

designed to be more portable which allows for use in a laboratory or a field environment. In this

configuration, the PM device was attached to a base plate and a manual modified Proctor

hammer (4.54kg mass falling 457.2mm) was utilized for compaction.

Sullivan et al. 8

FIGURE 1 Photos of PM Device and Associated Equipment.

Specimen Preparation and Testing Specimen Preparation with the PM-CF and PM-P approaches is illustrated in FIGURE 2. Mixing

(FIGURE 2a) was performed using a 19 liter stationary bucket mixer. Soil, water, and cement

were mixed for four minutes before compaction. Moisture content tolerances were ±0.5% of

OMC (optimum moisture content from treated Proctor) as determined by MT-9. Note that other

mixing methods could be used, if desired.

Specimens were compacted by three approaches (Proctor, PM-CF, and PM-P). For each

approach, specimens were compacted in three lifts, and scarification was performed after lifts 1

and 2 to mitigate compaction planes (FIGURE 2f is an example). Proctor compaction (not shown

in FIGURE 2) was performed with a mechanical hammer with standard effort (25 blows per lift

with a 2.5kg hammer).

PM-CF is a laboratory compaction approach developed prior to the PM-P approach. PM-

CF as performed herein was designed to re-create maximum dry density (γd) from a traditional

Proctor compaction curve (98 to 101% of γd was allowed). Note that PM-CF was not intended to

replace a Proctor mold and hammer to determine γd and OMC. Knowing γd, each lift was pre-

weighed to achieve the target density (see (2) for equation developed to determine lift quantities)

and introduced into the mold one lift at a time (FIGURE 2b). Specimens were compacted as

shown in FIGURE 2c to the desired height (etched marks shown in FIGURE 2e). On average,

PM-CF specimens required 10 to 13 blows per lift to achieve the target γd. From a compaction

energy standpoint, 7 blows is approximately standard compaction effort. FIGURE 2g shows a

specimen after compaction with the PM device open. After compaction, the top surface was

struck off with a straight edge (FIGURE 2h), and each specimen was covered with a plastic lid.

Aluminum Plate

Plastic Cut-Out

35 mm

(b) PM Device (c) PM-CF Compactor (e) PM-P Compactor

(a) Bottom View of Modified

Plastic Mold and Components

(d) PM-CF Compactor Base

Sullivan et al. 9

(a) Material Mixing (b) Measured Lift (c) PM-CF Compaction (d) PM-P Compaction

(e) PM-CF Head at Height (f) Scarifying Surface and Tool (g) Finished Compaction

(h) Striking off Surface (i) Specimen Extraction (j) Before Testing (k) After Testing

FIGURE 2 PM-CF and PM-P Specimen Preparation and Testing.

PM-CF PM-P

Etched Mark Plastic Mold

Plastic

Mold 1.97:1 h/d

Sullivan et al. 10

PM-P is a laboratory or field compaction approach. FIGURE 2d shows the device being

used on an MDOT project. For simplicity of field use, an equation was developed (see (2) for

details) to calculate the amount of soil needed per lift as a function of the soil’s γd and OMC. A

small experiment was conducted on laboratory prepared materials to determine how many blows

with a 4.54kg modified Proctor hammer would be needed to achieve target specimen density.

Five blows per lift yielded specimen densities between 96 and 100% of γd; therefore, 5 blows per

lift were used for data presented in this paper. For field prepared materials, 5 blows per lift

yielded 92 to 100% of γd indicating blow numbers may need to be considered on a project by

project basis.

Work omitted from this paper for brevity but provided in (2) developed density

correction relationships in field and laboratory testing for two MDOT soil-cement projects. The

resulting equations allow UCS adjustments as a function of specimen density while maintaining

ease of field use (these equations are not directly applicable to this paper). One PM-P

configuration and one operator can produce dozens of field specimens per day that can be

immediately handled. All that is needed is sufficient quantities of modified plastic molds.

Specimens can be left on site for traffic opening analyses, taken to the lab and cured in standard

conditions, or other as desired by an entities QC program.

