Transcript
CE 199 – Undergraduate Research Project in Civil Engineering Final Presentation of Research
17 October 2012 Construction Engineering and Management Group
Development of a Method for the Design and Performance Testing of Controlled Low-Strength Materials Containing Processed Class F Fly Ash KAZ MIKHAIL DAVID S. MAURILLO Undergraduate Student, B.S. Civil Engineering Program Institute of Civil Engineering, University of the Philippines Diliman E-mail:
[email protected] Advisers:
Dr. Nathaniel B. Diola Professor, Director, Building Research Service, National Engineering Center, University of the Philippines Diliman Abstract: The main purpose of the study is to create a design method in mixing Controlled Controlled Low-Strength Materials Materials (CLSM) using locally manufactured Class F fly ash by mixing actiual CLSM and testing for fresh (flowability and setting time) and hardened properties (unconfined compressive strength). Controlled Low-Strength Material is also commonly known as flowable fill or controlled-density fill. CLSM is a very versatile material made up of soil or fine aggregate, water, fly ash, and minimal cement. It is usually applied for pipe-bedding and filling, trench fills, backfills for retaining walls, slope stabilization, void fillings (e.g. walls, cracks, crevices), filling of abandoned spaces like underground tunnels and mines, and can also be used to support footings or shallow foundations. foundations. The main design target for CLSM is to e nsure the mixes made are flowable, self-leveling, easy to pump in place, will not settle, will not exert uplift pressures due to swelling, and will be excavatable at later ages. Mix proportions leading to inexcavatable specimens will noted, and will provide further information in improving the mix design methodology. methodology. The study aims to determine the optimum proportions of material components of CLSM, namely cement, Class F fly ash, fine aggregate (sand), and water to ensure proper density, flowability, satisfactory setting time, and ease of pumping, and excavatability in the long run. Mixes of increasing fly ash from the threshold content that satisfies flowability to the maximum allowed by ACI 229 are mixed, and tested for the main properties of flow (slump), density, setting time, and unconfined compressive strength. Specimen samples used were ASTM C109 50-mm cubes, as allowed by ACI 229R-99. New adjusted mix proportions designed for excavatability are then made and tested for performance based on interpolations from the satisfactory trial mixes. From the actual results obtained, a comprehensive guide and methodology methodology for mix design of CLSM were made that would be of use to future designers. A general design methodology, methodology, guide and recommendations for CLSM mixtures were also made based on the application of CLSM and the kind/type of materials that are utilized in the mix.
for the Canadian River Aqueduct Project, running 515 km from Amarillo to Lubbock, Texas [3]. The project cost was estimated to be 40% less than using standard conventional backfilling.
1. INTRODUCTION 1.1 Background and Problem Statement
Controlled Low-Strength Material (CLSM), commonly known as flowable fill, is a hybrid material sharing both properties of concrete/mortar and well-compacted soils. CLSM typically consists of minimal cement, fly ash or other pozzolans, fine aggregates, and water. CLSM is a cheap, economical, durable, and alternative construction material that has properties of both concrete/mortar and compacted soil, and can be classified as somewhere in between the latter two. CLSM is primarily used as backfill material, for structural fills, void fills, pipe bedding and fills, and slope stabilization of embankments. CLSM is an effective alternative material when insitu compacted fill proves to be unsatisfactory (e.g. too weak bearing pressure) and has undesirable properties (settlement, swelling, etc.).
