Artigo Solo Cimentado Artificialmente

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Parameters Controlling Tensile and Compressive Strength ofArtificially Cemented Sand

Nilo Cesar Consoli, Ph.D.1; Rodrigo Caberlon Cruz, D.Sc.2; Mrcio Felipe Floss3; and Lucas Festugato4

Abstract:The enhancement of local soils with 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. The present research aims to quantify the influence of the amount of cement, the porosity, and the

voids/cement ratio in the assessment of unconfined compressive strength qu and splitting tensile strengthqt of an artificially cemented

sand, as well as in the evaluation ofq t/qurelationship. A program of splitting tensile tests and unconfined compression tests considering

three distinct voids ratio and seven cement contents, varying from 1 to 12%, was carried out in the present study. The results show that

a power function adapts well q tand qu values with increasing cement content and with reducing porosity of the compacted mixture. The

voids/cement ratio is demonstrated to be an appropriate parameter to assess both qtand quof the sand-cement mixture studied. Finally, the

qt/ qu relationship is unique for the sand-cement studied, being independent of the voids/cement ratio.

DOI:10.1061/ASCEGT.1943-5606.0000278

CE Database subject headings: Tensile strength; Compressive strength; Soil cement; Compacted soils.

Author keywords: Tensile strength; Compressive strength; Soil cement; Compacted soils.

Introduction

Portland cement is used worldwide in the improvement of local

soils, particularly as a soil-cement mixture of a compacted layer

over a low bearing capacity soil. In such cases, Consoli et al.

2003has shown that the system failure mechanism usually start

up under tensile stresses at the base of the improved layer. Al-

though it would seem more reasonable to use the tensile strength

as a direct measure of the soil-cement strength, there are no dos-

age methodologies based on rational criteria considering the ef-

fect of different variables e.g., amount of cement and porosity

on the soil-cement tensile strength.

The first rational dosage methodology for soil-cement was de-

veloped by Consoli et al. 2007 considering the voids/cement

ratio /Cv, defined by the porosity of the compacted mixture

divided by the volumetric cement content, as an appropriate pa-

rameter to evaluate qu of the soil-cement mixture. Nowadays,

even though it is recognized that compressive and tensile

strengths are intimately related on artificially cemented soilse.g.,

Clough et al. 1981; Consoli et al. 2001, it is still not clear

whether there is a straight proportionality betweenquand qtand ifsuch relation is a function of porosity and cement content. Theunconfined compression test has therefore been used as the mostconvenient means to investigate the effect of different variableson the soil-cement strength and to carry out dosage methodolo-gies. Questions that remain unanswered are: is it correct to carryout dosage methodologies based on unconfined compression testsfor the cases where tensile stresses are the basic variables? Isthere a straight proportionality between qu and qt? Is the qt/quindex for a given soil-cement relationship a function of voids/

cement ratio? This study aims at approaching some of these ques-tions by quantifying the influence of the amount of cement andthe porosity on the tensile strength of an artificially cementedsand, as well as to evaluate the use of voids/cement ratio to assessits splitting tensile strength qt, unconfined compressive strengthqu and their ratio qt/qu. The main contributions of presentwork are showing that the voids/cement ratio /C

v is an appro-

priate index parameter to evaluate not only unconfined compres-sive strength qu of soil-cement mixtures, but splitting tensilestrengthqtas well, and that the q t/qu relationship appears to beunique for a given soil-cement studied, being independent of thevoids/cement ratio.

Experimental Program

The experimental program has been carried out in two parts. First,the geotechnical properties of the soil and cement were character-ized. Then, a number of splitting tensile and unconfined compres-sion tests was carried out. In addition, measurements of matricsuction were also performed in most specimens to check a pos-sible influence on the results.

