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15 capillary soak protocol, the sample is placed on a porous stone and wrapped in an absorptive fabric and allowed to absorb water through capillary rise until the moisture front ceases to move or for at least 24 hours. Additional Tests Involved in Stabilizer Selection Once an additive has been selected based on the index properties of plasticity index and percent of the soil mass smaller than 75μm, the possible impact of deleterious components of the soil must be considered. Organic contents in excess of one percent on a mass basis have been proven to be potentially deleterious (16). However, some soils with organic contents well over one percent have been successfully treated and stabilized with lime and Portland cement. The second deleterious component is high salt content. A high potassium or sodium content may negatively impact stabilization by competing with calcium cations. However, this can normally be overcome simply by adding the additional calcium-based stabilizer. However, salts containing sulfates have the potential to react with calcium and aluminum released from soil in the high pH environment formed during stabilization to form expansive minerals that can disrupt the stabilized layer. The mechanisms of these mineral formations and the associated volume changes in pavement layers are detailed elsewhere (13). Soil organic content should be measured following ASTM D 2974. Soils with an organic content of 1-2 percent as determined by ASTM D 2974 may be difficult to stabilize or may require uneconomical quantities of additives in order to stabilize. Stabilized soils, in some cases, may also not be able to meet the recommended strength criteria when excess amounts of organic matter are present. This is because the presence of organic materials in soils inhibits the normal hydration process and reduces the strength gain in stabilized soils. Sulfate contents in soil should be determined following Modified AASHTO test method T 290 or equivalent. Generally, water soluble sulfate levels greater than 0.3 percent (3,000 ppm) suggest the potential for expansive reactions to occur that may result in disruptive volume change in the stabilized layer. Recommendations outlined in Guidelines for Stabilizing Sulfate- Bearing Soils should be followed in stabilizing these soils with lime (13). VALIDATION OF STABILIZER SELECTION The procedure outlined below provides a guideline for mixture design for lime, Portland cement and fly ash. Lime Stabilization for Soils Lime is an appropriate stabilizer for most cohesive soils but the level of reactivity depends on the type and amount of clay minerals in the soil. The steps described in the following paragraphs ensure that the appropriate amount of lime is used to meet design expectations. If design expectations cannot be met with lime, that will become clear by following this protocol described in this section. Mix Design Considerations The mix design protocol presented here follows the National Lime Association protocol (21). The mix design protocol is designed to optimize the potential for long-term strength gain and durability of lime stabilized soils.
16 Soil Evaluation The first step in the NLA protocol is similar to the approach described in Figure 2 and in fact either the criteria described in Figure 2 or the criteria described in this section can be used. In this step, the soil fraction passing the no. 200 sieve is determined following AASHTO T-27. Liquid limit and plastic limit should be determined following AASHTO T 89 and AASHTO T- 90, respectively. Soils with a plasticity index of 10 or above and a minimum of 25 percent passing the no. 200 sieve are considered desirable for lime stabilization. The NLA protocol requires screening for organic contents above one percent following ASTM D 2974. The NLA protocol does not restrict or eliminate lime stabilization when the organic content of the soil is above one percent, but the protocol recommends that the designer maintain an awareness of this condition throughout the design process and also maintain an awareness of the fact that high organic contents may disrupt the pozzolanic reaction process and may require a greater lime content than normal for the soil in question to reach the desired strength. Water soluble sulfate should be evaluated following AASHTO T 290 (modified). The NLA protocol recommends that if the soluble sulfate content is greater than 3,000 ppm then the user should perform swell tests to verify the expected degree of expansion and take construction steps to mediate the potential expansive reactions. Additional steps to be followed in stabilizing soils with sulfate content above 3,000 ppm are detailed in the AASHTO draft recommended practice for stabilizing sulfate bearing soils (13). Optimum Lime Content The first step in assessing the optimum lime content to ensure optimal long term strength gain is to perform the Eades and Grim pH test. For reliable test results, the lime used in the pH test should be the same as that to be used in construction and this lime should be carefully stored to avoid carbonation. The lime used, whether it is in the form of CaO or Ca(OH)2, must meet AASHTO M 216 (ASTM C 977) or equivalent for purity requirements. The standard test method, ASTM D 6276, is used to determine the amount of lime