Concrete hardened in the drum: how to solve it?

Mixing drum is filled to the brim with concrete, and you are getting ready to unload it onto the site. However, you have a hardened concrete in a drum instead of liquid mortar.

This is a typical nightmare of any Carmix operator.

Concrete mixture can remain in mixing drum and still retain its properties for up to 3 hours at an optimum temperature of around 68 ºF with use of necessary chemical additives. It is important that drum blends the mixture continuously all the time, otherwise, the mortar will begin to set.

Hardening of mixture in the drum leads to high costs and difficulties. After all, hardened concrete puts the mixer out of work for good. This means a full stoppage of concrete production and pouring of sites for Carmix owners. Therefore, a delay in concreting process leads to a postponement of next phases of the construction project.

In the end, such situation results in a considerable expenditure of time, labor and economic resources.

Let’s find out why this situation may occur and how to avoid it?


Mixture hardening in the drum is a rare case.Usually, the concrete mixture stays in drum longer than it should and solidifies due to the following reasons:

  • Operator mistakes

In most cases, solidification of mortar occurs because of the human factor: driver didn’t inspect the machine before the shift, didn’t fully unload, or left the mortar inside the mixing drum for all-night.

Carmix concrete mixer must be operated by a competent worker who knows and strictly follows the operating and maintenance regulations.This prevents driver from making mistakes that would cause the solidifying of concrete mix in drum.

Operator should check the mixer performance before and after each shift, also inspect and clean Carmix at the end of each day.


Engine or hydraulic failure can lead to concrete not being unloaded from the concrete mixer.

Breakdowns occur due to a variety of circumstances:

Maintenance and inspection were not carried out in time. As a result, a concrete mixer that has been running continuously can stop in the middle of the working process.

The use of substandard parts and components for maintenance or repair work. Unoriginal spare parts may be made of weak materials which will quickly wear out during operation and lead to unexpected breakdowns.

Low level of service fluids. Operator may forget to check the correct amount of fuel and hydraulic oil in mixer. Thus, nodes can fail during prolonged work. It is also possible that concrete mixer will destroke due to lack of fuel. Since the diesel still needs to be delivered to mixer, concrete mortar may harden.

If you want to be sure that your mixer will produce concrete properly and won’t break down halfway to the pouring site, make sure that periodic maintenance is carried out by an authorized Carmix dealer service center in a timely manner.

Workplace accidents

When a concrete mixer goes sideways on a slope or falls into a ravine – it might look funny as an internet meme. Such situation is fraught with a lost day and solidified concrete right in mixer.

It is also worth mentioning that concrete sticking on the blades and walls of the concrete mixer can lead to hardening of a large volume of mixture in the drum.

This is a slow and implicit process, with minor mortar residues becoming larger and larger as the process progresses. This hardened concrete residue can lead to malfunctions and to inability to discharge the mix.

Concrete sticking, as opposed to curing of the entire mix, is the most common problem for Carmix. The small residual volumes of set mix will pick up larger amounts of concrete as mixing process progresses. The concrete sticking increases in size with every subsequent mixing and can lead to the following consequences:

  • Concrete in mixing drum weighs a lot and causes malfunctions or inaccuracies in weighing system
  • Stuck concrete causes drum to become de-centered, which in turn places an order of magnitude more stress on the engine and hydraulics than would otherwise be the case if the drum was not filled with hardened concrete.
  • The total strain on all components increases as total weight of mixer increases due to hardened concrete in the drum.

Carmix operator should check mixing drum at least once a week to ensure that concrete sticking is not a serious problem, and periodically clean the metal from concrete during maintenance.

Discharging liquid mortar

If you caught the moment when concrete is not yet hardened and is still in a liquid state, there is still a chance to reduce concrete mixer damage and the time for cleaning out the hardened mixture.

It is easiest when concrete mixer simply doesn’t have the ability to get to site of concreting before mixture hardens, while having no breakage. All you need to do is discharge concrete and rinse mixer.

Concrete discharging becomes problematic and virtually impossible if the engine or hydraulics fail.

There are several ways to discharge mixer in an emergency:

Use the hatch

Concrete mixers, like Carmix, have a special escape hatch for the emergency mix discharge. You need to unscrew bolts and turn drum so that hatch is at the bottom position. This way, you can pour most of the concrete out of mixer. Unfortunately, only the main volume will be discharged in this case, and concrete sticking on blades and walls of drum will remain. You will have to get rid of it using the cleaning methods described below.

Use an exterior hydraulic motor

If the self-loading concrete mixer fails on site, you can discharge mixture using the hydraulic system of another special-purpose equipment. For example, an excavator or front-end loader.

There are always other machines with hydraulics working nearby most of construction sites. You have to bring the work equipment as close as possible and connect Carmix with hydraulic hoses with fittings to the hydraulic line of another machine.