CLSM is divided into two categories based on its long-term unconfined compressive strength. CLSM can either be classified as excavatable or inexcavatable. Standard practice classifies excavatable CLSM as having strengths between 0.3 – 0.7 MPa (50 – (50 – 100 100 psi), easily removable using hand tools. Performance specifications impose a 1.4 MPa (200 psi) maximum strength [2] for excavatable mixtures while mixtures with strengths up to a maximum of 2.1 MPa (300 psi) are still considered excavatble with the use of mechanical equipment (backhoes, etc.). The main advantage of CLSM as a substitute material for compacted fill is due to its desirable properties. CLSM is self-leveling, thus does not need compaction unlike in-situ soil. CLSM is flowable, making it pumpable, readily flowing and filling hardto-reach spaces compacted soil cannot. When in its hardened state, CLSM also does not settle; preventing the cracking or shearing of structures it carries (e.g. pavement bases). Most importantly, when CLSM is no longer needed, it can easily be removed, in the case of excavatable mix proportions. Also, CLSM utilizes as little cement as possible, and recommends
The ACI Committee Report on Controlled LowStrength Materials (ACI 229R-99) defines CLSM as a self-compacted, cementitious material used primarily as backfill material substituting compacted soil. It is also defined as a resultant material having compressive strengths of 8.3 MPa (1200) or less [1]. The actual design and use of CLSM has been first reported in 1964; first used by the United States Bureau of Reclamation as pipeline bedding material 1
the use of fly ash as both pozzolan and enhancer of workability, which is much cheaper than using cement alone. CLSM also provides an economical and convenient alternative whenever conventional methods and materials prove to be unsatisfactory.
2. Optimize the mix proportions of CLSM by investigating the factors affecting flowability, setting time, and unconfined compressive strength. 3. Develop mix proportions that guarantee excavatability, reducing the time incurred by repetitive trial-and-error procedures. Determine effective means of suppressing the strength development of CLSM to ensure future excavatability of actual mixtures.
While CLSM is a versatile material most especially effective for highway construction works, its main drawback lies on its design. CLSM is a relatively new material, reportedly been first used in 1964 by the US Bureau of Reclamation for pipe bedding. Being a relatively new material compared to concrete (which has been used since ancient times), the design of CLSM is not yet defined and well-established. The ACI Committee Report on Controlled Low-Strength materials (ACI 229R-99) states that there is still no unified code on the design of CLSM. In the United States, each state has their own set of guidelines on the mixing and placement of CLSM. The variability of the constituent materials as well their properties also makes designing CLSM difficult to generalize. Trial-and-error in mix proportioning is performed until the appropriate mix proportions based on the intended construction application are achieved [1].
4. Based on the results of the experimental program, verifying the reliability and applicability of the existing guides reviewed and incorporate and improve the guides to create a simple, objective methodology for design and quality control specified for CLSM utilizing processed Class F fly ash. 6. Develop a general, organized design and performance directive for CLSM regardless of materials utilized to reduce the time, effort and resources expended by disorganized trial-and-error schemes in designing CLSM. 1.3 Scope of Work
The main disadvantage of CLSM over compacted soil is that CLSM gains strength over time, unlike soil due to the presence of cement and other pozzolans, in this case Class F fly ash. While CLSM mixtures within the excavatable category find a wide range of applications, this type of CLSM is more difficult to design. There are still no established guides and methods developed to consistently achieve the desired mix proportions of satisfactory performance properties in terms of flowability, setting time, density, unconfined compressive strength, and other design properties for CLSM.
The research aims to assess the feasibility and practicality of actual CLSM designs utilizing Class F fly ash and other locally available constituent materials. The study is limited to a single type of fly ash, locally produced Class F fly ash, compliant with ASTM C618 standards, marketed as ‘Maximizer’ and manufactured and distributed by Pozzolanic Philippines Inc. For convenience purposes the fly ash is referred to as PFA in the study. The study focuses on the investigation of three main properties of CLSM, namely: flowability, setting time, and unconfined compressive strength.
1.2 Objectives
With the urgency to use CLSM due to its versatility, it is important to be able to come up with an accurate, reliable methodology in the design and quality control of CLSM mixtures. While there are several guidelines for mix proportioning and actual placement of CLSM already available [1, 2, 4], they are very general and do not specify specific cases, nor do they give detailed instructions and cautions to the designers of CLSM.