Proctor specimens were cured under a damp towel for 2 hours and then moved to a moist

curing room. PM-CF and PM-P specimens were cured inside the covered plastic molds for 24

hours before being extracted (FIGURE 2i) and taken to the same moist curing room as Proctor

specimens. Calipers were used to measure diameter and height of all compacted specimens. Two

diameter measurements were taken orthogonal to each other at the top edge and bottom edge of

each specimen. Height measurements were taken at four locations equally spaced around the

specimen circumference. This measured data was used to calculate wet density that was

converted to dry density by measuring moisture content.

UCS testing was performed after 7 days of curing using compression machine equipment

and procedures described in ASTM D1633 as the basis. There was no soaking period prior to

testing. FIGURES 3j and 3k show representative PM-CF specimens before and after testing.

Elastic modulus testing was performed using equipment and procedures described in ASTM

C469 as the basis. The effective gage length from the compressometer collar was the middle

101.6mm of the specimen. Each specimen was first pre-loaded to approximately 40% of their

ultimate stress. Thereafter, specimens were loaded again and readings taken up to 40% of

ultimate stress were used to calculate elastic modulus.

TEST RESULTS

Results were divided into three categories in general accordance with the key items to be

integrated (laboratory mix design, pavement layer thickness design, and field QC). First,

feasibility of producing reasonable specimens is demonstrated, as this is necessary for all

activities. Second, UCS and density data is presented for each compaction method for purposes

of discussing relative unconfined compressive strengths and variability. This data set is mostly

presented from the perspective of laboratory mix design, though variability also plays into QC

activities. Third, elastic modulus data is presented for purposes of pavement layer thickness

design with the MEPDG. Of the key activities, field QC is given the least attention, though

between test results and descriptions presented in previous sections, reasonable QC support has

been provided.

Sullivan et al. 11

Feasibility of Producing Soil-cement Specimens Inside a Plastic Mold

FIGURE 3 shows relative frequency histograms of average top and bottom diameters of 752

compacted specimens. Averages include two orthogonal diameter measurements. The Mean,

standard deviation (Stdev), and coefficient of variation (COV) for the data are also reported.

Both PM-CF and PM-P approaches are represented in FIGURE 3. On average, there was a slight

taper from top to bottom of the specimens of about 0.4mm, and the majority of the data falls

above the nominal diameter of 76.2mm. Most of this variability may be contributed to dimension

variations of the plastic molds as allowed by AASHTO M205. In particular, AASHTO M205

allows the average inner diameter of the mold (i.e. the outer diameter of specimen) to vary up to

1% from the nominal measure (76.2mm). Therefore, the diameter of specimens could vary up to

77.0mm and still be considered acceptable. Approximately 7% of measured specimens had an

average top diameter greater than 77.0mm indicating that some specimen distortions occurred

during compaction. Further review of diameter data reveals that no two diameters differ by more

than 2%, which confirms the use of plastic molds are acceptable as per AASHTO M205.

FIGURE 3 Average Specimen Diameters.

Average specimen height was 150.6mm with a standard deviation of 0.24mm. Taking

into consideration the aluminum plate at the bottom of the mold, the expected height was

150.8mm. Deviations from the expected height could be contributed to mold manufacturing

variations and/or some specimen material sticking to the aluminum plate during specimen

extraction. From a volume perspective, compacted specimens were 100.9% of the expected

nominal volume (76.2mm diameter, 150.8mm height). This is another indication that minor

distortions occurred during compaction. The average specimen h/d aspect ratio was 1.97 with a

standard deviation of 0.005. Based on this data, specimens produced by PM-CF and PM-P are

considered suitable for soil-cement activities.

Unconfined Compressive Strength Results

FIGURE 4 shows 7-day UCS results for PM-CF, PM-P, and Proctor specimens. FIGURE 4 is a

direct raw comparison of measured UCS. Specimen UCS was not corrected for h/d aspect ratio

(Proctor h/d ratio is 1.15:1) or density in FIGURE 4. Outliers, marked by “X”s, were determined

using Tukey’s Method, and outliers were not included in averages or COV.

