September 2002
Demonstration Eases Sand Concerns
Agencies, associations, contractors, and suppliers participate in paving project using manufactured sand and high-fines concrete
by Mary Mccaig
Manufactured sand has a reputation of being difficult to work with in concrete. A demonstration project sponsored by Vulcan Materials’ Midwest Division at its Plainfield, Ill., quarry, helped ease some concerns of contractors, suppliers, and industry association officials.
With natural sand deposits dwindling, Chicago-area road contractors and their concrete suppliers face a problem that is becoming all too common in certain parts of the country. “We’re simply out of natural sand here,” said Melvin Kirchler, Illinois Department of Transportation (IDOT) District 1 mixtures control engineer.
Transportation costs to haul natural sand from farther locations are increasing. But what else can be done? An obvious answer is to use stone sand — manufactured sand — in concrete mixes.
There are a number of benefits to using stone sand. For example, when suppliers of crushed stone coarse aggregate also supply sand from the same mineral, it creates a homogenous concrete mixture. The appropriate use of manufactured sand can improve edge slump control during slip form paving. With proper water/cement ratios, concrete strengths may actually be higher with greater amounts of manufactured fines in the mix than with natural sand. And, of course, manufactured sand can fill the need in areas of the country where natural sand is becoming less available.
But use of manufactured sand has raised concern in the past for producers and contractors due to its reputation for being difficult to work with. In addition, there are a number of unknowns yet to be addressed with the use of manufactured sand in portland cement concrete (PCC).
In demonstration projects co-sponsored by Vulcan Construction Materials L.P. (Midwest Division) July 5 and Aug. 30, 2001, at its Plainfield, Ill., quarry, contractors, suppliers, and industry association officials found answers to some of these unknowns.
Vulcan co-sponsored the event with IDOT, K-Five Construction Corp. (paving contractor), Holnam (portland cement and slag supplier), Ozinga Illinois (central mix concrete plant located on site), and W.R. Grace & Co. (admixture supplier). Also participating in the demonstration were Airport Owners Resources; the American Concrete Paving Association; county and municipal officials; the Federal Highway Administration; the Illinois Association of Aggregate Producers; the Illinois Ready Mix Concrete Association; the International Center for Aggregates Research; the National Stone, Sand & Gravel Association; and Vulcan Materials’ Technical Services Laboratory. Invitations also were extended to area producers and contractors. More than 145 industry professionals attended the demonstration.
Vulcan Materials’ Plainfield, Ill., quarry is located southwest of Chicago. The quarry produces crushed dolomite, with a washing operation on site. It was originally a sand and gravel operation until the sand supply was depleted. Crushed stone has been produced for only a year.
During its busy season, the Vulcan Plainfield quarry handles about 200 to 250 customer trucks per day. During the winter months that number decreases by about half. The demonstrations involved paving the Plainfield quarry access road. In a preliminary event July 5, 2001, baseline mixtures contained natural sand and carbonate manufactured stone microfines for their fines content. Aug. 30, mixtures using high contents of manufactured fines were adjusted from the July 5 experience. Extensive testing followed in the months after the event.
Long-term Solutions
According to Mike Rorex, general supervisor for K-Five Construction, “We are looking for a manufactured sand (mix) that will hold up in the long-term. And while it’s true, if we can save money we become more competitive, performance is the key word.”
Rorex said that soon contractors will be required to assume responsibility for “design-mix-install-warranty” road projects. Contractors now can produce pavement with life expectancy exceeding 30 years. Soon that figure will jump to 40 years.
“We would specify our mixes in a heartbeat (versus the IDOT specifying the mixes),” he said. “We know what we’re doing and what works. Our experience gives us the confidence to take these projects on — and we will warranty it. Right now, we are responsible for our mixes with no control over their specifications. Naturally we would prefer responsibility with control over the mixes.”
Either way, Rorex said there are no secrets among pavers and concrete producers when it comes to mix designs. They are dependent upon each other — and upon IDOT, which would still have to approve mix designs even if they were developed by the paving contractors. And that’s where the level of cooperation for the Plainfield Quarry demonstration was born.
“For this project, even though it is on a private road, IDOT is here to see if we can take this mix design for public use,” said George Houston, P.E., IDOT mixture control area supervisor. “This site is ideal for this demonstration because there is a central lab here where we can conduct testing for durability (freeze/thaw), salt solubility, and more. It’s necessary due to the economics of using natural sand in this area that we conduct research on manufactured sand. But with the new material, new guidelines are also necessary.”
The Plainfield Quarry paving is important because of the number of people who have provided input, and the hands-on opportunity for aggregate producers, cement suppliers, concrete producers, and paving contractors to work together and be part of the process and testing, according to IDOT’s Kirchler. “We will have a better chance that these companies will buy into the technology and accept it, which is absolutely necessary,” said Kirchler. “We need continued cooperation like this to develop new products.