The study also is limited to the approach of designing excavatable CLSM without chemical intervention, such as the incorporation of admixtures that retard the strength development of CLSM mixtures, when future removal is of concern.
The main content of the methodology being developed contains the following, with the focus on utilizing processed Class F fly ash into CLSM mixtures:
Of importance in this study is the investigation of the effects of performance properties of CLSM due to the amounts of PFA incorporated into the mixtures. All the constituent materials utilized in the experiment mix proportions were satisfactory in accordance with the American Society for Testing of Materials Standards and other organizational standards. The first part of the methodology involves the investigation of constituent material properties utilized for CLSM.
2. LITERATURE REVIEW 2.1 Constituent Materials
1. Determine which mix proportions result in ‘lowstrength’ (excavatable) and ‘high-strength’ (inexcavatable) CLSM mixtures by actual design of CLSM and testing for the three priority properties, namely: flowability, setting time, and unconfined compressive strength.
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The following table shows the standards conformed by the actual materials in the study used to create trial mix proportions of CLSM.
2.2 Properties of CLSM
There are many properties of CLSM that are advantageous compared to conventional backfill materials. Three of these properties are given special interest and investigated in this study. The table below summarizes basic CLSM properties and their standard methods for investigation and quality control. The section discusses the priority properties investigated in the study. ACI 229R-99 as well as other documents [2] provides comprehensive information on the control properties of CLSM.
Table 1: List of material standards for CLSM Constituent Material Cement Fly Ash (Class F) Water
Fine Aggregates
Standard ASTM C150 [1] ASTM C618 [1] ASTM C94, AASHTO M 157 [1,3] ASTM C33 [1]
2.1.1 Cement
Table 2: List of control properties of CLSM
Cement generally acts as binder for CLSM aggregates, providing the basis for strength and cohesion. Its other purpose is to serve as an activator of pozzolans in PFA that contribute to the strength and cohesion of CLSM. Type I and II Portland cements generally compliant with ASTM C150. Blended hydraulic cements conforming to ASTM C595 can also be used.
Property Flowability Setting Time
2.1.2 Class F fly ash
2.2.1 Flowability
Class F fly ash, referred to as PFA in this study, is a coal-combustion by-product. It is produced from burning of harder, and older anthracite and bituminous coals. The hydration reaction of cement in water produces calcium hydroxide that reacts with PFA to form cementitious compounds. PFA generally acts as a cementitious substitute due to the impracticality of only using cement in CLSM, in terms of suppressing the strength development of CLSM. PFA also improves the workability and flow properties of CLSM.
Flowability gives the advantage of CLSM to be pumped and fill hard-to-reach spaces, reducing time and costs in placement. CLSM is of a thick liquid-like consistency in its fresh state, giving it its flowability.
Density Unconfined Compressive Strength Bearing capacity
Control Standard/s ASTM D6103 [1] ASTM C191, ASTM D6024, ASTM C403 [1] ASTM D6023 [1] ASTM D4832, ASTM C109, ASTM C39 [1] ASTM D1883 [1]
Flowability of flow consistency is measured using ASTM D6103: Test Method for Flow Consistency of Controlled Low Strength Material [1]. The test determines the water demand of a particular amount of PFA in CLSM required to achieve satisfactory flowability represented by a slump or flow diameter of 200 mm (8 inches) or higher. It can also determine the amount of water that can be added beyond the requirement to improve flow without segregation.
2.1.3 Fine Aggregates
Fine aggregates generally contribute to the strength, permeability, segregation control, and excavatability of CLSM. Generally, coarse aggregates are excluded in CLSM due to difficulty in suppressing strength in CLSM. Fine aggregates fill voids in CLSM, and effectively links with the binder to form more cohesive CLSM to avoid excessive segregation. ACI 229 primarily recommends the use of fine aggregates passing ASTM C33 specifications.