Materials

The Osorio sand used in the testing was obtained from the regionof Porto Alegre, in Southern Brazil, being classified ASTM

1Associate Professor, Dept. of Civil Engineering, Federal Univ. of Rio

Grande do Sul, Rio Grande do Sul 90035-190, Brazil corresponding

author. E-mail: [email protected] Fellow, Federal Univ. of Rio Grande do Sul, Rio Grande do

Sul 90035-190, Brazil. E-mail: [email protected]. Student, Federal Univ. of Rio Grande do Sul, Rio Grande do

Sul 90035-190, Brazil. E-mail: [email protected]. Student, Federal Univ. of Rio Grande do Sul, Rio Grande do

Sul 90035-190, Brazil. E-mail: [email protected]

Note. This manuscript was submitted on March 13, 2009; approved

on November 1, 2009; published online on November 5, 2009. Discus-

sion period open until October 1, 2010; separate discussions must be

submitted for individual papers. This technical note is part of the Journal

of Geotechnical and Geoenvironmental Engineering, Vol. 136, No. 5,

May 1, 2010. ASCE, ISSN 1090-0241/2010/5-759763/$25.00.

JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING ASCE / MAY 2010 /759

J. Geotech. Geoenviron. Eng. 2010.136:759-763.


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1993as nonplastic uniform fine sand with rounded particle shapeand specific gravity of the solids 2.65. Mineralogical analysisshowed that sand particles are predominantly quartz. The grainsize is purely fine sand with a mean effective diameter D50 of0.16 mm, being the uniformity and curvature coefficients of 1.9and 1.2, respectively. The minimum and maximum void ratios are0.6 and 0.9, respectively.

Portland cement of high initial strength Type IIIwas used asthe cementing agent. Its fast gain of strength allowed the adoptionof seven days as the curing time. The specific gravity of the

cement grains is 3.15. Distilled water was used for these charac-terization tests and tap water for molding specimens for the ten-sile and compression tests.

Methods

Molding and Curing of Specimens

For the splitting tensile and unconfined compression tests, cylin-drical specimens 50 mm in diameter and 100-mm high were used.Once established a given voids ratioe, the target dry unit weightd was calculated according to Eq. 1

e=s

d 1 1

where s =solids unit weight. A target dry unit weight for a givenspecimen was then established through the dry mass of soil-cement divided by the total volume of the specimen. In order tokeep the dry unit weight of the specimens constant with increas-ing cement content, a small portion of the soil was replaced bycement. As the specific gravity of the cement grains 3.15 isgreater than the specific gravity of the soil grains 2.65, for thecalculation of void ratio and porosity, a composite specific gravitybased on the soil and cement percentages in the specimens wasused.

After the soil, cement, and water were weighed, the soil andcement were mixed until the mixture acquired a uniform consis-

tency. The water was then added continuing the mixture processuntil a homogeneous paste was created. The amount of cement foreach mixture was calculated based on the mass of dry soil and themoisture content. The specimen was then statically compacted inthree layers inside a cylindrical split mold, which was lubricated,so that each layer reached the specified dry unit weight. The topof each layer was slightly scarified. After the molding process, thespecimen was immediately extracted from the split mold and itsweight, diameter, and height measured with accuracies of about0.01 g and 0.1 mm, respectively. The samples were then placedinside plastic bags to avoid significant variations of moisture con-tent. They were cured for 6 days in a humid room at 23 2 Cand relative humidity of above 95%.

The samples were considered suitable for testing if they met

the following tolerances: Dry unit weight d: degree of compaction between 99 and

101% the degree of compaction being defined as the valueobtained in the molding process divided by the target value ofd; and

Dimensions: diameter to within 0.5 mm and height of 1mm.

Splitting Tensile Tests

Splitting tensile tests followed Brazilian standard NBR 7222Brazilian Standard Association 1983. An automatic loading ma-chine, with maximum capacity of 50 kN and proving rings with

capacities of 10 and 50 kN and resolutions of 0.005 and 0.023 kN,

respectively, were used for the unconfined compression tests.

After curing, the specimens were submerged in a water tank

for 24 h for saturation to minimize suction. The water temperature

was controlled and maintained at 23 3C. 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 accep-

tance criteria, it was stipulated that the individual strengths of

three specimens, molded with the same characteristics, should notdeviate 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 ac-

cess the importance of influencing factors on the strength of ce-

mented 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 5739Brazilian Standard Asso-

ciation 1980, being simple and fast, while reliable and cheap.