0

10

20

30

40

50

60

76.0 76.2 76.4 76.6 76.8 77.0 77.2 77.4

Rel

ati

ve

Fre

qu

ency

(%

)

Specimen Diameter (mm)

- --n = 752

Mean = 76.8

Stdev = 0.16

COV = 0.2%

Average Top Average Bottom

n = 752

Mean = 76.4

Stdev = 0.19

COV = 0.2%

Overall

Average n = 752

Mean = 76.6

Stdev = 0.15

COV = 0.2%

Sullivan et al. 12

FIGURE 4 Comparison of Unconfined Compressive Strength (UCS) Results.

1200

1400

1600

1800

2000

2200

2400

2600

2800

PM-CF Proctor PM-P

UC

S (

kP

a)

(a) Pit A Cw = 4.2%

n = 29

UCS Mean = 2430

UCS COV = 5.3%

γd Mean = 99.3%

γd COV = 0.32%

n = 30

UCS Mean = 2201

UCS COV = 5.9%

γd Mean = 99.2%

γd COV = 0.22%

n = 30

UCS Mean = 2077

UCS COV = 11.7%

γd Mean = 98.6%

γd COV = 0.59%

1600

1800

2000

2200

2400

2600

2800

3000

3200

PM-CF Proctor PM-P

UC

S (

kP

a)

UCS Mean = 2461

UCS COV = 9.9%

γd Mean = 99.4%

γd COV = 0.41%

n = 30

UCS Mean = 2293

UCS COV = 6.9%

n = 28

UCS Mean = 2085

UCS COV = 6.5%

γd Mean = 97.7%

γd COV = 0.98%

(b) Pit B Cw = 4.3%

n = 30

γd Mean = 98.6%

γd COV = 0.85%

1600

1850

2100

2350

2600

2850

3100

3350

3600

PM-CF Proctor PM-P

UC

S (

kP

a)

(c) Pit C Cw = 3.2%

n = 30

UCS Mean = 2165

UCS COV = 10.1%

γd Mean = 99.6%

γd COV = 0.73%

n = 29

UCS Mean = 2279

UCS COV = 5.2%

γd Mean = 98.8%

γd COV = 0.62%

n = 30

UCS Mean = 3181

UCS COV = 5.6%

γd Mean = 99.2%

γd COV = 0.42%

Sullivan et al. 13

On average, the PM-CF approach produced higher UCSs for all three pit soils. PM-CF

strengths for Pits A and B were approximately 10% higher than Proctor, while PM-CF strengths

for Pit C were about 40% higher. Higher strengths for PM-CF are partly explained by higher

mean densities and nature of compaction. The PM-CF uses a compaction head that is the same

diameter as the interior of the plastic mold; therefore, the soil particles are pressed together to

achieve density, whereas, a Proctor hammer would have more of a kneading action to compact

soil particles. Based solely on h/d ratio, however, these results are not intuitive and could be an

area warranting future study. PM-CF UCS variability was similar to Proctor UCS variability.

On average, the PM-P approach produced UCSs that were similar to Proctor specimens.

This is likely due to similar kneading action from the Proctor hammer and a similar amount of

compaction energy which also yielded similar specimen densities. PM-P strengths for Pit A were

about 6% less than Proctor. PM-P strengths for Pit B were about 10% less than Proctor, and Pit C

PM-P strengths were about 5% higher than Proctor strengths. Likewise with PM-CF, the PM-P

UCS variability was similar to traditional Proctor UCS variability.

Statistical t-tests were performed on the UCS data using a level of significance of 0.05

with a two-tailed approach. TABLE 4 shows statistical comparison results. For a more direct

comparison, proctor specimen strengths were adjusted to reflect expected strengths of 2:1 aspect

ratio specimens according to ASTM D1633. For all soils, the difference between Proctor and

PM-CF was significant. The difference between Proctor and PM-P was not significant for Pits A

and B but was significant for Pit C. PM-CF and PM-P values were significantly different for all

soils. For the most part, average UCSs from the PM-CF and PM-P devices were statistically

different from Proctor specimens, but practically speaking in the context of MDOT soil-cement

practices discussed earlier, the design cement content of these mixtures would not change much

if the PM-CF or PM-P were utilized during mix design procedures. Between PM-CF and PM-P

approaches, the PM-P approach produced results most similar to traditional Proctor UCS results.

