CONSTRUCTION AGGREGATES

Construction aggregates include a broad category of coarse- to medium-grained materials used in construction, including sand, gravel, crushed stone, slag, recycled asphalt pavement (RAP) or recycled concrete, and geosynthetic aggregates. They are generally raw materials that are extracted from pits and quarries and are consumed by the concrete and asphalt industry. Stone is mined and crushed from many rock types. The U.S. Geological Survey (USGS) lists the types of rock sold for construction aggregates as limestone, dolomite, marble, calcareous marl, shell, granite (granite, gneiss, granitic-type rocks), traprock (basalt, diorite, diabase), sandstone/quartzite, slate, volcanic cinder, scoria and miscellaneous materials. Some rock types such as shell, slate, calcareous marl, volcanic cinder and scoria generally cannot meet the specifications required in most states to be used in plain cement concrete (PCC) and asphalt mixtures. Volcanic cinder and scoria may be used as lightweight aggregate sources or in concrete blocks if the material meets the specifications set by that industry, although the percentage of total aggregates supplied by recycled materials remained very small in 2023. Production The aggregates mining industry remains the largest mining sector in the United States in terms of production by weight, volume and number of active operations. In 2023, the USGS estimates there was aggregates production from 3,500 rock quarries and more than 6,200 sand and gravel pits. The top 10 leading states in crushed stone production were Texas, Florida, Missouri, Pennsylvania, Ohio, Georgia, North Carolina, Kentucky, Tennessee and Illinois. In 2023, the USGS estimated that 1.54 Gt of crushed stone was produced, valued at more than $24 billion, a decrease in production of more than 0.6 percent compared with 2022 but an increase of 9.5 percent in value. Specifications and tests Construction aggregates must meet various state, federal, municipal and sometimes local public-works and public-health department standards for septic sand filtration applications for use in projects for these agencies. Much of the physical testing required is set by ASTM International and the American Association of State Highway and Transportation Officials (AASHTO). Most states agencies base their specifications on those of ASTM and AASHTO. The Federal Aviation Administration and the U.S. Army Corp of Engineers (USACE) have their own specifications for airport runways and taxiways, jetties and armor stone. One test for armor stone used by the USACE is the so-called “drop test.” This test requires one 3- to 5-t stone to be dropped about 9 m onto another 3- to 5-t stone. After the drop, the stones are inspected for open fractures, and large losses due to breakage. Generally, it is professional judgement by the engineers or geologists whether the armor stone will be rejected or approved. In addition, railroads have their specifications for ballast stone and traction sand. Traction sand must be dried, so it is supplied by the industrial silica sand industry. One of the physical tests required by some state, federal and local regulators is the Los Angeles (LA) abrasion test. The LA method covers the procedure for testing coarse aggregates for resistance to degradation. The five-cycle magnesium or sodium sulfate soundness tests determine an aggregate’s resistance to disintegration by weathering, particularly freezing and thawing cycles. The LA and soundness tests are initial tests that should be performed on coarse aggregates to determine suitability of the rock to meet specification before other required tests. Additional tests include deleterious material; particle size and shape; mineral contaminants, especially iron sulfide minerals and asbestiform/nonasbestiform minerals; specific gravity and absorption; acidinsoluble residue for skid-resistance (friction) aggregates; hardness and freeze-thaw; alkali silica or carbonate reactivity; and petrographic analysis. Aggregates for skid-resistant or friction applications generally require all rock that passes physical specifications set by the agency to be noncarbonate. Therefore, limestone or dolomite cannot be used on the top course of most roads. Some states have a specification on the residue left after a certain-size carbonate aggregate is placed in concentrated hydrochloric acid and dissolved. If the residue left is significant enough, the material may be used on the top course of less-traveled roads. In recent years the Micro-Deval test has become popular in some states especially for testing aggregates for the asphalt industry. Officials report that it is a more accurate test for aggregate quality and durability than the LA abrasion test. States require different tests, or limits on some tests to accommodate the rock or sand and gravel in their area. For example, the Texas Department