Concrete and Mortar Washout on Residential Jobs

Why this matters

Every concrete or mortar job ends with tools, a wheelbarrow, a mixer and often a truck chute that need rinsing, and that rinse water is not dirty water. It runs around pH 12 or higher, close to the strength of a drain cleaner, carrying fine cement particles and trace metals. Down a storm inlet it reaches a creek untreated and kills what lives there; on a lawn it leaves a dead patch; on bare skin it burns. It is also the single most visible stormwater violation a residential crew can commit, because gray water in a gutter is unmistakable. A lined washout set up before the truck arrives costs about half an hour and a sheet of plastic.

Who regulates it, briefly

The federal floor is the Clean Water Act. Construction sites disturbing an acre or more need a construction stormwater permit, which requires managed washout, and EPA's guidance for those sites sets the practices most local rules copy, including keeping washout at least 50 ft from storm drains, ditches and water. Most residential jobs are under an acre and outside that permit, unless the lot sits inside a subdivision still under development, where the developer's permit can cover your work on that lot and its washout rules bind you there. Everywhere else the rule that reaches a small job is the city or county stormwater ordinance, which in most places prohibits washout water in the street, gutter or storm drain outright. The two-pipe system and the enforcement are covered in their own card. See related: Wash Water and the Storm Drain Rule.

The method, followed on one job

The steps below are the method. After each one, the same job shows what it looked like in practice: a 10 x 12 ft back patio poured 4 in thick, which is 10 x 12 x (4 / 12) = 40 cu ft, about 1.5 cu yd, from a ready-mix truck, finished by a three-person crew on a house whose driveway slopes to a curb inlet.

1. Decide the washout before the truck is scheduled

Choose one of three setups. A prefabricated washout bag or container, which is lined, quick, and hauled away or left to harden and lifted out whole. A lined above-ground frame or pit, built from boards or straw bales and plastic. Or no chute washout on site, where the ready-mix driver carries a washout bucket or bag and takes everything back to the plant. Then place it: on level ground the truck and crew can reach, at least 50 ft from any storm inlet, ditch or water, and out of the path of vehicles. Washing the drum itself is the plant's job; only the chute and hopper rinse happens on site.

Why first: once the truck is backing up, the crew washes wherever is closest, and on most residential lots closest is the driveway that drains to the street.

On the patio job: the driver confirmed when the pour was booked that the drum goes back to the plant and the chute is rinsed on site. The crew chose a lined board frame on the side lawn, measured 60 ft from the curb inlet, which clears the 50 ft distance.

2. Size it for the water plus 4 inches of freeboard

Estimate the rinse volume and give it room. The shop default is to keep 4 in of freeboard (empty height between the water and the top of the liner) at all times, so a rainstorm or a careless dump does not overflow it. A gallon is about 0.134 cu ft, so 7.48 gallons fill one cubic foot.

Why: an overflowing washout is a discharge with extra steps, and the overflow runs downhill toward the same inlet you set it up to protect.

On the patio job: the crew's own record from past pours gave an estimate of about 10 gallons for the chute and hopper rinse and 9 gallons for tools (three buckets at about 3 gallons), so 10 + 9 = 19 gallons, and 19 / 7.48 is about 2.5 cu ft. A 4 x 4 ft frame of 2x10 boards stands about 9.25 in tall; with 4 in of freeboard the usable depth is 9.25 minus 4 = 5.25 in, or about 0.44 ft, so 16 sq ft x 0.44 ft is about 7 cu ft, roughly 52 gallons. That holds the 19 gallons with room for rain.

3. Build it so it cannot leak

Clear the base of rocks and sticks, set the boards or bales square and stake them, and line it with a single sheet of 10-mil polyethylene, with no seams and no holes, lapped over the top of the frame and pinned or weighted outside. Walk the liner before use for punctures.

 SECTION THROUGH AN ABOVE-GROUND WASHOUT

  liner lapped over the frame, pinned outside
  |                                        |
 +--+                                    +--+
 |  | . . 4 in freeboard, kept empty . . |  |
 |  |   ~~~~~~~ washout water ~~~~~~~    |  |
 |  |   ####### settled fines #######    |  |
 |  +------------------------------------+  |
 +------------------------------------------+
  frame: 2x10 boards or bales, staked
  liner: one sheet 10-mil poly, no seams
  base: level, cleared, 50 ft from inlets

Why: a pinhole lets high-pH water into the soil, and a seam lets it run out under the frame where nobody sees it until the grass dies.

Hazard: stakes go in by hand; if they will reach below the topsoil, your state's one-call (811) notice rules apply before you drive them.

On the patio job: one sheet, no seams, stakes at the corners driven about 12 in, inspected and found clean.

