CONSTRUCTION AGGREGATES

Construction aggregates is a general term that includes crushed stone, sand, gravel, iron or steel slag, recycled asphalt pavement (RAP) or recycled concrete used to make portland concrete (PCC), bituminous concrete (asphalt), road base, railroad ballast, armor stone, concrete block/pavers, filter stone/gravel/sand, bedding sand and other construction materials. Most construction aggregates are consumed by the concrete and asphalt industry. Crushed stone is mined 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 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 forth by that industry. The aggregate mining industry remains the largest mining sector in the United States in terms of production by weight, volume and number of active operations. In 2022, the USGS reported aggregate production from 3,290 rock quarries and 6,200 sand and gravel pits. The leading states in crushedstone production were Texas, Florida, Missouri, Pennsylvania, Ohio, Georgia, North Carolina, Tennessee, Kentucky and Indiana. In 2022, the USGS estimated that 1.5 Gt of crushed stone was produced and valued at more than $21 billion. Specifications and markets Construction aggregate must meet the specifications of various state, federal and sometimes local public works departments or municipalities and public health departments for septic sand filtration to be used for projects by these agencies. Many of the physical testing required is set forth by ASTM and the American Association of State Highway Officials (AASHTO). Most state agencies base their specifications on ASTM and AASHTO. The Federal Aviation Administration (FAA) and the U.S. Army Corp of Engineers (USACE) have their own specifications for airport runways and taxiways, or 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 a judgement call by the engineers or geologists whether the armor will be rejected or approved. Railroads have specifications for ballast stone and traction sand. Traction sand must be dried so it is supplied by the industrial silica sand industry. A physical test required by state, federal and local regulators is the Los Angeles Abrasion (LA). The LA method covers the procedure for testing coarse aggregate 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 perhaps the tests that should be performed on coarse aggregate to determine suitability of the rock to meet specification before the many other tests that are needed are performed. 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-resistant (friction) aggregate, hardness and freeze-thaw, alkali silica or carbonate reactivity, and petrographic analysis. Skidresistant or friction aggregate generally requires all rock that passes physical specifications set forth 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. The residue left, if significant enough, may be used on the top course of less traveled roads. In recent years, the microdeval test has become popular in some states, especially for testing aggregate 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 the individual TXDOT regional engineer to decide whether to require a five-cycle sodium sulfate test or a fivecycle magnesium sulfate test on coarse aggregate. Magnesium sulfate is harsher on aggregate. In Oregon, a dimethyl sulfoxide (DSMO) test is used to test some aggregate, South Carolina has higher loss limits on the LA abrasion than most surrounding states, and the New York State Department of Transportation (NYDOT) Bureau of Materials requires a 10-cycle magnesium sulfate test on coarse aggregates. Construction aggregates’ markets can range from low-grade material such as fill, road base and miscellaneous products to washed and sized aggregate sold to the ready-mix concrete and asphalt producers. The market area is generally within trucking distances as trucks transport about 85 percent of aggregate in the United States. Rail and barge or ship together total about 15 percent. As an industry “rule of thumb,” trucks can generally travel 56 to 64 km before transportation costs increase significantly. These costs depend on many factors such as road conditions, 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 aggregate producers do not own their own trucks for delivery — many 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 have limited areas of service. The producer using rail generally owns or leases a siding on both production and a distribution end. Once the product is stockpiled in the distribution yard, extra costs include storage, loading and unloading. Those producers who have rail access generally serve the railroad ballast stone market as rail companies send their own rail cars to be loaded at that quarry. Barge and ship rates are generally lower than rail, and similar to rail service, the producer is generally on or near navigable water. A small river barge can carry approximately 1.4 kt to about 2.9 kt of aggregate. Larger barges can carry about 6,350 t. Generally, several river barges are grouped (tied) together and pushed by one 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. The American Bureau of Shipping (ABS) barges or ocean-going barges carry approximately 8.2 kt. Some of these barges deliver aggregate through the intercoastal waterways and the great lakes. Ocean-going vessels (ships) can carry approximately 32.6 kt and the larger Panamex ships can carry upward of approximately 54.4 kt of aggregate. The time it takes to unload or load the material (demurrage) and all other handling of material increase final costs at the dock. Construction aggregates are sold free on board (F.O.B.) quarry or sand and gravel pit. F.O.B. prices are listed at the scale house and provided by sales people. The F.O.B. prices do not reflect lower prices negotiated with contractors on a project, bid work, or those companies that buy in large volumes. When a producer is vertically integrated, its price for material to its asphalt plant, concrete batch plant or construction group may be the exact F.O.B. price or discounted. Where aggregate is needed, F.O.B. prices are generally higher than in areas where aggregate is abundant. For example, in intercoastal and coastal areas of the United States, prices for crushed coarse aggregate of any type are