of Transportation (TXDOT) allows an individual TXDOT regional engineer to decide whether to require a five-cycle sodium sulfate test or a fivecycle magnesium sulfate test on coarse aggregates (magnesium sulfate is harsher on aggregates). In Oregon, a dimethyl sulfoxide (DSMO) test is used to test some aggregates. South Carolina has higher loss limits on the LA abrasion test than most surrounding states, and the New York State Department of Transportation Bureau of Materials requires a 10-cycle magnesium sulfate test on coarse aggregates. Markets and transportation The results of some or all of the physical tests determine the marketability of the product. Aggregates that do not meet certain specifications are sold for low-grade construction products or as nonstate-specification aggregates. Being an approved aggregate source by the state, federal or other approval agencies is an advantage in the marketplace. Construction aggregates markets can range from low-grade material, such as fill, road base and miscellaneous products, to washed and sized aggregates sold to ready-mix concrete and asphalt producers. The market area is generally within trucking distance of the mine site, as trucks transport about 85 percent of aggregates in the United States. Rail and barge or ship transport together total about 15 percent. As an industry rule of thumb, trucks can generally travel 56 to 64 km (35 to 39 miles) before transportation costs increase significantly. These costs depend on many factors such as road conditions, weather, traffic, tolls, bridges, back hauls and weight laws. For example, relatively straight roads with limited traffic signals can allow trucks to carry material much farther than 64 km at low rates. Conversely, in areas near New York City, Atlanta, Dallas/Forth Worth, Los Angeles and other major cities, trucks are slowed by traffic congestion, traffic signals and tolls. Thus, freight rates are much higher. In these cases, many of the trucks charge an hourly rate. Most aggregates producers do not own their own trucks for delivery and use contract trucking companies. In recent years, the rule of thumb has changed as quarries and sand and gravel pits are being permitted farther from urban areas, where most of the material is consumed. Truck rates affect many operations on pricing and bidding projects. Rail rates are lower than trucking rates, but rail limits the areas of service. The producer using rail generally owns or leases a siding on both the production and distribution ends. Once the product is stockpiled in the distribution yard, extra costs can include storage, loading and unloading. Rail companies such as CSX that are in need of railroad ballast would get their material from producers that have railroad access presence on site. Many producers that have rail access generally supply the railroad ballast stone market, as rail companies send their own railcars to be loaded at that quarry. Barge and ship rates are generally lower than rail, and similarly with rail service, the producer is generally on or near navigable water. A small river barge can carry approximately 1,400 to about 2,900 t of aggregates. Larger barges can carry about 6,350 t. Generally, several river barges are grouped (tied) together and pushed by a tugboat. Producers on or near the Mississippi, Missouri, Ohio, Delaware, Potomac, Hudson and Columbia Rivers use river barges to deliver aggregates to markets in need of crushed stone and sand and gravel. American Bureau of Shipping (ABS) barges or oceangoing barges carry approximately 8,200 t. Some of these barges deliver aggregates through the Intracoastal Waterway and the Great Lakes. Oceangoing ships can carry approximately 32,600 t and the larger Panamex ships can carry approximately 54,400 t of aggregates. The time it takes to unload or load the material (demurrage) and all other handling of material increases final costs at the dock. Crushed stone, sand and gravel Sand and gravel as a category of construction aggregates is used for many of the same purposes as crushed stone. Geology plays a large role in determining the use of crushed stone or sand and gravel in geographic regions. In California, where large alluvial fan deposits are located, sand and gravel is widely used. Glacial deposits are generally mined in the Northeast and North Central states, colluvium and river deposits in Western states, and river deposits in the Mid-South. Where there are large deposits of rock, crushed stone is generally used. Natural sand is generally preferred over manufactured sand (stone crushed to sand size) by most ready-mix concrete and asphalt producers. Crushed stone is preferred over crushed gravel for the same use, if available. In both PCC and asphalt, crushed stone consists generally of one rock type while crushed gravel may contain several lithologies or rock types. In ready-mix concrete and asphalt, different