Foundry Sand in Flowable Fill
What is Flowable Fill?
Flowable fills are engineered and manufactured backfills that have a specified compressive strength £ 8300 kPa (1200 psi) at 28 days (ACI Committee 229). If future excavation is desired, however, the compressive strength should be £ 1040 kPa (150 psi) (Amon 1990). Flowable fills are self-leveling, liquid-like materials, and self-compacting to 95-100% of the maximum unit weight. Benefits of flowable fills include limited required labor, accelerated construction, ready placement at inaccessible locations, and the ability to be manually re-excavated. Applications for flowable fills include utility trenches, building excavations, underground storage tanks, abandoned sewers and utility lines, slab jacking, and filling underground mine shafts (Smith 1996, Naik and Shiw 1997).
Flowable fill is typically a mixture of sand, fly ash or cement, and water. Since sand is the major component of flowable fills, replacing the natural sand with foundry sand is an attractive beneficial reuse application. The research performed on the beneficial reuse of foundry sand in flowable fill applications is summarized on the Research Project Page. The Field Project Page contains a case-by-case summary of projects where foundry sand flowable fills were installed.
In 1993 and 1994 demonstration projects on flowable fill were initiated by the Ohio Cast Metals Association (OCMA), Ohio Ready Mixed Concrete Association (ORMCA), Ohio Department of Development (ODOD), Ohio’s Edison Materials Technology Centers (EMTEC), and the Institute of Advanced Manufacturing Sciences in Cincinnati, Ohio. This effort included placing foundry sand flowable fills next to virgin sand flowable fills for comparison. These projects were an opportunity for all industry sectors to be exposed to the use of foundry sand in flowable fills (Smith 1996). Laboratory and field evaluations showed that flowable fills using foundry sand perform as well as conventional flowable fills. As a direct benefit from these projects, the county of Hamilton, Ohio approved the use of foundry sand in flowable fill applications, which generated a substantial market for foundry sand reuse. Duritsch (1993) presents technical information describing the use of foundry sand in flowable fills in Ohio. Stern (1995) presents a detailed history of the use of foundry sand in flowable fill applications in Ohio.
Javed and Lovell (1994) conducted a research project at Purdue University on foundry sand reuse. The project was sponsored by the Indiana Department of Transportation (InDOT). The objective of the project was to identify the physical characteristics of foundry sands and to determine possible beneficial reuse applications of foundry sand in construction. The leachability of foundry sands was also studied, and found to be comparable to natural soils (Javed and Lovell 1994).
Javed and Lovell prepared flowable fill mixes using Class F fly ash, Type I cement, and a foundry sand (Mast 1997). Mix designs were tested for spread (flowability), set time, and compressive strength. Flowabililty is a critical parameter in flowable fill design because it ensures that all void spaces in the desired cavity are filled by the flowable fill material (Mast 1997). The material stops flowing when the set time is reached. The Purdue study concluded that foundry sand mixes perform better than conventional flowable fill mixes (Javed 1994). Javed (1994) also reports that the rate of strength gain was lower for foundry sand mixes than for conventional materials.
The project by Javed and Lovell was followed by a Joint Highway Research Project at Purdue University performed by Bhat and Lovell (1996). This project focused on the use of foundry sands in flowable fill applications. Mixes of Class F fly ash and foundry sands from three foundries were studied. Bhat and Lovell (1996) investigated the economy of foundry sand use, flow characteristics, hardening patterns, 28-day compressive strength, long-term strength, pore size distribution, hydraulic conductivity, pH of the pore water, and stress-strain characteristics. They conclude that flowable fill is an economic alternative to conventional compacted fills. They also conclude that mix design can be developed by trial and error depending on the physical characteristics of the materials. Bhat and Lovell (1996) also developed a step-by-step procedure for flowable mix design, and concluded that mixes containing up to 55.5% foundry sand can be used. Bhat and Lovell (1996) and Javed and Lovell (1994) conclude that Class F fly ashes and foundry sands from ferrous castings should be environmentally acceptable when used in flowable fill applications.
Naik and Shiw (1997) of the Center for By-Product Utilization (CBU) at the University of Wisconsin-Milwaukee have worked on developing flowable fill mix designs. Their early work involved the use of fly ash in concrete products such as flowable fills. Naik and Shiw (1997) prepared two reference flowable fly ash-slurry mixtures. These two mixes did not contain any sand. Class F1 fly ash was used for one mix and Class F2 fly ash was used for the other mix. Foundry sand was then added to these mixes. The mixtures using foundry sand as a replacement for fly ash were proportioned starting from the two reference mixes to obtain 28-day compressive strengths in the range of 350 to 700 kPa (50 to 100 psi). Naik and Shiw (1997) conclude that foundry sand can be used in flowable fills to replace up to 85% of the fly ash used in the reference mixes.
