What is Ready Mixed Concrete-How is it produced?
“Fresh concrete”, which is produced in concrete plant by being mixed with the materials at desired ratios with a computer control and delivered to the consumer, is called “Ready Mixed Concrete”. The distinguishing feature of ready mixed concrete from hand mixed concrete and machine mixed concrete is that it is produced in modern facilities with computer control. The qualities ready mixed concrete users will look for are stated in TS EN 206.
There two types of ready mixed concrete production according to the performance of water measurement and mixing processes in plant or transit mixer:
- Dry System
- Wet System
Dry mixture ready mixed concrete is the ready mixed concrete of which aggregate and cement are measured and mixed with transit mixer in the plant, and of which water and chemical admixtures, if any, are measured and mixed in delivery location. For dry mixture ready mixed concrete, special attention should be paid to water amount given to mixture(not exceeding the amount formulated) and mixing time(sufficient time for a homogenous mixture) in worksite.
Wet mixture ready mixed concrete is the ready mixed concrete of which all ingredients including water are measured and mixed in the concrete plant.
Ready Mixed Concrete Plant
The facilities where ready mixed concrete ingredients are stocked, mixed under control, ready mixed concrete is produced and transit mixers are filled up with ready mixed concrete are called “concrete plant”. According to mixing methods, concrete plants are classified into two types, which are “wet and dry mixture”.
Firstly, laboratory tests are performed in order to examine the quality of correctly selected ingredients(cement, aggregate, water, admixtures) and their compatibility with each other. In order to prevent the deterioration in the ingredients that have passed these tests, continuous quality inspections should be performed.
Production process of the ready mixed concrete starts with the computer system operation by plant operator, who determines the formula number identifying the concrete to be produced beforehand. After the first command, aggregate, cement and water stocked in different compartments are weighed at the same time. Then, weighed aggregate is carried with band and bucked and transferred to mixer tank and mixed.
Volume of the mixed concrete is generally 1-3 m3, although it may vary by plant. Standard mixing time has also been determined by standards in proportion to the volume of mixture. The mixture sufficiently mixed is poured into transit mixer and the same process continues until the filling is completed.
a) Compressive Strength Classes
Compressive strength of the concrete is measured on 28 day old cylinder(15 cm diameter, 30 cm height) or cube(15 cm edges) samples stored in standard curing conditions(In (20 °C ±2°C lime saturated water).
Compressive strength classes for ready mixed concrete, equivalent cylinder and cube strengths are summarized in the table below. (EN 206)
|Compressive Strength Class||F ck, cylinder ( N/mm2 )||f ck, cube ( N/mm2 )|
b) Consistency Classes
Processability of the concrete can be analysed with the consistency. Consistency is a feature that should be selected carefully based on the place of use(mold geometry, iron density, elevation), available methods for placement, compression, gauging and workmanship of the concrete, and concrete transmission methods available in the worksite(pump, bucket). In Ready Mixed Concrete Standard TS EN 206, concrete consistency is determined according to slump, vebe, compressibility and diffusion classes. This consistencies of which slump classes are defined with S1, S2, S3, S4 and S5 sysbols are measured with slump cone test.
Worksite delivery consistency for ready mixed concrete depends on transportation time and concrete temperature. Transportation time affecs the consistency, and when time entends and air temperature rises, consistency loss from the plant to the worksite increases. Balancing this consistency loss with the addition of water to the concrete decreases the strength.
|Consistency Class||Slump (mm)|
While Slump Test is performed ;
* Slump cone is placed on a flat surface.
* Standard slump cone is filled up at three equal levels, and rodding is performed for 25 times with standard rod for each level.
* When the hopper is fully filled, its upper surface is flattened with trowel.
* The hopper is slowly raised; meanwhile, fresh concrete slumps down with its own weight.
* Rod is put onto the hopper and the distance from the top of slumped concrete to the bottom is measured. This length is called slump value of fresh concrete.
It is inevitable to use vibrator during the placement process of concrete. Also, concrete placement without using vibrator is prohibited by “REGUALTIONS ON THE STRUCTURES TO BE BUILT IN DISASTER AREAS”. Manual concrete compacting methods such as rodding, ramming etc. can only be used as supplementary methods to the vibrator use.
Possible Reasons For Consistency Loss During Concrete Transport
- False set of concrete. This can be prevented by continuing the mixing process.
- Long transport distances. Setting of concrete can begin during the transportation. Mixing water can also evaporate.
- Excessive mixing time before placement.
- Hot weather conditions.
Because awareness and education level is low especially cosnstruction sector and concrete workmanship, it is very common to use very flowable concrete consistency with 18-22 cm slump value and adding water to the ready mixed concrete I nworksite due to the easiness of handling, placemnent and gauging processes. In order to eliminate the harmful effects of this tendency, which results in the strength reduction, Turkey Ready Mixed Concrete Association Board of Directors made a decision on recommending its members to keep the consistency of concrete delivered in worksite at K4 level(slump >16 cm) and meanwhile, drew the attention to water/cement ratio(thus strength). Customers who acquire knowledge and awareness on this issue order the concrete with K4 consistency.
c) Aggregate Maximum Particle Size Classes
They are classified according to the aggregate maximum aprticle size in accordance with TS EN 206-1. For example; Concrete with Dmaximum 22 cm.
Concrete should be selected as stated in TS 500 and in accordance with the narrowest mold size of the largest aggregate particle size to be used, placement depth, concrete cover and the firmest reinforcement gap factors.
The concrete commonly used in the market is the one with “Aggregate no. 2). In very firmly reinforced or rhin section elements, some conscious order ready mixed concrete with “Aggregate no.1”.
|Maximum Aggregate Particle Size Class||Dmax. (mm)|
|D1 (No. 1)||12|
|D2 (No. 2)||22|
|D3 (No. 3)||32|
|D4 (No. 4)|
a) Fresh Concrete Temperature
Temperature of fresh, ready mixed concrete delivered in worksite in accordance with Ready Mixed Concrete Standard should not be less than +5 °C.
b) Unit Weight
According to the values stated by ready mixed concrete producer, unit weight has ±2% tolerance. For example; if the stated value is 2350 kg/m3, 2350 x 0.02=47 kg/m3. Thus, tolerance limits are 2350±47 kg/m3.
Throught unit weight, quantity problems can be solved, concrete weight and volume can be found by weighing the transit mixer in empty and full state.