Making Top Quality Manufactured Concrete Sand
Natural sand is produced by natural processes including rock ground under a heavy burden of glacial ice over millions of years of time. This sand is most often found in and around rivers and streams. Many of the best sources of sand and rock are used to build homes, buildings, and highways. Sand on ocean beaches is contaminated with salt from the sea and sand from the desert is the wrong size to be used for concrete or asphalt. Transportation plays a significant role in the delivered price of sand to the consumer – truck haul cost is typical $ .25/ ton/mile whereas unit train costs are approximately $.10/ton/mile.
Sand extraction and usage is over 50 billion tons per year. Sand & gravel and concrete associations from all over the world forecast shortages of sand for concrete now and in the future. The USA, Canada, Australia. England, Europe, China and many other countries are hauling sand long distances to meet their concrete needs. Concrete mix designs have about 1.6 tons of aggregate per cubic yard of concrete. Mix designs very from 40 to 45 percent sand and many pump mixes use up to 50 percent sand. The average sand use is about .75 tons (1.5 tons x .45) per cubic yard of concrete. It is important to note that even if the sand meets the ASTM specification, it needs to have an FM of 2.5 to 3.00.
(The Finus Modulus (FM) is a rating of the fineness or coarseness of concrete sand. You start by looking at the gradation of the concrete sand with cumulative percent retained on each of the sieve sizes used in the gradation. The FM is the total percent retained on each of the following US sieve sizes: #4, #8, #16, #30, #50 and #100. To find the FM, add the cumulative percent retained of each of the sieve sizes and divide by 100. The results will vary between 2.5 and 3.5. The higher the number the courser the sand. For good quality concrete, the sand will not only fall into the range requirements of each sieve size established by the ASTM C33 requirements.
The FM should be in the range of 2.5 to 3.0. When the FM is greater than 3.00, it can affect the flow ability of the plastic concrete, lead to segregation of the motor from the sand and affect the finish ability of the surface of the concrete. When replacing natural concrete sand with manufactured sand keeping the FM below 3.0 is important since the natural grain sizes in the rock have a tendency to produce coarser sand.)
By-Product-Sand, Crusher Dust, Crushed Rock Fines
The by-product sand produced when crushing large rock of 6” plus using jaw crushers or horizontal shaft impact crushers as a primary crusher or cone crushers used to crush rock from 6” to 1” can be classified to meet the C-33 sand specification. The shape will have a large percentage of flat and elongated material. When tested by ASTM C1252, it will have a high voids numbers of 46 plus, indicating a high percentage of flat and elongated material. In cases in which the crusher fines are mixed with natural fines, the ratio is normally 70 percent natural fines and 30 percent crusher fines. The product can be used as concrete sand with a little negative effect. This material can be used as asphalt sand as-is. In a quarry application where there is no natural sand, it would not be suitable as concrete sand because of its poor particle shape.
You can, however, run this material through a vertical shaft impact crusher with rotor and rock box to improve the particle shape and then classify to the C-33 specification to use as Manufactured Sand.
During Classification approximately 50 percent of the #4 x #8, 75 percent of the #200 x 0 and 50 percent of the #100 x #200 is rejected.
Manufactured Concrete Sand, Manufactured Sand, etc.
Manufactured sand needs to be capable of replacing natural sand in concrete without any negative side effects. Two of the most common complaints regarding manufactured sand are a high of FM and excessive flat and elongated material in the finished product. For most materials, it means re-crushing part of the #4 by #8 material or wasting it. The first step in the production of manufactured sand is crushing the rock to a size by removing the excess material passing the #100 and #200 sieve. This will meet the C-33 sand specification on all the other sieves (#4, #8, #16, #30 and #50). Normal manufactured sand will result in sand with less than 80 percent passing the #8 sieve before classification and 70 to 75 percent after classification (less than the 80 to 100 required by the specification. This also results in a higher than 3.00 FM--one of the less than desirable manufactured sand conditions.
When using a compression-type crusher such as the cone crusher or high-pressure grinding role crusher (HPGR), the crushing is autogenous type which is rock on rock crushing. In autogenous crushing, the crusher receives the new feed plus the recirculating load with 75 to 100 percent less than the crusher-closed side setting. This forces the crushing action to be many layers of rock to rock with inter-particle commination. This produces a product with good quality shape.
More accurate results of the paddle abrasion test can be found by running the following test. A vertical shaft with a paddle of known weight is rotated through a metal pan holding 400 grams of rock ½” x ¾”. The paddle is run for 15 minutes and 4 separate tests are run with 4 separate paddles. By measuring the weight loss of the paddle, you use a wear-chart to establish an Abrasion Index number.
Highly Abrasive material have an AI of .75 to 3.00.
Abrasive material has an AI of .25 to .75.
Mildly Abrasive material has an AI 0f .05 to .25.
Nonabrasive material has an AI of .01 to .05.
Best wear analysis can be established by running a pilot test crushing 10,000 t of rock to sand size and determining the wear life in that application, with crusher type and material type. High wear cost is not the problem if you can incorporate the cost into the selling price of the material. As sand becomes in shorter supply, the price will increase making it possible to manufacture man-sand or replace natural sand. What can be a problem is not knowing the wear cost before hand.