generally higher as these areas are devoid of quality or friction aggregate. Inland, where many quarries are located, prices tend to be lower because of competition. Quality aggregates are railed from the southeast Piedmont area to rail yards on the East Coast or shipped from the Bahamas, Nova Scotia and New Brunswick, Canada. In late 2022, 36 kt of construction-grade limestone was shipped from Jamaica to Savannah, GA. Quarries along the Mississippi barge both crushed stone and slag to ports along the southern Mississippi to the Gulf Coast along the intercoastal waterway. Aggregate is shipped from Mexico to the Houston, TX market. Rail shipments from Oklahoma also supplies the Houston and Dallas, TX market. Sand and gravel aggregates are shipped from Vancouver Island, Canada to the West Coast cities, especially to San Francisco, CA. In much of the Western United States, rail is used to supply distant markets. The results of some or all physical tests determine the marketability of the product. Aggregate that does not meet certain specifications is sold for lowgrade construction products or nonstate specification aggregate. Being an approved aggregate source by the state, federal or other approval agencies is an advantage in the marketplace. Sand and gravel as construction aggregate is used for many of the same purposes as crushed stone. Geology plays a significant 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 the Western states, and river deposits in the Mid-South are utilized. Where there are large deposits of rock, crushed stone is generally used. There are two exceptions, 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, each different lithology may absorb more/ less water or react differently to the cement or cement additives. In addition, liquid asphalt added to the asphalt mix may not adhere (strip) to certain aggregate types, such as quartzite, and large mineral crystals in the rock matrix having a vitreous or glassy luster. 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 2022, the USGS estimates 960 Mt of sand and gravel were produced in the United States valued at $10 billion. The 10 leading producing states were California, Texas, Arizona Minnesota, Washington, Utah, Michigan, Colorado, Ohio and New York. Most of the sand and gravel production in 2022 was used by PCC aggregates. These leading states have an abundance of mines in the 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 of these 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. Recycling 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 aggregate. The material does not meet state specifications for approved PCC or asphalt aggregate. The USGS reports that recycling is a small percentage of aggregate 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 buying 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 was 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 site and are blending washed concrete grade sand with RAP for filler material. RAP has an abundance of fine material and therefore the producer uses washed concrete sand to meet specifications on the finer sieves. In some areas of the country, concrete sand producers are providing washed sand to both ready-mix concrete and asphalt producers creating a shortage of specification concrete sand. Crushed glass in asphalt (glassphalt) replacing some aggregate 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. Since the aggregate 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 environmental concerns of local citizens, the fact remains that opposition from not-in-mybackyard (NIMBY) citizens deny or slow the process of permitting or expanding aggregate mines. In 2022, denials of permits continue in several U.S. states. All the major companies and most of the other aggregate producing companies have environmental/compliance managers who handle all environmental issues and permitting. Many use open houses or environmental days at their mine sites to educate the general public and school children about the importance of construction aggregates in everyday lives. Because about 85 percent of aggregate production is transported by truck, truck traffic remains a concern to local citizens and is difficult to mitigate. Dust, noise, blasting, wetlands issues and safety issues have through the years been largely mitigated in most operations. In 2022, Heidelberg Materials (formerly Leigh Hanson) acquired JEV Recycling, a concrete and asphalt recycler in Woodinville, WA. Vulcan Materials Co. purchased Syar Industries of Northern California, a recycler, aggregates, asphalt and ready-mix producer. Blue Ridge Construction Capital purchased Mathews Sand and Gravel and Edge Materials in North Carolina. York Building Products acquired Union Quarries in Carlisle, PA and Martin Marietta opened a new quarry in Virginia. The Mine Safety and Health Administration (MSHA) is continuing its inspections and enforcement of safety of the industry. Most large aggregate operations have health and safety managers on staff to help comply with MSHA regulations and welcome practical inspections. However, MSHA inspectors often “write up” a violation based on rule with little flexibility. Each aggregate mine is different, and a one-size-fits-all approach to safety does not always work. MSHA continues to draft rules on respiratory crystalline silica. This issue is of great concern to producers because most igneous, metamorphic and sedimentary rocks mined for aggregate contain some coarsely crystalline or cryptocrystalline varieties of quartz (SiO2), and since quartz is extremely resistant to both mechanical and chemical weathering, it is found in most unconsolidated sand deposits and gravel deposits. Outlook The industry and its outside prognosticators seem reluctant to speculate on aggregate market growth in 2023. Most report that it will be a good year. The large urban areas such as Atlanta, GA, Dallas/Fort Worth, TX, New York City, Los Angeles and San Francisco, CA, Miami, FL and other large cities always need large amounts of construction aggregates that drive the market. References are available from the author.

CONSTRUCTION AGGREGATES

Authored by:
Mark J. Zdunczyk

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