Parameters Controlling Tensile and Compressive Strength of Fiber-Reinforced Cemented Soil

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Parameters Controlling Tensile and Compressive Strength

of Fiber-Reinforced Cemented Soil

by

Nilo Cesar Consoli, Ph.D.1; Rafael Rizzati de Moraes2 and Lucas Festugato3

ABSTRACT: The enhancement of local soils with fibers and cement for the construction of

stabilized pavement bases, canal lining and support layer for shallow foundations shows great

economical and environmental advantages, avoiding the use of borrow materials from

elsewhere, as well as the need of a spoil area. In previous studies, a unique dosage

methodology for cemented soils has been established based on rational criteria where the

porosity/cement ratio plays a fundamental role in the assessment of the target unconfined

compressive strength (qu). The present research has extended previous works by quantifying

the influence of the amount of cement, the porosity and the porosity/cement ratio in the

assessment on tensile strength (qt) and compressive strength (qu) of a fiber-reinforced

artificially cemented sand, as well as in the evaluation of qt/qu relationship. A program of

splitting tensile tests and unconfined compression tests considering four distinct dry densities

and five cement contents, varying from 1% to 7%, was carried out in the present study. The

results show that a power function adapts well qt and qu values with increasing cement content

and with reducing porosity of the compacted mixture. The porosity/cement ratio is

demonstrated to be an appropriate parameter to assess both tensile strength and unconfined

compressive strength of the fiber-reinforced sand-cement mixture studied. Finally, the qt/qu

relationship is unique for the fiber-reinforced sand-cement studied, being independent of the

porosity/cement ratio.

Keywords: fiber-reinforcement, soil-cement, porosity/cement ratio, tensile strength, compressive strength

1 Professor, Dept. of Civil Engineering, Federal University of Rio Grande do Sul, Brazil. E-mail: [email protected] 2 Research Assistant, Dept. of Civil Engineering, Federal University of Rio Grande do Sul, Brazil. E-mail: [email protected] 3 Research Fellow, Dept. of Civil Engineering, Federal University of Rio Grande do Sul, Brazil. E-mail: [email protected]

Journal of Materials in Civil Engineering. Submitted January 6, 2012; accepted July 13, 2012; posted ahead of print August 27, 2012. doi:10.1061/(ASCE)MT.1943-5533.0000555

Copyright 2012 by the American Society of Civil Engineers

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INTRODUCTION

Fiber-reinforcement and Portland cement are worldwide used in the improvement of local

soils, particularly as a fiber-reinforced soil-cement mixture of a compacted layer over a low

bearing capacity soil. In such cases, Consoli et al. (2003) has shown that the fibers seem to

inhibit tension crack formation and allow the distribution of stresses in a broader area, acting

similarly to plant roots and leading to the formation, at failure, of a thick shear band all around

the border of the plate. Although it would seem more reasonable to use the tensile strength as

a direct measure of the fiber-reinforced soil-cement strength, there are no dosage

methodologies based on rational criteria considering the effect of different variables (e.g.

amount of cement, porosity) on the fiber-reinforced soil-cement tensile strength.

The first rational dosage methodology for fiber-reinforced soil-cement was developed

by Consoli et al. (2007) considering the porosity/cement ratio ( /Civ), defined by the porosity

of the compacted mixture divided by the volumetric cement content, as an appropriate

parameter to evaluate the unconfined compressive strength, qu, of the fiber-reinforced soil-

cement mixture. Nowadays, even though it is recognized that compressive and tensile

strengths are intimately related on artificially cemented soils (e.g. Ingles and Metcalf, 1972;

Clough et al., 1981), it is still not clear whether there is a straight proportionality between

unconfined compressive strength, qu, and tensile strength, qt, for fiber-reinforced cemented

soils and if such relation is a function of porosity and cement content. The unconfined

compression test has therefore been used as the most convenient means to investigate the

effect of different variables on the fiber-reinforced soil-cement strength and to carry out

dosage methodologies. Questions that remain unanswered are: Is it correct to carry out dosage

methodologies of fiber-reinforced cemented soils based on unconfined compression tests for

the cases where tensile stresses are the basic variables? Is there a straight proportionality, for

fiber-reinforced cemented soils, between qu and qt? Is the qt/qu index for a given fiber-

reinforced soil-cement relationship a function of porosity/cement ratio? This study aims at

responding the above raised questions by quantifying the influence of the amount of cement

and the porosity on the tensile strength of a fiber-reinforced artificially cemented sand, as well

as evaluating the use of porosity/cement ratio to assess its splitting tensile strength (qt),

unconfined compressive strength (qu) and their ratio (qt/qu).



