How Crushing Concrete and Asphalt Works
Key takeaways:
- Crushing concrete and asphalt makes construction aggregate from recycled materials, reducing landfill waste
- Mobile crushing machines can process materials on-site for immediate reuse as aggregate
- Jaw crushers, cone crushers, and impact crushers break down concrete and asphalt in stages
- Screening and blending ensures aggregate meets specifications for size, shape, and composition
- Recycled concrete aggregate is sustainable, cost-effective, and performs similarly to virgin aggregate
The Importance of Sustainable Aggregate Supply
Construction projects require vast amounts of aggregate – crushed rock, sand, gravel – that form the base layers for buildings, roads, runways, and infrastructure. Traditionally, virgin materials from quarries and gravel pits provide aggregate. However, supplies are limited. Quarrying also causes land disturbance and habitat loss.
In the past decades, construction waste recycling has emerged as an important source of aggregate. Billions of tons of concrete and asphalt become construction debris each year after buildings and infrastructure are demolished or renovated. Crushing this waste makes aggregate for reuse in new construction projects.
Recycled concrete aggregate (RCA) and recycled asphalt pavement (RAP) offer sustainable, cost-effective solutions for the aggregate supply chain. More jurisdictions now allow high percentages of RCA and RAP in place of virgin aggregate. The recycled materials must meet strict technical specifications.
Crushing and processing equipment lets you transform solid waste from demolition projects into reusable aggregate products. With the right machines and workflow, you can recycle concrete and asphalt efficiently on-site, so it can be reused immediately as backfill, road base, or in concrete mixes.
Understanding Concrete and Asphalt Composition
Before operating processing equipment, it helps to know the composition of the materials you’ll be crushing.
Concrete consists of a gravel aggregate embedded in a cement and water paste binding matrix. The typical mix proportions by weight are:
- Aggregate: 60-75%
- Cement: 10-15%
- Water: 15-20%
Natural aggregate makes up most of the concrete volume. Crushed stone chips, gravel, and sand are common concrete aggregates. After concrete is crushed, these materials are recovered and sorted for size. The cement paste is discarded as slurry or separated for other uses.
Asphalt is made of aggregate coated with a bituminous binder. Hot mix asphalt concrete is typically:
- 95% aggregate (crushed stone, gravel, sand)
- 5% asphalt binder
The asphalt binder acts as the glue that holds the aggregate together in the pavement mix. During crushing, the asphalt coating breaks apart from the aggregate. The aggregate can be reused, while the asphalt can be recycled for the liquid asphalt or as a low-grade aggregate.
Both concrete and asphalt have high compressive strength. But the materials are brittle with low tensile strength. Cracking develops due to loading, weathering, and environmental factors over time. This makes the materials ideal feedstock for crushing equipment.
Mobile Crushing Enables On-Site Recycling
Traditionally, concrete and asphalt had to be hauled from demolition sites to a crushing facility. This required extra transportation costs and time.
With mobile crushing machines, the recycling process happens at the demolition site. The equipment crushes the rubble then screens, sorts, and stockpiles the recovered aggregates on-site. The material can be reused immediately, eliminating landfill fees and additional trucking.
On-site recycling with mobile equipment offers many benefits:
- Lower transportation costs since aggregate doesn’t have to be hauled away from site
- Reduced vehicle emissions from transportation
- Immediate reuse of crushed materials in the construction project
- Eliminates tipping fees for landfilling debris
- Saves costs by substituting recycled aggregate for virgin materials
- Provides sustainable construction practices with closed-loop recycling
Mobile jaw crushers, cone crushers, and impact crushers process concrete and asphalt quickly and efficiently. These machines can operate independently or combine as crush and screen trains to produce precise aggregate sizes and specifications.
Diesel-powered units operate on their own without external power. Track-mounted machines maneuver around sites and can crush material in place. Wheeled crushing plants are towed from site to site or set up in permanent locations.
When electricity is available, electric-powered crushers offer an even more eco-friendly crushing solution. They have advantages like lower noise and zero emissions. Stationary electric units can directly plug into the grid if adequate power is available.
Size Reduction in Stages in The Crushing Process
Crushing breaks down the concrete and asphalt into reusable aggregate pieces. The process typically happens in 2 or 3 stages to reduce the material to the ideal size, shape, and composition for the intended application.
Here is an overview of common crushing stages:
- Primary Crushing
- Secondary Crushing
- Tertiary Crushing
Primary Crushing
The first stage breaks larger chunks of concrete or asphalt into 3-4” pieces. This can be done at the demolition site with mobile equipment. Or at the landfill, where front-end loaders feed material into a hopper with a primary crusher.
Jaw crushers are often used for primary crushing. The jaws have a V-shaped opening with a fixed and moving jaw. Material enters the jaws and is crushed between the concrete surfaces as the jaws move back and forth. This action breaks pieces off the concrete by compression.
Jaw settings determine the size of the crushed product. The distance between the jaws narrows as the material moves through the chamber. This allows consistent reduction of the pieces as they descend. Jaw crushers have high production capacity and the wearing parts are easy to replace.
