Handbook Requirements

EPA Concrete Washout Handbook (May be revised at any time)

Minimum Measure: Construction Site Stormwater Runoff Control

Subcategory: Good Housekeeping/Materials Management

Description of concrete washout at construction sites

Concrete and its ingredients
 
Concrete is a mixture of cement, water, and aggregate material.  Portland cement is made by heating a mixture of limestone and clay containing oxides of calcium, aluminum, iron, magnesium, sodium, and silicon in a kiln and then pulverizing the resulting clinker.  The fine aggregate particles are usually sand less than ¼ inch in size.  Coarse aggregate is generally gravel or crushed stone ¼ inch or larger.  When cement is mixed with water, a chemical reaction called hydration occurs, which produces glue that binds the aggregates together to make concrete.  Supplementary cementitious materials can be used as a portion of the cement.  These include fly ash generated by coal burning power plants, slag cement produced from making iron, and silica fume particles captured from gases generated during the production of silicon or ferro-silicon metals.  By recycling these by-products, concrete producers can reduce the load on already burdened landfills and also make higher quality concrete.

Concrete washout

After concrete is poured at a construction site, the chutes of ready mixed concrete trucks and hoppers of concrete pump trucks must be washed out to remove the remaining concrete before it hardens.  Equipment such as wheelbarrows and hand tools also need to be washed down.  At the end of each work day, the drums of concrete trucks must be washed out.  This is customarily done at the ready mixed batch plants, which are usually off-site facilities, but large or rural construction projects may have on-site batch plants.  Cementitious washwater and solids also come from using such construction materials as mortar, plaster, stucco, and grout.

Environmental and Human Health Impacts

Concrete washout water (or washwater) is slurry containing toxic metals.  It has a pH near 12 and consequently is caustic and corrosive (Drano Liquid Clog Remover has a pH of 13.5).  Safe habitat pH ranges are 6.5 – 9 for freshwater aquatic life and 6.5 – 8.5 for marine aquatic life.  This caustic washwater can harm fish gills, eyes, and reproduction.  If the washwater is dumped on the ground [Fig. 1] or leaks out of washout containers [Fig. 2], it can run off the construction site to adjoining roads and enter roadside storm drains, which discharge to surface waters such as rivers, lakes, or estuaries.  Rainfall may cause concrete washout containers that are uncovered to overflow and transport the washwater to nearby surface waters.  Rainwater polluted with concrete washwater can also percolate down through the soil and alter the soil chemistry, inhibit plant growth, and contaminate the groundwater.  Its high pH can increase the toxicity of other substances in the surface waters and soils.  Construction workers should handle wet concrete and washout water with care because it may cause skin irritation and eye damage.

Figure 1.  Chute washwater

Figure 2. Chute washwater leaking from being dumped on the ground a bin being used as a washout container

Best Management Practice Objectives

The best management practice objectives for concrete washout are to (a) collect and retain all the concrete washout water and solids in leak proof containers, so that this caustic material does not reach the soil surface and then migrate to surface waters or into the ground water, and (b) recycle 100% of the collected concrete washout water and solids.  Another objective is to support the diversion of recyclable materials from landfills.  Table 1 shows how concrete washout materials can be recycled and reused.

a. Fine particles of cementitious material (e.g., Portland cement, slag cement, fly ash, silica fume).
b. Recyclable, if allowed by the concrete quality specifications.
c. Recycled after being treated to reduce the pH and remove metals, so it can be delivered to a municipal    
wastewater treatment plant.

Table 1.  How concrete washout materials can be recycled

Washwater recycling

Washwater from concrete truck chutes, hand mixers, or other equipment can be passed through a system of weirs or filters to remove solids and then be reused to wash down more chutes and equipment at the construction site or as an ingredient for making additional concrete.  A three chamber washout filter is
shown in Figure 3.  The first stage collects the coarse aggregate.  The middle stage filters out the small grit and sand.  The third stage has an array of tablets that filter out fines and reduces the pH.  The filtered washwater is discharged through a filter sock.  An alternative is to pump the washout water out of the washout container [Fig 4] and treat the washwater off site to remove metals and reduce its pH, so it can be delivered to a municipal wastewater treatment plant (MWTP), which provides additional treatment allowing the washwater to be discharged to a surface water.  However, the MWTP should be contacted to inquire about their pretreatment requirements and National Pretreatment Standards for Prohibited

Figure 3. Concrete washout filter	Figure 4. Vacumming washwater out of a washout container for treatment and reuse	Figure 5. Gel the water with Aqua-Solution granules in 5 minutes. Recycle with concrete.

