Peat Soil Stabilization, using Ordinary Portland Cement, Polypropylene Fibers, and Air Curing Technique Behzad Kalantari Department of Civil Engineering, University of Putra Malaysia
and Bujang B. K. Huat Professor, Department of Civil Engineering, University of Putra Malaysia
ABSTRACT This article describes a laboratory study on stabilizing peat soil using Ordinary Portland Cement (OPC) as binding agent and Polypropylene fibers as additive. Due to high initial water content of the stabilized peat soil samples and in-order to gradually reduce their moisture content, the stabilized peat soil samples kept in normal air temperature and out of water intrusions to drier condition during the curing period. This process of curing the stabilized peat soil with cement is been named Air Curing Technique. Laboratory tests used for the strength evaluations of stabilized peat soil are Unconfined Compression Strength (UCS), and California Bearing Ratio (CBR). Air Curing periods used are 28, 90, 180 days for the UCS tests and 90 days for the CBR (soaked, and un-soaked) tests. As the curing time for the stabilized peat soil continues the moisture content decreases, thus weight of Water/weight of Cement (W/C) reduces, and as a result stabilized peat soil is hardened and gains strength. The result of strength tests show significant strength improvement of stabilized peat soil through curing period. Also polypropylene fibers when added to the stabilized peat soil with cement, not only give more strength values to the stabilized peat, rather contribute a considerable amount of uniformity and intactness to the stabilized peat as well.
Ordinary Portland Cement, Polypropylene fibers, Unconfined Compression Strength, California Bearing Ratio, Air Curing.
INTRODUCTION Peat is one that contains a significant amount of organic materials. Peat is well known to deform and fail under a light surcharge load, and it is characterized with low shear strength, low compressibility, and high water content (Huat, 2004). Generally any ground that is to be subjected to additional loads which exceed its previous load condition or level, geotechnical requirements for design on that ground are to be established. These requirements include a set of
Vol. 13, Bund. J 2 standard laboratory tests and also some foundation design calculations in order to find the allowable bearing capacity. Usually these laboratory tests including the in-situ tests identify parameters which are essential for foundation design. If these parameters indicate that the in-situ soil is not capable of carrying the design load then there are two alternatives to choose, either the limitation imposed by the in-situ soil properties should be accepted, or use the following techniques enabling the loads to lay on the site (Huat and Ali, 2007).
Transfer the load to a more stable soil layer without improving the properties of the in-situ soil.
Improve in-situ soil properties with various techniques of ground improvement.
Remove the soft soil and replace it, fully or partially, with better quality fill.
Sometimes it may be possible to combine different methods to provide a suitable foundation for the imposed loads. Hebib, and Farrell (2003) provide a technique of surface stabilization combined with stabilized cement columns for foundation loads support. Also, Black et al. (2007) in their study used reinforced stone column that not only transfers loads to the lower and stronger layer rather receives lateral support from the weak soil along the way. In this study, method “ii” has been considered to strengthen the peat soil. Peat soil is stabilized with Ordinary Portland Cement (OPC) as binding agent , and also reinforced with polypropylene fibers as none chemically reactive additive. “Air Curing” method that is to strengthen the stabilized peat soil by keeping it in normal air temperature and out of water intrusions during the curing period has been used as curing procedure for the stabilized peat soil. Air Curing method causes the high moisture content of the stabilized peat soil to gradually decrease with time and during the curing process, and as a result strength values increase as the curing period become longer.
TEST MATERIALS Peat soil samples used for the study, were collected as disturbed and undisturbed according to AASHTO T86-70 and ASTM D42069 (Bowels 1978, and Laboratory Testing, 1980) from Kampung, Jawa on the western part of Malaysia. Table 1 presents the properties of the in-situ (field) peat soil. Binding agent used for this study was Ordinary Portland Cement (Table 2), and polypropylene fibers (Figure 1) as none chemically reactive additive used to reinforce the stabilized peat soil (Table 3).
