Testing of Materials for Cement or Cementitious Blend Stabilisation

Transcription

1 Technical Note 149 Testing of Materials for Cement or Cementitious Blend Stabilisation January 2016

2 Copyright State of Queensland (Department of Transport and Main Roads) 2016 Feedback: Please send your feedback regarding this document to: Technical Note, Transport and Main Roads, January 2016

3 1 Introduction The purpose of this technical note is to outline a mix design procedure which allows the optimum design cement or cementitious blend content for the modification or stabilisation of soils which are compatible with cement. This note provides guidelines, design criteria and test procedures for characterising the host soil and measuring the properties of the cement treated material. The three stages of this testing guideline are as follows: 1. sampling and characterisation of the host soil 2. mix design procedure 3. field control testing. In the remainder of this document the term cement can mean both cement and cementitious blends (that is, cement/flyash, cement/slag, cement/flyash/slag and so on). This process is also applicable to materials stabilised using slow setting additives such as lime/flyash and lime slag although these require a longer curing time before testing, usually 28 days. All test methods referenced in this document with the prefix Q are published by the Department of Transport and Main Roads #1 and those with a prefix AS are published by Standards Australia. Table 1 #2 Types of stabilisation Type of stabilisation/characteristic strength Modified 1.0 MPa < UCS* 2.0 MPa Bound Cat I 3.5 MPa < UCS* 4.5 MPa Bound Cat II 2.5 MPa < UCS* 3.5 MPa Anticipated performance attributes Flexible layer with improved shear strength, decreased permeability and significantly less tensile fatigue cracking. Bound pavement subject to tensile fatigue cracking and transverse shrinkage cracking. Bound pavement subject to tensile fatigue cracking and transverse shrinkage cracking. Generally cement is added to pavement materials to change its properties. This may be performed at a quarry to provide a plant-mixed material or in situ recycle an existing material. Generally the aim is to provide a modified material with a target unconfined compressive strength (UCS) of 1.5 MPa and to avoid making the material too stiff and susceptible to fatigue cracking. For more details refer to Section of the Pavement Rehabilitation Manual #5. 2 Sampling and characterisation The section under consideration must be sampled at regular intervals in order to identify material types and changes within any material. For more information refer to the departmental publications for Pavement Design #3,4 and Pavement Rehabilitation #5. It is important that each material type is sampled and tested at regular intervals in order to establish the range of material properties and to identify the presence of deleterious materials. Use of a soil classification system such as the unified soils classification system can assist in identifying the soil types #6. Technical Note, Transport and Main Roads, January

4 Host soil characterisation tests are as follows: Table 2 Test methods used to classify materials Property Test Method Title Particle Size Distribution (Grading) Atterberg limits (Liquid limit, plastic limit, plasticity index and linear shrinkage) Q103A Q103C Q104A or D Q105 Q106 Particle size distribution of soil - wet sieving Particle size distribution of soil hydrometer Liquid limit of soil Plastic limit and plasticity index of soil Linear shrinkage of soil Organic Content Q120B Organic content of soil - loss on ignition Sulfate content AS Determination of the sulfate content of a natural soil and the sulphate content of the groundwater Ferric oxide In-house FeO by classical method (grinding, digestion, titration) 2.1 Classification Initially, classification tests including particle size distribution and Atterberg limits should be undertaken #6. Depending on the variation in material properties as indicated by the particle size and Atterberg limit tests, either undertake further testing on samples representing material extremes, or where properties are similar, test the highest percentage passing the mm sieve. Generally materials that are not suitable for stabilisation are: poorly graded materials materials with a plasticity index (PI) > 10.0 where more than 25% passing the mm sieve materials where less than 25% passing the mm sieve may be suitable with a plasticity index higher (PI) than 10.0 Refer to Table 4.6 #5 of the Pavement Rehabilitation Manual for more details. 2.2 Deleterious materials This is usually not an issue for materials obtained from quarries or existing pavement materials originally obtained from quarries. In some situations is may be appropriate to test for deleterious materials where there may be a concern about the quality of the materials. These situations may include: Type 4 pavement materials Materials obtain from mine sites or adjacent to mine sites Materials that may contain sulphides or ironstone It may then be appropriate to test for deleterious materials. This includes the determination of sulfate, organic content and ferric oxide contents. Organic matter interferes with the hydration process as well as competing for available stabilising agent - water paste. Sulfates can also interfere with pozzolanic reactions due to the formation of very expansive hydrates which if formed after compaction, can result in heave. Ferric oxide also can also interfere with pozzolanic reactions; however there are no specific limits in the literature at this stage. Recent experience has indicated that contents in the range of Technical Note, Transport and Main Roads, January