                                         
Discharges (40CFR 403.5) before discharging the washwater to the MWTP.  The washwater can also be retained in the washout container and allowed to evaporate, leaving only the hardened cementitious solids to be recycled. Another alternative is to gel the washwater with Aqua-Solution. Broadcast the Aqua-Solution granules into the water and it turns to gel in about five minutes. Lift the gel out of the watertight container for recycling. [Fig. 5]

Solids recycling

The fine and course aggregate materials that are washed off concrete truck chutes into a chute washout bucket [Fig. 5]] can be either placed in aggregate bins to be reused as fresh concrete ingredients or returned to the truck’s drum to be taken back to the ready mixed plant and washed into a reclaimer [Fig. 6].  When truck drums are washed out into a reclaimer, the fine and course aggregates are separated out and placed in different piles or bins to be reused in making fresh concrete [Fig. 7].  Reclaimers with settling tanks [Fig. 8] separate cement fines from the washwater, and these fines can also be used in new concrete unless prohibited by the user’s concrete quality specifications. Biodegradable filter bags can also be used with a watertight vinyl container by lifting out the hardened concrete in the filter bag and taking it to a recycler. Remaining water that has been filtered into the vinyl container permeates up throug the last filter bag, gelled with Aqua-Solution, and recycled along with the hardened concrete. The gelled water will never go back to liquid form, helping the concrete recycler with dust control [Fig10]

Figure 8.  Reclaimer showing bins for and coarse cement aggregates	Figure 9.  Reclaimer showing settling tanks for separating fines from washwater	Figure 10.  Biodegradable Filter Bags

                                                     
                  
Hardened concrete recycling     

When the washwater in a construction site concrete washout container has been removed or allowed to evaporate, the hardened concrete that remains can be crushed [Fig. 9] and reused as a construction material.  It makes an excellent aggregate for road base and can be used as fill at the construction site or delivered to a recycler.  Concrete recyclers can be found at municipal solid waste disposal facilities, private recycling plants, or large construction sites.

Figure 9.  Crushed concrete stockpile and crusher

Wet concrete recycling

Builders often order a little more ready mixed concrete than they actually need, so it is common for concrete trucks to have wet concrete remaining in their drum after a delivery.  This unused concrete can be returned to the ready mixed plant and either (a) used to pour precast concrete products (e.g., highway barriers, retaining wall blocks, riprap), (b) used to pave the ready mixed plant’s yard, (c) washed into a reclaimer, or (d) dumped on an impervious surface and allowed to harden, so it can be crushed and recycled as aggregate.  However, unused wet concrete should not be dumped on bare ground to harden at construction sites.

Washout Containers

Different types of washout containers are available for collecting, retaining, and recycling the washwater and solids from washing down mixer truck chutes and pump truck hoppers at construction sites.

Chute washout buckets

After delivering ready mixed concrete and scraping the last of the customer’s concrete down the chute, the driver hangs the washout bucket shown in Figure 10 on the end of his truck’s chute.  Then he washes down the chute into the bucket to remove any cementitious material before it hardens.  After washing out the chute, he pumps the washwater, sand, and other fine solids from the bucket up into the truck’s drum to be returned to the ready mixed plant, where it can be either recycled into the next batch or washed into a reclaimer at the end of the day.  A removable screen at the bottom of the washout bucket prevents course aggregate from entering the pump.  This course aggregate can also be returned to the plant and added to the coarse aggregate pile to be reused.  All the materials are recycled.