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3 Table 1: Properties of the peat soil Standard Specifications*
Depth of sampling Moisture Content In-situ (natural) bulk density Specific gravity Classification/Von post Liquid Limit Plastic Index Organic content UCS (Undisturbed) CBR (Undisturbed)
ASTM D2216 BS 1337 BS 1337 ASTM D2974 ASTM 2166-6, AASHTO T208-706 ASTM D1883-73, AASHTO T193-63
Values 5 – 60 cm. 198 - 417 % 10.23 – 10.4 kN/m3 1.22 H4 –H5 160 % N.P. 80.23% 28.5 kPa 0.782%
*Bowles 1975, and BS 1337 Table 2: Main components of Ordinary Portland Cement (Nevile, 1999) Name of Components Tricalcium Silicate Dicalcium Silicate Tricalcium Aluminate TetracalciumAluminate ferrit Calsium Sulphate
Oxide 3CaO SiO2 2CaO SiO2 3CaO Al2O3 4CaSO4, Al2O3, Fe2O3 CaSO4 2H2O or CaSO4
Abbreviation C3S C2S C3A C4AF Gypsum
Table 3: Polypropylene fibers specifications (Polypropylene fibers, 2005) Property Color Specific gravity Fiber Length Fiber Diameter Tensile strength Elastic modulus Water absorption Softening point
Specification Natural 0.91 gr/cm3 12mm 18 micron – nominal 300 – 440 MPa. 6000 – 9000 (N/mm2) None 160ºC
Figure 1: Polypropylene fibers.
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EXPERIMENTAL PROGRAM In order to examine the effect of cement admixture and polypropylene fibers on the load bearing capacity of peat soil, index properties tests on the peat soil have been conducted. The tests include: Sieve Analysis, Water Content, Liquid Limit, Plastic Limit, Organic Content, specific gravity, Fiber Content. Strength values for the undisturbed peat soil as well as the stabilized peat soil (mixture of peat, OPC, and fibers) have been investigated by Unconfined Compression Strength (UCS) and California Bearing Ratio (CBR) tests. Peat soil used for the stabilized samples of UCS, and CBR tests were at their natural (field) moisture contents, and therefore no water was added or deducted from the peat soil during the mixing process of peat, cement, and fibers.
1. Unconfined Compression Strength (UCS) Unconfined Compression Strength tests have been conducted on the undisturbed peat soil as well as stabilized peat soil with OPC and fibers. Sample size used for the experiments was 38 mm diameter and 76 mm length. Disturbed samples used for the stabilized peat soil’s UCS tests were the peat soil samples at their natural (field) moisture content. The peat soil was screened in order to remove the larger size of vegetal fibers using sieve 6.3 mm (0.3˝) first and then, specified amounts of OPC and polypropylene fibers were added to screened peat soil, mixed well for their homogeneity. Then the mixture has been placed in three layers in UCS mould having inside diameter of 38 mm and L/D of 2. Each layer was given 10 constant full thumb pressures of approximately 10 seconds as used in Sweden for compacting stabilized peat soil samples in their mould described by Axelson et al. (2002), and trimmed at both ends, extracted by extractor jack, and wrapped in plastic sleeves for the following curing procedure. The UCS tests for stabilized peat samples were conducted immediately after mixing (0 day) and after being cured at ages of, 28, 90, and 180 days.
2. California Bearing Ratio (CBR) CBR tests have been conducted on the undisturbed peat soil as well as stabilized peat soil with OPC and fibers. For the stabilized peat soil with OPC (mixture of peat soil and cement) the soil samples used were samples at their natural (field) moisture content. Specified dosage of OPC and polypropylene fibers were mixed well with the peat soil for their uniformity and homogeneity, before molding the samples according to the specified standard. Stabilized peat soil samples with OPC and fibers at their CBR’s mold were prepared for the following curing procedure. Curing period used for the stabilized peat soil samples was 90 days. Un-soaked and soaked CBR tests were conducted on the stabilized peat soil samples.
3. Curing procedure (Air Curing Technique) In-order to cure the stabilized peat soil samples with OPC and fibers, Air Curing Technique has been used. In this Technique the stabilized peat soil samples for UCS and CBR tests were kept in normal air temperature of 30±2 ºC and out of reach of water intrusion during the curing period. Air Curing Technique is used to strengthen the stabilized peat soil samples by gradual
Vol. 13, Bund. J 5 moisture content reduction, instead of the usual water curing technique or water submergence method which has been a common practice of past experiments for stabilized peat soil with cement described by Axelson et al. (2002), Duraisamy et al. (2006), and Fei et al. (2007). The principle of using Air Curing Technique for strengthening stabilized peat is that, peat soil at its natural moisture content when mixed with cement has enough water (water content from198 to 417%) for curing process to take place, and does not need more water (submerging the samples in water) for the curing process to take place. This technique will cause the stabilized peat soils to gradually lose their moisture content through curing period and become drier and harder.
