DE4237543A1 - Prodn. of concrete with optimal consistency - involves monitoring of water addition - Google Patents

Abstract

During production of concrete or mortar, the water content is measured whilst the compsn. is in the form of a stiff consistent mixture and water is added using a standardised metering device. At the same time, other additives can be metered into the mixture. The water content is measured whilst in the stiff condition by passing a current via electrodes through the mixt. and measuring the resistance. Water is then added in response. The mixture consistency is measured by the force of resistance experienced by the mixing tool. This leads to a correction factor which is applied by metering in cement, size and/or other additive. Also more cement can be added. All additions are computer controlled. No more additives can be added once the concrete is being taken from the mixer. Mixing can only start when the mixer is completely empty or rinsing water of under 35l/m3 remains in the mixer. ADVANTAGE - By adding the optimal water amount, concrete of the required consistency and quality is assured.

Description

The invention develops a method for adjusting water content and Consistency in mortar and concrete in the plastic and soft consistency areas.

State of the art

Water is a determining factor for all basic properties of building materials, which are produced with the addition of hydraulic binders, namely For both up to processing, the building material is then fresh mortar or called fresh concrete, as well as for those in the finished state.

Installability

The term consistency stands for the assessment of mobility and thus the processability of an intended for installation in a building, under Add hydraulic binder with water mixed mixture.

The consistency of a mixture is crucial for proper installation at the time of processing; because the agility of the mixture is in Variable depending on numerous parameters. In the period between Concrete storage and processing usually occur stiffening on. If the stiffening has progressed too far, the Building material according to the consistency used Measures during installation no longer possible.

If the mixture is too liquid, you must also take into account beforehand safety surcharge, there is already a risk of segregation Mixing beyond the optimal mixing time, further on loading, transport, Transfer, conveying and installation in the form that the coarse stones by centrifugal force to be flung away or to fall by gravity and the Cement-water stillage floats up. The cement goes from the solid in this phase into a gel. In addition, the proportion of water in fresh concrete causes one Pore formation in hardened concrete. The more stillage or water, the more and the larger pores.

However, the wear on the mixer and the transport and conveyor equipment decreases with increasing consistency. Add to that the elimination of stiffening by additionally mixing water to simple and cost-reducing Way can be eliminated. But you have to, and you actually do it also, accepting that the fresh concrete is too far advanced stiffening is improperly refreshed.

The problem of stiffening is particularly pronounced with that for in-situ concrete a proportion of approx. 85% of the predominant preparation processes "mixed works Ready-mixed concrete "(15% are construction site concrete made on site). According to the German concrete standard, the delivery vehicles should at the latest Discharge 90 minutes after the water has been mixed into the cement. To In the same standard, the minimum mixing time is set to 30 seconds. This The minimum mixing time is generally a stationary one in the delivery plant Mixers common. For those chemically used when mixing cement with water In response to the setting, the two periods are outside each reasonable relation.  

Because the European pre-standard for concrete in Germany is called DIN V ENV 206 was introduced in October 1990 two parallel regulations. A pre-standard is on the level of one Standard. This allows you to temporarily have two standards side by side can provide for selection. Both are state of the art. Here is missing Guide value of 90 minutes, instead it says, "Concrete is as soon as possible after mixing to bring about a decrease in workability if possible to keep low. "" As soon as possible "is a legal, not a technical Down.

Practical probation

The water content absolutely and in relation to the binder weight as well as the Homogeneity in turn determines the properties hydraulically bound building materials in the state as a finished product.

• - The water content is absolute for the extent of that which takes place during the setting process Formation of pores essential.
• - The ratio of water to cement by weight is determined for a given type of cement basically the strength alone.
• - The homogeneity is a prerequisite that the inevitable pore volume as expected is present and the flow of force evenly in the building material runs.

In the poorly possible setting of water content and consistency today mortar and concrete have such a serious weakness that everyone Quality efforts in the construction sector are significantly affected by this; because In the basic building material of concrete, there is no fresh concrete from the base material assured quality control possible, so you see it as meaningless to practice strict quality assurance in construction at all. A clear one Assignment of responsibilities is excluded a priori: who makes which mistakes where and when?

problem

The risk of exceeding the specified water content is permanent given. The effects of a deviation are only in extremely extreme exceptional cases recognizable to the building owner during the acceptance of the building structure. The given The scope is great. There is a hidden defect in this case, which e.g. B. decides whether a bridge is 30-50 years or 80-100 years holds. However, this also includes the risk that they will be extreme Stress u. U. needs renovation after just 10 years.

