Klaus Holschemacher

RISK MANAGEMENT IN CONSTRUCTION PROJECTS

AbstractRisk management is an important field of construction industry and has gained more importance internationally due to the latest researches carried out on a large scale. However, this relatively new field requires more attention to bring some benefit. Construction projects are facing a number of risks which have negative effects on project objects such as time, cost and quality. This study is based on findings of a questionnaire-based survey on risk management in construction projects in Pakistan, reporting the significance of different type of risk, ultimate responsibility for them and the effectiveness of some most common risk management techniques practiced in the industry. Two types of risk management techniques were considered: preventive techniques which can be used before the start of a project to manage risks that are anticipated during the project execution; and remedial techniques that are used during the execution phase once a risk has already occurred. The study revealed that financial ...

Bond of Reinforcement in Ultra High-Strength Concrete

Ultra high strength concrete (UHSC) is undoubtedly an engineered high-tech material, which can be seen as the latest step in concrete technology development. Because this concrete type is relatively new, the structural behavior in the ultimate limit states as well as in the serviceability limit states is not fully investigated and understood, so is the bond behavior of reinforcement. An experimental program was set up to investigate the bond behavior of conventional reinforcement and of reinforcement with a new deep-ripped surface pattern in ultra high strength concrete. The assessment of the bond behavior concrete ages of 3, 7, 28 and 56 days using pull-out specimens was the subject of the investigation. Varying parameters in this research program were rebar diameter, type of reinforcement, surface geometry of the rebar, size of the concrete cover and the loading rate. The concrete comprehensive and splitting tensile strength was measured at the same test ages. In general, the bond of reinforcement in UHSC is characterized by very high maximum bond stresses as well as high bond stiffnesses, even at early ages. The reinforcing mesh bars with the “deepribbed” surface showed a remarkably ductility. This paper presents the details of the investigation and discusses the results obtained.

Hardened material properties of self‐compacting concrete

Abstract Self‐compacting concrete (SCC) is an innovative construction material with a favourable rheological behaviour, which is caused by its modified concrete composition. Based on this fact SCC offers improved fresh concrete but also hardened material properties and therefore many advantages regarding the productivity and the design potential compared with normal vibrated concrete. Consequently, the amount of SCC, used for structural purposes has strongly increased worldwide in the last years. In this context it is necessary to know, if it is possible to apply the current design rules, eg Model Code 90 and Eurocode 2, that are based on years of experience on normal vibrated concrete, to structural members made of SCC as well. This paper represents the analysis of own and internationally published test results of the compressive strength, tensile strength, modulus of elasticity, bond behaviour and the time‐dependent deformations of SCC in comparison with conventional concrete, in order to give a general...

Experimental Investigation of Full Scale Two-Layer Reinforced Concrete Beams

Two-layer beams (TLB), consisting of steel fibered high strength concrete (SFHSC) in the compression zone and normal strength concrete (NSC) in the tensile one, are studied experimentally. The current study is based on results of previous theoretical investigations and tests that showed high efficiency of such beams, carrying rather big bending moments. According to the previously developed concept, the fiber content for TLB should be calculated, corresponding to the required ductility level of an RC element. Following the results of a previous experimental study that were carried out to select the optimal fiber ratio, the current research is focused on testing full scale TLB. The study is aimed at experimental verification of the data related to interaction of concrete layers in TLB and proving the efficiency of two-layer bending element from beginning of loading and up to the ultimate load stages, including collapse. Development of Poisson deformations was studied to compare them with those obtained in the previous study for cylindrical specimens. The SFHSC layer was cast after the NSC hardening to study the influence of separate casting technology that is more convenient for TLB production in real construction. Three identical specimens with constant fiber content, corresponding to the proper ductility level, were prepared and tested by four-point loading. The results demonstrate the role of fibers in a high strength concrete layer and form a basis for development of TLB design provisions.