4. Mixtures dosages For UCS tests, each set of samples consist of peat soil having natural moisture contents plus 15, 30, and 50% of Ordinary Portland Cement(e.g. 15% cement means in each 100 gr. of peat soil having natural (field) moisture content, 15 gr. ordinary portland cement powder added) with and without polypropylene fibers. The Polypropylene fibers amounts used for the stabilized UCS soil samples were 0.1, 0.15, and 0.25 %( e.g. 0.15% fibers means for each 100 gr. of peat soil with its in-situ (field) moisture content, 0.15 gr. of polypropylene fibers added). For California Bearing Ratio (un-soaked, and soaked) tests, each sample consists of peat soil with natural moisture content plus 15, 25, 30, 40, and 50% of Ordinary Portland Cement (e.g. 25% cement means for each 100 gr. of natural peat soil at its in-situ moisture content 25 gr. of cement added), with and without additive ( polypropylene fibers). The polypropylene fibers amount used for the stabilized CBR soil samples was 0.15% (e.g. for each 100 gr. of peat soil with its in-situ moisture content, 0.15 gr. fibers added).
5. Optimum Polypropylene Fibers percentage determination tests Polypropylene fibers are usually used in concrete mixes to control cracks in hardened concrete (Mulli et al. 2006).The usual dosage recommended for concrete mixes differs from 0.6 to 0.9 kg/m3 ( Polypropylene Fibers, 2005). As for soil stabilization, fibers have been used to stabilize clayey soil, and according to Nagu et al. (2008) study on the strength of stabilized clayey soil reinforced with nylon fibers, 0.4% of fibers would provide the maximum UCS values for the stabilized clayey soil. For this study in order to find the optimum percentage of fiber contents for the stabilized peat soil that would provide the max. strength, peat soil samples at their natural moisture contents mixed with different percentages of OPC and polypropylene fibers cured in air for a period of 90 days prior to be tested for their UCS. The samples examined for this purpose consisted of 15, 25, and 30% of Ordinary Portland Cement as well as 0.1, 0.15, and 0.2% of polypropylene fibers. The sample which showed the maximum strength value for UCS after 90 days 0f curing was chosen as the optimum fibers mixture dosage for further strength evaluations of stabilized peat soil.
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6. Soaking CBR test procedure According to AASHTO T193-63,and ASTM D1883-73, the soaking period of CBR samples for normal soil is 96 hours or four days (Bowles 1978).For this study, in-order to investigate the CBR values of the soaked stabilized peat soil, a set of CBR samples made of different dosages of cement and polypropylene fibers(15, 25, 40, and 50% OPC, with 0.15% of fibers) at soil natural moisture content were cured in air for 90 days, and then soaked in water for a period of five weeks. During these five weeks of soaking period, the soaked CBR soil samples weighted for possible weight increase due to increased saturation periodically. After some times all soaked stabilized peat soil samples were 100% saturated and no weight increased occurred then after. For the first two weeks, the soaked CBR samples weights were recorded every 24 hrs. After first two weeks, their weights were controlled every two days for duration of one week. For the remaining last two weeks of the soaking period, their weights were recorded every five days.
RESULT OF OPTIMUM PERCENTAGE OF POLYPROPYLENE FIBERS DETERMINATION TESTS According to the results shown on Figure 2, the mixture consist of peat, cement and addition of 0.15% fibers would reach to its maximum or 100% Unconfined Compression Strength value when compared with the amount of 0.1% and 0.20% fibers after being cured for 90 days. Based on the obtained result, it is possible to conclude that 0.15% of fibers as none chemically reactive additive would provide the maximum Unconfined Compression Strength Value for the stabilized peat soil with cement. Also because of the obtained result of this test, 0.15% of polypropylene fibers have been chosen as an optimum amount for the stabilized pear soil samples to provide the maximum CBR values as well.
