Concrete is a composite material consisting of aggregate materials bonded with cement. When these materials are mixed together with water, they form a slurry that can be poured and molded into any shape. To get the best quality of concrete, these ingredients must be mixed properly and in a certain proportion. If not properly mixed, it can result in poor quality concrete.
Table of Contents
- What is a Concrete Mixer?
- Concrete Mixer Background
- Types of Concrete Mixers
What is a Concrete Mixer?
Concrete mixers are machines that mix cement, aggregates, and water together to make the concrete paste or slurry. A good concrete mixture should not only uniformly mix the ingredients, but they should also discharge the mix without disturbing its uniformity. This can be judged by the uniform consistency and color of the slurry.
In the absence of concrete mixers, concrete can also be mixed manually. However, this is applicable to only small amounts of concrete. Additionally, workers are prone to mix concrete poorly and, in many cases, end up over-mixing it. Hence, it is advisable to use machine mixers to mix concrete materials.
Concrete Mixer Background
The first motorized cement mixer was developed and patented by Stephen Stepanian in 1916 in a bid to replace the horse-drawn mixers that were used at that time. The horse-drawn mixer design consisted of wooden paddles that churned the ingredients in tandem with the turning of the cartwheels. However, it was a very slow and burdensome process. In fact, the trucks and engines of that time were hardly better.
However, by the 1940s, engines and trucks started being used for transporting wet, unset concrete. After World War II, mixer trucks became an essential part of construction work.
Today, the large drum mixers seen on our roads are not that much different from what Stepanian envisioned. These transit mixers consist of an engine, a truck frame and a rotating concrete mixer. These mixers are similar to those found on construction sites but are much larger.
The mechanism involves a large motor rotating the drum on the body of the truck and a series of blades which keep the mixture in constant motion and prevent the concrete from becoming hard.
Most cement manufacturers advise keeping the time between mixing and pouring the cement to preferably less than 6-10 minutes and no more than 90 minutes.
As technology advanced, most transit mixers do not just pick up the wet cement and transport it to the site, unless they are going to nearby roads or sites. Most of these mixers consist of a water tank in the trunk. The rotating drum keeps the dry aggregate and cement mixed during most of the journey. When the truck driver is within a few miles of the destination, water is added to the dry ingredients so that concrete is delivered fresh to the site.
This type of delivery is known as “batch” delivery, which requires mixing ingredients off-site and then transporting them to the location of the construction. Technological advancements have also made it possible to mix concrete on site, although transit truck mixers remain the primary mixers.
There are several types of commercial concrete mixers that make concrete production and delivery cost-effective and quick.
Types of Concrete Mixers
There are two main types of concrete mixers – continuous mixers and batch mixers. Batch mixers are further divided into categories and subcategories.
As the name suggests, continuous mixers feed the ingredients into the drum, mix them and discharge the slurry simultaneously and continuously until a break is called or until the work is completed. The continuous loading of material is done by screw feeders.
These mixers consist of on-tilting drums with screw-type blades spinning in constant motion in the middle of the drum. The drum is tilted downwards, usually at an angle of 15 degrees towards the opening and the mixing time is determined by its tilt.
Continuous mixers are used during large-scale projects like building bridges, dams or skyscrapers, which require a continuous flow of concrete. These mixers are also used for low–slump concrete like pavements. Since the mixing time is short and a continuous discharge of concrete is needed, the air
content and bubbles are not controlled easily even with air in the admixtures.
Batch Concrete Mixers
Batch concrete mixers are the most widely used types of mixers used to make concrete. Unlike continuous mixers, batch mixers mix and discharge concrete periodically, one batch after the other.
The batch mixers consist of rotating drums with blades inside. In this mixer, all the ingredients are fed into the drum or pan in the desired proportion and then the drum is spun at a fixed speed. The resulting mix is discharged and used. Once the first batch is done, the drum needs to be filled again. The ingredients are mixed and then discharged to make another batch. This process is repeated until the required amount of concrete has been made.
Batch concrete mixers are divided into two types depending on their rotational axis – drum, horizontal or inclined mixers and pan or vertical mixers.
Drum mixers consist of double-conical shape drums. They are classified into three main types, tilting drum mixers, non-tilting drum mixers, and reversing drum mixers.
Tilting Drum Mixers
Tilting drum mixers can be inclined at various angles. When the drum is almost fully horizontal, with no discernible tilt, concrete is mixed more efficiently as it is lifted by the rotating blades to the drum’s fullest diameter and dropped. It is during the drop that the ingredients mix together. This means the higher the drop, the better mixing of the ingredients. However, if the axis of the drum is almost vertical, the blades will not be able to lift up the concrete to the fullest and the ingredients are not fully mixed. Usually, the drum axis is tilted at an angle of 15 degrees towards the discharge opening.
