STUDY OF THE VARIABLES IN LABORATORY TESTING OF CEMENT STABILIZED MATERIALS

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1 STUDY OF THE VARIABLES IN LABORATORY TESTING OF CEMENT STABILIZED MATERIALS Pengpeng Wu * PhD student, Road and railway engineering, Delft University of Technology, Delft, The Netherlands * P.O. Box 548, 26 GA, Delft, The Netherlands P.Wu@tudelft.nl L.J.M. Houben Assistant Professor, Road and railway engineering, Delft University of Technology, Delft, The Netherlands Christophe Egyed PowerCem Technologies B.V., Moerdijk, The Netherlands ABSTRACT: Cement stabilized materials have been attractive in pavement construction due to the low construction cost and easy construction work. Extensive research on the properties and behaviors of the cement stabilized materials are documented in terms of laboratory testing. However, the variables in the testing conditions, such as the dimension of the sample and the testing procedure, can lead to a significant variability of the experimental results. In general, the experimental results from different researches can t be compared and analyzed even when the same type of material was investigated. More detailed studies in this field are leading to more knowledge. This research aims to evaluate the effect of variables in the laboratory conditions on the behavior of cement stabilized materials and to give the appropriate specification for laboratory tests. The variable factors including the dimensions of the specimen, the compaction time delay, the friction effect caused by the contact with the testing panel and the curing conditions, are evaluated. KEY WORDS: Cement stabilization; laboratory testing, mould size, compaction delay, barreling behavior 1. INTRODUCTION Use of cement stabilized materials is a cost effective option for pavement construction. Based on the literature survey, extensive research has been done to investigate the behavior of cement stabilized material [1, 2, 3; 4; 5]. Most of this research is conducted in terms of laboratory testing. However, the experimental results from these different resources can t be comparable due to the variable factors which essentially exist in the laboratory research, such as the size of the moulds used, the compaction process or the machine testing procedures, which may considerably influence the final test results. Therefore, it is important to identify the impacts of these factors for the experimental research and the appropriate testing specifications which can be achieved for cement stabilized materials. The primary objective of this research is to identify the variables that exist during the experimental testing of cement stabilized materials. Sand, sandy clay and clay are stabilized with cement to demonstrate the extensive results based on the different soil types. During the process a Nano-based additive (referred to as RC) is applied into the mixture. RC additive, based on the Nano technology, is specifically designed for application in road construction by enhancing the physical parameters like strength and flexibility of stabilized road layers. In research [6] nano-indentation and Rockwell micro-indentation measurements were conducted and it shows that the stabilized samples with use of RC have superior mechanical properties than those without using it. This particular study is a preliminary part of extensive research on the effects of RC on the performance of cement stabilized materials. Therefore, identification of the influences from the laboratory variables is essential for the ongoing test program. The variables investigated in this research are pointed to the size of the compacted specimen, the compaction time delay, the curing conditions and the friction in the interface between the specimen and the loading panel. These are the main factors related to the laboratory research. Mechanical strength tests, including the Copyright 213 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 1