Laboratory Tests of Two-Layer Beams Consisting of Normal and Fibered High Strength Concrete: Ductility and Technological Aspects

General concepts for the design of two-layer beams, consisting of fibered high strength concrete in a compressed zone and normal strength concrete without fibers in the tensile one, were developed about 2 years ago. It was shown that such beams are effective when the reinforced concrete (RC) section carries rather big bending moments. Steel fibers have little effect on beams’ elastic deformations, but increase the ultimate ones, due to the additional energy dissipation potential of fibers. Changing the fibers’ content, a required ductility level can be achieved. Providing proper ductility is important for design of structures to seismic, wind, and other dynamic loadings. The idea of two-layer beams was further developed for pre-stressed beams that become, in this case, high performance concrete elements. It was demonstrated that calculation of fibers’ content for such elements is important, like that of reinforcing steel bars for usual RC beams. The current study is focused on finding optimal fiber content, yielding the highest Poisson coefficient, and consequently higher ductility of the beams section. Fiber weight ratio is used as an alternative to fiber volume ratio as the first is a more accurate parameter for the definition of fiber content in the concrete mixture. Additionally, manufacturing technology and its influence on distribution of fibers in the beams cross sections were investigated, as this distribution has a direct influence on the sections ductility. The experimental results, obtained in the frame of this study, form a basis for general technological provisions, related to manufacturing of two-layer beams.

Composite Constructions of Timber and High-Performance Concrete

Currently Timber-Concrete Composite (TCC) Constructions are often applied for strengthening existing timber beam slabs. The load bearing capacity of the composite construction is primarily affected by the material properties of the timber beam and the concrete slab. But the type of bond between both parts is also of high importance. The concrete slab has to perform several tasks, not only in load carrying direction of the ceiling but also perpendicular to the direction of span or for stiffening the whole building. These tasks will be pointed out in this paper. Furthermore the working process (easy workable mixture and exchange of conventional reinforcement) and the dead load of the construction are of particular interest in the field of redevelopment. Several innovative concretes have been verified for the use in TCC constructions. Regarding their fresh and hardened concrete properties, they all can be described as High Performance Concretes (HPC). In this paper Self Compacting Concrete (SCC), Fiber Reinforced Concrete (FRC), Structural Lightweight Concrete (SLWC), High Strength Concrete (HSC) or combinations of them will be focused. Especially the advantages but also the disadvantages of innovative concretes for the use in TCC will be presented as well as the results of some experimental investigations.

Experimental Investigation on Ultra High-Strength Concrete under Concentrated Loading

This report presents the results of an experimental program dealing with ultra high strength concrete (UHSC) under concentrated load. The implemented tests should be used for the determination of the bearing capacity under concentrated load and for the observation of the failure behavior, Specimens made of plain and reinforced concrete were tested, whereby a helical reinforcement was used. The results were compared with an extrapolation of existing design rules established for normal and high strength concrete. In this context, two different UHSC and one high strength concrete for comparison were investigated in order to recognize the influence of load. The results showed that the bearable concentrated load depends on the compressive strength and also on the concrete composition. Furthermore the under-proportional increase of the tensile strength in relation to the compressive strength has an important influence on the bearing behavior of UHSC.

Pull-out Behaviour of Steel Fibres in Self-Compacting Concrete

Absract: Self-Compacting Concrete (SCC) is an upcoming building material which is characterized by many favourable hardened and fresh concrete properties. However, like all concretes, also SCC has the disadvantages of a low tensile strength and a large brittleness. To improve the tensile strength, the ductility and the toughness it is useful to reinforce the concrete with steel, glass or synthetic fibres. In practice, the often favoured steel fibres are available in a very wide variety regarding characteristics like strength, size, shape and surface properties. Until today there is an insufficient knowledge about the influence of the mentioned fibre characteristics on the hardened properties of steel fibre reinforced self-compacting concrete (SFRSCC). For this reason a research program was started, focused on the pull-out behaviour of different types of steel fibres in SCC. Within the experimental investigations a SCC and in comparison a normal vibrated concrete, both with a compressive strength of approximately 65 N/mm, in combination with two different steel fibres were used. The influences of the fibre end hook, the embedment depth and the concrete age on the pull-out behaviour of the fibres were determined. The results of the investigations can be summarized as follows: For fibres with end hooks the concrete age and the embedment length are negligible parameters. The load transfer from the fibre to the matrix is almost completely realized by the end hooks. Furthermore, it is to note that the scattering of the force-displacement curves is relatively high. Hence, a high number of pull-out tests were necessary to get clear tendencies. By the interpretation of the test results a better choice of a suitable fibre type regarding the application conditions is possible. The main consequence is a better and faster development of steel fibre reinforced self-compacting concrete mix designs for utilization in practice.