Strength increse (%)
0.10% 0.15% Fiber 0.20% 0.10% 0.15% Fiber 0.20% Fiber+15% +15% Fibers +15% Fiber+25% +25% Fibers +25% Cement+ Cement Cement Cement+ Cement Cement Peat +Peat +Peat Peat +Peat +Peat
Figure 2: Different percentage of fibers and cement mixed with peat soil versus percent strength increase of UCS after 90 days of curing.
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RESULT OF CBR SOAKING TESTS According to the result shown on Figure 3, the least contained Ordinary Portland Cement (15% of OPC) stabilized peat soil sample reaches 100% saturation and therefore constant weight during the first four days of soaking peiod. In the other hand the sample with highest amount of OPC (50%) reaches constant weight (100% saturation) within first six days of saturation or soaking process. Based on the obtained result of this test, all stabilized peat soil samples prior to soaked CBR tests submerged in water for at least six days. 100
Weight increse (%)
15%cement+peat+0.15%fibers 25%cement+peat+0.15%fibers 20
Number of days (soaked samples)
Figure 3: Number of days for soaked CBR samples versus percentage of weight increased during five weeks of soaking.
EFFECT OF POLYPROPYLENE FIBERS AND AIR CURING TECHNIQUE ON THE UNCONFINED COMPRESSION STRENGTH VALUES Results obtained from UCS tests shown on Figure 4 indicate that, addition of OPC to peat soil will increase the UCS values. As the curing time for the stabilized peat soil samples increases the UCS values increase. Also, addition of polypropylene fibers will increase UCS values further. Values of UCS increase considerably after 28 day of curing period. The strength gain continues through 6 months curing period as well.
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Unconfined Compresion Strength(kpa)
450 350 250 peat+15%cem ent
peat+15%cem ent+0.15 %fiber
a) 15% cement
Unconfined Compresion Strength(kpa)
500 peat+30%cem ent
peat+30%cem ent+0.15 %fiber
Curing tim e(days)
b) 30% cement
Unconfined Compresion Strength(kpa)
650 600 550 peat+50%cem ent 500
peat+50%cem ent+0.15 %fiber
Curing tim e(days)
c) 50% cement Figure 4: Curing time versus Unconfined Compression Strength values for the stabilized peat soil with different percent of Ordinary Portland Cement (15, 30, 50%) and 0.15% of polypropylene fibers.
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EFFECT OF CURING AGE AND WATER-CEMENT RATIO ON UNCONFINED COMPRESSION STRENGTH Concrete is a common civil engineering material that is made of cement and different sizes of aggregates. Generally concrete gains strength and hardens by time and gains most of its ultimate compressive strength at 28 days of age. One of the principal factors affecting the compressive strength of concrete is the Water – Cement Ratio (weight of water /weight of cement) or W/C of the fresh mixed concrete (Davis et al. 1983). Usually for normal concrete mixes a W/C of about 0.5 is suggested, and as W/C for fresh concrete increases the compressive strength of concrete at 28 days will decrease (Design of normal concrete mixes, 1975). Therefore, it is possible to conclude that, for the hardening process for concrete mixes to take place, not more than an average W/C of 0.5 is required and as the W/C is increased from 0.5, the compressive strength of concrete decreases. In this part of the study W/C character as well as the strength gain of the stabilized peat soil with cement during curing process is compared with concrete mix. According to the results obtained in this study for Water-Cement Ratio versus curing time (age) for the stabilized peat soil samples shown on Figure 5, W/C values decrease as the curing times are increased. The initial W/C of all the stabilized samples is high and above 2 and during the 180 days of air curing, the W/C of the stabilized peat soils never falls below 0.5. This suggests that the initial water content of the natural peat soil is more than sufficient to carry on the hardening process, and thus during the curing period there will not be any need for extra water to cure the stabilized peat soil with cement. 10 peat+15%cem ent
Water -Cement Ratio (W/C)
7 6 5 4 3 2 1 0 0
Figure 5: Curing ages of stabilized peat samples with different percentages of cement for Unconfined Compression Strength tests versus W/C. UCS test results of 0 (immediately after mixing), 7, 28, 90, and 180 days for stabilized peat with OPC in Figure 6 show that, as W/C values are decreased through curing process, the UCS values are increased. Therefore, Air Curing process cause the stabilized peat soil samples to gradually lose their moisture contents and become drier and as the stabilized peat soil become drier(water content is reduced), the UCS values increase.