5 10-13% have a deleterious effect on stabilised materials. As a guide advice should be sought if values greater than 2% are found. The following limits may be applied: Table 2.2 Deleterious materials limits Property Limit Sulfate Content (water soluble) < 0.3% Organic Content* < 1.0% Ferric Oxide * < 2.0% Note*: The limit shown is a guide; advice should be obtained before stabilising materials with ferric oxide contents greater than the limit or organic contents > 5.0%. 3 Mix design procedure This stage is to establish the optimum cement content for the host soil. Design test procedures are as follows: Table 3 Test methods used for design Property Test Method Title Unconfined Compressive Strength (UCS) Q115 Q135A Q135B Unconfined compressive strength of stabilised materials Addition of stabilising agents Curing of moulded specimens of stabilised materials Capillary rise Q125D Capillary rise of stabilised material Nominated working time limit Q136 Working time of stabilised materials Sulfate content of groundwater AS Determination of the sulfate content of a natural soil and the sulfate content of the groundwater 3.1 Cement and supplementary cementitious materials Cement, complying with the requirements of AS 3972: General purpose and blended cements, is to be used exclusively in laboratory testing. The cement should be sourced directly from suppliers. Bagged supplies from hardware stores/building suppliers should not be used as the age and condition of the cement is not known. Instructions for the storage and use of cement are included in Appendix 1. Supplementary cementitious materials (fly-ash, slag, silica fume and so on) should be sourced directly from suppliers. They should be kept dry in air tight containers. While they do not deteriorate with age they should be discarded after 12 months as the sample may no longer represent the materials currently supplied. 3.2 Lime Where slow setting additives are used, hydrated lime, complying with the requirements of AS Hydrated lime, is to be used exclusively in laboratory testing. The hydrated lime should be sourced directly from suppliers. Bagged supplies from hardware stores/building suppliers should not be used Technical Note, Transport and Main Roads, January

6 as the age and condition of the lime is not known. Instructions for the storage and use of lime are included in Appendix 1. A certificate with the available lime ( AL ) should be obtained from the supplier for the batch of lime. x As an alternative the lime may be sampled and forwarded to a laboratory to determine the AL. x The AL value should be forwarded along with other results of testing for the mix design procedure. If x a different source of lime is used for construction the quantity of lime ordered and spread rates on site may need to be adjusted for a lime that is more or less reactive than the lime used for the design procedure. The relationship between quicklime and hydrated lime content is: 0.76HrH Q= r Q ALI ALI where: Q r = application rate for quicklime (%) H r = application rate for hydrated lime (%) = available lime index of hydrated lime (%), used in laboratory mix design H ALI testing Q ALI = available lime index of quicklime (%), used in construction Quicklime is not to be used in the laboratory because of safety concerns, and the need to exercise very tight control over moisture during moulding. If quicklime is not fully hydrated before compaction, carbonation and localised expansion can occur. 3.3 Water Potable water (fit for human consumption) free from organics and with a sulfate content (AS ) less than 0.05% is the preferred mixing water. Where possible, the actual water source to be used in the field should also be used for the laboratory testing programme. While water high in soluble salts is suitable for mixing and curing, they can cause a build-up of salts which may migrate to the surface of the pavement. Excess concentration of salts can interface with the adhesion of seal coats. 3.4 UCS test analysis The UCS test is used to determine the optimum cement content. UCS tests (3 specimens) are to be undertaken at a range of cement contents (4) usually commencing with 0.5% cement. Suggested cement contents (%) for a target strength of 1.5 MPa are: 0.5, 1.0, 1.5 and 2.0 The strength of all compacted soil is greatly influenced by density. A 1% reduction in density rates can reduce the UCS by 5%. Small density variations can easily mask the effects of other variables such as cement content and curing conditions. Since compacted density has a direct influence on UCS, a strict control must be exercised on compaction moisture content. Suggested limits for achieved moisture content are: Achieved moisture content = target moisture content ± 0.3 percentile units Technical Note, Transport and Main Roads, January

