CONSISTENCY ASPECTS IN THE PRODUCTION OF CONCRETE USING AGGREGATES FROM RECYCLED DEMOLITION MATERIAL

von: Andrew Nealen und Marcus Rühl
aus: Darmstadt Concrete 97, Ausgabe 12

SUMMARY

Freshly mixed concrete made from recycled demolition material shows a different behaviour towards the development of rigidity, which results in workability problems, especially in the production of ready-mixed concrete. This is caused by the high water absorption capacity of the porous aggregates. Practical examples show, that the application of superplasticizer at any given time can resurrect the perfect workability consistency.

INTRODUCTION

Concrete used on site is produced to achieve a certain workability consistency during application time. It is therefore necessary to maintain this consistency over a maximum period of time. Especially in the area of ready-mixed concrete the time delay between production and application can amount up to 60 minutes and more.

Whereas the development of rigidity of concrete made from regular aggregates can still be controlled to a certain degree, the same development is the major factor in the production of concrete from recycled aggregates. The consistency change is much higher, especially within the first 10 minutes, which is a result of the high water absorptive capacity of the recycled material. This is related to different material properties of the recycled aggregates which are described further below.

By applying certain amounts of superplasticizer to the freshly mixed concrete the original consistency can be regained. The amount necessary is dependent on the consistency at application time. Laboratory research at the Technical University of Darmstadt (TUD) has shown, that the freshly mixed concrete can be ‚revived‘ up to 90 minutes after its initial production. Tests to maintain workability beyond these 90 minutes are currently in progress.

MATERIAL PROPERTIES OF RECYCLED AGGREGATES

The recycled aggregates absorptive capacity for water is mainly related to two aspects which have different mechanisms and influence ratios towards the development of rigidity (see Fig. 1) and the achievable compressive strength of the hardened concrete.

The comparison of the two freshly mixed concretes in Figure 1 is based on the same amount of water and cement used in both mixtures. Also, the aggregate grading curve is equal in both cases.

Porosity (a)

The porosity of the recycled aggregate causes water to be absorbed within the aggregates core where it is enclosed and withdrawn from the cement paste. The result is the reduction of the effective water-cement ratio and a higher achievable compressive strength of the hardened concrete. This aspect is the reason for the fast development of rigidity within the first 10 minutes. Laboratory tests have shown that the aggregates reach 90% of water saturation within the first 10 minutes of absorption. Beyond the 10 minute value the curves of regular concrete and concrete from recycled aggregates run mostly parallel (see Fig. 1).

Change of the grading curve during the mixing process (b)

Variable amounts of recycled aggregates are made up of two compontents. These hybrid grains are rocks with parts of the cement matrix still binded. During the mixing process these grains are mechanically split, thus redefining the aggregate grading curve. The aggregates size distribution factor is changed so that a higher amount of water is necessary to achieve the aimed consistency. This aspect has no influence on the development of rigidity but changes the original consistency of the mixture and must be put into consideration. In general though, the porosity of the aggregate has the stronger influence (as seen in Fig. 1).

CONSISTENCY CORRECTION

Correction can be accomplished by two methods which differ in effectivity and outcome towards achievable compressive strength of the hardened concrete.

Adding extra water and superplasticizer at t = 0 minutes (Method 1)

By adding extra amounts of water, the water which is enclosed in the aggregates can be compensated for without loss of compressive strength since the aggregates core water is not part of the cement paste and does not effect the water-cement ratio. The amount necessary is dependent on the absorptive capacity of the aggregates, which is directly related to the density of the aggregate. Although, since most aggregates are not completely dry when used, the absorptive capacity w‘_a must be defined as:

Gravimetric measurement of water content is the precise method, but also costs time and is not effective in the production of ready-mixed concrete. It is therefore a value which is to be assumed by the engineer, based on the climatic conditions of storage.

The consistency loss due to the change of the grading curve cannot be compensated with extra water since this would have a direct impact on the water-cement ratio. This loss is made up for by adding small amounts of superplasticizer at t = 0 minutes (see Fig. 2). One advantage of this method is also the fact that all technological measures take place at the production facility, not on site. Of course, this is only possible if the ready-mixed concrete is delivered promptly, depending on the original consistency, outside temperature and many other factors.

Adding Superplasticizer at any given time (Method 2)

By adding Superplasticizer at any given time, depending on the concrete‘s compacting factor at that moment, the workability consistency (KR) can be achieved without adding any extra water after the initial mixing process. Although this means a higher dosage of superplasticizer than in method 1, the extra cost ist compensated for by the lower water-cement ratio and therefore higher compressive strength than in method 1.

This later dosage must be performed preferrably shortly before application to the concrete forms and requires manpower on site to perform the necessary consistency correction. In laboratory tests, the development of rigidity after adding superplasticizer runs mostly parallel to the original curve (see Fig. 3).

This method compensates for both effects, but results in different effective water-cement ratios from mixture to mixture depending on the amount of absorbed water in the aggregates since the water absorption is not taken into account in the design of the concrete mixture. The mixture is designed with w’_a=0 (wet aggregates, no surface moisture), so that if dry weather creates a w’_a³ 0, it will reduce the effective water-cement ratio and improve the hardened conrete’s quality. This water content must be considered in the value of apparent specific gravity of the aggregate.

EXAMPLE AND CONCLUSION

A production facility for ready-mixed concrete has been established near the Technical University of Darmstadt. It is part of the R&D project "Concrete construction regarding building material cycles" (German:"Baustoffkreislauf im Massivbau", abbreviated:"BiM") which is financed by researching institutions and economical partners on one side and the German government on the other (Fig. 4). It is exclusively equipped with recycled aggregates from concrete rubble, producing for various demonstration sites planned. One of these demonstration buildings is currently under construction in Darmstadt. In this case method 2 is being used, with manpower on site to measure consistency degree and dose the amount of superplasticizer necessary for consistency correction.

Quality assurance is being supervised in a more accurate way than it is in the case of regular concrete to secure the standards set by german building codes. Still, variation of results (compressive strength) is inevitable, since water content and therefore absorptive capacity is not fully considered at this time. To limit variation, the knowledge of moisture amounts, both in the core and on the surface of all grains must be a known factor in the dosing and mixing process to guarantee a constant effective water-cement ratio.

This can be achieved in future by modern methods of moisture measurement, which are already in use for measuring the complete moisture content of aggregates, but not differing between core- and surface moisture. At this time, moisture measurement is installed for measuring the surface moisture of the sand fraction (0/2 mm). The other fractions (surface) moisture is set in the facilities computer control unit, considering weather conditions. These are values, which should be electronically measured in future to automate the production process to the complete. This way the effective water-cement ratio would be limited to a small variation and the original consistency at t=0 minutes would remain mostly constant. Further research in this area is necessary and already in progress.