Another aspect of their study dealt with hydraulic conductivity, because more permeable fills have greater leaching potential. Results of the test program showed that a minimum hydraulic conductivity occurs when 30% of the fly ash is replaced with foundry sand. Flowable fills having 70% of the fly ash replaced with foundry sand do not have significantly different hydraulic conductivities (Naik and Shiw 1997). However, when 85% of the fly ash is replaced with foundry sand, the hydraulic conductivity increases dramatically. Low hydraulic conductivity is also correlated to water to cementitious material ratios ranging from 0.4 to 0.6.
Controlled Low-Strength Material / Flowable Fill
Summary
Flowable fills are engineered and manufactured backfills that have a specified compressive strength £ 8300 kPa (1200 psi) at 28 days (ACI Committee 229). If future excavation is desired, however, the compressive strength should be £ 1040 kPa (150 psi) (Amon 1990). Flowable fills are self-leveling, liquid-like materials, and self-compacting to 95-100% of the maximum unit weight. Benefits of flowable fills include limited required labor, accelerated construction, ready placement at inaccessible locations, and the ability to be manually re-excavated. Applications for flowable fills include utility trenches, building excavations, underground storage tanks, abandoned sewers and utility lines, slab jacking, and filling underground mine shafts (as well as sink holes) (Smith 1996, Naik and Shiw 1997). It has also been used for abutments, embankments, bases and subbases.
Flowable fill is typically a mixture of sand, fly ash or cement, and water. It goes by several names, but is generally the same material. Other names include: controlled density fill, controlled low strength material, fly ash slurry, lean mix backfill, unshrinkable fill, and soil cement. Since sand is the major component of flowable fills, replacing the natural sand with foundry sand is an attractive beneficial reuse application. The research performed on the beneficial reuse of foundry sand in flowable fill applications is summarized on the Research Project Page. The Field Project Page contains a case-by-case summary of projects where foundry sand flowable fills were installed.
Foundry sand does not have to meet ASTM C33 gradation specification requirements as a concrete fine aggregate to be suitable in flowable fills. In fact, it has been demonstrated that foundry sand with up to 20% fines has produced successful mixtures. Foundry sand with organics may be suitable as well because low strength development is desirable. These flowable fills can be used in dry or moisture conditioned form.
Mix Specifications
Foundry sand can be the major ingredient in flowable fills. Water content varies from 50-200 gallons per cubic yard in most material combinations. Portland cement, normally from 50-200 pounds per cubic yard, is added to provide the weak cementitious matrix.
Although the upper compressive strength limit of flowable fills is roughly 1200 psi, it can be as low as 50 psi if desired (50-70 psi has about two to three times the bearing capacity of compacted earthen backfill). A large portion of the flowable fill mixes are designed for a 28-day maximum unconfined compressive strength of 100-200 psi. The goal is to support early loads without settling, while still being able to excavate at a later date.
Important Physical Characteristics of Flowable Fills and Foundry Sand
- Strength Development
- Directly related to water-cement ratio
- Water is added for flowability, and larger amounts will usually result in lower compressive strength
- The coarser the sand, the higher the bearing capacity of the hardened fill
- Flowability
- Mainly a function of water content and aggregate gradation
- Greater water content and a more uniform and rounded sand will produce an increasingly flowable mix
- A highly flowable mixture allows for maximum self-compaction (good)
- Time of Set
- A factor of the following: sand content, cementitious material content and type, water content, and current weather
- Construction equipment can generally move across the fill within 24 hours without apparent damage
- Time of set also a factor of the type of foundry sand used: green sand with low clay content and foundry sand that is chemically bonded usually require less water and take less time to harden
- Bleeding and Settlement
- This is possible in high water content flowable fills
- Caused by evaporation of bleed water, leading to settling
- Cracks on the surface can lead to water infiltrating later on
Research
In 1993 and 1994 demonstration projects on flowable fill were initiated by the Ohio Cast Metals Association (OCMA), Ohio Ready Mixed Concrete Association (ORMCA), Ohio Department of Development (ODOD), Ohio’s Edison Materials Technology Centers (EMTEC), and the Institute of Advanced Manufacturing Sciences in Cincinnati, Ohio. This effort included placing foundry sand flowable fills next to virgin sand flowable fills for comparison. These projects were an opportunity for all industry sectors to be exposed to the use of foundry sand in flowable fills (Smith 1996). Laboratory and field evaluations showed that flowable fills using foundry sand perform as well as conventional flowable fills. As a direct benefit from these projects, the county of Hamilton, Ohio approved the use of foundry sand in flowable fill applications, which generated a substantial market for foundry sand reuse. Duritsch (1993) presents technical information describing the use of foundry sand in flowable fills in Ohio. Stern (1995) presents a detailed history of the use of foundry sand in flowable fill applications in Ohio.
Javed and Lovell (1994) conducted a research project at Purdue University on foundry sand reuse. The project was sponsored by the Indiana Department of Transportation (InDOT). The objective of the project was to identify the physical characteristics of foundry sands and to determine possible beneficial reuse applications of foundry sand in construction. The leachability of foundry sands was also studied, and found to be comparable to natural soils (Javed and Lovell 1994).
