german trends.pdf

143

GERMANY –

CONCRETE CONSTRUCTION INDUSTRY - CEMENT BASEDMATERIALS AND CIVIL INFRASTRUCTURE (CBM - CI)

Dr. Klaus Holschemacher Prof. and Dean of the Department of Civil Engineering and ArchitectureLeipzig University of Applied SciencesLeipzigGERMANY

CBM-CI International Workshop, Karachi, Pakistan Dr. K. Holschemacher

ABSTRACT: This paper summarizes significant trends that have influenced the developmentof the German concrete industry within the last 10 years. Among these trends are advancesin cement-based materials, the increased importance of quality assurance, changes in almostall design codes, and new or improved construction methods.

In spite of the fact that there was a strong economic recession, some outstanding construction projects were realized in the last decade, especially in the field of civil infrastructure. For example, in the context of the construction of new federal highways and railway lines, theerection of large-span bridges undtunnels was necessary. A few past and future projects are presented in the paper.

Finally, a short outlook on expected future developments of German concrete industry isgiven. In the next years, the main tasks of construction industry are the maintenance andreevaluation of existing structures.

1. INTRODUCTION

To understand the current situation of the German concrete industry, it is necessary to knowsome statistical aspects.

1.1 Population

At present, 82.5 million people live in Germany. For several years the population growthrate has been a value near zero, with a slightly negative tendency. The area of Germanyamounts 357,093 km2 (adequate 137,874 square miles). The ratio of population age 65 andover to the total population is 18.9% today, but it will increase to 21.9% in the year 2020.Even more disadvantageous is the ratio of the inactive population aged 65 and over to thetotal labour force. This ratio is 36.7% today, and it will rise to 43.7% in 2020. However,the problems arising in this context are the same as in the most industrialized countries.

144

After the German reunification in 1991, there was an unbalanced distribution of economic power between the eastern, formerly Soviet controlled districts and the western part of thecountry. The bad economic situation in the eastern districts led to a high unemploymentrate up to 20% or more in some regions, whereas this rate was just 4 to 9% in the westerndistricts. The consequence was a migration of more than two million people from east towest within a period of only few years. This presented large challenges to the constructionindustry. In particular, the need for efficient traffic routes and housing had to be satisfied.

1.2 Economy

The German gross domestic product amounted 2,900 billion USD in 2005. About 250 billion USD were invested in construction projects that year. The number of employeesin the construction industry was 717,000. However, there was a contraction of the Germanconstruction industry for several years. In Figure 1.1, this unfavorable circumstance isexemplarily shown for cement production. This tendency ended in 2005. Since 2005 therehas been a significant upturn in the construction industry.


Figure 1.1 Development of cement production in Germany.

1.3 Study of Civil Engineering

At present, there is a reorganization of the courses of civil engineering and architecture atall German universities. The old German degree “Diploma-Engineer” is being replaced bythe international common degrees “Bachelor” and “Master.” Students are encouraged tostudy part time at a foreign university. Numerous federal and state programs financiallysupport the students in their foreign activities.

145

Furthermore the number of students of civil engineering is insufficient now. There were just 50% of students in civil engineering in 2005 in comparison to the year 1995. This ledto a considerable lack of young engineers in the construction industry in the context of theeconomic upturn in 2005.

2. RECENT TRENDS IN THE CONCRETE CONSTRUCTION

INDUSTRY

2.1 Tailor-made Concrete and New Structural Materials

Modern concrete technology is characterized by the application of admixtures and variouscementitious materials (fly ash, silica fume) in addition to ordinary portland cement. Byan appropriate composition, it is possible to create a tailor-made concrete with fresh andhardened properties corresponding to the requirements of a special construction task.

The use of fly ash and superplasticizers is common for almost all types of concrete inGermany. In most cases the demanded cylinder compressive strength is in the range between20 N/mm2 (2,900 psi) and 45 N/mm2 (6,525 psi). However, the utilization of high performanceconcrete is increasing. Some trends in the development of special concretes are discussedin the following sections.

2.1.1 Fiber Reinforced Concrete

Steel fiber reinforced concrete (SFRC) is often used for slabs on the ground and for industrialfloors. The usual fiber content amounts 20 to 50 kg/m3 (1.25 to 3.12 lb/ft3) for this purpose.For other applications, a combined reinforcement composed of steel bars or meshes andsteel fibers is favored. In this case the load bearing capacity is mainly influenced byreinforcing bars. The steel fibers predominantly limit crack width, resulting in better structural behavior in the serviceability limit states. Additionally, new design conceptsallow the consideration of steel fibers in the calculation of shear resistance.

