USE OF FLY ASH IN MAKING CONTROLLED LOW STRENGTH MATERIAL (CLSM) FOR USE AS SELF-COMPACTED STRUCTURE BACKFILL

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1 USE OF FLY ASH IN MAKING CONTROLLED LOW STRENGTH MATERIAL (CLSM) FOR USE AS SELF-COMPACTED STRUCTURE BACKFILL Suresh Kumar, Dinesh Kumar and Nancy Mittal National Council for Cement and Building Material ABSTRACT The paper presents the study of Controlled Low Strength Materials (CLSM) utilizing Fly Ash from Thermal Power Plant. The properties of CLSM that had been investigated included Bleeding, Density of hardened CLSM, Permeability and unconfined Compressive strength of CLSM at 7 days and 28 days age. The CLSM specimens were prepared by varying the proportion of cement, fine aggregate, fly ash and water. CLSM mixtures were cast into cube moulds with the size of (100mmx100mmx100mm). In order to achieve good flowability the water/cement ratio used in CLSM was found to be very high. With the increase of filler s especially fly ash, bleeding and segregation conditions of CLSM are improved. With various mix design of CLSM, compressive strength of CLSM is controlled between 1.00MPa to 8.3MPa, in order to suit the requirement of different application, such as excavatable backfill and structural backfill. With this range of strength, CLSM can be used as alternative materials to solve the soft soil problems. The paper describes the use of CLSM with an emphasis on the use of waste Materials like fly ash for infrastructure applications. Keywords- CLSM; Fly ash; Bleeding and Segregation; Density; Permeability; Compressive strength; Backfill; Soft Soil Problems. 1. INTRODUCTION CLSM is a self-compacting, flowable, low-strength cementitious material used primarily as backfill, void fill and utility bedding as an alternative to compacted fill. Several terms are currently used to describe this material, including flowable fill, unshrinkable fill, controlled density fill, flowable mortar, plastic soilcement, soil-cement slurry, K-Krete and other various names (1). Controlled low-strength materials are defined by Cement and Concrete Terminology ( ACI 116R) as materials that result in a compressive strength of 8.3MPa or less. However, most current CLSM applications require unconfined compressive strengths of 2.1MPa or less. This lower-strength requirement is necessary to allow for future excavation of CLSM. The term CLSM can be used to describe a family of mixtures for a variety of applications. For example, the upper limit of 8.3MPa allows use of this material for applications where future excavation is unlikely, such as structural fill under buildings (2). CLSM can replace compacted soil as structural fill or backfill in many applications. Because CLSM flows and needs no compaction, it s ideal for use in tight or restricted- access areas where placing and compacting soil or granular fill is difficult or even impossible. CLSM is used as a backfill material for utility trenches containing ducts and/or pipes, around manholes and other excavations in streets, around foundations, or as a fill for abandoned tunnels, sewers, storage tanks, etc. Utility companies often specify CLSM instead of soil for backfilling around pipes or conduits. The material flows under and around pipes, providing uniform support without leaving voids. Self- leveling, CLSM also eliminates the chance of workers accidentally damaging pipes by operating compaction equipment near them. If easy access to utility lines is essential for maintenance or repairs, CLSM compressive strengths can be specified at or below 0.70Mpa. At these strengths, the material can be excavated easily with a backhoe or other digging equipment. CLSM also has applications for pavement

2 construction and maintenance. Used under roadways, it serves as a strong, stable subbase (3). Generally, CLSM mixtures are not designed to resist freezing and thawing, abrasive or erosive forces, or aggressive chemicals 2. These factors should be taken into consideration when designing CLSM mixtures. Here are some factors that should be considered: 1. Removability (Excavatability) 2. Flow ability 3.Available Materials 4. Load Transfer (Bearing capacity). Many waste materials were successfully used to develop CLSM. To name a few, there are flue gas desulphurization material, foundry sand, wood fly ash, dry scrubber ash and glass cullet (4). We have taken up this study further to use fly ash conforming to IS: 3812 (Part-2), which is a coarse fly ash and replaced part of cement and fine aggregate to a greater extent. Attempt is made to look into the maximum usage of coarse fly ash in CLSM. 2.0 EXPERIMENTAL WORK 2.1 Materials for CLSM Conventional CLSM mixtures usually consist of water, Portland cement, fly ash or other similar products, fine or coarse aggregates or both. It is not necessary to use the standardized materials as set by many available standard requirements (2). Selection of materials should base on availability, cost specific application, and necessary characteristics of mixture, such as strength, flowability, excavatability, and density Cement Cement is a binder material that holds all the other materials together and contributes to the strength and cohesion for CLSM mixtures. Ordinary Portland Cement 43 Grade conforming to IS 8112:1989 is used in our experiment work. The Chemical composition and physical properties of Cement are given in Table 1&2 respectively. Table.1 Chemical composition of Cement Material CaO SiO 2 Al 2 O 3 Fe 2 O 3 MgO K 2 O Na 2 O SO 3 Cl LOI Cement (%) Table.2 Physical Properties of Cement SI No. Physical properties Cement 1 Fineness Blaine, m 2 /kg Specific Gravity Setting Time, minutes Initial Final 4 Compressive Strength, N/mm 2 3-day 7-day 28-day 5 Soundness Autoclave, % Le Chaterlier Exp. (mm)