TABLE 4 t-test Comparison of Average Unconfined Compressive Strengths

Pit Comparison n

Mean

(kPa) t-statistic t-critical Sig Different?

A Proctor 30 2000* -13.36 2.00 Yes

PM-CF 29 2430

Proctor 30 2000* -1.54 2.02 No

PM-P 30 2080

PM-CF 29 2430 7.01 2.02 Yes

PM-P 30 2080

B Proctor 30 2090* -7.30 2.01 Yes

PM-CF 30 2460

Proctor 30 2090* 0.00 2.00 No

PM-P 28 2090

PM-CF 30 2460 7.36 2.01 Yes

PM-P 28 2090

C Proctor 30 1970* -24.85 2.00 Yes

PM-CF 30 3180

Proctor 30 1970* -7.32 2.01 Yes

PM-P 29 2280

PM-CF 30 3180 22.92 2.01 Yes

PM-P 29 2280

*Proctor unconfined compressive strengths were adjusted to 2:1 h/d ratio by dividing by 1.10 as per ASTM D1633.

Sullivan et al. 14

Elastic Modulus Results FIGURE 5 plots ASTM C469 elastic modulus (E) results versus UCS. Data includes PM-CF and

PM-P compaction approaches, and overall, E values varied between 3.3 and 11.8GPa. A linear

fit was drawn through the data with an intercept set at 0, and upper and lower boundaries were

visually drawn to capture 98% of data points. Also shown in FIGURE 5 are the UCS and E

correlation equations from the literature review. The majority of measured E and UCS data

plotted close to or between Equations 1 and 2. This data suggests the PM device can produce

acceptable specimens with E values that are in line with other research findings. The PM device

could be utilized to produce E data for level 1 MEPDG inputs, or it could be used to collect data

to build a database of typical E values for a level 2 MEPDG input for soil-cement mixtures.

Figure 5 Elastic Modulus Results.

DISCUSSION OF POTENTIAL IMPLICATIONS OF PM DEVICE

Currently, the PM device is being evaluated by MDOT for possible statewide implementation.

Specific details are envisioned to evolve over the next few construction seasons. The remainder

of this section briefly describes concepts under consideration.

MDOT Materials Division is considering replacing Proctor sized specimens with

specimens made with the PM device for laboratory soil-cement mix design. Envisioned

application is to make strength specimens with the PM device to determine optimum cement

content, but not replace the Proctor test for determining OMC and γd.

For designing pavement layer thickness, the PM device may be used to determine elastic

modulus values for level 1 input in the MEPDG. The PM device can also be used to create a

database of modulus values for level 2 inputs, or the data in this paper supports the use of

Equation 2 as a conservative estimate of E values for soil-cement mixtures.

The PM device offers a couple of options that are currently being explored by MDOT for

quality control activities. First, PM device specimens could be used to monitor soil-cement UCS

after field mixing to assess uniformity and verify design properties. MDOT sees a great benefit

0

2

4

6

8

10

12

14

2000 2500 3000 3500 4000 4500

Ela

stic

Mod

ulu

s (G

Pa

)

UCS (kPa)

Equation 1 (8) for Pits A, B, and C

Equation 2 (MEPDG level 2 input)

Upper/Lower Data Boundary

and Linear Data Fit

PM-CF

PM-P

Trendline and Boundary Equation E (GPa) = Ci * 10-6 * UCS (kPa)

Sullivan et al. 15

in evaluating the UCS of the soil-cement mixtures after field mixing because (14) revealed

considerable variability in UCSs of soil-cement field cores taken from Mississippi highways.

Second, PM device specimens could be utilized for traffic opening of constructed soil-cement

layers. Specimens could be field cured inside the plastic molds and tested to evaluate layer

capacity for supporting traffic loads.

SUMMARY AND CONCLUSIONS

Based on data presented in this paper, the PM device has been shown to be feasible and

advantageous to produce soil-cement specimens. A review of literature and soil-cement practices

has revealed a disconnection between laboratory mix design, pavement thickness design, and

construction QC that could be at least partially addressed by the PM device. The main purpose of

the PM device is to bring continuity to soil-cement practices and interface laboratory and field

operations.