The efficiency of SFRC is evaluated based on the results of four-point bending tests. Thespecimens used in Germany differ from the usual international ones in their geometry andthe fact that there is no notch at the bottom of the specimen (Figure 2.1). By analyzingthe results of four-point bending tests, it is possible to determine the equivalent bendingtensile strength at two deformation levels. The equivalent bending tensile strength is the basis for the classification of SFRC in fiber classes.


146

Figure 2.1 German test specimen for SFRC.

When designing an SFRC member, it is necessary to select or determine the required fiber class. Because there is no clear correlation between fiber amount and fiber class, numerousresearch projects were conducted. Of course, steel wire fibers with end hooks achieved the best test results (Figure 2.2). However, these fibers are relatively expensive, so other fiber types are used in practice too. Recognizing that the efficiency of alternate fibers may not be as good as that of steel fibers, this must be compensated for by using a higher fiber amount.


147

Figure 2.2 Load-deflection behavior of SFRC in four-point bending tests. Influence of fiber type [1].

In few cases, the application of steel fibers is unfavorable. Particularly, floors of agriculturalconstructions are appropriate for using fiber reinforced concrete with synthetic fibers. Thesurface of the concrete is subjected to abrasion caused by vehicles and, sometimes, byanimal feet. If fibers stick out of the concrete, they can potentially cause injuries or physical


148

damage. In these specific applications, synthetic fibers such as polyvinylalcohol fibers (PVAfibers) are an alternative to steel fibers. The basic properties of PVA fibers are listed inTable 1. It is remarkable that polyvinyl fiber reinforced concrete (PVAFRC) exhibits anacceptable post cracking behavior. The influence of the fiber content on the load deflection behavior of PVAFRC is shown in Figure 2.3. The test results are for a four point bendingtest.

Table 1 Basic Properties of PVA Fibers in Comparison to Steel and Gas Fibres.


Figure 2.3 Average load-deflection curves of PVAFRC [2].

149

2.1.2 Self-Consolidating Concrete

Self-consolidating concrete, (SCC) also termed as self-compacting concrete has been usedin Germany for about 10 years. Initially, this material caused an enthusiastic response inthe concrete construction industry. Currently, it has changed to a more realistic assessmentof the advantages of the SCC and identification of applications where these advantages can be realized. SCC is predominantly used for reinforced structural members with high amountsof reinforcement and in the precast concrete industry. Because of the flow characteristicsof SCC, it flows through congested reinforcement areas without segregation. The maintype of SCC used in Germany is the stabilizer type SCC, which is characterized by moderatefiller content and the application of a stabilizer. Rarely, self-consolidating steel fiber reinforced concrete and self-consolidating lightweight aggregate concrete are used inconstruction. A successful example of the usage of self-consolidating lightweight aggregateconcrete was a pedestrian bridge with a span of 20 m (66 ft) erected in 2001 near Leipzig[3]. The lightweight aggregate used was expanded clay; the resulting cylinder compressivestrength was 45 N/mm2 (6,525 psi). A special problem was the gradient of the superstructure,which posed a particular challenge for casting. Based on intensive concrete technologyresearch, the bridge construction succeeded and was a basis for the further application of SCC in Germany (Figure 2.4).


Figure 2.4. Application of self-consolidating lightweight aggregate concrete for a pedestrian bridge near Leipzig. Germany [3].

150

2.1.3 Ultra-High Performance Concrete

Based on its outstanding fresh and hardened properties, ultra-high performance concrete

(UHPC) offers new possibilities for concrete construction industry. In addition to the high

compressive strength, UHPC offers outstanding durability. Several studies on the fresh and

hardened properties of UHPC have been conducted. An extension of the European design

code for structural concrete is being drafted. This will provide better guidance for the

application of UHPC in construction practice.

In Germany the most famous structure made of UHPC is the bridge “Gärtnerplatzbrücke”

in Kassel, which will be completed in 2007 (Figure 2.5). This bridge is a 140 m (460 ft)

long pedestrian bridge with a main span of about 36 m. The superstructure consists of a

steel framework and 8 to 12 cm (3.1 to 4.7 in) thick precast UHPC slabs. The concrete has

a cylinder compressive strength of more than 165 N/mm2 (23,930 psi).