lithologies may absorb varying amounts of water or react differently to the cement or cement additives. In addition, liquid asphalt added to the asphalt mix may not adhere to certain aggregate types, such as quartzite, and large mineral crystals in the rock matrix having a vitreous or glassy luster, and may strip. The producer may have to add antistripping chemicals, thus increasing costs. Sand for use in PCC has been increasingly difficult to find. Sand may seem ubiquitous, but many deposits of natural sand are too fine-grained to meet ASTM C-33 gradation specifications. In 2023, the USGS estimates that 920 Mt of sand and gravel were produced in the United States, valued at over $11 billion. The 10 leading producing states of sand and gravel were California, Texas, Minnesota, Arizona, Michigan, Utah, Washington, Colorado, Ohio and New York. Most of the sand and gravel production in 2023 was used by PCC aggregates. These leading states have mines in alluvial fans, colluvium, river deposits and glacial deposits. Most specifications required by states for crushed stone are the same for sand and gravel aggregates with a few exceptions; one is that natural gravel must be crushed to meet PCC and asphalt specifications. Natural gravel or gravel that is not crushed is generally used for landscape products, flat commercial roofing material and filtration media. This is generally low-volume, high-price material. Commodities that saw declines in November 2023 from November 2022 included crushed stone and sand and gravel, down approximately 8 percent year over year. Recycling and substitutes RAP and recycled concrete usage are becoming more prevalent in the industry. This material is generally crushed and sized for use as base materials and other low-grade construction aggregates. The material does not meet state specifications for approved PCC or asphalt aggregate. The USGS reports that recycling is a small percentage of aggregates consumption. Substitutes for natural aggregates such as iron or steel slag have quality and environmental issues. Steel slag can cause expansion cracks in concrete, and the stockpiling of slag may release pollutants over time. Some asphalt producers are recycling asphalt roofing shingles to limit or replace the buying of liquid asphalt for the mix. The oil from the shingles is extracted during the heating process at the plant and added to the mix. Sand is used as a filler in asphalt. The specifications are less rigid than sand used for PCC. One major difference is that asphalt sand is not washed. Many asphalt producers have stockpiles of RAP on their sites and are blending washed concrete-grade sand with RAP for filler material. RAP has an abundance of fine-grained material and therefore the producer uses washed concrete sand to reduce the proportion of finer material. In some areas of the country, concrete sand producers are providing washed sand to both readymix concrete and asphalt producers, creating more of a shortage of specification concrete sand. Crushed glass in asphalt (glassphalt) replacing some aggregates has gained acceptance as a substitute with some producers. Recycled tire rubber and cellulose fiber are also used in asphalt. The main issue with recycled materials in asphalt is implementing a proper and acceptable mix design to the customer. Issues Construction aggregates have many issues facing the industry. As the aggregates industry is the largest mining sector in the United States, the general public is exposed to their operations more than any other mining industry. Although the industry has come a long way in mitigating many of the environmental concerns of local citizens, the fact remains that opposition from NIMBY (not in my backyard) citizens denies or slows the process of permitting or expanding an aggregates mine. In 2023, denials of permits continued in several U.S. states. All the aggregates producing companies have environmental/compliance managers who handle environmental issues and permitting. Many use open houses or environmental days at their mine sites to educate the general public and schoolchildren on the importance of construction aggregates in everyday lives. Because about 85 percent of aggregates production is transported by truck, truck traffic remains a concern to local citizens and is difficult to mitigate. Dust, noise, vibrations, wetlands issues and safety issues have been largely mitigated through the years in most operations. Industry news Caterpillar and Luck Stone announced a collaboration to deploy Caterpillar’s autonomous solution to Luck Stone’s Bull Run Plant in Virginia. CRH Ventures is to explore autonomous hauling technology with SafeAI, with a pilot program at its most prominent facilities. Women in Aggregates is a new group dedicated to supporting women working in the aggregates industry. Aggregates producers are encouraging employee participation