    Figure 10.  Chute washout system                                       
Hay bale washout pits with a plastic lining

A washout pit made with hay bales and a plastic lining is shown in Figure 11.  Such pits can be dug into the ground or built above grade.  The plastic lining should be free of tears or holes that would allow the washwater to escape.  After the pit is used to wash down the chutes of multiple ready mixed trucks and the washwater has evaporated or has been vacuumed off, the remaining hardened solids can be broken
up and removed from the pit.  This process may damage the hay bales and plastic lining.  If so, the pit will need to be repaired and relined with new plastic [Fig. 12.  When the hardened solids are removed, they may be bound up with the plastic lining and have to be sent to a landfill, rather than recycled.  Recyclers usually accept only unmixed material.  If the pit is going to be emptied and repaired more that a few times, the hay bales and plastic will be generating additional solid waste.  Ready mixed concrete trucks can use hay bale washout pits, but concrete pump trucks have a low hanging hopper in the back that may prevent their being washed out into bale-lined pits.    

                         
Figure 11.  Hay bale and plastic                                   Figure 12.  Leaking washout pit that has not been
washout pit                                                                      well maintained

                                           
Vinyl washout Containers

The vinyl washout containers shown in Figure 13 is portable and easier to install than a hay bale washout pit.  The filter bag [Fig.  14] assists in extracting the concrete solids, while the vinyl container is reusable.  When the biodegradable filter bag is lifted, the water is filtered out and the remaining concrete solids and the bag can be disposed of or recycled together.   Once the vinyl container is full of waste water, the last filter bag is placed into the vinyl container allowing the water to permeate up in the filter bag. Aqua-Solution is broadcasted into the water. The water turns to gel in about 5 minutes. The filter bag is then lifted and recycled in the same manner as the concrete solids.

Figure 13.  Vinyl washout container with filter bag

Figure 14.  Extracting the concrete solids and/or gelled water

Metal washout containers

The metal roll-off bin shown in Figure 15 is designed to securely contain concrete washwater and solids and is portable and reusable.  It also has a ramp that allows concrete pump trucks to wash out their
                    
Figure 15.  Mixer truck being           Figure 16.  Pump truck using the              Fig 17.  Delivering hardened
washed out into a rolloff bin             ramp to wash out into a rolloff bin            concrete to a recycler, rather
                                                                                                                                                  than a landfill

hoppers  [Fig. 16].  Roll-off providers offer recycling services, such as, picking up the roll-off bins after the washwater has evaporated and the solids have hardened, replacing them with empty washoff bins, and delivering the hardened concrete to a recycler [Fig. 17].  Some will vacuum off the washwater, treat it to remove metals and reduce the pH, so it can be accepted by a municipal wastewater treatment, and deliver it to the treatment plant for additional treatment, so it can be discharged to surface water.  Everything is recycled.

Another metal, portable, washout container, which has a rain cover to prevent overflowing, is shown in Figure 18.  It is accompanied by an onsite washwater treatment unit, which uses a forced weir tank system to remove the coarse aggregate, fine aggregate, and cement fines.  The washwater can then be reused at the construction site to wash out other mixer truck chutes and equipment.  The solids are allowed to harden together and can be taken to a concrete recycler to be crushed and used as road base or aggregate for making precast products, such as retaining wall blocks.  All materials are recycled.

 
Figure 18.  Washout container with rain
cover and filter system

Green Star Participation

The Green-Star program of the National Ready Mixed Concrete Association has been designed to support the efforts of the ready mixed concrete industry that lead to continual environmental management improvement.  Participating facilities are required to develop and implement an Environmental Management System.  The program serves as a mechanism for recognizing those within the industry who have achieved or are actively working towards significant reduction of environmental impacts by following the principles of sustainability.  Today more types of customers favor doing business with organizations that are known to be protective of the environment, and they consider doing so to be a genuine best management practice.

The antithesis of a best management practice is shown in Figure 19—hardened and wet concrete being dumped into a canal that flows into the Caribbean on an island in the Bahamas near Nassau within view of the Atlantis Hotel.

Figure 19.  Dumping concrete into the Caribbean