Analytische Chemie
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Today's civil engineering NDT needs are for quantitative, fast to apply and easy to interpret methods. They are required on site both for evaluation of existing structures/infrastructure and for quality control in new construction. With this purpose, BAM has carried out in-situ feasibility studies, using radar, impact-echo and ultrasonic-echo methods. Two examples are given of their combined application on different railway slab–track constructions, using high frequency radar, scanning impact-echo and ultrasonic array methods. The aim was to evaluate limitations and advantages, comparability and complementarity of these methods. The on-site measurements demonstrated the possibility to investigate, with good accuracy via NDT, the bonding conditions between sleepers and slab, to locate voids and measure layer thickness. The acoustic and electromagnetic methods used, were found to be complementary.
The changes in the sonic surface wave velocity of concrete under stress were investigated in this paper. Surface wave velocities at
sonic frequency range were measured on a prismatic concrete specimen undergoing several cycles of uniaxial compression. The
loading was applied (or removed) gradually in predefined small steps (stress-controlled). The surface wave velocity was measured
at every load step during both loading and unloading phases. Acoustic Emission (AE) test was conducted simultaneously to
monitor the microcracking activities at different levels of loading. It was found that the sonic surface wave velocity is highly stress
dependent and the velocity-stress relationship follows a particular trend. The observed trend could be explained by a combination
of acoustoelasticity and microcracking theories, each valid over a certain range of applied stresses. Having measured the velocities
while unloading, when the material suffers no further damage, the effect of stress and damage could be differentiated. The slope
of the velocity-stress curves over the elastic region was calculated for different load cycles. This quantity was normalized to yield a
dimensionless nonlinear parameter. This parameter generally increases with the level of induced damage in concrete.
Amplitudes variation of GPR rebar reflection due to the influence of concrete aggregate scattering
(2015)
Dense GPR measurements of rebar reflection amplitudes Show relative variations, which can be in the order of more than 10% - 20%. Former investigations demonstrated that these variations are caused by the heterogeneity of concrete, i.e. due to the inclusion of aggregates in concrete. These amplitude variations make it difficult to analyse single reflection amplitudes in order to determine the rebar diameter or to estimate the concrete deterioration state. In a systematic study we have quantified the statistical variation of the rebar reflection amplitude for concrete covers of 6 cm, 9 cm, 12 cm, 15 cm and 18 cm, for two different grading curves and for the rebar diameters 12 mm and 28 mm. Also the influence of the wavelength has been investigated by using antennas with different centre frequencies in relation to the aggregate size. The results are discussed with regard to a quantitative amplitude evaluation of GPR measurements and also the potential of using these variations for a characterization of concrete material properties.
Supplementary cementitious materials (SCMs) are largely used all over the world. The leading SCMs are fly ash (FA) and slag. However, such materials originate from industries that are not well established in sub-Saharan Africa (SSA). Therefore, it becomes extremely challenging and expensive to build up sustainable cement and concrete. For SSA, agriculture is one of the leading economic sectors. Agricultural by-products such as rice husk ash (RHA), can be a potential binder material beneficial for use in a cementitious system. RHA combined with limestone filler (LSF) is an option that can be practised for rice producing countries in Africa.
Recently developed new transducers for ultrasonic transmission, which can be embedded right into concrete, are now used for non-destructive permanent monitoring of concrete. They can be installed during construction or thereafter. Large volumes of concrete can be monitored for changes of material properties by a limited number of transducers. The transducer design, the main properties as well as installation procedures are presented. It is shown that compressional waves with a central frequency of 62 kHz are mainly generated around the transducer's axis. The transducer can be used as a transmitter or receiver. Application examples demonstrate that the transducers can be used to monitor concrete conditions parameters (stress, temperature,
) as well as damages in an early state or the detection of acoustic events (e.g., crack opening). Besides application in civil engineering our setups can also be used for model studies in geosciences.