needed to achieve the design pH at 250C (770F), which is about 12.45, depending on specific soil characteristics. The goal of this test is to identify the amount of lime necessary to satisfy immediate lime-soil reactions and also provide a sufficient quantity of calcium to maintain a high residual pH and sustain significant long-term pozzolanic reactions. The pH test is only a first step. The optimum lime content must be validated based on strength testing. Moisture Density Relationship The addition of lime changes the optimum moisture content (OMC) and maximum dry density (MDD) of soils because the effects of cation exchange and short-term pozzolanic reactions between lime and the soil results in flocculation and agglomeration of clay particles leading to textural changes that are reflected in the moisture-density relationships. For this reason it is necessary to verify the moisture-density relationship of the lime-soil mixture when the amount of lime identified by the Eades and Grimm pH test has been added. The moisture-density relationship of lime-soil mixtures should be determined in accordance with AASHTO T 99. Fabrication and Curing of Samples for Compression Testing Lime-soil mixtures should be fabricated following ASTM D 3551 for compressive strength testing. The samples should be prepared at the moisture content and density expected in the field. Normally, for compressive strength testing, samples are not allowed to mellow before samples
17 are fabricated. However, if it is difficult to achieve satisfactory homogeneity during laboratory mixing, it is reasonable to consider a mellowing period (between initial mixing and final mixing before compaction) of up to 24 hours to simulate field mellowing. However, with the high efficiency of lab mixing compared to field mixing, it is assumed that lab mellowing will not be necessary in most applications. Triplicate samples are prepared for compressive strength testing following ASTM D 5102 procedure B with the lime content determined from the pH test. Samples are fabricated at between optimum moisture content (OMC) and OMC ± 1 percent. Additional mixtures with lime contents one and two percent higher than the optimal lime content identified by the Eades and Grim pH test as optimum should also be fabricated and tested following ASTM D 5102 to verify the optimum lime content, which may be greater than that identified by Eades and Grim pH test. After compaction the test specimens should be wrapped in a plastic wrap and stored in an air tight moisture proof bag with about 10 ml of free water to ensure proper moisture for pozzolanic reactions. The specimens are then cured at 40oC (1040F) for 7 days before compression testing. Since the accelerated cure is not always a good approximation of strength gain by long term normal cure, it is appropriate to subject one set of lime soil samples to normal cure for 28 days before compression testing. After the curing period, the specimens are removed from the storage bags and plastic wraps are removed. The specimens are then wrapped with a wet absorptive fabric or geotextile and placed on a porous stone for capillary soak. Capillary soaking should continue for as long as it takes for the moisture front to move to the top of the sample or until the moisture front ceases to move. A soaking period of at least 24 hours is recommended. Research work by Thompson (22) and Little (23) demonstrated that the reduction in compressive strength due to soaking is not substantial (less than about 10 percent) for stabilized soil with a significant level of pozzolanic reaction. But the deleterious effects can be significant (up to 40 percent) if soaking occurs prior to significant pozzolanic strength gain. During capillary soak, the water used in soaking should never come in direct contact with the specimen (24). The water level should be maintained to the top of the porous stone and kept in contact with the fabric wrap. Unconfined Compression Strength Testing Following capillary soak, unconfined compression strength testing should be performed in accordance with ASTM D 5102 procedure B. The results of compression tests are compared with the suggested minimum requirements given in Table 2. If more than one lime contents are considered in compression testing, the lowest lime concentration that meets the compression strength requirement is considered as the required lime content for stabilization purposes. If the specimens do not meet the strength criteria, then the soils can be considered as modified soils and not stabilized soils. Higher lime content may be used in these soils and the mix design procedure, starting from moisture density relationship, should be repeated. It should be noted that the compressive strength values given in table below are suggested minimum values and field requirements may vary depending on purpose of stabilization, exposure conditions, expected freeze thaw cycles and cover material over stabilized soil. Table 2. Compressive strength recommendations for lime stabilized sections (22). Anticipated Use of Stabilized layer Compressive strength recommendations for different anticipated conditions
