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Physical and Engineering Properties of a Cement
Geotechnical Engineering Division, Department of Civil Engineering and Geotechnical Engineering Division, Department of Civil Engineering |
ABSTRACT
The soil mixing, a ground modification technique, has been used for many diverse applications including building and bridge foundations, retaining structures, liquefaction mitigation, temporary support of excavations, water control and structures that protect the natural environment. This method has a basic target, to find the most efficient and economical method of mixing cement with soil, so that, the soft soil obtains properties more like to those of a soft rock, such as a clayey shale or lightly cemented sandstone.
Soft soils are well known for their low strength and high compressibility. Usually, due to sedimentary process on different environments, both physical and engineering properties (namely void ratio, water content, grain size distribution, compressibility, permeability and strength) show a significant variation. Further, they exhibit high compressibility (including an important secondary consolidation), reduced strength, low permeability and compactness, and consequently low quality for construction. Thus, deep mixing has recently been used to improve the strength and deformation characteristics of these soft clays.
For the aforementioned reasons, a comprehensive laboratory testing programme was carried out in order to study the effect of inclusion of cement and acrylic resin on physical and engineering behaviour of a soft clay. Series of unconfined compression, oedometer consolidation, durability, porosity, permeability tests were conducted with cement content varying from 5% to 30% w/s.w.(weight by soil weight) and acrylic resin of 5%w/c.w.(weight by cement weight) and having curing periods of 7 and 28 days.
KEYWORDS: stabilization, cement, acrylic resin, compressive strength, slaking, permeability, compressibility.
INTRODUCTION
A technique of stabilization and reinforcement of a soft soil is mixing by cement, ground modification. This technique has been used in Japan (mid 1970’s), and in Scandinavia (Sweden, Finland), for many diverse applications. Names such as Jet Grouting, Soil Mixed Wall (SMW), Geojet, Deep Soil Mixing (DSM) or in present work Cement Deep Mixing (CDM) has recently gained wide acceptance in the United States (Kaushinger, 1992). The CDM technique is used at the following cases:
BUILDING FOUNDATION: to reduce settlement by ensuring volume stability or controlling permeability or increasing strength.
EXCAVATION WORK: to provide better support by strengthening the surrounding soil or varying its permeability.
PAVEMENT CONSTRUCTION: to provide more durable and stronger pavements by eliminating volume changes and preventing weathering deterioration.
SLOPE STABILITY: to prevent slips by strengthening the soil.
WATER RETENTION: to ensure safe structures against water erosion
ENVIRONMENTAL CONSERVATION: to prevent or repair erosion damage by increasing the resistance of the soil to natural weathering from water (Lee, 1974; Ingles et al, 1972).
Each of these aforementioned methods has a basic target; the engineering structures remain intact over a long period of time without any required repairs or reconstruction in the future.
During the last decades some limitations have been imposed to constructions: limits on the space occupied and on time for construction; safety, growing environmental concern; shortage of traditional natural material. Soft soil deposits are problematic soils for construction and that is why they have been ignored for long time, on the contrary with soils of higher quality with reduced technical difficulties and lower construction costs. Alternative areas for construction become however more and more important during the last decades, due to growing shortage of better quality soils for construction. These limitations could be overcome with the introduction of new mixing materials (such as cement-acrylic resin) and new construction techniques (such as CDM), (Pinto et al, 2003).
Several minimum requirements must be satisfied in order to improve adequately the physical and engineering properties of soft soils. If a soft soil is stabilized by cement, factors as compressive strength (bearing capacity), durability under environmental conditions of wetting-drying (strength of cement-soil bonds), porosity (water or waste leakage through the pore grains), permeability (containment walls can be constructed with permeability of approximately 5x10-7 cm/sec), compression index (consolidation-settlement of foundations) should be taken into consideration (Bell, 1975; Kawasaki et all, 1981 and Kamruzzaman et all, 2000).
For these reasons, tests of compressive strength and slake durability index were carried out, and parameters such as porosity, compression index Cc, apparent swelling index Cas, permeability coefficient, were estimated for soft clay stabilized with 5, 10, 20 and 30% cement addition with/without 5% acrylic resin, cured for 7 and 28 days. The main concept in this research work is the study of the influence of cementing agent on the stabilization-engineering parameters (Anagnostopoulos et al, 2003).