“This (high-fines mix design) could provide a way for us to control costs,” continued Kirchler. “And we can maintain a quality product that lasts longer.”
Concerns about Manufactured Sand
Possibly the largest obstacle participants had to overcome in the Plainfield demonstration was a prejudice against the use of manufactured sands in concrete mixes. Manufactured sand is regarded by some as being difficult to work with. The particles are more angular, and have raised concerns among the concrete producers and pavers about their tendency to bridge in aggregate bins and batch plants, and to catch or tear under the trowel during hand finishing.
“In addition, water demand appears to be a bit higher with 100 percent manufactured sand (than with natural sand),” noted Charles Sanders, director of Technical Services (Midwest Div.) for Vulcan Materials. “However, some additional water can be tolerated because of the higher strength potential of these mixtures. The traditional slump test is a very poor indicator of workability with manufactured sand. Alternatives such as rheological measurements to properly assess the fluidized state of the concrete need to be pursued.”
According to Sanders, the primary unknowns in the use of manufactured fines with PCC include freeze/thaw resistance implications, set time considerations due to the higher water demand in conjunction with different admixtures and types of cementitious material, and amounts and types of water reducers needed to achieve workability when producing the concrete in a central mix plant versus a laboratory setting. “Admixture manufacturers will play a vital role in the future and viability of these mixtures,” said Sanders.
Testing would determine the effects of various types of cementitious materials with various manufactured sands and typical concrete paving admixtures. The testing and effects noted would include:
• Ease of handling raw materials
• Charging and mixing times in the plant
• Discharge and placement at the paving operation
• Finishability
• Set times versus ambient temperature
• Strength progressions
• Other physical properties of the hardened concrete
• Durability
Tests performed on site and during the following months included air content (taken on the first series of mixes, then every three to four trucks), water-cement ratios, slump, strength, and durability/freeze-thaw.
Trial By Paving
The July 5, 2001, trials tested two mixtures. Mix 1 used natural sand; Mix 2 used manufactured sand. These are considered baseline trials.
“The intent of the project was to incorporate a typical natural sand that has traditionally been used in one trial and compare it with two different manufactured sand gradings in other trials,” said Sanders.
Sand products incorporated into Mix 1 included FM-02 natural sand approved by IDOT for PCC use. In the first mixture trial, a FM-02 control section was placed using three IDOT-approved cementitious materials (ternary mixtures), including Type I portland cement; Class C fly ash; and ground, granulated blast furnace slag. In the second mixture trial, an FM-21 manufactured sand was used with the same three cementitious materials. The sand had a maximum limit of 18 percent minus 200 mesh. It was not approved by IDOT for PCC use, but met criteria for “A” quality sand and having all fines made up of mineral dust of fracture.
Sand products incorporated into Mix 3 Aug. 30, 2001, included FM-21 manufactured sand, using Type I portland cement and Class C fly ash as the cementitious materials. The mortar factor and water/cement ratio was lowered based on the experiences with Mix 2. In Mix 4, an FM-20 manufactured sand (maximum limit of 8 percent minus 200 mesh; approved by IDOT for PCC use) was used with Type I portland cement and ground, granulated blast furnace slag. Mix 5 used FM-20 manufactured sand and a combination of Type I portland cement, Class C fly ash and the slag.
Managing the water/cement ratio was the key to success with the manufactured sand mixes, according to Mark Korduck, quality control and technical service manager for Ozinga. The batch plant did not experience bridging problems with the manufactured sand.
“Physical flow through the plant was a concern for us because of the angularity of the stone sand, and the higher amount of fines being used in these mixes overall,” said Korduck. “But it has not been an issue for us so far. We may find there could be a one-time cost involved with bulk handling of the manufactured fines, but this cost is small compared to the ongoing, long-term costs of transporting natural sands — especially when we have access to the manufactured sands in our back yard…literally.”
The demonstrations confirmed that the manufactured sand has more friction. “Contractors have to work with it to compensate for the additional surface area, but they can do it,” said Randall Riley, P.E., engineering consultant with the Illinois Chapter of the American Concrete Pavement Association.
Riley said that in many respects, use of manufactured fines is causing industry professionals to relearn technology from 50 years ago. “The asphalt people were quick to take the belly of the gradation curve for their product, and the concrete industry was left with more coarse aggregates,” Riley said. “(Manufactured fines) are just giving us back the full curve of materials we once had. Any time we can fill the gaps with something other than cement — the most expensive component in the mix — we’re ahead. The contractors are saving money.”
In spite of increased labor initially, Riley predicted labor requirements will decrease as contractors and suppliers learn to work with manufactured sand. “And consider the alternative of hauling natural sands for many miles,” he said.
It was predicted the strength of the mixes using the high content of manufactured sand should rise. The curing curve for natural sand is initially steep, but then it levels off. With manufactured sand, the curing curve is more gradual — curing takes more time — but the final strength is greater.