The PM device is capable of producing repeatable specimens with minimal dimensional

distortions, similar unconfined compressive strength variability as compared to Proctor

specimens, and similar elastic modulus values as compared to other published research. The PM

device (through use of plastic molds) can produce many test specimens that can be handled

immediately. In the field, this capability offers many advantages over the use of traditional

Proctor equipment. Overall, the PM-P configuration of the PM device may be favored over the

PM-CF because this configuration is more portable for field applications and utilizes a modified

Proctor hammer which is readily available. Additional investigation may be required to

determine an appropriate density correction for PM-P specimens. Additionally, data presented

within this paper suggests a h/d aspect ratio correction (according to ASTM D1633) may not be

advantageous within an overall framework since PM-P unconfined compressive strengths were

similar to Proctor unconfined compressive strengths. Currently, MDOT is working towards

incorporating the PM device (specifically PM-P) into their soil-cement practices both in the

laboratory and field.

ACKNOWLEDGEMENTS

MDOT funded State Study 206. Bill Barstis, James Williams, and Caleb Hammons of MDOT

provided data and project guidance. Tim Cost of Holcim Cement, Inc. provided cementitious

materials and project guidance. Joe Ivy of Mississippi State University supported equipment

development efforts.

REFERENCES

1. Scullion, T., Sebesta, S., Harris, J.P., and Syed, I. Evaluating the Performance of Soil-

Cement Modified Soil for Pavements: A Laboratory Investigation. Publication RD120,

Portland Cement Association, 2005.

2. Howard, I.L., Sullivan, W.G., Anderson, B.K., Shannon, J., Cost, T. Design and

Construction Control Guidance for Chemically Stabilized Pavement Base Layers.

Publication FHWA/MS-DOT-RD-13-206, Mississippi Department of Transportation,

2013. www.gomdot.com/portal/research.aspx

3. Soil-Cement Laboratory Handbook. Publication EB052.07S, Portland Cement

Association, 1992.

Sullivan et al. 16

4. Soil Stabilization of Pavements. Publication TM5-822-14, United States Army Corps of

Engineers, Department of the Army, the Navy, and the Air Force, 1994.

5. Soil-Cement Inspector’s Manual. Publication PA050.03, Portland Cement Association,

2001.

6. Report on Soil-cement. Report No. ACI 230.1R-09, ACI Committee 230, American

Concrete Institute, Farmington Hills, MI.

7. Griffin, J.R. and Tingle, J.S. In Situ Evaluation of Unsurfaced Portland Cement-

Stabilized Soil Airfields. Publication ERDC/GSL TR-09-20, U.S. Army Engineer

Research and Development Center, 2009.

8. Kolias, S. and Williams, R.I.T. Estimation of the modulus of elasticity of cement

stabilized materials. Geotechnical Testing Journal, Vol. 7, No. 1, 1984, pp. 26-35.

9. Felt, E.J. and Adams, M.S. Strength and elastic properties of compacted soil-cement

mixtures. Papers on Soils, STP 206, American Society for Testing and Materials,

Philadelphia, 1957, pp. 152-178.

10. Williams, R.I.T. and Patankar, V.D. The effect of cement type, aggregate type and mix

water content on the properties of lean concrete mixes. Roads and Road Construction,

Vol. 46, 1968, pp. 542-543.

11. Fossberg, P.E., Mitchell, J.K., and Monismith, C.L. Load deformation characteristics of a

pavement with cement-stabilized base and asphalt surfacing. Proceedings of the Third

International Conference on the Structural Design of Asphalt Surfacing, Vol. 1, 1972, pp.

795-811.

12. James, R.S., Cooley, Jr., L.A., and Ahlrich, R.C. Chemically Stabilized Soils. Publication

SPR-1(51), Mississippi Department of Transportation, 2009.

13. Sullivan, W.G. Investigation of Compaction and Corresponding Thermal Measurement

Techniques for Cementitiously Stabilized Soils. M.S. Thesis, Mississippi State University,

2012.

14. Varner, R.L. Variability of Cement-Treated Layers in MDOT Road Projects. Publication

FHWA/MS-DOT-RD-11-227, Mississippi Department of Transportation, 2011.