The final accepted basic strategy of this study for the above mentioned engineering parameters was:
(1) The influence of variability percentage of cement on strength, slaking (potential weakening of bonds between the grains of cement-acrylic resin stabilized soft soil due to wetting-drying or due to stresses), porosity, and compressibility parameters.
(2) The characterization, classification and study of higher values of the above stated parameters for this improved soft clay in order to use it as reinforced material in soil-improvement techniques such as soil mixing.
Additionally, the acrylic resin used as additive with cement paste increases significantly the bonds within the grains as well as durability, cohesion, engineering strength and decreases porosity.
Finally the acrylic resin prevents, by making the clay-cement system more impermeable, from the adsorption of deleterious substances such as sulphates or probably organic compounds with small chains.
DESCRIPTION OF MATERIALS
Soil
Soft soil sample was collected from excavations at a depth of 10 to 15m. The basic characteristics of the in-situ soft soil are listed in Table 1.
Table 1. Properties of the in-situ soft soil
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| Properties | Characteristics | Properties | Characteristics |
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| Liquid Limit LL (%) | 43.54 | Grain size distribution Clay (%) Silt (%) Sand (%) |
27 40 33 |
| Plastic Limit PL (%) | 25.32 | ||
| Plasticity Index PI (%) | 18.22 | ||
| Water Content (%) | 25.16 | ||
| Activity | 0.67 | Initial Void Ratio (eo) | 1.653 |
| Bulk Unit Weight (kN/m3) | 16.68 | pH (Soil:water = 1:5) | 8 |
| Dry Unit Weight (kN/m3) | 13.33 | X-Ray Diffraction Analysis |
Montmorillonite, Kaolinite, Illite, Chlorite, Quartz, Calcite. |
| Specific Gravity | 2.7 | ||
| Compression Index (Cc) | 0.311 | ||
| Swelling Index (Cas) | 0.093 | ||
| Shear Strength (kPa) | 18.6 | ||
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Cement
Microfine Portland cement was used with Blaine over 4500cm2/g., produced by Titan Co., Greece and its chemical composition is shown in Table 2.
Table 2. Chemical composition of the cement used
| Chemical Component | SiO2 | Al2O3 | Fe2O3 | CaO | MgO | SO3 | K2O | Na2O | Ignition loss |
| Amount (%) | 30 | 7,5 | 2 | 52 | 2 | 3 | 1,5 | 0,5 | 1,5 |
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Acrylic Resin (A.R.)
The acrylic resin used was an emulsion of a synthetic elastic chemical substance which as additive in cement paste increases significantly the bonds within the substrate particles as well as the cohesion and the engineering strength. It improves also the durability in chemicals intrusion, the impermeability and finally the durability in cycles of freezing and thawing.
The amount of A.R. added in cement-treated soil samples was 5%w./c.w. (per weight of cement) for all cement/soil ratios.
SAMPLE PREPARATION
Cement and cement-acrylic resin treated soil samples were prepared as follows:
A required amount of water was added to the soil sample and mixed thoroughly by a high speed-rotating stirrer to obtain saturated conditions. Afterwards a quantity of cement in powder form was added to the saturated soil and the whole mixture was stirred in short time (5 min) to avoid hardening of the soil-cement mixture. Then, the quantity of A.R. (5% w/s.w.) was added and another mixing followed for 5 min. Quantities of the added cement to the soil slurry were 5, 10, 20, 30% w/s.w. The samples for unconfined compression tests were kept in plastic bags during the curing time. Oedometer specimens were prepared by placing the treated soil directly into the oedometer ring and kept them in airtight plastic bag to prevent moisture loss until the day of their test.
The samples for compressive strength (ASTM D 1632-96) and slake durability test were kept in plastic bags during the 7 and 28 days of curing. The strength was measured using a commercially available device named Versa Tester/Soil test Inc. The dimensions of the cylindrical specimens tested were 35.5mm in diameter and 71mm in length. The rate of strain was 0.6604mm/min. The slaking (100 – Id2) was measured using the device and testing procedure developed by Franklin (Franklin and Chandra, 1972). Finally the slake durability index (Id2 – second cycle) was calculated as the percentage ratio of the final to initial dry sample weight. The dimensions of the cylindrical specimens tested in slake durability test were 35.5mm in diameter and 23.7mm in length.