Testing at the Plainfield demonstration site after the Aug. 30 trials indicated the following:
Mix 1: The control mixture, which included natural sand, had the lowest Relative Dynamic Modulus of Elasticity after 300 cycles of freezing and thawing. It also had the worst salt scaling rating at 60 cycles. Compressive strengths were the highest at 28 days and flexural strengths were the highest and equaled Mix 5 (see below) at 14 days.
Mix 2: The second mixture on the first day of paving was the first attempt with the highest level of micro-fines for the FM-21 manufactured sand. The higher mortar factor (the volume of mortar per volume of dry-rodded coarse aggregate) caused a very sticky mix that was difficult to finish without the addition of excess water. As expected, Mix 2 had the lowest strengths of all the mixes. Despite this, the compressive strength at seven days exceeded the IDOT specification minimum (3,500 psi) set for 14 days. The flexural strength was nearly equal. This mix and the other FM-21 mix had the highest freeze/thaw resistance (300 cycles) of all the fly ash mixtures. Salt Scaling Rating at 60 cycles was better than the control mix (above), but poorer than the other manufactured sand mixtures.
Mix 3: This FM-21 mix had high levels of micro-fines and was similar to Mix 2. The biggest difference is that the mortar factor was dropped from 0.90 to 0.78, based on the experience with Mix 2. Water demand dropped from a water/cement ratio of 0.56 to 0.47. At five days, the compressive strength rose to nearly the specification level and the flexural strength increased to more than 18 percent above the IDOT minimum 14-day requirement. Freeze/thaw durability at 300 cycles exceeded Mix 1 and Mix 5. The Salt Scaling Rating at 60 cycles was much better than Mixes 1 and 2 and equaled Mix 5. It was only slightly lower than Mix 4. The high amount of mineral fines helped this mix close and finish well. Some of the concrete finishers said this mix finished better than all the others, except for Mix 1 with natural sand.
Mix 4: This mixture was the only one created with ground granulated blast furnace slag instead of fly ash. The manufactured sand was an IDOT-approved FM-20 gradation. Both compressive and flexural strengths at five days exceeded the IDOT minimum 14-day requirement. This also was the only mix that exceeded IDOT minimum freeze/thaw expectations of 80 percent at 300 cycles. It also had the best Salt Scaling Rating at 60 cycles. The mix placed and finished well at an average water/cement ratio of 0.48. The motar factor was reduced from 0.90 to 0.83.
Mix 5: This was the third ternary mix and had FM-20 manufactured sand. The mortar factor was reduced from 0.90 to 0.83. The mix had the second best water/cement ratio at 0.46, although it was above the theoretical ideal ratio of 0.42. At 14 days, Mix 5’s compressive and flexural strength exceeded that of all of the other manufactured sand mixes. Mix 5 equaled the flexural strength of, but showed a lower compressive strength than, Mix 1 at 14 days. Once start-up problems were resolved, this mix stabilized, placed, and finished well.
In the end
The manufactured sand posed no major problems in handling through the central mix plant or in placing and finishing. Everyone acknowledged that the natural sand mixtures were the easiest to work with, which is not surprising because it is the material with which the people involved have the most experience. Manufactured sand mixtures were most homogeneous at 75 seconds mixing time, which is the minimum time required under IDOT specifications.
One of the greatest difficulties was convincing people that high micro-fines mixtures could be placed and finished adequately at less than 1-1/2 in. of slump. It was equally difficult, according to Sanders, to convince people that slump determinations above that level were meaningless and not an adequate measure of water of convenience. Large amounts of water are needed to show increases in slump above that level, he said.
These trials prove that manufactured sand can be used successfully in PCC paving mixtures. Some of the idiosyncrasies that arose in these trials can be easily dealt with, Sanders said. Others will require further research. He recommended that before further use of ternary mixtures, government and industry professionals must learn the ramifications for cases where properties of any of the three cementitious materials could vary.
Probably the biggest success of the Plainfield demonstration is that it drove away many doubts about the viability of using manufactured sand in PCC mixtures — and fears of failure in trying.
| More Fines Info
Initial research, which led to the the Vulcan High Fines Concrete demonstration, was conducted by the International Center for Aggregates Research (ICAR) and the National Stone, Sand & Gravel Association (NSSGA). ICAR’s mission is research, education, and the advancement of knowledge through conducting scientific and technical research related to aggregates. For more information about the use of high fines and manufactured fines in concrete, contact Charles A. Pryor, Jr., P.E., NSSGA vice president of engineering, at (800) 342-1415; email: cpryor@nssga.org. Pryor is also the NSSGA’s technical liaison for ICAR. |
| Project Parameters
The Plainfield pavement was designed to sustain 10 million ESALs (18,000 lb. equivalent single axle load) over a 20-year period, and is configured as follows: |
Mary McCaig is a freelance writer who specializes in the construction materials industries.