Lateral Load Bearing Behavior of Timber-Concrete Composite Constructions

The idea to combine the building materials timber and concrete is well known since the 20th of the last century. While timber is beneficial in bearing tensile forces, concrete on the contrary sustains compressive forces very well. Thus, connecting both materials in the way that loads can be transferred between them, their advantages can be used to improve the load bearing capacity of structures. A lot of researches were carried out to investigate different problems of timber-concrete composite (TCC) constructions during the last years. Yeoh et al. [1] for example gave an overview of different full-scale short-term collapse tests in order to evaluate the load bearing behavior of different TCC systems. Nevertheless, some questions are still very few explored, among them is the lateral load bearing behavior of TCC constructions. These structures can be considered as 3 dimensional plane structures, being able to distribute loads along and perpendicular to the span of the timber beams. A concentrated load (applied to one beam of the TCC construction) will deflect it due to bending. All beams are connected with each other because of the concrete slab. For this reason adjacent beams also deflect even if no loads are applied to them directly. These beams contribute to the load bearing of the loaded beam and relieve it partly from the loads, reducing stress in this beam. The described behavior, of distributing loads perpendicular to the span, is called lateral load bearing behavior. This paper will report on short-term bending testes of TCC slabs, carried out at Leipzig University of Applied Sciences. Several slabs have been tested. Each of them had a span of 3.9 m, consisting of three timber beams (10/20 cm) and a connecting concrete slab (thickness 6 cm). Only the middle beam was loaded in the third part points. During the test deflections, as well as strains, at different points were recorded. Additionally, the concrete properties were determined and push out tests were carried out. Overall, seven TCC slabs were manufactured and tested. With the slabs several parameters have been varied, e.g. the type of concrete, the stiffness of the connection system, and the center distance of the timber beams. By varying these three parameters, their effect on the lateral load distribution was examined by measuring deflection and strain at different points of the composite slab. A timber beam ceiling, as reference sample, without concrete topping was tested additionally. The experiments and its results will be described in detail in the paper.

Smart CFRP systems - Fiber Bragg gratings for Fiber Reinforced Polymers

Since 1970 the development of optical fiber measurement systems has begun. Especially the measuring of strain and temperature with Fiber Bragg Gratings (FBG) is interesting, because there are a large number of advantages in opposite to electrical measuring methods. Examples are small dimensions, low weight, high static and dynamic resolution of measured values, corrosion resistance, variable forms, multiplex behavior, long distance monitoring and high durability. In the past there were several attempts to use optical systems for monitoring of concrete structures. One example is the strain measurement in bar reinforcement. Another possibility is the embedding of optical fibers with FBG in Carbon Fiber Reinforced Polymers. Fiber Reinforced Polymers (FRP) have got more and more important during the last decade. In civil engineering the main usage of FRP is the repair of concrete structures. CFRP systems for retrofitting of concrete structures with optical sensors have already been discussed in several publications. It could be shown, that the reinforc- ing function of the CFRP can be ideally connected with the measurement and monitoring functions of the op- tical sensors like FBGS. The main problem is the fixing of the glass fiber and the small FBG at the designated position. In this paper the possibility of setting the glass fiber with embroidery at the reinforcing fiber mate- rial will be presented. Experiments will show the functionality of the method.