2 compressive strength test, the indirect tensile strength test and the flexural tensile strength test, are performed to evaluate the mechanical behavior of cement and modified cement stabilized materials. Based on this comprehensive research, the mechanical properties are correlated with these influencing factors to define an appropriate specification for laboratory testing of cement and modified cement stabilized materials. 2. TEST PROGRAM 2.1 Overview of this study In this research, the investigated variable factors are presented as follows: Friction occurred during compression: Under compression the specimen may tend to horizontally expand which will cause an increased cross sectional area, and meanwhile the friction between the specimen and the loading panel will occur in the opposite direction. The induced friction force provides additional confinement for the top and the bottom of the specimen, which is known as the barreling behavior [7]. This extra confinement can lead to increased compressive strength results. Therefore, in order to obtain reliable test results, the friction force between the specimen and the loading panel should be minimized. In this study, the friction reduction system is obtained by use of two polyethylene foils and applying a certain type of soap between the foils. This type of system was investigated in previous research [8] which showed that by utilizing the friction reduction system, a uniform lateral deformation over the height of the cement stabilized samples was obtained. Size of the specimen: Since the size and shape of the specimen is chosen according to different testing manuals or standards, the test results vary significantly. For instance, study [9] indicates that the cube compressive strength of cement treated materials may range 1.25 and 1.5 times the cylindrical compressive strength. With such a correlation factor the equivalent results can be predicted or compared. In this study, cylindrical samples having the size of ASTM and Proctor size, which are both widely used in research, are compared. Compaction delay: It is well known that a cement stabilized mixture should be compacted as soon as the mixing procedure is accomplished. Delayed compaction will result in reduced strength and performance of the cement stabilized material. In an experimental study [1] it is shown that the compressive strength significantly reduces when the time between mixing and compaction increases, the density and the compressive strength significantly reduced. But in reality, in the field, immediate compaction is not always practical. So it is necessary to examine the influence of delayed compaction hours on the final strength and hence an acceptable number of hours delay can be provided. In this study, the time between mixing and compaction is specified at 1, 2 and 3 hours. Curing conditions: The appropriate curing regime is essential for the strength development of cemented materials. A too dry curing environment may cause insufficient cement hydration of the materials due to lack of sufficient moisture. While excessive moisture will fill in the porous structure of the samples, leading to the reduced early strength. In this study, specimens cured in the moisture chamber with temperature of 18.5 o C and constant humidity of 85% are compared with samples cured in a normal room with temperature of 18.5 o C and average humidity of 25%. 2.2 Soil materials Three types of soil are chosen to represent typical grain sizes. Manufactured sand, clay and sandy clay are stabilized in this experimental research. Sandy clay is obtained by mixing sand and clay (1: 1 by dry mass). The grain size distributions of these three types of soil are shown in Figure 1. The general properties of these three types of soil are summarized in Table 1. Copyright 213 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 2

3 Percentage passing (%) Clay Sandy clay Sand ,1,1 Grain,1 size (mm) 1 1 Figure 1 Grain size distribution of 3 types of soil Table 1 Soil properties. Soil property Sand Sandy clay Clay Particle density (g/cm 3 ) Coefficient of uniformity (C u ) Curvature index (C c ) Liquid limit (LL) Plastic Index (PI) Poorly Sandy Lean Classification (USCS) graded lean clay clay sand 2.3 Sample preparation The mix composition with 25 kg/m 3 cement and 1.9 kg/m 3 RC (related to the dry soil) is used. After mixing the components uniformly, the homogeneous mixture was filled in steel moulds with diameter of 11.6 mm and height of mm and compacted by an electronic vibrating hammer (Hilti TE1-AVR). The specimen size is commonly used in ASTM standards according to D To compare the results for different specimen sizes, additional specimens with diameter of 15 mm and height of 12 mm required in NEN-EN are prepared. This size is commonly used by the Proctor compaction and in this study it is written as Proctor size. For ASTM and Proctor size, the height to diameter equal 1.15 and.8, respectively. The vibration method is performed by impacting a vibrating hammer with an attached tamper. The diameter of the tamper is designed as 9 mm, which is less than the internal diameter of the cylindrical moulds. The mixture is subjected to the vibratory load at a frequency of 6 Hz for 6 seconds per layer (three layers in total). Compaction of each sample took approximately 5 minutes. To indicate the compaction delay, the target samples were compacted 1, 2 and 3 hours after mixing. After compaction and hardening for 24 hours, the specimens were extracted from the moulds and were placed in a moisture chamber with a temperature of 18.5 o C and a constant humidity of 85% for 28 days. To investigate the effect of the curing conditions on the properties of cement stabilized materials, specimens with dimensions of 4 by 4 by 16 mm were manually compacted. These beams were cured in two different environments: the moisture chamber and the normal room. The latter environment has a room temperature of 18.5 o C and a humidity of 25%. After curing during the specified 3, 7, 14 and 28 days, these beams were Copyright 213 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 3

4 Compressive strength (MPa) subjected to the monotonic three-point bending test. After the bending test the broken specimen portions were employed for the compression test. This test procedure on the compacted beams was only carried out on the stabilized sand. 2.4 Laboratory testing method Prior to the compression testing, the friction reduction system was fixed by applying two layers of polyethylene foils on the top and bottom of the specimen and with a certain soap applied above each layer of foil. The compression and indirect tensile strength tests are widely used to characterize the properties of cement stabilized materials. The unconfined compressive strength test and indirect tensile strength test were carried out according to NEN-EN and NEN-EN , respectively. The specimen is loaded at a specified constant loading rate (.5 (N/mm 2 )/s for compressive loading; 1.77 kn/s for indirect tensile loading) until failure. The compressive strength is defined as the peak load (F) divided by the area of the cross section of the specimen. The indirect tensile strength is defined as the stress at failure of a cylindrical specimen subjected to a compression force applied on two opposite directions (NEN-EN ). The cement stabilized sand beams were subjected to three-point bending test at a constant loading rate of.5 (N/mm 2 )/s. All the experimental results are based on the mean value of three tested samples. 3 ANALYSIS OF TEST RESULTS 3.2 Effect of the friction during testing Figure 2 and Figure 3 show the compressive strength of the cement stabilized samples with and without friction reduction system in the compression tests With friction reduction systerm Without friction reduction systerm Figure 2 Compressive strength of specimen with diameter of 11.6 mm and height of mm Copyright 213 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 4