Figure 2.5 Bridge “Gärtnerplatzbrücke” in Kassel [4].

2.1.4 Textile Reinforced Concrete

Textile fabrics made of alkaline-resistant continuous glass fibers are innovative materials

that can be used to reinforce cementitious matrices. Because of the fact that glass fibers

consist of many fine filaments, the bond behaviour of this material is problematic. The

bond between the inner filaments and the bond between the outer filaments and matrix

influence the load bearing capacity of textile reinforced concrete members. Typically it is

not possible to stress the textile reinforcement up to the material strength of the textile

fibers. In most cases, the exploitation ratio is only 50%. The application of textile reinforced

151

concrete is just starting in Germany. There are few examples for usage of this material in

construction practice. It is expected that textile reinforcement may be most commonly used

in precast concrete members. A report outlining the use of SCC with textile fabrics and

GFRP-bars as reinforcement for producing a double wall concrete member was published

in [5], [6].

2.2 Other Trends

Other trends in the development of concrete construction industry include the following:

• All design codes have changed in the last years. These days, the basis of design codes

is a safety concept with partial safety factors. That means that the safety factors for

materials are different than the safety factors for the imposed loads.

• The percent portion of precast members in concrete construction has decreased. A

possible reason is the increase in the steel price.

• The importance of arrangements for quality assurance has increased.

• More and more composite structures are being used in construction practice. This

recognizes that the favorable properties of different building materials can be utilized

more effectively.

2.3 Examples of Outstanding Structures

In the last few years, many interesting structures have been constructed. Only a few

examples are presented. The enhancement of the existing highway system required the

construction of several large-span bridges. Remarkable was the rebirth of arch bridges in

this context. One of the most spectacular is the bridge across the river “Wilde Gera”(Figure

2.6) with a main span of 252 m (276 yd). For the same highway project, the largest German

tunnel “Rennsteigtunnel” with a length of 7,900 m (8,640 yd) was constructed.


152

3. FUTURE OF THE CONCRETE CONSTRUCTION INDUSTRY

The next decade will bring large challenges for the German concrete construction industry. The main challenges include:

• The need for improved durability of concrete structures. Due to this need, the use of high performance concrete is increasing and will continue to increase in the constructionindustry.

• Limitations on the available raw materials in some regions, especially coarse aggregates.Due to this, the importance of sand-rich concrete as an alternative to concrete withnormal grading will increase in the future.

• Maintenance of existing constructed facilities. The evaluation and re-evaluation of existing structures will be one of the important challenges for the German constructionindustry in future. The development of useful composite structures, such as timber-concrete composite members, will be highly desirable.

REFERENCES

[1] Holschemacher, K. & Müller, T., Effect of fibre type on properties of steel fibre reinforcedconcrete. Proceedings of The Tenth East Asia-Pacific Conference on Structural Engineering& Construction, Bangkok, Thailand 2006, S. 383–388.

[2] Holschemacher, K., Pachow, U. & Höer, S., Polyvinylalcohol fibre reinforced concrete for

industrial floors. Proceedings of The Tenth East Asia-Pacific Conference on StructuralEngineering & Construction, Bangkok, Thailand 2006, S. 481–486.


Figure 2.6 Arch bridge across the river “Wilde Gera.”

153

[3] König, G., Holschemacher, K., Maurer, Dehn, F., Pedestrian and Cycling Bridge as a Pilot

Project in the Use of Self-Compacting Concrete. 6 th International Conference on Short andMedium Span Bridges. Vancouver 2002.

[4] Bornemann, R., Teichmann, T. & Schmidt, M., Influence of mix design on fresh properties,strength and durability of UHPC. König, G., Holschemacher, K. & Dehn, F. (Ed.), Ultra HighPerformance Concrete, Bauwerk, Berlin 2002, pp. 55-65 (in German).

[5] Klug, Y. & Holschemacher, K., Textile reinforced slabs and prefabricated double walls. The3rd International Conference on Structural Engineering, Mechanics and Computation, CapeTown, South Africa 2007.

[6] Holschemacher, K. & Klug, Y., Development of self-compacting concretes for textile reinforcedstructural members. The 3rd International Conference on Structural Engineering, Mechanicsand Computation, Cape Town, South Africa 2007.


german trends.pdf

Documents