3 2.1.2 Fly Ash Fly ash obtained from thermal power plant was used in the investigation. The fly ash used in this investigation conforms to IS: 3812 (Part 2) The Chemical composition and physical properties of fly ash are given in Table 3 & 4 respectively. Table.3 Chemical composition of Fly Ash Material CaO SiO 2 Al 2 O 3 Fe 2 O 3 MgO K 2 O Na 2 O SO 3 Cl LOI Fly ash (%) Table.4 Physical Properties of Fly Ash SI No. Physical properties Fly Ash 1 Fineness Blaine, m 2 /kg Specific Gravity Lime reactivity, N/mm Soundness Autoclave, % Fine Aggregates The water absorption and specific gravity of the sand used is 0.80% and 2.61, respectively and conforms to grading Zone I. The testing of sand was done as per Indian Standard Specifications IS: 383:1970. The sieve analysis results are shown in Table 5. Table.5 Physical Properties of Fine Aggregate IS Sieve designation Percentage passing Grading Limit for Zone I as per IS: mm mm mm mm µm µm µm Zone I 2.2 Test Procedure Concrete Mix Design The mix formulation used in the investigation is shown in Table 6. The proportions for the CLSM mixtures were designed for cement contents ranging from 30 to 70 kg/m3 and fly ash contents between 460 and 1200 kg/m 3.The CLSM was designed for 28-day characteristics strength of 0.52MPa with the slump requirement of mm. The Target mean strength is calculated by using the equation i.e. f ck *S, where S is assumed standard deviation as 20% of required characteristics strength at 28-day.

4 Mixing, Slump Test, Casting, De-moulding and Curing The mixing was done in a pan mixer for 2 to 3 minutes and workability of CLSM mix was checked using slump cone method as per IS 1199:1959. The cube specimens of 100mmx100mmx100mm size were cast without any vibration. The specimens were stored in a place free from vibration, in moist air of > 90% relative humidity and at a temperature of 27 C±2 C until the fourth day after preparation. Humidity chamber were used to maintain these condition. On the fourth day, specimens were placed in a water curing tank Bleeding The bleeding test of CLSM was carried out as per the method given in IS: A cylindrical container of approximately 0.01m 3 capacities was filled with fresh CLSM and the test specimen was kept at 27 C temperatures. The water accumulated at the top was drawn off by means of a pipette, initially at 10min intervals during the first 40min and after that at 30 min intervals subsequently till bleeding ceases. The bleeding water was collected in a graduated jar and accumulated quantity of bleeding water was recorded at the end of test and the percentage of bleeding water was calculated Density of Hardened CLSM Density of hardened CLSM specimens was measured by measuring the mass of the hardened CLSM, and then divided by the volume of CLSM measured Compression test Compression test was carried out on compression testing machine having maximum load capacity of 150 KN. Cubes of size 100mmx100mmx100mm were tested Permeability of CLSM Permeability of most excavatable CLSM is similar to compacted granular fills. Cylindrical specimens with the size of (150mmx150mm) were cast for Permeability test. Finer constituent materials and mixtures of higher strength can achieve permeabilities as low as 1.00 x 10-7 cm/sec (2). Permeability is increased as cementitious materials are reduced and aggregate contents are increased. Permeability of hardened CLSM was tested as per IS: and pressure head was maintained as 1kg/cm 2. Water Content Cement Content Fly ash Content Table.6 Trails conducted for CLSM Fine aggregate content Workability of concrete obtained in terms of slump (mm) 7-Day comp. strength of cube (N/mm 2 ) 28-Day comp. strength of cube (N/mm 2 ) Density of Hardened CLSM