The automatic loading machine and proving rings were thesame used for the splitting tensile tests. Curing of specimens and

acceptance criteria were exactly the same as for splitting tensile

tests.

Matric Suction Measurements

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 porosi-

ties and cement contents.

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

inside the sample, was measured using the filter paper technique.

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 29 MPa. The calibration

equations for this filter paper are those presented by Chandler et

al. 1992.

Program of Splitting Tensile and Unconfined Compression

Tests

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 voids/cement ratio on the mechanical

strength of the artificially cemented soil.The molding points were chosen considering voids ratio of

0.64, 0.70, and 0.78 corresponding, respectively, to high, me-

dium, and reduced relative densities, with the same moisture

content 10%. Each point was molded with seven different

cement percentages: 1, 2, 3, 5, 7, 9, and 12%. These percentages

were chosen following Brazilian and international experience

with soil-cement e.g., Mitchell 1981; Consoli et al. 2003, 2006,

2007, 2009; Thom et al. 2005. Because of the typical scatter of

data for both splitting tensile and unconfined compression tests, a

minimum of three specimens were tested for each point.

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Results

Effect of the Cement Content and Porosity on Tensileand Compressive Strengths

Fig. 1 shows the raw data for the three studied voids ratio andthe fitted lines for the splitting tensile strength qt as a functionof the cement content C, the latter defined by Eq. 2 as

C=Wc

Ws2

where Wc =mass of cement and Ws =mass of dry soil. It can beobserved in Fig. 1 that the cement content has a great effect on thetensile strength of this sand-cement mixture, where a small addi-

tion of cement is enough to generate a significant gain in strength.The lines shown on the figure are best fit lines, demonstrating thata power function adapts well the relation q t C.

Fig. 2 shows how the porosity affects the splitting tensilestrength of the sand-cement mixture. The tensile strength in-

creases with reducing porosity of the compacted mixture. Thebeneficial effect of a decrease in porosity on the tensile strengthhas been reported by several researcherse.g., Moore et al. 1970.In particular, Chang and Woods 1992 have already shown,through electron microscopy on different sands with various kindsof cement, that the existence of a larger number of interparticlecontacts and, consequently, a greater possibility of the cement topromote effective bonds at these contacts, explains the increase inthe rate of tensile strength gain with the reduction in the porosity.

The unconfined compression strength qu variation with theamount of cement is shown in Fig. 3. With a similar pattern to thesplitting tensile tests, a power function also fits well to the rela-tion qu C. Besides that, the soil-cement mixtures present clearincrease in the unconfined compressive strength gain rate withdecreasing voids ratio Fig. 4.

The process of submerging the specimens for 24 h before thesplitting tensile tests was found to be satisfactory to ensure a highand repeatable degree of saturation. An average degree of satura-tion of 92% was obtained for specimens after submersion, irre-

Fig. 1.Variation of splitting tensile strengthqtwith cement content

Fig. 2. Variation of splitting tensile strength qt with porosity

Fig. 3. Variation of unconfined compressive strength qu with ce-

ment content

Fig. 4. Variation of unconfined compressive strength qu with po-

rosity




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spective of the initial porosity or cementitious material content.The values of suction measured were low with values rangingfrom about 14% of the tensile strength. These measurementswere made on the specimens after failure in the splitting tensiletests and are therefore likely to overestimate the real value, be-cause a slight drying of the sample may have occurred during thefew minutes from the start of the test until the measurement wasmade. Given the small values of matric suction measured in thesespecimens, the small effect arising from the unsaturated naturewere disregarded.

Effect of Voids/Cement Ratio on Tensile and Compressive

Strengths

Fig. 5 presents the splitting tensile strength as a function of the

voids/cement ratio /Cv expressed as porosity divided bythe volumetric cement content C

v, the latter expressed as a per-

centage of cement volume regarding total volume, defined by Eq.3

Cv

=

VvVtotal

Vc

Vtotal

=Vv

Vc3

where Vv

=volume of voids water+air of the specimen; Vc=volume of cement of the specimen; and Vtotal=total volume ofthe specimen.

