IMPLEMENTATION OF LONG TERM MEASUREMENTS AT A BUILDING MADE OF CONCRETE WITH AGGREGATE DERIVED FROM CONCRETE RUBBLE

Andreas Garg, Gerhard Koster, Markus Rühl

SUMMARY
A measuring equipment was installed in two buildings of the demonstration building project "Vilbeler Weg" in Darmstadt, in order to determine deformations of the bad bearing structure. The buildings under observation had been built symmetrical, with identical structural design and nearly the same use. Both structures are reinforced concrete constructions, one erected with concrete made with natural dense aggregates the other with aggregates derived from recycled concrete rubble. The measuring set up is an integral part of the architectural design of the building designed to measure the expected long-term difference in deformation of the two structures. The analysis of data captured during the first six months after completation of the building verify the laboratory results regarding deformation behaviour of concrete with natural dense aggregate and recycled aggregate under practical conditions.

1 INTRODUCTION

The research project called "Recycling of Mineral Building Materials" (Baustoffkreislauf im Massivbau, BiM) financed by the German Governement and the building industiry, shall give an answer to the question, how to regulate the use of the recycled aggregates. Parallel to this research project, the Bauverein AG in Darmstadt erected an office building with parking areas. Two sections of this building are symmetrical. One of these sections is made of concrete with recycled aggregate derived from concrete rubble. The other one is constructed by using concrete with natural dense aggregates. So it is possible, to observe the different deformation behaviour of bearing structures made of the two types of concrete. Between November 1997 and February 1998, about 480 m³ concrete with recycled aggregate were built in.

2 MIX PROPERTIES

Different deformations can be explained by the difference of the concrete mixes. Table 1 shows the design mixes used in building 1 (natural dense aggregate) and building 3 (aggregate derived from concrete rubble).

The actual grading curve of both aggregate mixes is AB 16 according to DIN 1045.
The aggregate derived from recycled concrete rubble were produced at a preparation company near Darmstadt (Baustoffaufbereitungsanlage K&S GmbH, Blittelborn).

3 PROPERTIES OF THE FRESHLY MIXED AND HARDENED CONCRETE

The compressiv strength, the static elastic modulus, the tensile splitting strength, the air quantity, the consistency and the apparent dry density of the two different concrete mixes were measured and are shown in Table 2.

The differences of the freshly mixed and hardened concrete properties can be explained with the differences between natural dense aggregate and aggregate derived from recycled concrete rubble. Because of the higher cement ratio, the compressive strength of both mixes is nearly the same. The elastic modulus is about 34 % lower than the elastic inodulus of the concrete made with natural dense aggregates.

The lower value of the elastic modulus of the concrete made with recycled aggregate, causes a different deformation behaviour. The deformation under bad in the range of working stress, shows a proportional behaviour to the elastic modulus. In this case, the modular ratio of 1,5 between concrete with recycled aggregate and concrete made with dense aggregate can be found.

4 MEASURED PROPERTIES

To investigate the different deformation behaviour of concrete made with recycled aggregate, following attributes were measured:

• Bending under bad (4. Floor)
• Concrete Shrinkage (4. Floor)
• Horizontal and Vertical Deformation (Wall, 4. Floor)
• Concrete Temperature (Wall, 4. Floor)
• Outdoor Temperature

5 MEASURING POINTS

The compound units of the bearing structure are made of reinforced concrete. The floors were built as flat ceilings and the bad transfer is assured by pads and walls. So these are points of interest. Also there are deformations in the flat ceilings. The deflection between the supporting points is the result of the combination of dead boads and live boads. With the help of the FE-Program "Infograph, Version 4.62", the internal force distribution and the expected deformations were calculated. Figure 1 shows the calculated deformations of the flat ceiling over the third floor in building one.

This figure shows, that the maximum elestic deformations will occure between the pads of the non strutted edge design. The calculated value of deformation is about 4,18 mm in the construction made of concrete with recycled aggregate and about 4,07 mm in the construction made of concrete with natural dense aggregate. (Remark: The FM-Program is only able to calculate the deformation of reinforced concrete in state 1). The maximum of concrete compression is expected near these points. These values will be observed of the upper side of the flat ceiling. The calculated tensile stresses are about 4.42 MN/m² and 4.31 MN/m². To determine the influence of the environmental and the concrete temperature, these values were captured too.