Javed and Lovell also prepared flowable fill mixes using Class F fly ash, Type I cement, and foundry sand (Mast 1997). Mix designs were tested for spread (flowability), set time, and compressive strength. Flowability is a critical parameter in flowable fill design because it ensures that all void spaces in the desired cavity are filled by the flowable fill material (Mast 1997). The material stops flowing when the set time is reached. The Purdue study concluded that foundry sand mixes perform better than conventional flowable fill mixes (Javed 1994). In addition, Javed (1994) reports that the rate of strength gain was lower for foundry sand mixes than for conventional materials.
The project by Javed and Lovell was followed by a Joint Highway Research Project at Purdue University performed by Bhat and Lovell (1996). This project focused on the use of foundry sands in flowable fill applications. Mixes of Class F fly ash and foundry sands from three foundries were studied. Bhat and Lovell (1996) investigated the economy of foundry sand use, flow characteristics, hardening patterns, 28-day compressive strength, long-term strength, pore size distribution, hydraulic conductivity, pH of the pore water, and stress-strain characteristics. They conclude that flowable fill is an economic alternative to conventional compacted fills. They also conclude that mix design can be developed by trial and error depending on the physical characteristics of the materials. Bhat and Lovell (1996) also developed a step-by-step procedure for flowable mix design, and concluded that mixes containing up to 55.5% foundry sand can be used. Bhat and Lovell (1996) and Javed and Lovell (1994) conclude that Class F fly ashes and foundry sands from ferrous castings should be environmentally acceptable when used in flowable fill applications.
Naik and Shiw (1997) of the Center for By-Product Utilization (CBU) at the University of Wisconsin-Milwaukee have worked on developing flowable fill mix designs. Their early work involved the use of fly ash in concrete products such as flowable fills. Naik and Shiw (1997) prepared two reference flowable fly ash-slurry mixtures. These two mixes did not contain any sand. Class F1 fly ash was used for one mix and Class F2 fly ash was used for the other mix. Foundry sand was then added to these mixes. The mixtures using foundry sand as a replacement for fly ash were proportioned starting from the two reference mixes to obtain 28-day compressive strengths in the range of 350 to 700 kPa (50 to 100 psi). Naik and Shiw (1997) conclude that foundry sand can be used in flowable fills to replace up to 85% of the fly ash used in the reference mixes.
Another aspect of their study dealt with hydraulic conductivity, because more permeable fills have greater leaching potential. Results of the test program showed that a minimum hydraulic conductivity occurs when 30% of the fly ash is replaced with foundry sand. Flowable fills having 70% of the fly ash replaced with foundry sand do not have significantly different hydraulic conductivities (Naik and Shiw 1997). However, when 85% of the fly ash is replaced with foundry sand, the hydraulic conductivity increases dramatically. Low hydraulic conductivity is also correlated to water to cementitious material ratios ranging from 0.4 to 0.6.
Critical Questions
1. How are foundry sands being used in CLSM / Flowable Fill?
· As a fine aggregate:
o Used as a partial replacement for natural sands
o Mixed with water and cement (and sometimes fly ash) to create a self-leveling backfill that can be excavated at a later time
- What are the primary technical issues involved in using foundry sands in CLSM / Flowable Fill?
· Type of foundry sand affects physical properties
o Clay-bonded or chemically bonded
· Mix design
o Water content, amount of foundry sand, use of admixtures (fly ash and accelerators are most common) are all important for obtaining desired properties
- What are the performance issues relating to foundry sand in CLSM / Flowable Fill?
· Compressive strength
o American Concrete Institute defines CLSM as having maximum compressive strength of 1200 psi
o Foundry sand typically lowers compressive strength in CLSM mixes, keeping them under the limit
· Flowability
o High flow is desirable as the material is supposed to be self-setting
o Must be careful to avoid bleeding
· Setting time
o Can depend on type of foundry sand
o Accelerating admixtures can remedy slow setting
- Can foundry sands provide SUPERIOR performance to conventionally used materials in CLSM / Flowable Fill?
· If foundry sand contains rounded grain characteristics, it can aid in flowability
· Foundry sand typically lowers compressive strength, making excavation at a later time easier
· CLSM that contains foundry sands which used a bentonite binder system can reduce the need to add fly ash or other fines due to the presence of bentonite clay
- Are there minimum or maximum technical limitations to use of foundry sands in CLSM / Flowable Fill?
· Ratio of 1:3 to 1:1 of foundry sand to commercial sand is recommended
· CLSM that is intended to be excavated should have a compressive strength between 20-100 psi
· Permanent CLSM mixtures should be between 290-1200 psi
- Are there particular issues relating to QA/QC that the foundry and/or end user needs to be aware of?
· If bleeding is a critical factor, admixtures or air entrainment may need to be added
· Clay-bonded sands should be blended with rounded sands to aid in flowability
- What types of physical testing on the sands are needed for CLSM / Flowable Fill?
· Absorption
· Gradation
· Roundness for foundry sand
· Bulk specific gravity
- Are there any known environmental issues relating to foundry sand in CLSM / Flowable Fill?
· Sands from ferrous foundries have been found to be non-hazardous
· Some restrictions may apply for non-ferrous sands as some have been found to have metal leachate concentrations above RCRA standards