18 Extended Soaking for 8 Days (psi) Cyclic Freeze-Thaw 3 Cycles (psi) 7 Cycles (psi) 10 Cycles (psi) Sub-base Material Rigid Pavements/Floor Slabs/Foundations 50 50 90 120 Flexible Pavement (>10 in.) 60 60 100 130 Flexible Pavement (8 in. - 10 in.) 70 70 100 140 Flexible Pavement (5 in. - 8 in.) 90 90 130 160 Base Material 130 130 170 200 For cyclic moisture conditioning the samples should be made to reach 80 percent saturation upon âwettingâ followed by 50 percent saturation upon âdryingâ. This is satisfactory to represent the damaging cyclic environment. Volume Change Measurements for Expansive Soils Samples prepared for compression testing can be used to evaluate volume changes in lime stabilized expansive soils. Vertical and circumferential measurements of samples before and after soaking should be made to calculate volume changes between the dry and soaked conditions. A three dimensional volumetric expansion of up to 2 percent is typically regarded as acceptable (24). If the expansion in the treated soil is higher than the recommended value, then additional lime of 1 to 2 percent should be evaluated. This step is applicable only for expansive soils. Although this test can be used to validate swell in sulfate bearing soils, the period of exposure to moisture for sulfate bearing soils is considerable longer than 7-days. In the case of sulfate-bearing soils the period of swell should continue until swell ceases. Lime Treatment of Base Courses The protocol described above addresses lime-soil mixtures. In the event that lime is used as a stabilizer for base materials, it is important to understand that the purpose of lime is to interact with the fine material, normally finer than 75 μm, to form a matrix that will provide improved strength for the aggregate base. It is assumed that the candidate aggregate base material is of at least moderate quality, otherwise, the material should be treated as a soil. Moderate quality is defined as: (1) not more than 20 percent finer than the no. 40 sieve (0.425 mm or 0.0165 in.), (2) a maximum plasticity index of 12 percent, and (3) a maximum liquid limit of 40 percent. Since in aggregate base courses, the fine material (smaller than about 75 μm) comprises no more than about 10 percent of the of the entire mixture by weight, the amount of lime used by weight of the total aggregate base will be considerably less than that used in soils. Normally, the amount of lime used in base stabilization is between about 1 percent and 3 percent by total weight of the aggregate base. Adding lime to the fines matrix will decrease plasticity as well as increase strength, and it can generally be surmised that if an acceptable target strength is achieved, that the plasticity of the
19 fines will be appropriately altered as well. However, it is prudent to test the plasticity of the minus no. 40 sieve fraction with the target lime content to verify the impact of lime on the plasticity of the fines. The steps for stabilization of a base course are: (1) add the appropriate target percentages of lime (generally starting with 1 percent by weight of the entire mixture and increasing in 1 percent increments to 4 percent), (2) determine moisture density relationships for each aggregate-lime blend following either AASHTO T-99 or AASHTO T-180 based on agency requirements, and (3) determine unconfined compressive strengths of the lime-aggregate blends following curing for 7-days at 400C (1040F) followed by capillary soak as described in the NLA protocol for lime- soil mixtures. The compressive strength testing procedure and target compressive strength requirements should be based on specifications defined by the user agency. Cement Stabilization The American Concrete Institute (ACI) defines soil cement as a mixture of soil and a measured amount of cement and water mixed to a high density (25). Soil cement has been classically defined as a stabilized soil in which the coarse aggregate, sand size and larger (coarser than 75 μm) is surrounded and bonded by a matrix of cement paste and fine soil particles. The goal of mix design for this type of soil is to float the coarse aggregate in the matrix. The durability of this matrix is determined by durability tests such as AASHTO T 135 and T 136 (or by their ASTM equivalents D 559 and D 560) or by compressive strength testing. However, Portland cement has also been successfully used to stabilize fine grained silt and clay soils. In fact cement stabilization of silty soils provides perhaps the most dramatic improvement of any soil type (when the properties of the cement treated silty soil are compared to the properties of untreated soil). However, the amount of cement required to stabilize fine grained soils can be substantially more than that required to stabilize coarse grained soils because of the higher surface area of fine grained soils. The transition from silt to clay means that the particle surface area increases by orders of magnitude. However, in actuality cement does not need to coat all particles for successful stabilization and substantial improvement of moderately plastic clay soils, plasticity indices of below 30, has been achieved with about the same amount of Portland cement as would be required of hydrated lime. This is primarily because the cement forms a stabilized matrix around agglomerates of clay particles. Obviously if the integrity of cement matrix surrounding the agglomerates is compromised, then the durability of the matrix will begin to degrade. The ability to stabilize soils with plasticity indices