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EXPERIMENTAL PROGRAM

The experimental program has been carried out in two parts. First, the geotechnical

properties of the soil were characterized. Then a number of splitting tensile and unconfined

compression tests was carried out. In addition, measurements of matric suction were also

performed in most specimens to check a possible influence on the results. The splitting tensile

and unconfined compression tests constituted the main part of this research. The program was

conceived in such a way as to evaluate, separately, the influences of the cement content,

porosity and porosity/cement ratio on the mechanical strength of the fiber-reinforced

cemented soil.

Materials

The soil used in this study was derived from a weathered sandstone and was obtained

from a borrow site in the region of Porto Alegre, southern Brazil. The sample was collected in

a disturbed state, by manual excavation, in sufficient quantity to complete all the tests.

The results of the characterization tests are shown in Table 1 and the grain distribution

size curve is shown in Figure 1. This soil is classified as non-plastic clayey sand (SC)

according to the Unified Soil Classification System.

Portland cement of high early strength [Type III according to ASTM C150-09 (2009)]

was used as the cementing agent. Its fast gain of strength allowed the adoption of seven days

as the curing time. The specific gravity of the cement grains is 3.15.

Monofilament polypropylene fibers were used throughout this investigation to reinforce

the cemented soil. The fibers were 24 mm in length and 0.023 mm [dtex=3.3 - where dtex is a

unit of measure for the linear mass density of fibers (mass in grams per 10,000 meters)] in

diameter (consequently aspect ratio of 1043), with a specific gravity of 0.91, tensile strength

of 120 MPa, elastic modulus of 3 GPa and linear strain at failure of 80%. The fiber content

used in the experiments was 0.5% by weight of the sum of dry soil and cement. The fiber used

in present research (fiber type, percentage, diameter and length) was chosen based on previous

studies by Consoli et al. (2002, 2007b, 2007c, 2009a and 2009b).

Distilled water was mixed with dry soil, Portland cement and fibers. Such mixture was

used for molding specimens for the tensile and compression tests.



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Methods

Molding and Curing of Specimens

For the unconfined compression and splitting tensile tests, cylindrical specimens, 50mm

in diameter and 100mm high, were used. The fiber-reinforced compacted soil specimens used

in the tests were prepared by hand-mixing dry soil, cement, water and polypropylene fibers.

During the mixing process, it was found to be important to add the water prior to adding the

fibers, to prevent floating of the fibers. The amount of fibers for each mixture was calculated

based on the mass of dry soil plus the mass of cement. Visual and microscope examination of

exhumed specimens showed the mixtures to be satisfactorily uniform.

After mixing sufficient material for one specimen, the mixture was stored in a covered

container to avoid moisture losses before subsequent compaction. Two small portions of the

mixture were also taken for moisture content determination.

The specimen was then statically compacted (putting the mold assembly in a static load

frame and compacting using a displacer disc) in three layers inside a cylindrical split mold,

which was lubricated, so that each layer reached the specified dry density. The top of the first

and the second layers were slightly scarified. After the molding process, the specimen was

immediately extracted from the split mold, and its weight, diameter and height were measured

with accuracies of about 0.01g and 0.1mm. The samples were then placed within plastic bags

to avoid significant variations of moisture content before testing. They were cured in a humid

room at 23º±2ºC for six days.

The samples were considered suitable for testing if they met the following tolerances:

Dry Density ( d): degree of compaction between 99% and 101% (the degree of compaction

being defined as the value obtained in the molding process divided by the target value of d);

Moisture Content ( ): within ± 0.5% of the target value and Dimensions: diameter to within

±0.5mm and height ±1mm.

The molding points were chosen considering dry densities of 17.3 kN/m3, 18.0 kN/m3,

19.0 kN/m3 and 19.7 kN/m3, with similar moistures contents ( = 10%). Each condition was

molded with five different cement percentages: 1%, 2%, 3%, 5% and 7%. These percentages

were chosen following Brazilian and international experience with soil-cement [e.g., Mitchell

(1981), Consoli et al. (2006, 2007a, 2009c, 2009d, 2012a, 2012b, 2012c), Estabragh et al.