Tilting drum mixers are usually used for low workable concrete that contains large-sized aggregates that are usually more than 7.5 cm in size. However, the mixing efficiency depends on several factors like the shape and angle of the drum and the size and angle of the vanes.
One disadvantage of the tilting drum mixer is that concrete can stick to the bottom of the drum. To prevent this, a method called “buttering” of mixers is applied in which some cement is pre-mixed in the mixture before the drum is used for its first batch of concrete.
Non-Tilting Drum Mixers
As the name indicates, these types of mixers cannot be tilted and their axis always remains at 0 degrees. The concrete is discharged by inserting an inclined chute inside the drum or by reversing the direction of the rotation and in some very rare cases, splitting the drum. In the case of the inclined chute, the drum is opened at both ends, and the ingredients are poured through one end while the mix is collected through the other end.
Because of the horizontal orientation of the drum, a rapid discharge of concrete is not possible; hence the slurry may become vulnerable to segregation. Large-sized aggregates cannot be discharged easily with the mix so aggregates which are smaller than 7.5 cm are used in this type of mixers. Because of this, non-tilting drum mixers are used in small-scale projects.
Reversing Drum Mixers
The reversing drum mixers are similar to the non-tilting mixers, except that the drum is opened only at one end and the same opening is used to fill the drum, as well as, discharge the material.
With the help of a motor, the drum rotates in one direction to mix the material and in the other direction to push out the prepared concrete. This is achieved by two types of blades connected to the inner walls of the drum. One set of blades hauls the concrete upwards towards the center of the mixture when the drum revolves in one direction while the other set of blades pushes the concrete towards the opening for discharge when the blades rotate in another direction. The blades are arranged in a spiral so that they can discharge and mix the material, based on the rotation.
Reversing drum mixers are very economical as they have low operating and maintenance cost and can produce medium-quality concrete in a short time. These types of mixers are mainly used in the markets where production cost is essential.
Truck mixers belong to the reversing drum mixer categories. These mixers allow the driver to regulate the speed of the drum rotation with a clutch. The speed is determined by whether the concrete was pre-mixed before being placed in the truck or the truck had to do the majority of the mixing. Typically, the speed is 15 revolutions per minute (rpm) while pre-mixed concrete requires only 6 rpm. In the U.S., most concrete is mixed inside the truck and does not come pre-mixed from the manufacturing plant.
All types of pan mixers work on the same principle. They contain a circular pan which contains the concrete to be mixed and a set of blades that rotate inside the pan to mix the ingredients and scrape the stuck concrete mixture from the walls. The prepared mixture is discharged from the bottom of the pan.
There are two main types of pan concrete mixers. In one case, the circular pan remains stationary and only the blades move in planetary or anti-current motion. In the second case, the blades, arranged in a star shape, remain fixed, while the pan revolves. In both these cases, the concrete mixing is very efficient and the mixture is discharged through a central hole in the pan. Pan mixers also contain a special scraper blade, positioned at an angle that dislodges the stuck ingredient from the sides of the pan and incorporates it into the rest of the material.
The revolving blades can also be raised and lowered so that there is no room for the concrete to stagnate on the sides of the pan. Pan mixers are considered to be the most efficient type of mixers out of all the batch mixers.
Other Varieties of Concrete Mixers
Twin Shaft Concrete Mixers
These mixers are used for their high intensity mixing in a very short time. They are used in projects where high strength is needed and are often used to make self-consolidating concrete.
Vertical Axis Concrete Mixers
These mixers are used for precast and pre-stressed concrete. Vertical axis concrete mixers clean well between batches and are preferred for their precast and colored batches. They are also more suitable for smaller areas and consist of more than one discharge points. The planetary or counter-current mixers in this category are particularly popular.
Volumetric and Metered Mixers
These mixers measure out precise proportions of cement, stone, sand, and water on site, which means trucks do not have to mix in-transit. This reduces the risk of concrete hardening if the truck mixers get stuck in traffic. It also produces the exact amount of concrete needed and prevents waste.
Special Purpose Concrete Mixers
Hopper concrete mixers and conveyor belt mixers are two special purpose mixers that are used for precast forms, block making, and batching.
There are also special self-loading mixers which providing mixing and transportation at the same time. These mixers are used in-transit or when batch mixers are unavailable or if workers are limited.
In batching plants, there are large batching concrete mixers which have storage, discharge, and delivery capabilities. These are sold for large batching projects, usually for a large block or ready-mix manufacturers. Batching plants connect conveyor belts and storage silos to the mixing equipment itself.
Some concrete mixers come with built-in lifts to easily apply the material to upper levels.
Concrete preparation is a complicated process which is affected by the type of mixers, the loading method, and the duration and the energy of mixing. Machine mixers make it easier by providing various advantages and controlling the water to cement ratio, regulating the mixing process, and reducing wastage.