above about 20 with cement is based on the ability to intimately mix cement with the soil to a degree that will produce a reasonably homogeneous and continuous, stabilized matrix of the agglomerates. This requires a certain efficacy of mixing, which is in turn associated with the energy imparted to the soil by the mixing equipment and by the time span over which mixing occurs. The limitation associated with mixing Portland cement with plastic clay soils is the short time of initial set of the cement, usually not more than 2 hours is provided for mixing before compaction. However, this mixing time has been extended under certain circumstances. During the extended mellowing period, the release of free lime during cement hydration alters plasticity and textural properties of the clay soil, which can improve workability. However, mixing following this extended mellowing must be performed with equipment that has the ability to impart sufficient energy to mix the soil and cement after the cement has reached a final set, which normally occurs within 8 hours. It must be
20 understood, when extended mellowing is adopted, that all the strength lost during remixing may not be recovered with additional curing. Hardened soil cement mixtures must withstand adverse environmental conditions. Other stabilization objectives include reducing plasticity index, increasing shrinkage limit, meeting strength thresholds, and improving resilient modulus. Soil cement can provide a strong and uniform support for pavement layers and provide a firm and stable working platform for construction. In summary, most soil types, except those with high organic content, highly plastic clays and poorly reacting sandy soils, are amenable to stabilization with Portland cement. General gradation specifications limit the nominal maximum size at 2-inches with at least 55 percent passing the no. 4 sieve. For uniformly graded materials, the addition of non plastic fines like fly- ash, aggregate screenings, cement and lime kiln dust may help fill the voids in the soil structure and help reduce the required cement content Mix Design Considerations As with lime stabilization, soils must be screened for organic content and sulfate content prior to verifying whether Portland cement is an acceptable stabilizer. Soils with higher organic content may require a higher cement content as the organic matter can inhibit normal hardening processes. A pH test, as recommended by the U. S. Army Corps of Engineers, using a mixture of 10 parts soil to one part cement (by weight) is used to verify if organic matter might interfere with the hydration process (6). If the pH of the paste after 15 minutes of mixing is 12.0 or higher then it is probable that organics will not interfere with the normal hardening process. If not, then a higher cement content than that recommended based on AASHTO soil groups (Table 3) may be needed. Again the required cement content must be confirmed based on strength testing. The following procedure outlines the steps to be followed in developing an effective mix design for cement stabilized soils. Preliminary Estimate of Cement Content The first step in determining the required cement content is to classify the soil, AASHTO M 145. Table 3 defines a starting point to be considered in treatment. These cement contents are based on a data base of empirical evidence of soil cement mixtures that have proven to be able to meet the durability requirements established in AASHTO T 135 and T 136 or their respective ASTM equivalents D 559 and D 560. In Table 3, the cement quantities are proportioned on a weight basis in terms of the percent of oven dry soil. Table 3. Cement requirement for AASHTO soil Groups (26). AASHTO Soil Group Usual Range in Cement Requirement Estimated Cement Content, Percent by Weight Percent by Volume Percent by Weight A-1-a 5-7 3-5 5 A-1-b 7-9 5-8 6 A-2 7-10 5-9 7 A-3 8-12 7-11 9 A-4 8-12 7-12 10 A-5 8-12 8-13 10 A-6 10-14 9-15 12 A-7 10-14 10-16 13
21 These cement contents are only preliminary estimates and must be verified or modified based on additional test results. Additional cement requirement for soils with higher organic contents should be considered based on pH test of soil cement mixtures (6). It is important to understand that the requirements in Table 3 are based on durability tests, ASTM D 559 and D 560, and that many soils can be successfully stabilized with considerably lower cement contents. Determine the Moisture Density Relationship Changes in optimum moisture content and dry density with addition of cement are not always predictable (4). Flocculation of clay particles by cement can cause an increase in optimum moisture content and decrease in maximum dry density for cement-soil mixes whereas the higher density of cement relative to soil can result in a higher density for mixes. Therefore, it is appropriate to use the median cement content as estimated in Table 3 for determination of moisture density relationships as the maximum dry density varies only slightly with modest changes in percent cement content (26). However, as previously discussed, if it is expected that acceptable treatment can be achieved with considerably lower cement contents than those in Table 3, then that cement content should be used to determine the moisture-density relationship. After the required amount of cement is