in this group and fostering their mission. In September 2023, Summit Materials announced a deal to purchase Cementos Argos’ operations in the United States, combining Summit and Argos North America Corp. Holcim US continued to strengthen its position in Colorado with its third acquisition (Tezak) in the rapidly growing state. California-based Granite Construction acquired the Brunswik Canyon quarry and asphalt plant in Carson City, NV, and also acquired Memphis-based Memphis Stone & Gravel along with its partner company Lehman-Roberts. SGI Acquired Panamint Valley Limestone, based in California. BMC Enterprises Inc., the largest producer of aggregates in the St. Louis region, acquired Raineri Building Materials, Eureka Materials and Blue Grass Ready Mix. CRH has completed its acquisition of a portfolio of cement and ready-mix concrete assets from Martin Marietta Materials. SRM Concrete announced the strategic acquisition of Vulcan Materials Co.’s ready-mix assets in Texas. Overall, all the public companies reported at least 5 percent growth in revenues for 2023. Private, nonreporting companies are also ready to participate in merger-and-acquisition opportunities. For example, the new Armada Materials, which was formed with the backing of private equity group Kelso & Co., acquired Tennessee-based Volunteer Materials. Producers remain confident in the industry despite inflation, elevated interest rates and other dynamics that could otherwise limit the growth of their businesses. Health and safety The Mine Safety and Health Administration (MSHA) continues its inspections and enforcement of safety in the industry. Most large aggregates operations have health and safety managers on staff to help comply with MSHA regulations and welcome practical inspections. However, MSHA inspectors often cite violations based on rule with little flexibility. Each aggregates mine is different and a one-size-fitsall approach to safety does not always work. MSHA drafted a rule on respiratory crystalline silica that would require mine operators to maintain miners’ permissible exposure limit at or below 50 μg/ cm3 of air for a full-shift exposure, calculated as an eight-hour time-weighted average. This issue is of great concern to producers because most igneous, metamorphic and sedimentary-type rocks mined for aggregates contain some amount of coarsely crystalline or cryptocrystalline varieties of quartz (SiO2), and because quartz is extremely resistant to both mechanical and chemical weathering, it is found in most unconsolidated sand deposits and gravel deposits. Although 2023 was a good year in terms of construction aggregates production, the industry had its deadliest year in a decade in 2023, as 40 miners died in accidents across the United States. The 40 fatalities were the most in a single year industrywide since 2014. MSHA provided details about the fatalities in reports on its website, attributing the accidents to nine general causes. It characterized 16 of the fatalities as involving machinery, with 10 classified as powered haulage accidents. Combined, machinery and powered haulage accounted for 65 percent of mine fatalities in 2023. The other fatalities were classified in seven different ways: electrical accidents (3); slip or fall of a person (3); drowning (2); fall of roof or back (2); falling, rolling or sliding rock or material of any kind (2); fall of face, rib side or highwall (1); and handling material (1). Outlook Forecasters seem reluctant to speculate on aggregates market growth in 2024 but most report that it is predicted to be a good/decent year. The industry continued to address health and safety regulations, permitting and zoning issues, and environmental restrictions in 2023. The 2021 Infrastructure Investment and Jobs Act reauthorizes surface transportation programs for five years and invests in additional funding to repair roads and bridges and support major, transformational projects. Shortages of construction materials in some urban and industrialized areas are expected to continue to increase owing to local zoning regulations and land-development alternatives. These issues are expected to continue and to cause new crushed-stone quarries to be located away from large population centers. Resultant regional shortages of crushed stone and higher fuel costs could result in higher-thanaverage price increases in industrialized and urban areas. Large urban areas such as Atlanta, GA; Dallas/ Fort Worth, TX; New York City, NY; Los Angeles and San Francisco, CA; Miami, FL and other metro areas always need large amounts of construction aggregates and this will continue to drive the market.

CONSTRUCTION AGGREGATES

Authored by:
Venkata Aneesh Kona

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To view the past issues of the UCA magazine you must be a member of SME/UCA.

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