The Production and Use of Recycled Concrete in the USA Thomas Van Dam, Ph.D., P.E., FACI Principal Engineer CTL Group
Introduction In the United States, concrete is the most commonly used recycled material 140 million tons annually Multiple Uses Erosion protection (rip-rap), fill, and embankment Unbound aggregate layers Aggregate in new asphalt and concrete 4
Why Use Recycled Concrete? It makes economic sense Recycled aggregates are cheaper than natural aggregates In urban areas natural aggregates are becoming scarce It makes environmental sense Minimizes the mining of new material and the disposal of waste Reduces carbon footprint It makes societal sense Less societal impact, better for communities 5
What is Recycled Concrete? Reclaimed construction debris that can be used as rubble or aggregate in new construction Demolished structures or pavements Returned concrete Recycled concrete aggregate (RCA) is typically classified by size Fine RCA or coarse RCA Will focus on coarse RCA 6
Production of RCA Step 1: Separation of contaminants Mainly a concern if RCA to be used in new concrete Contaminants include asphalt, sealants, bricks, building waste Step 2: Concrete breaking and removal Broken into manageable sizes Loaded onto trucks and transported Note that it may be recycled in-situ
Potential Contaminants Contaminant How Removed Effect on PCC Mixtures with RCA Reinforcing steel and dowel bars On grade Electromagnet during No effect (steel effectively removed) crushing operations Oil Not removed Small quantity believed to have no effect Joint sealant Soil/base course materials Removed prior to demolition by some; not removed by others Careful loading operator Scalping screen ahead of primary crusher Small quantity believed to have no effect May introduce clay balls in mix, reducing PCC strength Building Rubble Limited by Specification Small quantity believed to have no effect 8
HMA Removal: Cold-Milling 9
Concrete Breaking Photos ACPA EB043P
Concrete Removal Photos ACPA EB043P 11
Removal of Embedded Steel Photo ACPA EB043P 12
Transporting to Crusher Photo ACPA EB043P 13
Alternative: In- Place Recycling Photo by Steve Gillen, Illinois Tollway Authority
Alternative: In- Place Recycling Photo by Steve Gillen, Illinois Tollway Authority
Production of RCA (Continued) Step 3: Crushing and Sizing Various crushers can be used Same basic equipment as used to process natural aggregate Modifications can be made to increase efficiency Most RCA crushing plants use a primary and secondary crusher Primary reduces particles to 8-10 cm Secondary crusher creates desired maximum size 16
Crusher Types Jaw crushers use large steel plate to compress concrete against stationary plate Often used as primary crusher Cone crushers uses eccentric rotating cones to trap and crush concrete against housing wall Most often used as secondary crusher Impact crushers use rotating heavy steel bars to impact concrete and hurl it against break plates Removes most mortar but produces most fines 17
Material Prior to Crushing
Illustration of Crushing and Sizing Operation Pavement rubble Hopper feeder Primary (jaw) crusher Sizing screen Spring supported conveyer Magnet for rebar salvage Secondary crusher stacker Sizing screens Typical sizes 38 mm max 19 mm max stacker stacker 19 10 mm max
Stockpile Management Run-off can be highly alkaline (ph from 9 to 10) Will dissipate within weeks Run-off not considered hazardous Secondary cementing will occur in stockpile Fine RCA stockpiles may bind together Coarse RCA must be washed and kept moist Plant must be set up with sprinklers if used in concrete 20
Stockpiled Coarse RCA
Coarse RCA
Properties of RCA Crushing operation produces both fine and coarse particles The properties of the RCA is controlled by the reclaimed mortar (hydrated cement paste, unhydrated cement, and sand) which is inherently porous The finer the RCA, the higher the reclaimed mortar The more reclaimed mortar that is present, the larger the impact on RCA properties Absorption and specific gravity significantly affected
Properties of RCA Property Natural Aggregate RCA Particle Shape and Texture Well rounded, smooth (gravels) to angular and rough (crushed stone) Angular with rough surface Absorption Capacity 0.8 3.7 percent 3.7 8.7 percent Specific Gravity 2.4 2.9 2.1 2.4 L.A. Abrasion Test Mass Loss 15 30 percent 20 45 percent Sodium Sulfate Soundness Mass Loss Magnesium Sulfate Soundness Mass Loss 7 21 percent 18 59 percent 4 7 percent 1 9 percent Chloride Content 0 1.2 kg/m 3 0.6 7.1 kg/m 3
RCA Applications: Base or Subbase Use same specifications as for natural aggregate Can have high fine content will actually selfcement If fine RCA is used, will not drained and can clog up drainage systems If used for drainage layer, only use coarse RCA Might consider stabilizing with cement or asphalt
Cement-Treated RCA Drainage Layer
RCA Applications: Asphalt Mixtures The angularity provides good mixture stability The rough surface texture results in good skid resistance Coarse RCA is more commonly used than fine RCA High absorption requires additional asphalt binder and stockpiles must remain dry Must meet same quality requirements as natural aggregate
RCA Applications: New Concrete Can be effectively used as aggregate in new concrete Most often, coarse aggregate RCA only (retained on the No. 4 sieve (4.76 mm) When fine RCA used, blended with natural fine aggregate Must be aware of and compensate for changes in properties Strength, thermal expansion, shrinkage MUST keep coarse RCA stockpiles moist during batching
Expected Changes in Fresh Concrete Properties Property Range of Expected Changes Compared to Concrete Made with Natural Aggregate Coarse RCA Only Coarse and Fine RCA Workability Similar to slightly lower Slightly to significantly lower Finishability Similar to more difficult More difficult Bleeding Slightly less Less Water Demand Greater Much greater Air Content Slightly higher Slightly higher From ACPA EB043P)
Expected Change in Hardened Properties Property Range of Expected Changes Compared to Concrete Made with Natural Aggregate Coarse RCA Only Coarse and Fine RCA Compressive Strength 0 to 24 percent less 15 to 40 percent less Tensile Strength 0 to 10 percent less 10 to 20 percent less Strength Variation Slightly greater Slightly greater Modulus of Elasticity 10 to 33 percent less 25 to 40 percent less Thermal Expansion 0 to 30 percent greater 0 to 30 percent greater Drying Shrinkage 20 to 50 percent greater 70 to 100 percent greater Creep 30 to 60 percent greater 30 to 60 percent greater Permeability 0 to 500 percent greater 0 to 500 percent greater Specific Gravity 0 to 10 percent less 5 to 15 percent less
Paving with RCA Concrete
Concluding Remarks Recycling concrete make economic, environmental, and social sense A sustainable practice Processes exist to effectively break, crush, and size RCA for various purposes Properties of RCA are controlled largely by the amount of reclaimed mortar For new asphalt and concrete mixtures, it is most common to use coarse RCA
Concluding Remarks Success requires paying attention to differences between RCA and natural aggregates Fine RCA stockpiles must remain dry Coarse RCA stockpiles for new concrete must be maintained in a moist condition Impact on mixture properties must be accomodated With care, the use of recycled concrete is a viable alternative to natural materials
Resources American Concrete Pavement Association (ACPA). 2009. Recycling Concrete Pavements. Engineering Bulletin 043P. American Concrete Paving Association, Skokie, IL. Thomas Van Dam, Ph.D., P.E., Principal Engineer, CTL Group tvandam@ctlgroup.com