Permeability coefficient was measured according to A.S.T.M D 5084-00e1 regulation and porosity was measured according to the referenced method of Grimshaw (Grimshaw, pp. 421-422, 1971).
RESULTS AND DISCUSSION
Unconfined compression tests were performed to determine the stress-strain and stiffness characteristics of cement treated soft soil. The experiments were conducted with cement content varying from 5 to 30% with/without 5% A.R. having curing periods of 7 and 28 days. Figures 1 and 2 illustrate the stress-strain behaviour of treated soil for different cement content with/without A.R. after 7 and 28 days curing period, respectively. It was found that all soil-cement stabilized samples cured for 7 days and treated with acrylic resin revealed lower compressive strength and stiffness compared with soil-cement stabilized samples without A.R. treatment. The difference in strength was 18, 28, 33 and 48% for 5, 10, 20 and 30% w/s.w. cement respectively. The reason of the above reported difference was related to the retarding action of the A.R. on the pozzolanic reactions and consequently on the cement hardness. This adverse action is going to be diminished by time.
Strength values of soil-cement stabilized samples cured for 28 days and treated with A.R., were higher than values of soil samples treated only with cement. In case of 5% w/s.w. cement addition the increment of strength was 26.5% while in larger amount of cement addition this increment was almost permanent between 18.5 to 19% (figure 3).
Figures 1 and 2 also reveal that higher cement content treated soil exhibits more ductile behaviour. More brittle type of failure with low values of failure strain was observed for lower cement content treated soil in both curing periods (Tatsuoka et al, 1996).
Figure 1. Stress-strain behaviour of cement and cement acrylic resin treated soil cured for 7 days
Figure 2. Stress-strain behaviour of cement and cement acrylic resin treated soil cured for 28 days
Figure 3. Compressive strength vs. cement percentage for treated soil cured for 7 and 28 days
Figure 4. Compression index Cc vs. cement percentage for treated soil cured for 7 and 28 days
Figure 4 shows the Compression index Cc in relation to cement content of treated soil aged for 7 and 28 days. Consolidation characteristics were improved strongly as the cement content was increased. Curing time had only marginal effect on this improvement. The addition of 5, 10, 20, 30% w/s.w. cement, decreased the compression index Cc to about 0.17, 0.11, 0.017 and 0.016 correspondingly, in 28 days, from 0.311 of the untreated soil. Acrylic resin caused a small increment of Cc for all percentages of cement due to initial water absorption from the hydrophilic groups of acrylic resin polymeric chains and to its final water loss during loading.
The portion of (e-logs´v) relationship before apparent pre-consolidation pressure is defined as apparent swelling index (Cas). Figure 5 illustrates the relation between Cas and cement content. In case of 5%w/s.w. cement addition, the curing time and A.R. influenced the Cas values. This influence is limited for higher amounts of cement added. Generally A.R. reduced slightly the Cas values due to prevention of water intrusion in the soil mass.
Figure 5. Apparent Swelling Index Cas vs. cement percentage for treated soil cured for 7 and 28 days
Figures 6-8 show the relation of cement content with slaking, permeability coefficient and porosity of treated soil cured for 28 days. The above results indicate that the addition of A.R. decreases the values of slaking, permeability coefficient and porosity. Also, the increase of cement content decreases strongly the values of slaking, permeability coefficient and porosity. In relation to the above, the slaking decreases because of the potential strengthening of bonds between the grains of cement-acrylic resin stabilized soil. Additionally, A.R. forms polymer films, which partially fill the voids and micro cracks between the cement-soil grains resulting in the decrease of permeability coefficient and porosity.
Figure 6. Slaking vs. cement percentage for treated soil cured for 28 days
Figure 7. Influence of cement content permeability coefficient of treated soil cured for 28 days
Figure 8. Influence of cement content on porosity of treated soil cured for 28 days
Figure 9. Compressive strength vs. slaking of treated soil cured for 28 days
Figure 10. Porosity vs. slaking of treated soil cured for 28 days
Figures 9 and 10 show the influence of A.R. on slaking-compressive strength and slaking-porosity, respectively. Slaking (durability) is governed by both cement and A.R. film formation process in their binder phase with soil particles. The above improves significantly durability, strength and decreases slightly the porosity of treated soil cured for 28 days (Stavridakis, 1997).
CONCLUSIONS
The conclusions from the compression and oedometer consolidation tests are described below.
references
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