A simple observation of this figure suggests that the voids/cement ratio is useful in normalizing results. A good correlationR2 =0.97 can be observed between this ratio /C

v and the

splitting tensile strength qt of the sand-cement studied see Eq.4

qtkPa= 4,266 Cv

1.30 4Fig. 6 presents a good quality correlation R2 =0.98 between

/Cv

and the unconfined compressive strength qu of the sand-cement studied see Eq. 5

qukPa= 28,327

Cv1.30

5

Fig. 7 summarizes all measurements from Figs. 5 and 6. Byexamining this figure, as well as Eqs. 4 and 5, it can be seenthat they present rather similar trends. In order to check whether aqt/qu relationship for the sand-cement mixture is a function ofporosity, cement content, or voids/cement ratio, Eq. 4is dividedby Eq. 5 which yields the ratio

qt

qu=

4,266 Cv

1.30

28,327 Cv

1.30= 0.15 6

It can be seen in Eq. 6 that qt/qu is a scalar for the sand-cement blend, being independent of porosity, cement content, orvoids/cement ratio. So, there is a straight proportionality betweentensile and compressive strengths, which is valid for the whole

Fig. 5. Variation of splitting tensile strength qt with voids/cement

ratioFig. 6.Variation of unconfined compressive strengthquwith voids/

cement ratio

Fig. 7. Variation of both splitting tensile qt and unconfined com-

pressive strengths qu with voids/cement ratio

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range of voids ratio and cement content studied in the presentresearch program. As a consequence, it is possible to concludethat any rational dosage methodology considering the effect ofdifferent variables can be centered on tensile or compression tests,once they are intimately related through a scalar 0.15 for thesand-cement studied in the present research. Finally, the resultspresented in this paper suggest that the voids/cement ratio can bean extremely useful index for practitioners from which an engi-neer can choose the amount of cement appropriate to provide amixture that meets the strength required by the project at the

optimum cost. The voids/cement ratio can also be useful in a fieldcontrol of soil-cement layers. Once a poor compaction has beenidentified, it can be readily taken into account in the design beingadopted through the qt versus / Cv or qu versus /Cv curves,with corrective measures accordingly such as the reinforcementof the treated layer or the reduction in the load transmitted.

Conclusions

From the data presented in this technical note, the following con-clusions can be drawn: A power function adapts well to both qt C and qu Csand-

cement mixture relations;

The reduction in porosity of the compacted mixture increasesboth the tensile and compressive strengths;

The voids/cement ratio /Cv has been shown to be an ap-

propriate index parameter to evaluate both splitting tensile qtand unconfined compressivequstrength of sand-cement mix-tures. Both q tand q ureduce with increasing / Cv values; and

The q t/ qu ratio is a scalar 0.15 for the sand-cement mixtureevaluated in the present study, being independent of voids/cement ratio. As a consequence, dosage methodologies basedon rational criteria can concentrate either on tensile or com-pression tests, once they are interdependable.

Acknowledgments

The writers wish to express their gratitude to Brazilian ResearchCouncil CNPq/MCT Projects Produtividade em Pesquisa GrantNo. 301869/2007-3, Edital Universal Grant No. 472851/2008-0,PNPD Grant No. 558474/2008-0, and INCT, to Brazilian Elec-trical Energy Agency ANEELProject P&D Grant No. 0089-036/2006-CEEE-GT/9936455, and to PRODOC CAPES for theirfinancial support to the research group.

Notation

The following symbols are used in this technical note:

C cement content expressed in relation to massof dry soil;

Cv

volumetric cement content expressed inrelation to the total specimen volume;

D50 mean effective diameter;e voids ratio;

qt splitting tensile strength;qu unconfined compressive strength;

R2 coefficient of determination;Wc mass of cement;Ws mass of dry soil;d dry unit weight;s solids unit weight; porosity;

/Cv voids/cement ratio; and moisture content.

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