5 Compressive strength (MPa) Compressive strength (MPa) 25 2 With friction reduction systerm Without friction reduction systerm Figure 3 Compressive strength of specimen with diameter of 15 mm and height of 12 mm As can be seen from Figure 2 and 3, testing of the specimens with ASTM size with and without the friction reduction system nearly shows the same results. However, as the diameter of the specimen increases approximately by 5% (from 11.6 to 15 mm), the influence of the friction reduction system is obvious. In the graph 2 for the specimens with Proctor size, the obtained compressive strength of the sand-cement materials increases by 24% without using the friction reduction system. Similarly, the strength of sandy claycement sample increases by 38%. The difference of compressive strength is caused by the friction force. The larger the loading area, the more influence the friction force has. However, the friction doesn t have a big effect on the test results of stabilized clay material. That is because the stabilized clay samples have smooth surface sides which wouldn t cause much friction from the contact with the loading panel. From these results it can be suggested that when conducting the compression test, it is essential to reduce the friction in the interface between the specimen and the loading panels, especially on the samples with relatively large loading areas. 3.3 Effect of the size of the specimen The compressive strength of cement stabilized samples with different dimensions is demonstrated in Figure 4 and Figure D: 15 mm H: 12 mm D: 11.6 mm H: mm 1 5 Figure 4 Comparison of compressive strength of specimen with different sizes Copyright 213 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 5

6 Indirect tensile strength (MPa) 2,5 2, D: 15 mm H: 12 mm D: 11.6 mm H: mm 1,5 1,,5, Figure 5 Comparison of indirect tensile strength of specimen with different sizes In Figure 4 and Figure 5 it can be clearly seen that the compressive strength and indirect tensile strength indicate the similar trend which shows that the strength of the samples with Proctor size is generally larger than the strength of specimens with ASTM size. The compressive strength of sand-cement samples with Proctor size is 1.5 times higher than that of samples with ASTM sizes, while the splitting strength exhibits a factor of 1.3. Similarly, for sandy clay-cement materials, the conversion factors for compressive strength and splitting strength are 1.3 and 1.2, respectively. In contrast, the size of the specimen doesn t have an obvious influence on the strength of clay-cement materials. Apparently, the larger the soil particle size, the larger the influence of the specimen size on the strength. Therefore, for cement stabilized materials the conversion factor of 1.3 can be adopted to correlate the strength of the specimen with ASTM size and the strength of the specimen with Proctor size based on the cement stabilized sandy materials. The strength of samples with ASTM size multiplied by 1.3 is equivalent to the strength of samples with Proctor size, which is applicable for both indirect tensile strength and compressive strength. The mechanical strength of stabilized clay samples nearly remains the same for these two sample sizes. Besides, it can be noted that the ratio of indirect tensile strength and the compressive strength is in the range of 8% ~ 1%. The soil type has no big influence on this ratio. 3.3 Effect of the time of compaction delay Figure 6 presents the results of the compressive strength related to the delayed compaction time. The compaction was conducted at the time of 1, 2 or 3 hours after the completion of the mixing. Copyright 213 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 6

7 Flexxural tensile strength (MPa) Compressive strength (MPa) h 2h 3h Figure 6 Compressive strength with delayed compaction time In Figure 6, it can be seen that the compressive strength of samples compacted within two hours after completion of the mixing is almost constant. However, the strength of samples compacted at 3 hours after mixing is lower with exception of sand-cement. The compressive strengths of clay-cement and sandy clay-cement significantly reduce by 5% and 3%, respectively. So it is clear that the delayed compaction has a detrimental effect on the final compressive strength. That is because the cement reaction starts in the presence of water, resulting in cementitious materials. But as soon as the delayed compaction is done, the bound materials will be broken and hence lead to reduced strength. Additionally, the moisture loss of the mixture which is caused by the evaporation after mixing will have a negative impact on the cement hydration and hardening process. Therefore, in order to achieve the required strength, the compacting process should be essentially accomplished within 2 hours after mixing. 3.4 Curing conditions Figure 7 and Figure 8 give the results of the strength development in different curing regimes. The moisture curing represents the moisture chamber with a relative humidity of 85%, and the dry curing indicates the normal room with an average humidity of 25%. 3,5 3 2,5 2 1,5 1,5 Mosture curing Dry curing Curing time (days) Figure 7 Flexural tensile strength development in different curing conditions Copyright 213 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 7