(2012), Fatahi et al. (2012)]. Because of the typical scatter of data for both splitting tensile



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and unconfined compression tests, a minimum of six specimens having identical

characteristics were tested (three under compression and three under tensile conditions).

Splitting Tensile Tests

Splitting tensile tests followed Brazilian standard NBR 7222 (1983). An automatic

loading machine, with maximum capacity of 50kN and proving rings with capacity of 10kN

and resolution of 0.005kN was used for the splitting tensile tests.

After curing, the specimens were submerged in a water tank for 24 hours for saturation

to minimize suction. The water temperature was controlled and maintained at 23 ± 3ºC.

Immediately before the test, the specimens were removed from the tank and dried

superficially with an absorbent cloth. Then, the splitting tensile test was carried out and the

maximum load recorded. As acceptance criteria, it was stipulated that the individual strengths

of three specimens, molded with the same characteristics, should not deviate by more than

10% from the mean strength.

Unconfined Compression Tests

Unconfined compression tests have been systematically used in most experimental

programs reported in the literature in order to verify the effectiveness of the stabilization with

cement or to access the importance of influencing factors on the strength of cemented soils.

One of the reasons for this is the accumulated experience with this kind of test for concrete.

The tests usually followed Brazilian standard NBR 5739 (1980), being simple and fast, while

reliable and cheap.

The automatic loading machine was the same used for the splitting tensile tests and the

proving rings with capacities of 10kN and 50kN and resolutions of 0.005kN and 0.023kN

were used for the unconfined compression tests. Curing of specimens and acceptance criteria

were exactly the same as for splitting tensile tests.

Matric Suction Measurements



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At their molding moisture contents, all specimens were in an unsaturated state

exhibiting a certain level of suction. Suction measurements aimed to verify its magnitude and

examine if there was significant variation between specimens of different porosities and

cement contents.

The matric suction, i.e. that arising from the capillary forces inside the sample, was

measured using the filter paper technique (Marinho, 1995). The filter paper used was

Whatman Nº 42. Its initial moisture content in the air dried state is approximately 6%, which

allows measurements of suction from zero to 29MPa. The calibration equations for this filter

paper are those presented by Chandler et al. (1992).

RESULTS

Effect of the Cement Content and Porosity on Tensile and Compressive Strength

Figure 2 shows the raw data (for the four studied dry densities) and the fitted lines for

the splitting tensile strength (qt) as a function of the cement content (C). It can be observed

that the cement content has a great effect on the tensile strength of this fiber-reinforced sand-

cement mixture, where a small addition of cement is enough to generate a significant gain in

strength. The lines shown on the figure are best fit lines, demonstrating that a straight line

adapts well the relation qt – C.

Figure 3 shows how the porosity affects the splitting tensile strength of the fiber-

reinforced sand-cement mixture. Porosity is calculated using Eq. (1),

s

F

sd

C

sd

S

sd

V

Gs

FC

V

Gs

CC

V

Gs

SC

V100

1001

100100

1100

1001

100

100

(1)

where d is the dry unit weight of the specimen, Vs is the volume of the specimen, C, S and F

are the cement, soil and fiber contents, and GsC, GsS and GsS are the specific gravities of

cement, soil and fiber, respectively.



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The tensile strength increases with reducing porosity of the compacted mixture. The

beneficial effect of a decrease in porosity on the tensile strength has been reported by several

researchers (e.g. Moore et al. 1970). In particular, Chang and Woods (1992) have already

shown through electron microscopy on different sands with various kinds of cement that the

existence of a larger number of inter-particle contacts and, consequently, a greater possibility

of the cement to promote effective bonds at these contacts, explains the increase in the rate of

tensile strength gain with the reduction in the porosity.

The unconfined compression strength (qu) variation with the amount of cement is shown

in Fig. 4. With a similar pattern to the splitting tensile tests, straight lines also fit well to the

relation qu – C. Besides that, the soil-cement mixtures present clear increase in the unconfined

compressive strength gain rate with decreasing porosity (Fig. 5).

The process of submerging the specimens for 24 hours before the splitting tensile tests

was found to be satisfactory to ensure a high and repeatable degree of saturation. An average

degree of saturation of 89% was obtained for specimens after submersion, irrespective of the

initial porosity or cementitious material content. The values of suction measured were low

with values ranging from about 1% to 7% of the tensile strength. These measurements were

made on the specimens after failure in the tests and are therefore likely to overestimate the

real value, because a slight drying of the sample may have occurred during the few minutes

from the start of the test until the measurement was made. Given the small values of matric

suction measured in these specimens, the small effect arising from the unsaturated nature was

disregarded.