4. Brief the driver and the crew on one rule

Every drop of concrete or mortar rinse goes into the washout, and nothing goes on the ground, the driveway or the street. Tell the driver where the washout is before the chute swings; tell the crew that wheelbarrows, screeds, floats and the mixer drum all rinse there too.

Why: a driver new to the site will rinse the chute where the truck stops, out of habit, in about thirty seconds.

On the patio job: the lead walked the driver to the frame on arrival. The chute rinse went in at the end of the pour.

5. Scrape, then rinse, and keep the cement off skin

Scrape tools and the wheelbarrow back into the pour or into the washout before any water touches them, then rinse in buckets and pour the buckets into the washout. A mason's mortar mixer and tubs follow the same rule. Brick cleaning with acid is a different job with an acid runoff, and it gets its own containment.

Hazard: wet cement and washout water burn skin slowly and without much warning, and cement's chromium content causes an allergic dermatitis that gets worse with every exposure. Wear alkali-resistant gloves (nitrile or butyl, not cotton or leather), safety glasses, long sleeves, and waterproof knee pads for kneeling on fresh concrete; tuck pant legs over boots so it cannot get inside. Wash any contact off promptly with clean water. Splash in the eyes is flushed for 15 minutes at the jobsite water supply and then seen by a clinician.

On the patio job: tools scraped into the last of the pour, three buckets rinsed and emptied into the frame. One finisher took a splash on a forearm, washed it off within a minute, no burn.

6. Watch the freeboard and the weather until it is gone

Check the water level at the end of each day. If it is within 4 in of the top, stop adding to it and add capacity (a second bag or frame). Cover it with a tarp when rain is forecast.

Why: the washout stays a hazard until it is dry, and the days after the pour are when nobody is looking at it.

On the patio job: end of day one, the 19 gallons (about 2.5 cu ft spread over 16 sq ft, so about 0.16 ft or 2 in deep) leave 9.25 minus 2 = 7.25 in of freeboard, over the 4 in minimum. Rain was forecast for the second night, and the crew covered the frame with a tarp. The check on the morning of day two found the tarp off on one corner and about 1 in of rainwater added, for about 3 in of water and 9.25 minus 3 = 6.25 in of freeboard, still over the minimum; the tarp was re-weighted with blocks.

7. Let the water evaporate or set, and do not pour it out

Leave it to evaporate, which in warm weather takes days, or use a washout product that solidifies the liquid. Do not pump it to the street, the lawn or a ditch, and do not add acid to "neutralize" it in the field, which produces a new chemical hazard and rarely lands at a safe pH. If the job ends before it dries, the options are a hauler or vacuum truck, or a sanitary sewer connection only with the sewer utility's written permission.

On the patio job: by day four, when the crew returned to strip forms, the water had evaporated, leaving a hard gray crust of fines on the liner.

8. Remove the solids without making dust

Lift the liner by its edges and fold it around the hardened solids. Hardened concrete goes to a concrete recycler as clean material or out with construction debris; the liner goes to the trash. If the solids need breaking to fit a bag, wet them first and break them with a sledge.

Hazard: breaking or scraping hardened concrete releases respirable crystalline silica, the lung hazard covered for construction work by 29 CFR 1926.1153. Keep it wet, break it outdoors and upwind, never dry-sweep or blow it with compressed air, and anyone whose task calls for a respirator wears one only within the shop's written program under 29 CFR 1910.134; a crew with nobody in the program does not break dry material at all.

On the patio job: the crust came up in two pieces inside the folded liner with no breaking needed, and went into the debris load. See related: Construction and Demolition Debris: What Goes Where.

9. Leave the ground as you found it

Pull the stakes, rake out the base, and look at the turf where the frame stood. A healthy patch means the liner held. A yellowed or dead patch means it leaked, and the frame and the liner both need checking before they are reused.

On the patio job: green grass under the frame, flattened but alive.

What changes the method

A lot inside an active subdivision brings the developer's permit into play, often with a designated washout area on another lot you must use instead of your own. A small job with no ready-mix truck, say a set of fence posts in bagged concrete or a repointing job, shrinks this to two or three lidded buckets that settle at the shop, with the same rule for the water and the solids. And a site with no room 50 ft from an inlet is the case for a prefabricated container hauled away the same day, rather than a frame that has to sit for a week beside the gutter.

References

• EPA construction stormwater guidance on concrete washout, and the Construction General Permit
• 29 CFR 1926.1153, respirable crystalline silica in construction; 29 CFR 1910.134, respiratory protection
• Cement manufacturer safety data sheets for alkalinity and chromium sensitization
• See related: Wash Water and the Storm Drain Rule
• See related: Rinsing Tools and Equipment Without a Discharge Violation