8 Compressive strength (MPa) Moisture curing Dry curing Curing time (days) Figure 8 Compressive strength development in different curing conditions As can be observed in Figure 7 and 8, similar strength development trends are obtained for the flexural tensile strength and the compressive strength, which generally indicates that strength obtained from the samples cured in the moisture chamber is approximately 2 times higher than the strength of the samples cured in the dry environment, except the strength after only 3 days curing. That is because the appropriate moisture curing environment can alleviate the moisture loss and hence ensure the sufficient cement hydration, while the dry environment can accelerate the moisture loss by the evaporation during the curing time which inevitably results into a reduced strength. Additionally, both strengths at curing age of 3 days remain the same within these two curing conditions. That is because during the first hours the moisture loss is mainly caused by the cement hydration. So the premature curing condition has no big influence on the strength of the samples. Subsequently, as the curing continues, moisture loss is more in the terms of evaporation. Therefore, in order to achieve satisfactory performance of cement stabilized materials, a curing environment with appropriate high humidity is quite essential. 4 CONCLUSIONS This study describes the effect of variables encountered in laboratory research on testing cement stabilized materials. The presented experimental results in this study can be outlined as follows: 1) During the compression test, the friction induced from the interface between specimen and loading panel may increase the compressive strength. For cylindrical specimens with diameter of 15 mm, without friction reduction system the compressive strength is 3% ~ 4% higher than in case such a friction reduction system is applicable. So utilizing the friction reduction system is very essential in conducting compression tests. 2) A conversion factor of 1.3 can be adopted for cement stabilized sandy materials to correlate the mechanical strength of a specimen with ASTM size and the strength of a specimen with Proctor size. It applies to both the compressive strength and the indirect tensile strength. 3) In order to achieve the required strength, the compacting process should be essentially accomplished within 2 hours after mixing, because the strength of cement stabilized materials decreases in general when compaction is done 3 hours after mixing. 4) A curing environment with appropriate high humidity ensures sufficient cement hydration and considerably helps to enhance the strength of cement stabilized materials. Copyright 213 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 8

9 ACKNOWLEDGEMENT: The authors appreciate the knowledge and expertise from PowerCem Technologies. All the tests were carried out in cooperation with PowerCem Technologies. This preliminary study is just a part of research on cement stabilized materials with the use of additive RoadCem in various soil types, that has been proven in practice to be successful. REFERENCES: [1] Shihata, S. A. & Z. A. Baghdadi. 21. Long-term strength and durability of soil cement. Journal of materials in civil engineering 13: 161. [2] Kolias, S., V. & Kasselouri-Rigopoulou, et al. 25. Stabilisation of clayey soils with high calcium fly ash and cement. Cement and Concrete Composites 27(2): [3] Bjorn Birgisson & Christophe Egyed, et al. 28. New Nano crystalline structure leads to viscoelastic behaviour of cement based materials. [4] Park, S. S. 21. Effect of Wetting on Unconfined Compressive Strength of Cemented Sands. Journal of geotechnical and geoenvironmental engineering 136(12): [5] Consoli, N. C. & A. V. da Fonseca, et al Voids/Cement Ratio Controlling Tensile Strength of Cement Treated Soils. Journal of geotechnical and geoenvironmental engineering 1(1): 36. [6] Lemoine P., 213. Nanoindentation of cement samples with and without RoadCem additives. University of Ulster, UK. [7] Mier, J.G.M. v Fracture Process of Concrete: Assesment of Materials Parameters for facture models. Boca Raton: CRC Press. [8] Xuan D.X Cement treated recycled crushed concrete and Masonry aggregates for pavements. Delft University of Technology, The Netherlands. [9] Williams, R.I.T., Cement-Treated Pavements: Materials, Design and Construction. Elsevier Applied Science Publishers LTD, London. [1] TRH Cementitious Stabilizers in Road Construction South Africa. TRH 14, Pretoria, South Africa. Copyright 213 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 9

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