added to the soil, the blend should be mixed thoroughly until the color of the mixture is uniform. Fabrication and testing of samples for moisture density relationship should be done in accordance with AASHTO T 134 or its ASTM equivalent D 558. Sample Preparation for Compressive Strength and Durability Testing Two types of tests are typically used to evaluate the efficacy of a soil cement mixture: strength tests and durability tests. The Portland Cement Association (PCA) considers the ability to withstand adverse environmental conditions as the primary requirement for soil cements (26). The PCA manual recommends durability tests based on weight loss under wet-dry and freeze- thaw conditions for evaluating usability of soil cement mixtures. Both PCA and ACI determine the weight loss in samples subjected durability tests in accordance with ASTM D 559 or ASTM D 560 as appropriate. These methods are highly subjective and carry significant user variability. In addition, these test methods may not reflect field conditions that are applicable to all stabilized pavement layers. In flexible pavements the soil cement base is protected at the surface by a hot mix bituminous wearing surface and in rigid pavements by a concrete slab. Hence the extent of damage in pavement layers due to freeze-thaw activity will vary significantly depending not only on climate but also the pavement structure. Healing of micro cracks in the stabilized layers with time may also influence the extent of damage in field (27). This effect is not reflected in the recommended freeze thaw test criteria. It is most important to consider that the depth of penetration and the number of freeze-thaw cycles to which the pavement layer is exposed varies considerably from site to site. Since the results of freeze thaw testing does not simulate field conditions, many state departments of transportation currently recommend minimum unconfined compressive strength testing based on ASTM 1633 in lieu of durability tests (3). The research work by Thomson and Dempsey in lime stabilized soils has shown that compressive strength of samples subjected to freeze thaw can be used as a criteria in deciding durability issues in soil cements (28). Thompsonâs data demonstrate that the compressive strength decreases by approximately 8-10 psi for every freeze thaw cycle endured. The U. S. Army Corps of Engineers recommends using 12 freeze-thaw cycles as described by ASTM D 560 (but omitting the wire brushing part) for
22 cement modified soils. This method may also be considered an alternative method by which to assess the durability of cement stabilized soils. Whether the cement requirements in Table 3 are used or alternative cement requirements are used, cement contents above and below the nominal value of cement should be considered. Therefore, the accepted approach is to prepare mixtures at the nominal stabilizer content and two percent above and below the nominal content. Again, the samples should be prepared following AASHTO T 134. Unconfined Compressive Strength Testing Compressive strength is indicative of the degree of reaction in the soil-cement-water mixture based on the rate of hardening of the mixture. Since the compressive strength is directly related to density, it is affected by the degree of compaction and water content in soil cement. Similar to lime stabilization, moisture conditioning of cement-soil mixtures is recommended prior to testing as most soil cement structures are either intermittently or permanently saturated during their service life. Preparation and curing of samples compressive strength testing should be performed in accordance with ASTM D 1632 which recommends moist cure for soil cement samples. Testing of cured samples should be done following ASTM D 1633 that requires the cured samples to be immersed in water for 4 hours prior to testing (6). Typical ranges of unconfined compressive strength criteria of moisture conditioned soil cement specimens for varying soil classifications are given in Table 4. Table 4. Range of compressive strength in soil cements (29). Soil Type AASHTO Classification Soaked Compressive Strength (psi)7 Days 28 Days Sand and gravelly A-1, A-2, A-3 300-600 400-1,000 Silty A-4, A-5 250-500 300-900 Clayey A-6, A-7 200-400 250-600 Strength requirement for stabilized layers may vary considerably from agency to agency. The required compressive strengths for soil cements shown in the Table 4 are based on ACI and the U. S. Army Corps of Engineers recommendations (4, 6). Strength criteria are presented in Table 5 are based on the experience of the U. S. Army Corps of Engineers and the ACI. The lowest cement content in the mixture design that meets the requirements in Table 5 should be used as the design content. If the selected samples does not confirm to the recommendations, then higher cement contents may be added to soil and strength and durability tests may be repeated till the strength values confirm to the requirements. Table 5. U.S Army Corps of Engineers unconfined compressive strength criteria (6). Purpose of Stabilized Layer Minimum 7 day Unconfined Compressive Strength (psi) Flexible Pavement Rigid Pavement Base Course 750 500 Sub base, select material or subgrade 250 200 The typical minimum requirement varies from around 200 psi for sub base layers to around 750 psi for base layers (6).