5 3.0 RESULTS AND DISCUSSION Properties of CLSM such as bleeding, density, unconfined compressive strength and permeability are discussed. 3.1 Bleeding In this experiment, there is no significant bleeding or segregation observed. This is because only fine aggregates and fillers are used in all the mixtures instead of coarse aggregate. Fine particles have smaller void between the particles. With less voids, particles are less likely to be dislocated. Unless excessive free water exists or terrible bleeding occurs, segregation of CLSM designed with fine aggregates only is very unlikely to happen (1). 3.2 Density of Hardened CLSM The density of CLSM in this experiment obtained varies from 1736 kg/m 3 to 2126 kg/m 3 as given in Table 6.The results found by Horiguchi, Okumura, and Saeki (5), the range of CLSM density varies from 1338 kg/m 3 to 2056 kg/m 3. Comparing to those results, the results obtained seem to be very similar. 3.3 Unconfined Compressive Strength of CLSM Unconfined Compressive Strength is the parameter that determines the load-carrying ability of CLSM. The compressive strength for all the specimens was tested for 7 days and 28 days (Fig-1). From the figures as given in Table 6, it can be concluded that the higher quantity of cement used will produce CLSM with higher compressive strength as it has lower water/cement ratio. This is very rational as cement is the major source of cementitious materials within the mixture that is used to bond the aggregates and particles. The Compressive strength of CLSM is depending on the bond formed between these particles. Thus, it is reasonable that more cement used can generate more strength as particles are more effectively bonded together (6). Besides cement, fly ash content also causes minor effects to the compressive strength of CLSM specimens. 3.4 Permeability of CLSM Permeability in an existing concrete structure is an essential and important step for the definition of its durability, performance, and lifetime. Permeability regulates the speed of aggressive water penetration inside concrete besides, controlling the movement of the water during the freeze-thaw process. The coefficient of Permeability of CLSM when tested as per IS: (Fig-2) is 1.00 x 10-8 cm/sec which is lower than clay that has relatively low permeability with a coefficient of Permeability of 1.00 x 10-7 cm/sec. Fig-1: Compressive Strength Test Fig-2: Permeability Test

6 4.0 CONCLUSION Based on the experiments conducted and analysis of experimental results, the following conclusions are drawn: i. No significant bleeding or segregation observed. ii. Higher fly ash content generates lower density value of CLSM concrete. iii. Higher water-cement ratio produced lower density value of CLSM concrete. iv. Fine aggregate, especially fly ash, was the most significant factor affecting the water demand of CLSM. v. The workability of concrete decreased with the increases in volume of fly ash. It can be due to extra fineness of flyash as the volume of flyash in CLSM mix is increased. Thus, increase in the specific surface due to increased fineness and a greater amount of water needed for the mix ingredients to get closer packing, results decrease in workability of mix. vi. Fly ash from 400 Kg/m 3 to 1000 Kg/m 3 can be used successfully to achieve the require strength and Workability. However higher value of fly ash up to 1200 Kg/m 3 can also be considered in some cases. vii. The compressive strength of CLSM mixes tested is in the range of 1.0MPa to 3.68MPa. This shows that CLSM using fly ash can be designed for excavatable and structural filling application like soil and structural filling, utility bedding and sub-base. Based on this investigation, it is concluded that industrial waste fly ash can is successfully used in CLSM. viii. Fly ash used as fine aggregates replacements enables the large utilization of waste product. Furthermore, the application of CLSM is as structural fill or backfill in place of compacted soils and granular fill. Per cubic meter of material, CLSM costs more than conventional granular backfill. The savings in time and money as well as versatility and consistent quality of CLSM typically outweighs the use of conventional fill materials. Moreover, in future it will be very difficult to find soil for backfilling purposes and for this CLSM is a very good as well as eco-friendly alternative. 5.0 REFERENCES (1) A. Katz and K. Kolver, Utilization of industrial by-products for the production of CLSM, Waste Management, vol. 24(5), pp , (2) American Concrete Institute, Committee 229, Controlled Low-Strength Materials (CLSM), ACI 229R-94 Report, (3) Joseph A. Amon, Controlled Lowstrength Material, The Construction Specifier, December 1990, Construction Specifications Institute, 601 Madison St., Alexandria, VA (4) Jenny L. Hitch, Amster K. Howard and Warren P. Bass Innovations in Controlled Low Strength Material (Flowable Fill), ASTM STP 1459,2002, pp. 3,15,31,41,51. (5) Horiguchi, T., H. Okumura, and N. Saeki, 2001a. Optimization of CLSM mixes Proportion with Combination of Clinker Ash and Flyash, In ACI Special Publication SP-199. ACI: Farmington Hills, p (6) Neville, A.M; Properties of concrete.4 th Edition ed. Edinburgh Gate, Horlow: Longman. ACKNOWLEDGEMENT This paper is based on R&D studies carried out at National council for cement and Building Materials, Ballabgarh, Haryana, India. This paper is published with the permission of the Director General of the council.