7 The cement/soil mixture should be conditioned using short term conditioning, that is, where the cement/soil mixture is allowed to condition in an air-tight container for 45 minutes before further mixing and compaction. A standard curing regime comprising moist curing at 23 ± 2ºC for seven days has been adopted as detailed in Q135B. All specimens are to be tested using the standard UCS test, long term immersion testing is not usually undertaken for cement stabilised specimens. Plot the UCS data versus cement content and determine the cement content corresponding to the target strength (MPa). In Figure 1 the cement content corresponding with the target strength of 1.5 MPa would be the optimum cement content of 1.65%. Figure 3.4 Cement content v UCS UCS (MPa) Cement Content (%) 3.5 Working time The working time test is undertaken to determine the nominated working time limit for the stabilising agent and host soil. A moisture/density relationship (MDR) and UCS test are performed with the design cement content to establish a reference MDD and reference UCS. Further specimens are compacted with a delay between mixing and compaction which for cements will typically be 1, 2, 4, 6 and 8 hours. These delays reduce the achieved MDD and UCS due to the cement hydrating and reacting with the host soil, reducing the moisture content and increasing the friction between particles. These effects inhibit the reorientation of particles during compaction and therefore reduce the achieved MDD and the lower density also reduces the achieved UCS. The working time limit is defined as the delay time that produces a 3% reduction in achieved MDD (that is, 100% to 97%) or a 20% reduction in achieved UCS (that is, 100% to 80%), whichever is the shorter delay time. Technical Note, Transport and Main Roads, January

8 The determination of working time will need to be repeated before the project starts with samples of the actual materials and proportions (for example, host soil with 3% GB cement with 30% flyash) to be used in the project. Plot the maximum dry density (MDD) versus time delay and the UCS versus time delay for the cement content and determine the delay at 0.97 of 0 hour delay MDD (0.97 x = 2.134) and 0.80 of 0 hour delay UCS (0.80 x 1.5 = 1.2). In Figure 2 the delays corresponding 0.97 MDD and 0.80 UCS are 3 and 2 hours respectively. Therefore the nominated working time limit would be 2 hours. Figure 3.5 MDD/UCS v delay MDD/UCS Working time MDD (t/m3) MDD MDD WTL UCS WTL UCS Delay (hr) 4 Field control testing #1, 7 Field control testing is undertaken to ensure that particular design parameters, such as cement content, relative compaction and unconfined compressive strength are achieved. Table 4 Test methods used for field control Property Test Method Title Spread rate Q719 Field spread rate of solid stabilising agents - fabric mat Cement content Q134* Stabilising agent content - heat of neutralisation Mixing uniformity (cement content) Q134* Stabilising agent content - heat of neutralisation AS * Lime or cement content of stabilised pavement materials (EDTA method) Compacted density Q141A Compacted density of soils and crushed rock (nuclear gauge) Q141B Compacted density of soils and crushed rock (sand replacement) Technical Note, Transport and Main Roads, January

9 Property Test Method Title Reference density Q142A Dry density-moisture relationship (standard compaction) Q144A Assignment of maximum dry density and optimum moisture content for soils and crushed rock Relative Compaction Q140A Relative compaction of soils and crushed rock Field UCS Q115 Unconfined compressive strength of compacted materials Nominated working time limit Q136 Working time for stabilised materials * These tests measure cement content. 4.1 Cement Content and Mixing Uniformity Insitu stabilisation For insitu stabilisation a simple mat test (Q719) is used to monitor stabilising agent content and distribution. One or more 1 m 2 mats are placed so as to catch the discharge from the spreader. While modern high performance equipment is capable of providing uniform mixing, the need arises from time to time to measure mixing efficiency. The cement content can be readily measured anywhere within the stabilised soil profile using one of two tests. The Heat of Neutralisation test (Q134) is a rapid low cost field test while the EDTA Titration method (Q117A) is a high cost, time consuming analytical test which is normally performed in a specialised laboratory. Samples of both the unstabilised material and the stabilising agent are required prior to performing either of these tests. 4.2 Cement content Plant-mixed stabilisation For plant mixed stabilisation the Heat of Neutralisation test (Q134) is a rapid low cost field test. Samples of both the unstabilised material and the stabilising agent are required prior to performing this test. 4.3 Compacted density and reference density insitu stabilisation The inherent variability of insitu stabilised materials usually means that a testing regime of one for one testing must be employed. Such testing involves taking a sample from each insitu density location and determining a reference density. The sand replacement test is the accepted test for measuring insitu density. A nuclear gauge technique can be used for cement modified/stabilised materials and has the potential to be a rapid, low cost alternative to sand replacement. Provided a wet density and moisture content bias is determined as detailed in the Nuclear Gauge Testing Manual #7.The reference density is measured using the traditional dry density - moisture relationship test. It is important that the reference density sample is taken and compacted before the allowable working time for cement has elapsed. Refer to Q140A for standard working times. An alternative is to determine a project/material specific nominated working time limit using Q136. The Hilf density ratio is not applicable for on-site compaction control. The Hilf method assumes the insitu density is determined at the same time the reference density is sampled and that both specimens are at the same moisture content. For insitu stabilisation the reference density is sampled from uncompacted material and then insitu density is determined later in the compacted material. The moisture content of the material can change significantly during the time between sampling and testing. Technical Note, Transport and Main Roads, January