2.2.2 Setting Time
When time constraints in construction need to be addressed, such that pavement bases or other works to be supported by CLSM must be placed as early as possible, the setting time of CLSM is of utmost importance. Setting time generally takes 3-5 hours after placement of CLSM, empirically measured as the time when in-place CLSM can support the weight of a person. Shorter setting times indicate shorter waiting time before CLSM can carry design loads (e.g. traffic loads, footings, pipes etc).
2.1.4 Water
Water that is acceptable for mixing concrete is acceptable for mixing CLSM as well. Water used in CLSM must be non-toxic, almost pure, and free of any impurities. Drinking water is usually very suitable for mixing CLSM. ASTM C94 and AASHTO M 157 provides specifications for water used in CLSM mixes.
Setting time for this study is measured by a modified Vicat penetration test (ASTM C191), adapted for CLSM mixtures. The initial setting time of CLSM as defined by the test is the first time duration in hours when the Vicat needle penetration is less than 25 mm.
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2.2.3 Unconfined Compressive Strength
3. EXPERIMENTAL PROGRAM
The projected construction application of CLSM is determined by the unconfined compressive strength. Excavatability is guaranteed by very ‘low -strength’ CLSM mixtures when intended for general purpose backfilling and pipe beddings, while inexcavatable mixtures are designed when CLSM is utilized as a base course for pavements and structural fills.
Trial mixtures with the materials selected for CLSM mix proportions are subject to tests in order to obtain information on the factors that affect the properties of CLSM the study aims to investigate. Secondly, it also tests the applicability and reliability of following existing design guides in actual mixing of CLSM using the materials specified within the study. Fresh properties of flow consistency and setting time was first investigated before sampling specimens subjected to unconfined compressive strength tests. Sample specimens used were ASTM C109 50-mm (2 in) cubes particularly used for mortar specimens, as included by ACI 229R-99 in the list of allowable test specimens.
Table 3: Categorization of unconfined compressive strengths of CLSM mixtures Category
Excavatable
Inexcavatable
Strength (Range) 0.3 – 1.0 MPa (2.1 MPa max)
2.1 – 8.3 MPa
Uses
Pipe bedding, void fill, general purpose backfills Highway base course, roadside trench fills, structural fills
3.1 Mix Proportioning
Mix proportions involved two batches. The first batch aims to determine the effect of the amount of PFA in CLSM on the compressive strength development of the mixtures, in order to give an estimate on the possible amounts of PFA that would guarantee excavatability. The second batch of mix proportions makes an attempt on designing excavatable CLSM based on first-hand information obtained from the first mix proportioning scheme. Material quantities were expressed in kg/m3 of CLSM (Weight of components per 1 cubic meter of CLSM).
Studies on CLSM that D. Hardjito, et. al. [4] suggest that cement content typically determines the unconfined compressive strength of CLSM. Strength increases as the cement content increases. It is thus crucial to optimize the amount of cement used in order to ensure excavatability of CLSM. Cement contents of 1-3% by weight per unit volume of CLSM provides high chances of excavatability by keeping the strengths low due to cementitious cohesion.
Table 4.1 Mix Proportions I of the experiment
Table 4.2 Mix Proportions II of the experiment
3.2 Flowability
Flowability studies were performed to determine the relationship between fly ash volume in CLSM and its corresponding water demand to achieve good flowability (200 mm or 8 inch slump according to ASTM D6103). The following table shows the observed water requirement to achieve a flow diameter of 200 mm and above (8 inches) based on ASTM D6103 flow cylinder tests performed.
Figure 1: relationship of strength development of CLSM with amount of cement in the mixtures (D. Hardjito, et. al. [4]). 4
Table 5: Water requirement to achieve good flow consistency, Mix Proportions I
Mean compressive strengths, Mix Proportions I i 500 s p , h 400 t g n 300 e r t S e 200 v i s s e 100 r p m 0 o C
460 307.67 132.92
182.92
114.54
mean f'c psi
43.75 0
200
400
600
800
1000 1200 1400
Class F fly ash content, kg/m3
3.3 Setting time
Figure 2: Strength development of CLSM as amount of fly ash increases in the mixture.