23 Cement Treatment of Base Courses The protocol described above addresses cement-soil and cement-base mixtures. However, in certain situations a lower level of cement is used to achieve a target increase in compressive strength and/or modulus for structural performance reasons. In that case target quantities of Portland cement should be added to the aggregate material and the compressive strength or modulus of the cement-soil mixture should be evaluated. As in the discussion of lime treatment of aggregate bases, it is assumed that the candidate aggregate base material is of at least moderate quality, otherwise, the material should be treated as a soil. Moderate quality is defined as: (1) not more than 20 percent finer than the no. 40 sieve (0.425 mm or 0.0165 in.), (2) a maximum plasticity index of 12 percent, and (3) a maximum liquid limit of 40 percent. Since in aggregate base courses, the fine material (smaller than about 75 μm) comprises no more than about 10 percent of the of the entire mixture by weight, the amount of cement used by weight of the total aggregate base will be considerably less than that used in soils. As with lime, adding cement to the fines matrix will decrease plasticity as well as increase strength, and it can generally be surmised that if an acceptable target strength is achieved, that the plasticity of the fines will be appropriately altered as well. However, it is prudent to test the plasticity of the minus no. 40 sieve fraction with the target cement content to verify the impact of cement on the plasticity of the fines. The steps for stabilization of a base course are: (1) add the appropriate target percentages of cement (generally starting with 1 percent by weight of the entire mixture and increasing in 1 percent increments to 3 percent), (2) determine moisture density relationships for each aggregate-cement blend following AASHTO T-99 or AASHTO T-180 based on agency requirements, and (3) determine unconfined compressive strengths of the cement-aggregate blends following most cure for 7-days followed by 4 hours soak as recommended by ASTM D 1633. The compressive strength testing procedures and the target compressive strength requirements should be based on specifications defined by the user agency. Fly Ash Stabilization for Coarse Grained Soils and Aggregates Fly ash typically contains at least 70 percent glassy material with particle sizes varying from 1μm to greater than 1 mm. Based on AASHTO M 295 (ASTM C 618), fly-ash can be classified into two groups: class C and class F. Class C refers to as a self cementing or cementitious fly ash that has enough available calcium to react with soil in the presence of water. Most of the calcium in class C fly ash is combined with the silica and/or alumina so that when water is added, a hydration reaction similar to the hydration reaction in Portland cement occurs. Some free lime is produced in the hydration process, as it is in the hydration of Portland cement. This free lime can participate in the pozzolanic reaction process between silica and/or alumina released from clay or silica and/or alumina from the fly ash, which are not combined with calcium. Class C fly ash is a by-product of burning lignite or sub-bituminous coal in power plants. Class F fly ash on the other hand is more of a pure pozzolan, with a low concentration of available calcium. Therefore stabilization with class F fly-ash requires the use of an activator like lime or cement to initiate hardening processes during stabilization (5). Low lime ash or class F fly ash is formed during burning of anthracite or bituminous coal. Although these fly ash types are known to induce cementitious reactions in stabilized soils, mix properties cannot be predicted solely from chemical composition of the ash. Due to the complex nature of ash hydration, the utility of fly ash for stabilization applications must be based on physical properties of ash treated materials.