Impact crushers use striking to shatter concrete and asphalt chunks. The material enters a chamber where a rapidly spinning rotor propels it against steel aprons. The force of the impact breaks up the pieces. Impact crushers produce a nice cubical aggregate shape.
Cone crushers squeeze material between an oscillating cone head and a fixed bowl. The final aggregates exit at the bottom when they achieve the target size. Cone crushers are effective across all crushing stages and for crushing hard, abrasive materials. The cone maintains consistent discharge particle size by compensating for wear.
Primary crushing produces a manageable feed size for additional grinding. It also removes concrete and asphalt pieces unsuitable for the final product like metals and odd shapes.
Secondary Crushing
The second stage further reduces the material to 1.5” size or smaller using a secondary crusher. Impact crushers or cone crushers are used at this stage to achieve the particle size and gradation required for the final product.
A scalping screen removes any oversized material before the secondary crusher. This ensures the machine is not overloaded. Scalping also increases the capacity of the plant to process maximum tonnage.
Secondary crushing pulverizes the concrete and asphalt chunks into smaller pieces. The materials may pass through additional screening after secondary crushing to remove fines, soils, metal, and other contaminants. Air classifiers separate fine particles. Magnets pull out any steel pieces from reinforcing bars.
The secondary crushing stage produces uniform, cubical aggregate at the desired size to meet specifications. For road base, this is usually around 1 – 1.5” particles. Concrete sand involves even finer crushing.
Having an impact crusher for secondary crushing is beneficial. Impact crushing creates the nice aggregate shapes and gradation for concrete applications. The crusher’s ability to shape aggregate can reduce the harshness of concrete while providing better workability and long term strength.
Tertiary Crushing
A third stage of crushing and screening may also be required, especially for concrete sand making.
Tertiary crushing can create very fine aggregate sizes down to 3/16” or less. Vibrating screen decks with small mesh openings remove the fines. Sand washing equipment can further clean the material and classify it into consistent grades for concrete mixes.
At the tertiary stage, precise gradation control is necessary. Crushed fines and dust can be moved from screening decks to stockpiles to prevent buildup. This keeps the screening equipment from blinding or clogging and reduces product loss.
Mobile Crushing Improves Recycling Economics
On-site recycling of concrete and asphalt can provide an income stream for demolition contractors while producing much needed aggregate. Mobile equipment improves recycling business profitability in many ways:
- Eliminates landfill and dump fees
- Lowers project costs by immediately reusing crushed product on-site
- Reduces need for purchasing virgin aggregate
- Allows demolition contractors to offer additional aggregate supply services
With the right equipment, used concrete and asphalt can be turned into income-generating products instead of costly waste. Mobile crushers and screener trains process materials to create precisely sized and graded aggregate on-site. The recycled products can be reused immediately to offset purchases of aggregate for construction projects.
Recycled Concrete Aggregate (RCA) Performance
With processing, demolished concrete can be turned into recycled concrete aggregate for reuse in building projects. This keeps concrete debris out of landfills and reduces the use of virgin aggregate.
However, there have been questions around whether RCA performs as well as natural aggregates in concrete mixes. Extensive research demonstrates RCA can be used reliably in many applications:
- Unbound base and subbase layers under foundations, roads, and parking lots
- As aggregate in lean concrete construction
- For non-structural precast concrete products and blocks
- Up to 100% for low-grade pavements and shoulders
- 10-30% mix with virgin aggregate for standard ready mix concrete
The ideal RCA application depends on aggregate size and quality. Smaller RCA works for making concrete mixes and precast products. Larger RCA recycled from pavements or other high-strength concrete is suitable for road base and fill material.
With good materials management and advanced crushing processes, the properties of concrete made with RCA compare well with natural aggregate concrete. Studies found comparable compressive strength, modulus of elasticity, flexural strength, and abrasion resistance.
The attached mortar and cement particles from the original concrete can increase water absorption. With adjustments to the mix design, this does not impact performance. Adhered mortar also gives older, weathered RCA superior bonding properties that improve the strength of fresh concrete.
Recycled Asphalt Pavement (RAP) Uses
Like concrete, asphalt pavement can be recycled back into usable products after it has served its useful life. The reuse of RAP provides aggregate for new asphalt mixes and other construction needs.
According to the Asphalt Pavement Alliance, over 100 million tons of RAP are reused in new asphalt pavements each year. The Federal Highway Administration permits up to 50% RAP in new asphalt pavement mixes.
With proper processing, RAP offers many advantages:
- Reduces costs of paving mixes by replacing virgin aggregate
- Lowers energy use and emissions by decreasing need for extracting and transporting virgin aggregate
- Conserves landfill space by reusing old asphalt instead of disposal
In addition to asphalt mixes, RAP can be used for:
- Road base and subbase material
- General fill or embankments
- Shoulders and dikes
- Driveway gravel
- Cold patch mixes
The aggregate quality and shape, asphalt binder content, and gradation of RAP determine its best uses. We quantify these properties through extraction and recovery testing when designing crushing operations. This ensures the product meets application specifications.