Effect of Porosity/Cement Ratio on Tensile and Compressive Strength

As seen in the results presented above (Figs. 2 to 5), both the splitting tensile (qt) and

the unconfined compressive strength (qu) are dependents on both the porosity and the cement

content of the mixtures. Rising values of porosity cause reduction of both qt and qu while

increasing values of cement content ends up in larger values of both qt and qu. It is being

proposed herein the existence of specific relations between qt and porosity/cement ratio

(η/Civ) and qu and porosity/cement ratio (η/Civ), being η/Civ defined by Eq. (2).

ContentCement VolumetricPorosity

ivC (2)



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The relations qt versus η/Civ and qu versus η/Civ suggest that η/Civ joins the distinct

effects of both variables (η and Civ) in a unique factor controlling both qt and qu. It means that

η and 1/Civ affect separately both qt and qu and that the effect on both qt and qu of increasing

values of porosities can be counter acted by increasing values of volumetric cement contents,

ending up in η/Civ governing both qt and qu.

Figures 6 and 7 present, respectively, the splitting tensile strength and the unconfined

compressive strength as a function of the porosity/cement ratio (η/Civ) for the fiber-reinforced

cemented sand studied, highlighting distinct cement contents used. Best fit curves for both qt

and qu studied present reasonable correlations (coefficient of determination — R2=0.78 and

0.77, respectively for qt - Eq. 3, and qu - Eq. 4) with η/Civ for the studied fiber-reinforced

cemented sand. Some scatter of data around the best fit curves can be seen in both Figs. 6 and

7.

00.1

31023.3)(iv

t CxkPaq (3)

88.0

31050.16)(iv

u CxkPaq (4)

Figures 6 and 7 distinguish the plotted points by their cement contents, respectively for

qt and qu versus η/Civ. In both figures, it can be observed that points with similar η/Civ, but

obtained by different combinations of cement content and density, show distinct strengths for

each cement content, supposedly due to substantial differences in rates of change of both qt

and qu with porosity (η) and with the inverse of the volumetric cement content (1/Civ). A way

to make the variation rates of η and 1/Civ compatible is through the application of a power to

one of them (in the present work the application of a power is suggested to be on Civ - the

optimum fit was found to be applying a power equal to 0.28) as shown in Fig. 8 for both qt

and qu. The coefficients of determination (R2) of 0.95 and 0.97, for tensile strength - Eq. 5 and

compressive strength - Eq. 6, can be observed between η/(Civ)0.28 and the splitting tensile

strength (qt) and unconfined compressive strength (qu), respectively.

90.2

28.061055.2)(

ivt

CxkPaq

(5)



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90.2

28.061096.17)(

ivu C

xkPaq

(6)

Examining Fig. 8, as well as Eqs. (5) and (6), it can be seen that qt and qu present rather

similar trends. In order to check whether a qt/qu relationship for the fiber-reinforced cemented

sand mixture is a function of porosity, cement content or porosity/cement ratio, Eq. (5) is

divided by Eq. (6) which yields the ratio Eq. (7):

14.0

1096.17

1055.2

90.2

28.06

90.2

28.06

iv

iv

u

t

Cx

Cx

qq

(7)

It can be seen in Eq. (7) that qt/qu is a scalar for the fiber-reinforced sand-cement blend,

being independent of porosity, cement content or porosity/cement ratio. Figure 9 presents

plotting of the qt/qu ratios using the actual experimental data points for each porosity-cement

ratio [η/(Civ)0.28]. Best fit line (qt/qu=0.14) is presented together with upper bound (qt/qu=0.17)

and lower bound (qt/qu=0.11) values. So, there is a straight proportionality between tensile and

compressive strengths of fiber-reinforced cemented sand mixtures, which is valid for the

whole range of voids ratio and cement content studied in the present research program. As a

consequence, it is possible to conclude that any rational dosage methodology for fiber-

reinforced cemented sand, considering the effect of different variables (porosity, cement

content or porosity/cement ratio), can be centered on tensile or compression tests, once they

are intimately related through a scalar (0.14 for the fiber-reinforced sand cement studied in the

present research). Finally, the results presented in this paper suggest that the porosity/cement

ratio can be an extremely useful index for practitioners from which an engineer can choose the

amount of cement appropriate to provide a mixture that meets the strength required by the

project at the optimum cost. The porosity/cement ratio can also be useful in a field control of

fiber-reinforced soil-cement layers. Once a poor compaction has been identified, it can be

readily taken into account in the design being adopted through the qt versus /Civ or qu versus

/Civ curves, with corrective measures accordingly such as the reinforcement of the treated

layer or the reduction in the load transmitted.