When all the mortar has been emptied, you only need to pour water into mixer and make a few revolutions of the drum. This way, you will get rid of concrete sticking, which can also harden and have to be removed.

To ensure that Carmix owners are prepared for such accidents, we recommend picking up 2 or 3 couplings and 2 straight and return hydraulic hoses in advance to connect to exterior machinery.

Discharged by hand

You will have to use a lot of workers, buckets, and shovels to discharge the entire volume of concrete from the drum. This option is time-consuming, but it saves you from an even more complicated process of cleaning up the hardened mortar.

Disconnect the drum

Here you will also need additional special-purpose equipment, more specifically a crane or another lifting equipment.

On Carmix, you have to remove back plate where hydraulic motor is attached and disconnect hoses from it. The drum is then lifted with crane and turned over to get all concrete out of it.

You can also use a cone to pour water and rinse the mixing drum from the remaining mortar on the walls. Then you won’t have to clean the drum of hardened concrete sticking.

Ways to clean up hardened concrete

If you allow the concrete mixture to harden in the drum, you will be in for hours of cleaning.

There are situations when the concrete doesn’t harden all but only sticks to the walls. For example, road to the site or waiting for discharging took too long. This can lead to a reduction in volume of mixing drum.

Such an outcome is not the worst.

If this has happened to your concrete mixer, there are several ways to clean it. We will tell you more about them.


One of our customers allowed concrete to harden inside Carmix. In order to completely clean the drum, we had to cut the sidewall and use a hydraulic hammer to break up the hardened mortar.

This is the essence of the mechanical method.

It uses all the tools to manually break down the hardened mass:

  • concrete breaker
  • club hammer
  • air hammer
  • pneumatic hammer

This method is most common and is not as costly as others. However, it takes a long period of time, since cleaning work is done manually.

The duration of cleaning from the concrete mixture depends on three factors

  • concrete grade
  • hardened volume mass
  • hardening time

Concrete gains its maximum strength in 28 days. Therefore, it will take much less time to hammer out concrete in the first three days than it will take a month after it hardens. Also, the higher the grade of the mortar, the harder it will be to break down the concrete.

It is necessary to perform the cleaning carefully, as it is possible to damage blades and concrete mixer walls.

Usually a hole is drilled in the hardened mass so that concrete can be broken away more easily. It can then be broken away more quickly with a hydrohammer.

In order to perform such cleaning, safety precautions must be clearly observed and personal protective equipment must be worn. Workers directly involved in the cleaning process must wear:

  • А respirator with a special valve to filter the air from finely-divided concrete dust
  • A helmet to prevent direct contact with flying concrete debris
  • Safety goggles
  • Special protective gloves
  • Earplugs or earphones

Mechanical method is a very labor- and energy-intensive process. Therefore, workers must be changed every 15-20 minutes. Also, to prevent dehydration during the work, they need to drink plenty of water.

Powder destruction

In this method, the main tool is an alkaline powder. It acts as an explosive when mixed with water and splits concrete from inside.

It is a dense, finely dispersed substance whose solution begins to expand over time and, under pressure, destroys the hardened mass. It is also called “silent explosive”.

In order for the solution to work, it must be mixed with a liquid at a ratio of 1 pound per 5.4 ounces and make holes in the concrete with a perforator to pour the substance into it.

The holes must be 3 inches in diameter and 33 inches apart for a concrete to crack accurately from the expansion of the mortar. After the active ingredient is poured, you have to wait 2 days and concrete will split on its own.

After that, all you have to do is disassemble split concrete stones and clean the walls and mixer blades of any small concrete sticking.

Powder is a dangerous chemically active substance that can seriously harm human health. Therefore, all safety precautions must be observed when working with it, workers must wear a respirator and gloves.


The method affects the concrete structure. It destroys crystallized lattice, dissolves a number of concrete components and converts the fossilized mass into a loose substance.

Acidic solutions are used to remove the concrete: muriatic acid and hydrochloric or phosphoric solutions.

Entire cured mortar is soaked in a large amount of acid to initiate the reaction

Chemical reaction is rapid and only takes up to 24 hours, depending on the amount of cured mortar.

At the end, only a loose substance remains, which can easily be removed from mixing drum.

Method comes out very expensive because of the amount of acid solution required, so it is not common.

Stickings and streaks from the body and glasses of mixer are removed with acid solutions. This is more cost-effective than removing a huge volume of hardened concrete.

Chemical methods of cleaning concrete can be accompanied by the release of heat and dangerous gases. Therefore, safety precautions must be followed when using solutions or powders.

There are several other methods that are new and not so well established in the market. For example, destroying the structure of the cured concrete mixture with electrodes or using real explosives in small quantities.

The most effective method that can be advised to everyone: It is better not to let the concrete harden in the mixing drum in the first place, so you do not have to use the described methods.