The setting time range for the actual mix proportions were determined using the Vicat needle test previously discussed. The goal of this section is to verify whether the actual setting times determined agrees with the 3-5 hours range of satisfactory setting time for CLSM [1]. Table 6: Setting times for trial mix proportions
3.4 Unconfined Compressive Strength
Suppression development excavatability time caused compounds.
or retardation of the strength of CLSM is necessary to ensure due to CLSM gaining strength over by the presence of cementitious Figure 3: Compressive strengths, Mix Proportions I
The first trial mix proportions involve the determination of the suitable PFA amounts guaranteeing excavatability and to determine a trend of strength development with respect to the amount of fly ash used in CLSM. To determine potentially excavatable mix proportions in as early as 28 days (especially due to timeconstrained projects), ACI as well as other organizations [2, 7, 8] set the 28-day strength maximum limit as low as 1.4 MPa (200 psi), on the premise that relatively weak CLSM still has enough allowance for strength development and will still be excavatable at later ages. The following figures show the strength development of the mix proportions during the first and second batches to determine amounts of fly ash leading to inexcavatability.
Figure 4: Strength development, Mix Proportions II
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4. RESULTS AND DISCUSSION
Boxplots, Mix Proportions I PFA-CLSM 600
Based on the experiments and analysis performed; the following conclusions were made:
i 500 s p , h t g 400 n e r t S 300 e v i s s e 200 r p m o C 100
1. In the flowability experiment, as PFA increases in the mixture, the water demand to achieve good flow consistency decreases. 2. Increasing the water content for a particular fly ash volume beyond the water demand for flowability increases the flow diameter further, and retards the strength development of the mixture. 3. Mixtures with low water-to-binder ratios, implying threshold water requirement, pose the risk of inexcavatability due to higher strength developments. 4. In the setting time experiment, the amount of fly ash did not have an effect on the setting time of concrete. Only the amount of water established a relationship with setting time. The more saturated CLSM is, the more time it requires to drain out excess water. 5. For relatively high amounts of PFA (300-400 kg/m3), setting the water content to the minimum required for good flowability did not retard the strength development, leading to inexcavatable mixtures 6. Segregation was not a serious problem with the highly saturated mixtures as PFA as well as the fine aggregates were not hygroscopic and easily drained out excess water out of the mixtures. 7. Adding the water content effectively retarded the strength gains of CLSM in order to achieve excavatable mixtures. It is important that discretion must be observed in adding water so as not to result in lack of component cohesion and segregated mixtures, leading to disintegration when immersed in water after 24 hours (parallel study performed).
460
307.667
300 282.05
200 114.542 87.7547
182.917
132.917
43.75
31.375
35 0
7 7 8 8 8 1 8 2 8 2 8 2 8 5 0 - 1 - 1 - 1 2 5 2 4 0 2 6 0 0 0 0 0 0 0 0 0 0 0 2 3 4 2 6 8 2 a a 1 1 a a a a a p f p f a a p f p f p f p f p f p f p f
Figure 6: Batch I compressive strengths. Note that inexcavatability increases with the amount of fly ash in the mixtures.