Vol. 13, Bund. J 10 Actually, a considerable strength gain of the stabilized peat soil is occurred after 28 days, and the strength gain continues through 90 and 180 days of curing. 700
Unconfined Compresion Strength (kpa)
peat+15%cem ent 600 peat+30%cem ent 500
400 300 200 100 0 0
Water-Cem ent Ratio (W/C)
Figure 6: Water-Cement Ratio versus Unconfined Compression Strength of stabilized peat with different percentage of cement. Also the results obtained from Figure 5 and 6 justify the Air Curing Technique used for the stabilized peat soils with cement, instead of the usual curing technique of water submergence method used for stabilized peat soil samples with cement practiced in the past.
EFFECT OF AIR CURED STABILIZED PEAT SOILS ON CALIFORNIA BEARING RATIO (CBR) The result of CBR tests for stabilized peat soil samples with Ordinary Portland Cement (OPC) and polypropylene fibers after being air cured for 90 days are shown on Figure 7. The results indicate that as cement amount in the mixture is increased, the CBR values are increased and addition of polypropylene fibers content will increase the CBR values even further. Also CBR values are decreased for soaked (saturated) samples compared with the un-soaked samples. Polypropylene fibers as additive contributes more strength to the lower dosage of OPC (15%) mixed with peat soil samples compared with higher dosage of OPC (50%) in the mix. The Air Curing Technique as well as OPC and polypropylene fibers used for peat soil stabilization will increase the general rating of the in-situ peat soil from very poor (CBR from 0 to 3%) to fair and good (CBR from 7 to above 20%), (Bowles, 1978). Also, visual inspection of soaked CBR samples depict that, Polypropylene fibers not only increase the CBR values rather contribute a considerable amount of uniformity and intactness to the stabilized peat soil samples as well, when compared with the soaked stabilized peat soil samples having only Ordinary Portland Cement.
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be tur di s
t t at eat eat eat ea ea pe r+p r+p r+p t +p t +p e e e nt + n n b b b e e e i i i %f %f %f em em em .15 .15 .15 %c %c %c 15 30 50 t +0 t +0 t +0 n n n e e e em em em %c %c %c 15 30 50 t ea
Figure 7: CBR values for the undisturbed peat and different percentage of OPC and fibers for the stabilized peat soil cured for 90 days. Conclusion In this study, peat soil has been stabilized with Ordinary Portland Cement (OPC) as a binding agent alone, and also stabilized with, OPC, and Polypropylene fibers as none reactive chemical additive. Air Curing Technique that is to keep the stabilized peat soils with OPC in normal air temperature and out of water intrusion during the curing period is been used. This technique will cause the stabilized peat soils to gradually lose their moisture content through curing period and become drier. The binding agent and the additive as well as the curing technique have proved to increase Unconfined Compression Strengths(UCS) values of the stabilized peat soil samples at 28, 90, and 180 days. The result of UCS tests for stabilized peat soil with OPC with different percentage of polypropylene fibers show that 0.15% is the optimum percentage to provide maximum UCS values. Curing the stabilized peat soil samples in air, will cause the Water – Cement Ratio (W/C) of the samples to reduce. As the W/C for the stabilized peat soils are reduced through air curing period, the UCS and CBR values increase. Peat soil at its natural state, when mixed with OPC has high initial W/C (W/C > 0.5) and it maintains enough W/C during the air curing process even through 180 days of curing period for hardening process to take place, and does not need extra moisture (being submerged in water) for the hardening process. Air Curing Technique of stabilized peat soil during six months with 15% of OPC (less than 190 kg/m3), will increase the UCS of in-situ peat soil by over 13 folds (from 28.5 Kpa. to 380 Kpa.), and increases the UCS value of in-situ peat by over 14 folds when 0.15% (less than 2 kg/m3) polypropylene fibers added to the mixture of peat and OPC.
Vol. 13, Bund. J 12 Also Air Curing Technique of three months for the stabilized peat soil with 15% OPC increase the CBR values of in-situ peat soil by over 24 folds (from 0.782% to 19%) for the unsoaked samples, and by over 9 folds for the soaked samples. Addition of 0.15% of polypropylene fibers to the stabilized peat soil with 15% OPC increase the CBR values of in-situ peat soil by over 29 folds(from 0.782% to 23%) for the un-soaked samples, and by over 19 folds for the soaked samples.
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