10 4.4 Compacted density and reference density plant-mixed stabilisation The plant-mixed stabilised materials are usually more uniform which means that a testing regime using an assigned reference density may be employed. Such testing involves taking a sample from the quarry stockpile and adding the target additive content to the materials and determining a reference density. An initial value based on the average of six tests is used. This is then updated every 10,000 t using one new test and calculating a rolling average. The project may use an allowable working time as specified in Q140A or as an alternative determine a project/material specific nominated working time limit using Q136. A nuclear gauge technique can be used for cement modified/stabilised materials provided a wet density and moisture content bias is determined as detailed in the Nuclear Gauge Testing Manual #7. The sand replacement test may be used for measuring insitu density but it is more destructive to the finished pavement. 4.5 Field UCS The UCS test provides additional information on expected field performance relative to design. The field mixed material is moulded at field moisture content using either standard or modified compaction effort. Moulded specimens are then cured under standard conditions prior to testing. When sampling and making UCS specimens in the field it is assumed the insitu moisture content will be close to the optimum moisture content of the material. If not then the achieved dry density and dry density ratio will be likely to be well below the target. This will mean the UCS results for the samples will be lower than target due to the moisture and density being lower than the targets. Therefore care is required in interpreting these results. 4.6 Nominated working time limit As detailed in Sub-Section 3.5 this will need to be re-determined with samples of the project cement, supplementary cementitious materials, lime, water and host soil (that is, in situ and/or quarried). In future it may also be necessary to adjust this limit for the local temperature. 5 Future developments Since 2010 amendments to the Materials Testing Manual have allowed major changes in the testing of cement stabilised materials. Various methods have been published or amended to include the greater use of nuclear gauges in stabilised works, laboratory determination of allowable working times and testing of cores for unconfined compressive strength. There are some issues to be addressed in future amendments/edition of the Materials Testing Manual and departmental technical specifications, including: use of delay factors for compaction control accelerated curing of laboratory specimens effect of temperature on working time triple blend cements (that is, cement, fly-ash and lime). Some of these issues still require significant research to be performed. Technical Note, Transport and Main Roads, January

11 6 References 1. Materials Testing Manual, Queensland Department of Transport and Main Roads, Edition 4, Lime treatment of clay sub-grades, Queensland Department of Main Roads, Technical Note 39, December 2000, Jothi M Ramanujam 3. Guide to Pavement Technology Part 2: Pavement Structural Design, AustRoads, Pavement Design Supplement, Supplement to Part 2: Pavement Structural Design of AustRoads Guide to Pavement Technology, Queensland Department of Transport and Main Roads, November Pavement Rehabilitation Manual, Queensland Department of Transport and Main Roads, April Geotechnical Site Investigations, Standards Australia, AS , Appendix A Description and classification of soils and rocks for geotechnical purposes 7. Nuclear Gauge Testing Manual, Queensland Department of Transport and Main Roads, Edition 2, October 2003 Technical Note, Transport and Main Roads, January

12 Appendix 1 Storage of cement in the laboratory Importance of proper storage Cement will react with products in the atmosphere and degenerate with age. With proper handling, the degeneration can be held to a minimum. It is important to keep all cement dry and in airtight containers. Storage 1. It is best to obtain no more than a one - two month supply of cement at a time. 2. Upon receipt, the total supply should be transferred from bags into airtight stock containers, Ergotainers are recommended. The date received should be marked on each container. 3. It is helpful to tap the full containers on the ground to achieve some degree of cement settling. This compaction will help limit atmospheric exposure of the cement below the surface. 4. A smaller lab container is needed to hold the cement used in day-to-day testing. 5. The lab container must also be airtight and should hold a one - two week supply. The use of a lab container avoids exposing the cement supply to the atmosphere and limits the number of times a stock container must be opened. The stock container only needs to be opened a couple of times a month rather than daily. 6. Cement in the stock container should be discarded 12 weeks after it is received. Use/sampling 1. Each time cement is transferred from the stock container to a lab container the top mm of cement in the stock container should be discarded. When cement is added to the lab container the date should be marked on the container. 2. Before each testing job, the top mm of cement should be discarded from the lab container. 3. Prior to transferring cement from the stock container to the lab container, all cement remaining in the lab container should be discarded. 4. No container should be left open when not being used. If the lab container is to be unused for more than 10 minutes, close it tight. 5. Cement in the lab container should be discarded if it has been in the lab container for more than 14 days. 6. If care is taken to follow these storage methods, and stock is rotated often, the cement quality should remain at acceptable levels. Technical Note, Transport and Main Roads, January

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