PFA-CLSM Mixtures II, 5 and 14-day strength 300
300
i s 250 p , h t g200 n e r t s e 150 v i s s e 100 r p m o C 50
200
150
77.2083 31.5
35.375
44.1667
42.625
35.25 22.1667
66.5
38.75
55.8333
33.7917
35 34.5417
0
5 4 5 4 5 4 5 4 5 4 5 4 0 - - 1 7 5 - 5 - 1 0 0 - 0 - 1 2 5 - 5 - 1 5 0 - 0 - 1 7 5 - 5 - 1 2 5 2 5 0 2 7 3 0 3 2 3 5 3 7 a a 2 a 3 a 3 a 3 a 3 p f p f a p f p f a p f p f a p f p f a p f p f a p f p f a
Figure 7: Early strength development retardation of CLSM mix proportions (2 nd batch) by increasing water-binder ratio. 3. CONCLUSION
By using the approach of designing CLSM that is excavatable, compromises were made when a relatively high amount of Class F fly ash is pr esent. Increasing the amount of Class F fly ash in CLSM definitely decreases the water demand for satisfactory flowability. However, due to the decreased water demand, adding water beyond the requirement causes saturation, exhibited by bleeding, and in worst cases, segregation and disintegration due to lack of cohesiveness between CLSM solid particles. In ensuring excavatability by adding water beyond the requirement, fresh properties are being compromised.
Figure 5: Low water-to-binder ratio (w/b) with increasing fly ash increases the strength of CLSM, leading to inexcavatable mixtures. Increasing w/b effectively retards strength gains.
The data collected enabled the development of a simplified mix design procedure for CLSM, based on what CLSM will be used for. The following tables show the developed methodology for designing CLSM that utilizes Class F fly ash to improve the 6
workability and complement the minimum amount of cement in the mixtures. This method can be further expanded and improved into a local design book for CLSM specifications in a Philippine setting.
Table 11: Allowable sample specimens for testing hardened CLSM Table 3.1 Sampling Specimens of CLSM for Compressive Strength Specimen Description 50-mm cube ASTM C109 mortar cube specimens 75 x 150 mm cylinder ASTM D4832 CLSM plastic cylinder specimens 150 x 300 mm cylinder ASTM C39 concrete cylinder specimens CBR cylinder ASTM D1883 California bearing ratio specimens
Table 7: Step-by-step procedure for CLSM design
Table 12: Procedure for determining minimum good flow consistency Table 4: ASTM D6103 Summary to determine water demand of CLSM
Equipment: mortar mixer, 75 x 115 mm cylinder, tape measure 1. Adjust mix proportions to fill 0.001 cubic meters of volume (1 L). 2. Select mix proportions (excluding water), place into mixi ng bowl. 3. Add 100 mL of water, turn mixer on for 3 minutes. 4. Pour contents into cylinder and raise immediately, measure slump. 5. If slump diameter is 200 mm (8 inches), water demand is attained. 6. If slump diameter is less than 200 mm, add water, and remix. 7. Add water in increments of 10 mL until a slump of 200 mm occurs.
Table 8: 1st phase, determination of the type of CLSM for the project
Table 13: Decision table for evaluation of setting time
Table 9: Materials selection table
Table 14: Unconfined compressive strength requirements Table 6: Strength Requirements 28-day strength (MPa) 28-day strength (psi) Category 0.2 - 1.0 30 - 150 Very Excavatable 1.0 - 1.4 150 - 200 Excavatable > 1.4 > 200 Inexcavatable > 2.1 > 300 Inexcavatable
REFERENCES
Table 10: Recommended mix proportions 1.
2.
3. 4. 5. 6.
7.
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American Concrete Institute Committee Revised Committee Report on Controlled Low-Strength Materials, ACI 229R-99, American Concrete Institute, 1999 Guideline for Flowable Fill or CLSM-Controlled Low Strength Material, Concrete promotional Group, Kansas Cit y, United States Guide Specification for Controlled Low Strength Material, National Ready-Mixed Concrete Association Controlled Low Strength Materials (CLSM) Utilizing Fly Ash and Bottom Ash, D. Hardjito, C.W. Chuan, J. Tanijaya Sustainable development using Controlled-Low Strength Material, D. Trejo, K. Folliard, L. Du Development of a Recommended Practice for use of Controlled Low Strength Material in Highway Construction, National Cooperative Highway Research Program (NCHRP Report 597), Foliard, Du, Trejo, et. al. 2008 Supplemental Technical Specification for Flowable Fill, South Carolina State Department of Transportation (SC-M210, 06/07)
