Have you ever wondered what happens to all the hardened concrete residue that builds up over time inside the rotating drums of those big concrete mixer trucks?
Well, someone has to get rid of it periodically and here’s why:
- As the concrete residue inside the drum builds up, the mixer’s capacity is gradually reduced. This means you can’t transport as much fresh concrete as you could with a clean drum.
- All that dry concrete residue lining the inside of the drum weighs a lot and heavier trucks use more fuel. Fuel costs money.
- The longer the concrete residue stays in the drum and the more it builds up, the harder it is to get out.
So, how are concrete ready-mix drums cleaned?
Before we start, let’s talk briefly about ready-mix concrete. What is it exactly? First of all, concrete is not cement although the two words are often used interchangeably. Cement is an ingredient in ready-mix concrete. Ready-mix concrete is the stuff they mix up at a manufacturing plant (using cement, water, and aggregates like sand or crushed stone) and then transport to construction sites using a concrete mixer truck.
Now that we have that out of the way…
Those big, rotating concrete drums are cleaned via a process called concrete truck chipping (You’ll also see it called cement truck chipping.) and it’s a pretty nasty job as you’ll soon see. Let’s take a look at what’s involved…
Everything you ever wanted to know about concrete drum cleaning
The process of cleaning a ready-mix concrete drum
The process itself is fairly straightforward. Someone enters the drum and uses a jackhammer to chip away at the dried concrete. The real story is what happens before the person gets into the drum…
As you might imagine, using a jackhammer on concrete in a small, enclosed space stirs up a lot of dust. But, this isn’t ordinary dust. This is something called respirable crystalline silica (more on this in a bit). It’s carcinogenic and a respirator alone is not enough to protect the worker from it. So, the mixer drum needs to be ventilated first. Now, it’s time for the worker to get suited up.
A special respirator with a strong seal around the face prevents the worker from breathing in the respirable crystalline silica. Safety goggles and a face shield protect the eyes. Hearing protection requires both ear plugs and ear muffs. Both should have a Noise Reduction Rating (NRR) of at least 30dB as it gets loud in there.
The worker’s skin must be fully covered because an ingredient in concrete called hexavalent chromium can cause an allergic reaction. A hardhat is necessary because pieces of concrete on the top of the drum (i.e. over the worker’s head) might break off and fall. The worker also needs a pair of safety boots with steel toes and a pair of leather gloves.
Before getting into the drum the worker eyeballs the inside of it to see if there are any areas of heavy concrete buildup or cracked concrete that might come loose. If there are, the drum is rotated so those areas are not on top.
Other safety measures include making sure the truck won’t be moved or the drum rotated while the worker is inside.
Hazards and health problems of concrete truck chipping
The hazards of concrete truck chipping include working in a confined space with limited ventilation, noise levels that can permanently damage your hearing, the danger from the drum accidentally turning while you’re inside, and heat stress if the chipping is done during warm weather.
But, the real danger is from silica poisoning. So, let’s talk more about that.
The aggregates that go into making concrete contain large amounts of crystalline silica, a component of granite, quartz, sand, and other minerals. In fact, crystalline silica is found in just about every type of rock. It’s one of the most common minerals on the planet and is literally everywhere. Don’t worry though. You’re not going to be harmed by picking up a rock. Crystalline silica only becomes a problem when we start drilling, crushing, grinding, or cutting it. That’s when it turns into very fine dust and becomes respirable, or breathable.
Ready-mix drum cleaning and respirable crystalline silica (RCS)
Respirable crystalline silica is very dangerous to breathe. Concrete chippers who inhale RCS on a regular basis are at high risk of developing a non-curable and sometimes fatal lung disease called silicosis. Some people who work for years breathing RCS also develop lung cancer, chronic obstructive pulmonary disease(COPD), and kidney disease.
The only cure for silicosis is not to get it.
And RCS exposure during concrete truck chipping is very concentrated. Ventilation is not enough. Respirators must be used at all times as well. (Just make sure you don’t use a respirator as your only protection because the amount of silica present during chipping often exceeds the respirator’s Assigned Protection Factor (APF)). Jackhammers can also be retrofitted with a water spray nozzle for dampening the dust.
New OSHA Standards for working with respirable crystalline silica
If you’ve read this far you know that respirable crystalline silica is bad stuff. Very bad stuff. Well, OSHA(Occupational Safety and Health Administration) agrees.
On June 23, 2017, OSHA finally got tough and significantly lowered the Personal Exposure Limit (PEL) for working with respirable crystalline silica in the construction industry from 250 µg/m3 all the way down to only 50 µg/m3. That’s an 80% decrease and employers need to comply with it or face a significant fine. Criminal charges are even possible if OSHA finds that the violation was willful and an employee died as a result of silica poisoning.
In addition to lowering the Personal Exposure Limit (PEL)from 250 µg/m3 to 50 µg/m3, the requirements for OSHA’s new rule (11926.1153) regarding exposure to RCS in the construction industry include implementing engineering controls, providing personal protective equipment (PPE) to employees, and developing a written exposure control plan. Employers are also required to offer free medical exams, information, and training.
Some critics feel the new rule is unnecessary and will kill jobs. However, U.S. Secretary of Labor Thomas Perez feels it will save lives. What’s for sure though is that employers are going to have to be on top of things if they want to meet OSHA’s strict new requirements for America’s dirtiest job.
Hydroblasting: An alternative to manual concrete truck chipping
As you can see, ready-mix drum cleaning really is a nasty job. However, it isn’t the only way to clean a concrete drum. Hydro blasting gets the job done from outside the drum and in a fraction of the time.
Let’s look at the difference…
It takes a manual chipper several hours to clean just one concrete mixer drum while hydro blasting can remove up to 4000 lbs. of dried concrete from a drum in less than 90 minutes. And this is without anyone getting inside the drum and exposing themselves to respirable crystalline silica. Hydro blasting only requires one worker outside the drum using a remote control.
Given OSHA’s tough new rule regarding respirable crystalline silica, concrete manufacturing companies should consider hydro blasting as a safe and efficient alternative to manual chipping.