One is never beyond approaches to limiting the problem of concrete watering came. A dozen ways of water enrichment in the treatment phase must be considered:

• - Rinsing water residues remaining in the mixer drum
  - Mixing a partial load into another type of concrete
  - Spray water for cleaning the inlet when handling trucks
  - Unrecognizable stratified water in the additional landfill
  - insufficient measurement of the aggregate moisture (e.g. too great inaccuracy of the device)
  - Drip water from the additional landfill on weighing equipment
  - Rainwater in the central system on a conveyor belt
  - Correction of consistency in the mixer via force absorption (because water demand is unknown)
  - Permissible measurement inaccuracy of the weighing devices
  - Separation in the truck mixer by floating
  - Replenishment of water in the truck mixer
  - inconsistent water distribution due to improper re-mixing

To make the facts clear, reference is made to the attached picture. It shows the dependence of the concrete compressive strength on the preferred one Cement type Z 35 from the weight ratio water to cement, expressed with the technical term "water cement value". The picture shows that according to the European standard ENV 206 for the prevailing concrete grade B 25 with the specification for concrete outdoors, the water cement value must be the same or less than 0.55, almost the quality of the concrete type above B 35 is reached. This is a wasteful reserve measure. Therefore, even if everything goes well, the concrete must therefore continue for a long time don't get better. Depending on the circumstances, concrete is possible to optimize exactly.

Inaccurate workflow is secured by increased use of building materials; because with low water cement you have to get a reliable one Technically, a correspondingly high amount of water and processability Add cement.

If the conditions are unfavorable, the water cement value doubles to 1.1. there is a concrete in the quality of the former B 160. This concrete has in accordance with the standardization from 1943, also outdoors until the 1960s Well proven in years. The labor was cheap. The concrete preparation happened in small batches on site and the processing took place artisanal. Today, production takes place under different conditions. In addition the outside air has become much more aggressive and the same has happened with porous concrete Rusting of the steel reinforcement accelerated.

Appropriate for water content and consistency in mortar and concrete To be able to adjust, the moisture of the aggregates and the water demand be known to the mixture. Large fluctuations from batch to batch for the same Concrete types in one and the same mixing plant can be used by each of these both influencing factors.

The water requirement of a mixture depends on numerous individual parameters, that influence each other. The related technology has not even been sufficiently researched. According to today's knowledge it can never be controlled reliably enough. Sudden Consistency changes on both sides are possible. A reasonably Satisfactory control is only with great effort, especially on the part of laboratory provision and process control possible. As soon as negligence in the slightest tears, the quality gets out of control. In other words: Application security is questionable.

With today's possibilities of measurement technology is a proper determination the water content of a mixture in the mixer only in the area of the stiff Consistency ensured. In this area, the consistency is measured by the slump for the classification of processability as an absolute measure without Importance. It cannot be recorded as an exact parameter. That is why  Consistency cannot be set here using a precisely defined value.

Different in the plastic area. This area is with the slump 350 to 410 mm fixed. However, only mixtures that are considered to be easy to process Have a slump of at least 380 mm or more. What is underneath is procedurally to move to another area. Technically correct the range should be as follows: 400 ± 20 mm. In the usual procedure can this area cannot be controlled, so you get this narrow limitation was not allowed to be included in a standard.

Similarly, the soft range of consistency is standard with one Spread of 420 to 480 mm too wide. The mixture is in the upper range so fluid that only slight compression is allowed, and that carefully, it is already plastic in the lower area and makes of therefore intensive compaction is essential.

The standards recommend the use of concrete in the most liquid form possible, because even today these broad ranges of consistency are not adhered to is ensured!

To date, people have shown themselves generous compared to the addition of water. Man believes itself from the entrepreneurial point of view when it comes to concrete due to its large lead dimensions and short guarantee periods for the guarantee adequately secured to have. The mostly valid VOB only provides a 2-year guarantee. At best 5 years are agreed in accordance with the German Civil Code.

For the evaluation of concrete, these time periods are from the perspective of the building owner completely insufficient.

This situation changes when the EC internal market takes effect. Intended are a 20-year guarantee with the entrepreneur's obligation to provide evidence, if he wants to reject a fault that gives rise to liability. Of the Entrepreneurs are liable in the same way for suppliers they use and subcontractors.

In addition, the product liability out of the time development for tightened the contractor more and more. A dangerous criterion is thereby the malicious deception. In this case, the warranty period is 30 years. It doesn't even have to be a shortage. Detectable Defect approaches or proof of non-existence of more legitimate claims have claims for reduction made.