24 Mix Design Considerations Prior to stabilization, the cementitious properties of fly ash should be characterized following ASTM D 5239-04. But, it should be noted that ASTM D 5239-04 does not evaluate the interaction between fly ash and soil or aggregate which must to be verified separately based on mix design procedures outlined in the following paragraphs. Self Cementing Fly Ash Class C fly ash can be used as a stand alone material. At present there are no standard test procedures available for design of materials stabilized with self-cementing fly ash. The American Coal Ash Association recommends using moisture density and moisture strength relationships for developing effective mix designs in soils (5). Design Considerations For self cementing fly ash, one of the primary design considerations is the rate at which fly ash hydrates upon exposure to water. Hydration reactions can start immediately on exposure to water and hence the time delay in mixing and compaction of the specimens needs to be accounted for and included in laboratory mix designs (5). As hydration progresses, soil particles are bonded in a loose state and a portion of the compaction energy used in densification is lost in breaking bonds in the mix. Maximum dry density achieved for a given compaction energy therefore decreases with increase in compaction delay. In addition, compaction delay can cause a significant reduction in compressive strength. This is most likely due to the inability to maximize the impact of cemetitious and/or pozzolanic product development at lower densities. In other words, if a soil mass is under-compacted, the cemetitious/pozzolanic product does not have the same opportunity to develop âbondsâ among soil particles (or agglomerates of particles) as they would if the soil mass were compacted to within a reasonable range of target density. This effect is much more likely to be significant in class C fly ash mixtures due to the faster rate of reaction. An additional design consideration when selecting the optimal fly ash content is to determine the optimum moisture content at which maximum strength gain is achieved. Optimum moisture content for strength gain may typically be 1â8 percent below optimum moisture content needed to attain maximum dry density. This value may vary with soil type and the mineralogy of ash particles. Mix Design Addition of fly ash alters the compositional characteristics of soils and hence the moisture density relationship must be established for each soil type and fly ash content. These can be measured based on adaptations of ASTM C 593 and ASTM D 1633. Once the optimum moisture content for the mix is determined, the moisture-strength relationship is evaluated. In order to evaluate strength, specimens are prepared by blending soil, fly ash, and water and molded after the specified compaction delay. Test specimens are compacted at different moisture levels below optimum to determine the moisture content that will produce the maximum compressive strength. Test specimens are cured for 7 days at 38oC (100oF) in accordance with ASTM C 593 before compression testing. ASTM C 593 recommends moisture conditioning for 4 hours after curing period where the specimens are allowed to cool down to room temperature and are then immersed in water for 4 hours. However, as with lime mixtures, the authors recommend an alternative moisture conditioning regime of capillary soak until the moisture front ceases to migrate or for a minimum of 24-hours. The strength requirements
25 typically vary based on objectives and requirements specified by the agency and these requirements should be followed in selecting the mix design for field application. Non Self Cementing Fly Ash For stabilization with non self cementing fly-ash, the addition of activators such as lime or cement is required to initiate stabilization reactions. These materials typically continue to gain strength after a curing period due to pozzolanic activity. The slow strength gain in these materials helps reduce shrinkage cracking and improves healing of micro cracks forming in the stabilized layers. The methodology given below is adapted from coal ash association mix design procedure. Typical fly ash contents in granular mixes vary from about 10-15 percent with activator contents varying from about 2-8 percent lime by weight of the mixture. These materials are similar to cement stabilized base in production, placement and in appearance. Strength development depends on curing time and temperature and is typically measured after accelerated curing of 7 days at 380C (1000F) (30). Selection of Optimal Fly Ash Content The first step in selecting the optimal fly ash content is to determine the utility of the stabilized product and the target level of strength required based on the utility of the product. The purpose of using fly ash in soils can broadly be divided into two categories: to achieve maximum strength for the mix or to achieve a target level of strength for the mix. If an aggregate base course is to be stabilized and the goal is to achieve maximum strength and durability then the strategy is to fill the voids with fly ash to achieve maximum density, then to determine the moisture density relationship for this optimal blend. This is followed by the addition of the amount of activator that will produce the