6 INSTRUMENTATION

The concrete shrinkage is measured by improved strain transducers developed at the Institut für Massivbau. The principle is based on strain gauges in full bridge configuration, see Figure 2. Anchor plates at both ends provides a good bonding between the measurement elements and the concrete. The sensors were prevented against humidity and rough handling during casting by a special sealing. To eliminate thermal effects, the concrete temperature is measured by thermocouples nearby.

The deformation is measured by inductive displacement sensors, with a related temperature sensor (thermocouple).
The temperature sensor for the environmental temperature is placed on the northem side of building 3 to avoid the effect of direct radiation. In addition, there is a temperature sensor on the front side of the building 1, behind the sunprotectors.
Fig. 3 shows the instrumentation scheme. The main measurement station with the personal computer is located in building 3. In each building there are 8 measurement points. The sensor signals were fed to the measurement system spider8 from Hottinger Baldwin Meßtechnik. These devices provide the necesary signal conditioning/amplification for 8 channels and the analog to digital conversion. They were controlled by an personal computer, connected either via RS 232 or the printer port.

One problem in distributed measurement systems is the timing. If each system is using its own clock, differences will occur caused by drifting and lack of accuracy. To avoid these, both measurement systems were controlled by a single personal computer. Therefore, the station in building 1 is connected to the station in building 3. The distance between the two buildings is 100m, due to the wiring through the basement, the actual cable-length is approximately 200m The specifiation of the printer port allows only a cable of max 2m, whereas the R5232 specification allows cables up to 25m. For this reason the two stations were connected via line modems, connected to the personal computer by R5232. Figure 3 shows the measurement station in buiding 3. The spider8 and its power supply can be seen in the upper part of Figure 3. Beneath there is the wiring box in the left and the line modem in the right. The personal computer is not shown here, for the situation in building 1 looks alike.

The system runs on the self developed software package MW described in [1], with a newly instrument driver for the spider8. The software does not require a high-performance personal computer. This leads to the interesting aspect in this project: making use of a "recycled PC" , too slow for office aplications. The line modems had been used in the past to connect teletype terminals with a host computer.

The data are stored during the measurements simultaniously on the harddisk. For the evaluation of the data, the software provides several export filters for standard Software. For Microsoft EXCEL an import filter is now available, therefore the measuring data captered can be loaded directly.

7 FIRST RESULTS OF THE MEASUREMENTS

In figure 5 the different deflections of the flat ceilings are shown. Also the temperature difference beetween two measurements is given. The deformation of the flat made of concrete with natural dense aggregate and the flat made of concrete with recycled aggregate, are showing the same characteristic behaviour in relationship to the change of temperature. Compared to the flat made of natural dense aggregate, the absolute changes of deformation depending on the variation of temperature, are higher. This can be explained with the lower density of the concrete made with recycled aggregate and the lower coefficient of thermal expansion of the aggregate derived from recycled concrete rubble. The amount of deformation of the flat in building 1 is about 12,5 mm, the value of deformation of the flat ceiling in building 3 (concrete with recycled aggregate) is about 20 mm. The quotient of these values is about 1,6. This factor can also be found by deviding the the amount of the elastic modulus of die concrete made with natural dense aggregate by the value of die elastic modulus from concrete made of aggregate derived from recycled concrete rubble.

The first results show, that the expected differences in the deformations have been occured. The deformations depend on the different elastic moduli of the used concrete mixes. The deformations in the flat ceiling made of concrete with recycled aggregate resulting from temperature changes, are not so extreme because of the lower coefficient of thermal expansion. In constructions in which deformations have to be considered, the hardened concrete's properties, resulting from the use of recycled aggregate, have to be evaluated by tests. Conventional building components made of concrete with recycled aggregate, can be designed with the same characteristic values as components of concrete made with natural aggregate.

References