Inexpensive dilution of concrete in the above listed Extent must be considered a malicious deception.

In order to make the quality of fresh concrete verifiable, it is a prerequisite the water content and the consistency at the time of delivery of the concrete from To be able to correctly record the mixer in the conveyor. The consistency should also be easy to estimate visually. It is that Interface between concrete preparation and concrete processing. The clear separation These two areas of responsibility are organizational and legal Importance.  

invention

The vulnerability shown is eliminated by means of an at least two-stage Mixing process with at least three measurements in such a way that with a mixture in the rigid area is initially produced from the starting materials becomes. The water content is then measured. According to this value, it will still be missing mixing water added. His measurement, e.g. B. on the water meter is quite possible. The consistency fine adjustment then takes place by measurement e.g. B. the force absorption of the mixer or its mixing devices instead, this process being controlled separately. Possibly still necessary Corrections then result in compliance with special measures ahead. The consistency can then be set very precisely.

By mixing in at least two stages with a total of at least three measurements otherwise becomes a crucial area of consistency for in-situ concrete Unachievable accuracy when adjusting water content and Consistency made possible. Modern computer control allows the entire process to automate.

Mixing can be done in different mixers, e.g. B. in a stationary installed mixer and in the mixing drum of the transport vehicle. It can also take place in succession in one and the same mixer.

Measured values from the stationary processing plant can be wirelessly transferred to the Transfer computer of the transport vehicle. Water is allowed in the mixer or mixers only initiated via a measuring device with documentation of the work phase become. If a mixer is partially emptied, the water supply must lock yourself automatically.

The total water content is known. It is calculated from the measured values on the mixture in the mixer or mixers, the consistency can be immediately determine before handover. It is conveniently only there exactly set. This process is measured by itself.

The four essential, overlapping influencing factors can thus be identified take control:

• - Water demand of the mixture
  - Natural moisture of the aggregates
  - improper addition or enrichment of water
  - Mixing water metering

The areas of consistency can be put on a foundation that is structural overcomes today's unsatisfactory state. Suitable for building materials Claims and determinations are possible. Not only can they be tested, but distinguish them from each other simply by looking at them. This effect alone proves the novelty of the invention:

Plastic range: 400 ± 20 mm
Soft range: 460 ± 20 mm

The visual assessment possibility is both for the concrete supplier also important for the processor; because after a little while like that of 10 minutes, the consistency may have dropped significantly.

Claims (8)

1. A method for adjusting the water content and consistency in mortar and concrete in the plastic and soft consistency range, characterized in that by producing a mixture in the stiff consistency range with subsequent determination of the water content and a subsequent subsequent water addition by means of a calibrated measuring device in the production of fresh mortar or Fresh concrete in the consistency areas that are important for processing on site provides an otherwise unattainable accuracy in the metering of the desired water quantity. In addition to the addition of water using a calibratable measuring device, additives with a liquefying effect can also be added. 2. The method according to item 1, characterized in that the determination of the Water content in the stiff area by measuring the current resistance Electricity is introduced via electrons and the related water metering is regulated by a water meter. 3. The method according to item 1, characterized in that after measuring the water content in the stiff area and then adding water the consistency achieved in the mixer z. B. by measuring the force absorption the mixer tools is determined and possibly related to it Correction that becomes necessary as a separate process with water as part of a specified W / Z value maximum, with cement paste and / or with an additive he follows. In addition to the high accuracy when dosing the target water volume this also means maintaining the desired consistency with a Accuracy not guaranteed today. 4. The method according to item 1, characterized in that after measuring the water content in the stiff area and then adding water the consistency achieved in the mixer z. B. by measuring the force absorption the mixer tool is determined and, based on this, one may be necessary correction as a separate process with water with the addition of a related amount of cement. 5. Claim according to items 1, 3 and 4, characterized in that any Dosing of water into the mixer only via a measuring device with documentation the working phase of the mixer can take place. The mixer is partial emptied, the water supply must automatically shut off. One more The intended subsequent correction of the consistency can then only be made using additives respectively. 6. Claim according to items 1, 2, 3, 4 and 5, characterized in that the Overall process is controlled by computer. 7. Claim according to items 1, 2, 3, 4, 5 and 6, characterized in that After emptying has started, no more water can be added to the mixer can. When the mixer is empty, rinse water can then be introduced again. that is recorded in terms of quantity. 8. Claim according to items 1, 2, 3, 4, 5, 6 and 7, characterized in that that a mix can only be started when the mixer is either is completely empty or the documented amount of rinsing water remaining in the mixer less than 35 liters per cubic meter of nominal content of the mixer.