Ultrasonic echo testing is widely used in non-destructive testing in civil engineering to investigate concrete structures, to measure thickness, and to locate and characterise built-in components or inhomogeneities. Currently, synthetic aperture focusing techniques are mostly used for imaging. These algorithms are highly developed but have some limitations. For example, it is not possible to image the lower boundary of built-in components like tendon ducts or vertical reflectors. We adopted reverse time migration for non-destructive testing in civil engineering in order to improve the imaging of complicated structures in concrete. By using the entire wavefield, including waves reflected more than once, there are fewer limitations compared to synthetic aperture focusing technique algorithms. As a drawback, the required computation is significantly higher than that for the techniques currently used. Simulations for polyamide and concrete structures showed the potential for non-destructive testing. The simulations were followed by experiments at a polyamide specimen. Here, having acquired almost noise-free measurement data to test the algorithm, we were able to determine the shape and size of boreholes with sufficient accuracy. After these successful tests, we performed experiments at a reinforced concrete foundation slab. We obtained information from the data by reverse time migration, which was not accessible by traditional imaging. The imaging of the location and structure of the lower boundary of the concrete foundation slab was improved. Furthermore, vertical reflectors inside the slab were imaged clearly, and more flaws were found. It has been shown that reverse time migration is a step forward in ultrasonic testing in civil engineering.
The laser-induced breakdown spectroscopy (LIBS) is a fast method to provide multi-elemental analysis of any sample. At the Federal Institute for Materials Research and Testing (BAM) the LIBS technique is applied on building materials to measure ingress profiles of harmful species like chloride and alkalis. The ingress depth and the quantitative amount is important for the evaluation of the potential for damage processes like the alkali-silica reaction or chloride-induced corrosion. Concrete as an example is a highly heterogeneous material with 1/7 cement (major component CaO) and 6/7 aggregates (SiO2) with different grain sizes. Due to a scanning procedure a two dimensional element distribution of a concrete surface can be measured. In order to have an automated Separation method to evaluate heterogeneous materials, different cluster algorithm have been tested. Best results have been achieved with the Expectation-Maximization-Algorithm (EM-Algorithm).
As groundwork for thickness determination of polymeric surface protection systems for concrete, we present a method for measuring the thickness of isolated semitransparent solids using pulse thermography both in Transmission and reflection geometry. Since standard models do not capture semitransparency, an advanced analytical model by Salazar et al. is applied. Physical material parameters are deduced by fitting experimental data from samples of well-known
thickness. Using those, the thickness of samples of the material can be obtained by fitting, as demonstrated for different semitransparent polymer materials.
Chemical admixtures like superplasticisers or stabilising agents are of ever increasing importance for modern concrete technology. They liberate the workability of concrete from its dependency on water content, and thus, open the gate towards innovative and future oriented concrete technologies such as self-compacting concrete. Today, admixture addition has become common practice in concrete technology, but the understanding of their highly complex mode of operation is extremely difficult and demands for understanding of processes within the range between nanometres and centimetres. Due to its complex time-dependent, multi-phase and multi-scale behaviour, flowable concrete systems are highly complicated and cannot be described comprehensively by simple models. It is therefore extremely challenging to identify the relevant parameters that predominantly control flow phenomena on different size scales, since these may occur on any scale between the nano scale (e.g. superplasticizer adsorption) and macro scale (e.g. grading of the aggregates). The present study discusses fundamental mechanisms at the interface between particle or hydrate surfaces and the fluid phase at a very early stage of concrete formation, and links these effects to macroscopic flow phenomena. Methods are discussed that appear promising interdisciplinary tools for enhancement of the understanding of the relevant interactions that are responsible for the macroscopic flow of flowable concrete.
Interactions between waste paper sludge ashes and superplasticizers based on polycarboxylates
(2016)
In many industrial nations, about two third of the paper demand is covered by recovered paper. A major process step within the treatment of waste paper is the de-inking. It is a floating process yielding paper sludge as a waste product. About 50 % of this residue is used as a fuel. In several cases it is burnt at temperature of about 850 °C and thereafter the accrued ashes are collected in the flue gas filter. During the combustion, kaolinite and calcium oxide generate gehlenite and larnite. Calcite is the main component of waste paper sludge ash (PA).The chemical and mineralogical composition of PA suggests using it as a supplementary cementitious material.
In modern construction materials technology, workability aspects gain importance, since for most modern materials the rheology and compaction ability are relevant for the operation at a hardened state. It was observed that PA significantly increases the water demand of powder systems, which can cause serious problems during the casting of mineral binder systems containing PA. It is therefore obvious that binder systems containing PA might demand for the use of superplasticizers. Superplasticizers are polymers with anionic backbone that cause electrostatic and steric repulsion effects upon adsorption on surfaces of particles and hydration phases.
In this paper interactions between superplasticizers and waste paper sludge ashes are discussed and analysed. Based on observations of changes in the zeta potential and the dispersion of the particle system, the influence of the charge density of superplasticizers is observed and time dependent effects are demonstrated.