maximum level of strength. If the goal is to achieve a target level of strength for either base courses or soils, then the strategy is different. In this case, experience or a trial and error process is required to identify trial fly ash percentages and activator contents. These estimates are used to establish moisture density relationships and to determine compressive strengths. Five different samples are prepared with varying fly ash proportions typically starting at about 6 percent and ranging up to as high as about 20 percent by weight of the coarse aggregate fraction. Mixes are molded at estimated optimum moisture content in accordance with ASTM C 593 to determine the dry density of each mix. A two percent fly ash concentration above the proportion that gives the maximum dry density is selected as the optimum content for the mix. Optimum moisture content and maximum dry density are determined for the selected blend. Sample Preparation for Selection of Optimal Activator Content Determination of the optimal activator content is best achieved on a trial and error basis realizing that the required lime content or Portland cement content to activate the fly ash is typically between one part lime to three parts fly ash (1:3 ratio) to one part lime to four parts fly ash (1:4 ratio). Compressive strengths of the resulting mixture should then be compared to target values in order to judge whether or not the blend produces acceptable strengths for loading and environmental conditions. If lime kiln dust (LKD) or cement kiln dust (CKD) is used as an activator, then higher activator ratios are required based primarily on the CaO content of the kiln dust. ASTM C 593 requires
26 preparation of three replicate samples for compressive strength testing for each blend of fly ash and activator. Curing of Samples for Compression and Durability Testing Fly-ash soil mixes are cured for 7-days in sealed containers. Samples prepared with lime and kiln dust activators are cured at 37.8oC (100o F) for seven days. Portland cement activator fly ash mixes are cured at a 100 percent relative humidity environment at 22.8oC (73o F) for seven days. ASTM C 593 recommends moisture conditioning following curing period in which the samples are subjected to 4 hour soak after cooling to room temperature. Then the compressive strength of the samples is measured. However, the authors recommend the NLA capillary soak described under the NLA recommendations for lime mixtures as an alternative moisture conditioning regime. Compression and Durability Testing The three replicates prepared are tested for compressive strength testing should be subjected to vacuum saturation or strength testing without moisture conditioning as recommended in ASTM C 593. Durability testing in fly-ash soil mixes can also be performed in accordance with AASHTO T 136/ASTM D 560. But the issues discussed earlier regarding the effectiveness of AASHTO T 136/ASTM D 560 are applicable in this case also. In areas where there is no freeze- thaw effect, durability testing may be waived in accordance with local practice. Acceptability Criteria A 7-day compressive strength of 400 psi is considered acceptable for field applications (30). A mix that attains the required properties with the lowest percentage activator is selected as the design mix for use in field. Lime-Fly Ash Treatment of Soils to Achieve a Target Strength Lime and fly ash may be used to achieve mixtures with a target strength instead of in an attempt to optimize strength of a mixture. This approach may be applied to any soil (coarse-grained or fine-grained). In this case various ratios of lime and fly ash should be tried until the target strength is achieved. A reasonable guideline is to begin with is to use four percent fly ash and increase the ash content in two percent increments for various trials. The initial trial activator ratio (lime content) added to each should be one part lime to two parts fly ash as a general rule, but this can be varied based on experience. Approximately six trial ratios of lime and class F fly ash (three ash contents and two activator contents per ash content) should be used. A moisture-density relationship should be developed for each ratio to determine optimum moisture for each blend. Samples should be prepared at the target moisture content following ASTM C 593. Strength testing on each candidate mixture should be used to establish the acceptable mixture design. The authors recommend the same curing regime as described for lime stabilized soils. On occasion, the goal is for fly ash to provide a strength increase to lime treated, fine-grained soils that are not sufficiently pozzolanically reactive. This normally occurs in clay soils where the lime is effective in reducing plasticity and improving workability but not in providing the target strength. In this case an acceptable approach is to determine the lime content required based on the Eades and Grimm pH test. This content should provide sufficient lime to modify the soil and still provide sufficient residual lime to provide pozzolanic reaction. Next trial
27 quantities of fly ash should be added to the blend beginning with four percent fly ash and increasing in two percent increments until acceptable strength is achieved. A separate moisture- density relationship is required for each blend.