Sand Production Crusher Wear Cost
The chemical analysis and the paddle abrasion test can be used to help predict the wear cost of the crusher used. By finding the chemical content of the calcium carbonate (CaCo3), silicon dioxide (SiO2), iron oxide (Fe2O3), aluminum oxide ((Al2O3) and magnesium carbonate (CaCO3), the silica equivalent number can be established. The Silica Equivalent Number = the percent of (CaCo3) + (SiO2) + 2 x [(FeO3) + (Al2O3)] + (MgO).
Example Silica Equivalent Number = (15 %) + (40 %) + 2 x [ (12 %) + (15 %)] + (3 %) = 112
The higher the number, the more abrasive the feed material. When the paddle abrasion test is not known, you can estimate the result with Abrasion index = .6 x Silica Equivalent number.
Other tests that can be used to help predict crusher wear cost are:
Compressive Strength (Mega pascal x 145 = PSI)
Specific Gravity (for aggregate + 2.65)
Los Angeles Abrasion Test (Range of 10 to 50)
Paddle Abrasion Test (1.5 to .02)
Metso Crushability (CR) 10 to 50 + and Abrasiveness (ABR) (0 to 2500)
ASTM C1252 Particle Shape Test
The ASTM 1252 Voids test uses a relationship of measured voids in non-compacted sand and the particle shape. The test consists of putting a dried sand sample into a container of known volume and comparing the calculated weight of the material using the specific gravity of the sand sample to the measured weight in the test. Natural sand with a result of 35 to 45 percent voids and are considered acceptable for concrete sand. Sand with results of 45 to 50+ percent are acceptable for asphalt mixes. Natural sand with no crushed sand particles added will give results of 35 to 40 percent and manufactured sand with crushed material produced by a compression crusher in autogenous crushing (the feed material, new feed plus recirculating load, is smaller than the crusher closed side setting) will produce a sand product with 40 to 45 percent voids.
New Zealand and ASTM C939 flow cone tests can also be used to measure the particle shape of sands. The more alluvial the material with rounded particles the faster the material will flow through a funnel with fixed opening.
Another way of decreasing the effect of particle shape on the concrete mix is to replace 50 percent of the natural sand with manufactured sand.
By-product sand can be used as concrete sand if you pass it through a VSI rotor and rock box as a shaping operation. It is recommended to send a sample of the material to determine if a single pass in an open circuit will improve the ASTM C1252 test results enough to consistently provide acceptable results.
International Manufactured Sand Specification
In most countries outside the USA, manufactured sand is used on a regular basis and a specific specification has been adopted that allows for more passing the #50, #100 and #200 sieves in the specification. This change in gradation will also lower the FM by .15, in most cases, and reduce the amount of plus #8 that needs to be added to keep the gradation greater than 80 percent passing the #8 sieve.
For most of the concrete work that is performed for federal and state regulated projects, the manufactured sand must be classified to the C-33 specification. The mix could consist of 50 percent manufactured sand and 50 percent natural sand if particle shape requires it. The classified manufactured sand can meet all the requirements of the C-33 specification. A separate set of mix designs could be developed that would include the use of the international manufactured sand specification by reducing the minus 200 mesh to less than 10 percent with air classification.
More frequently, sand is hauled in from long distances to satisfy demand and the haul cost frequently approaches the FOB price at the quarry or pit. To test a source material to manufacture sand for concrete, first test the existing crusher discharge material and determine the size fractions. Unlike normal crushing where the feed material is greater in size than the crusher setting, the crusher discharge gradation will change with the setting. The sand gradation, #4 x 0 will increase in percentage of the crusher discharge but the sand gradation between #4 and 0 will remain about the same. No two sources will produce the same gradation and sometimes different elevations in the quarry may produce different results. Manufactured sand (#4 x 0) tends to produce a coarser sand with a high percentage of #4 x #8 material.
This is the most difficult size to crush and in most cases, the crusher discharge will contain 25 to 35 percent of #4 x #8 fraction when crushed to #4 x 0 product. To produce a finished product that meets the C-33 specification, 80 to 100 percent passing the #8 sieve the excess #4 x #8 sieves to be either rejected or send back for re-crushing into the recirculating load. The goal is to produce a finished product with a target gradation of dry crushed sand that will allow only the removal the excess #100 and #200 material during classification to meet the product specification. The classified sand should meet the concrete sand specification and keep the FM (Fineness Modules) under 3.00. Sand with a FM of plus 3.00 is too coarse and leads to problems with finish ability, segregation of the mortar and the workability of the plastic concrete. The material size and gradation to feed the crusher to produce the manufactured sand must be determined next.
It is recommended to crush some of your material through a pilot plant for two or three months to establish the gradations, production rate and wear cost before investing in larger size equipment at higher production rates. Alternatively, rent a portable plant that can be setup at the plant site and try different crusher types, such as cone crusher, HPGR, VSI rotor and rock box and VSI rotor and anvil, hammer mill or double reversing cage mill with different gradations of material and material. After crushing the sand to minus #4 the sand must be re-crushed to address the excess amount of #4 x 8 that is typically present (normally 30%) and the specification only allows 15 to 20 percent. Many concrete producers that have used manufactured sand in the past have had bad experiences because the sand they were furnished had a combination of poor particle shape, coarse gradation, and FM above 3.00 to 3.5.
Aggregate & Mining Consultants, LLC
Applications Engineer Crushing, screening, and mineral processing.