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CONCLUSIONS

From the data presented in this manuscript the following conclusions can be drawn:

A straight line adapts well to both qt – C and qu – C fiber-reinforced sand-cement

mixture relations;

The reduction in porosity of the compacted mixture improves greatly both the tensile

and compressive strengths of the fiber-reinforced sand-cement mixture;

The porosity/cement ratio (η/Civ) has been shown to be an appropriate index parameter

to evaluate both splitting tensile (qt) and unconfined compressive (qu) strength of fiber-

reinforced sand-cement mixtures. Both qt and qu reduce with increasing η/Civ values;

The qt/qu ratio is a scalar (0.14) for the fiber-reinforced sand-cement mixture evaluated

in the present study, being independent of porosity/cement ratio. As a consequence,

dosage methodologies based on rational criteria can concentrate either on tensile or

compression tests, once they are interdependable.

ACKNOWLEDGEMENTS

The authors wish to express their gratitude to Brazilian Research Council/Brazilian

Ministry of Science and Technology (CNPq/MCT) (projects PNPD, Produtividade em

Pesquisa and INCT-REAGEO) for their financial support to the research group.

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Consoli, N.C.; Heineck, K.S, Casagrande, M.D.T. & Coop, M.R. (2007c). “Shear strength

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parameters for the stiffness and strength control of artificially cemented sand”. Journal of

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cement-voids ratio on stress-dilatancy behavior of artificially cemented sand”. Journal of

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(DOI: 10.1061/(ASCE)GT.1943-5606.0000565)

Consoli, N. C., Viana da Fonseca, A., Silva, S. R., Cruz, R. C., and Fonini, A. (2012b).

“Parameters controlling stiffness and strength of artificially cemented soils”.

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NOTATION

C cement content

Civ volumetric cement content

D50 mean effective diameter

qt splitting tensile strength

qu unconfined compressive strength

R2 coefficient of determination

STS splitting tensile strength

UCS unconfined compressive strength

η porosity

η/Civ porosity/cement ratio

d dry unit weight

moisture content



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TABLES



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TABLE 1: Physical Properties of the Soil Sample

PROPERTIES VALUE

Liquid Limit 23%

Plastic Limit 13%

Plasticity Index 10%

Specific Gravity 2.64

Medium Sand (0.2 < Diameter < 0.6 mm) 16.2%

Fine Sand (0.06 < Diameter < 0.2 mm) 45.4%

Silt (0.002 < Diameter < 0.06 mm) 33.4%

Clay (Diameter < 0.002 mm) 5.0%

Mean Effective Diameter (D50) 0.12 mm

Coefficient of Uniformity 50

Accepted Manuscript Not Copyedited



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LIST OF FIGURE CAPTIONS

FIGURE 1: Grain Size distribution.

FIGURE 2: Variation of splitting tensile strength (qt) with cement content.

FIGURE 3: Variation of splitting tensile strength (qt) with porosity.

FIGURE 4: Variation of unconfined compressive strength (qu) with cement

content.

FIGURE 5: Variation of unconfined compressive strength (qu) with porosity.

FIGURE 6: Variation of splitting tensile strength (qt) with porosity/cement ratio

(η/Civ).

FIGURE 7: Variation of unconfined compressive strength (qu) with

porosity/cement ratio (η/Civ).

FIGURE 8: Adjusted porosity/cement ratio for both splitting tensile strength

(qt) and unconfined compressive strength (qu) fiber-reinforced specimens.

FIGURE 9: Plotting of the qt/qu ratios of fiber-reinforced sand-cement blend

using the actual experimental data points for each adjusted porosity-cement ratio

[η/(Civ)0.28] as well as presenting the best fit line (qt/qu=0.14) together with upper

bound (qt/qu=0.17) and lower bound (qt/qu=0.11) values.



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Parameters Controlling Tensile and Compressive Strength of Fiber-Reinforced Cemented Soil

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