One key aspect of prestressed carbon fibre-reinforced polymer (CFRP) tendons in direct bond with high performance concrete (HPC) are the properties of the bond in the transfer zone. Bond-slip-behaviour has been widely studied in the past by pull-out experiments with a focus on the surface treatment of the tendons, e.g. smooth surface, ribbed, braided or sand-coated or of stranded tendons. In those studies, mainly the force and the slip at the free end was measured. In contrast to that, we focused in this research project on sand-coated CFRP tendons prestressed up to 1700 MPa, where distributed fibre optical sensors (DFOS) were integrated in the CFRP tendons. The integration of the optical fibre did not affected the bond. This allowed to measure the strain distribution in the transfer zone with a high spatial resolution. Furthermore, the axial tendon strain of the prestressed structures were monitored for up to 1 year. Prismatic samples with different concrete cover, prestress level and time to release were produced and their load transfer zones monitored. In addition, by varying the dimensions of the prisms, the minimal required concrete cover to sustain the complex stress field in the transfer zone was determined. This allows to fully exploit the advantage of the non-corroding CFRP tendons, resulting in lighter structures. The reliability, accuracy and robustness of the integrated optical fibre was proven. This study gives a novel insight into the long-term stability of the bond in the transfer and anchorage of pretensioned, sand-coated tendons in combination with prisms made of HPC.

Pull-out tests are standard procedures to assess the bond between reinforcement and concrete. They allow to compare the effectiveness of the bond of different rebars or tendons. Those mostly quasi-static tests investigate mainly the short-term behaviour for small embedment lengths, where a reasonably uniform bond stress is assumed. This, however, does not reflect the situation in prestressed structures, where non-uniform bond stresses and longer transfer lengths result and where the pretensioning load needs to be sustained for the whole service life of the structure. For that reason, the transfer length of sand-coated carbon fibre-reinforced polymer (CFRP) tendons was studied here using embedded distributed fibre optic sensors (DFOS). The strain distribution of superior measurement quality enabled the calculation of the local bond stress and the local slip. This allowed a comparison of bond stress vs. slip curves of several experiments together with pull out results. Prisms of different size and beams were studied, all pretensioned using sand-coated CFRP tendons with tendon stresses up to 1700MPa. Different bond mechanisms were identified, leading partially to high variations in the transfer length. Furthermore, the long-term prestress transfer length of prisms was measured for more than 500 days. The transfer length increased by approximately 8% after 500 days vs. 28 days. Based on those results, a logarithmic creep law was proposed, which allows to predict the transfer length as a function of time. Finally, the bond between tendon and concrete was analysed using visual inspection of split prisms, X-ray tomography and optical microscopy.

Blending biochar with cement is considered as a potential strategy for compensating for the CO2 emissions of cement production. We studied two commercially available cements (a Portland-limestone cement and a Portland-limestone-calcined clay cement) blended with different types of biochar (obtained from different sources and pyrolysis processes). The finely-ground biochars were blended with cements at 5 mass-%. The progress of cement hydration was studied using isothermal calorimetry, X-ray diffraction and thermogravimetric analysis. The results show that biochars have a diluting effect on cement, similar to other inert mineral fillers, with no clear chemical effect. Standard mortars with biochar-cements achieved good mechanical properties, with a slight reduction primarily due to the dilution effect. No significant impact on hydration and mechanical properties of different types of biochars was found, despite their significantly different composition.

Concrete-filled fibre reinforced polymer (FRP) tubes (CFFTs) offer alternatives to traditional steel reinforced concrete columns or concrete-filled steel tubes, primarily due to their enhanced structural capacity and ductility, which are achieved by FRP hoop confinement, and by their reduced maintenance needs especially in coastal areas. However, the service load capacity of such elements is limited by the considerable damage to the concrete core that is necessary before the FRP confinement mechanism can be substantially mobilised. Prestressing of CFFTs in the hoop direction is proposed in this paper as a means of enhancing the service load capacity of these elements when loaded in compression, through early mobilisation of confinement stresses; this is a topic which remains comparatively unexplored in the research literature. An experimental campaign is presented using a novel, self-prestressing, self-compacting concrete developed at Empa to generate hoop prestrain of the CFRP tubes during the early stages of concrete curing. The research aims to quantify improvements in the compressive behaviour of CFFTs as a result of hoop prestress, and to compare this against otherwise identical non-hoop-prestressed CFFTs. Continuous distributed fibre optic sensors (DFOS) are used to provide comprehensive continuous measurement of hoop and axial strains. Using this method, a hoop prestress of 3.55 MPa was achieved after 28 days, resulting in a 25

Magnesia-based binders are emerging as sustainable alternatives to Portland cement. A special class are blends of MgO with hydrated magnesium carbonates, such as hydromagnesite (HY), which can reach mechanical properties comparable to Portland cement. However, the moisture sensitivity of these binders raises concerns for practical use. This study investigates the water sorption behavior and microstructural evolution of MgO/HY cement pastes with varying blend ratios (70/30, 80/20, 90/10 by mass), alongside pure MgO and Portland cement references. Samples were exposed to relative humidity conditions of 0-100% RH and characterized using 1H Nuclear Magnetic Resonance (NMR) T2relaxometry, supported by chemical analysis and mercury intrusion porosimetry measurements. Results reveal a very high water uptake at high RH and corresponding significant widening of water-filled pores. The water adsorption and its impact on the pore structure explain the (reversible) decrease of strength and stiffness at high RH reported in previous studies.

MgO-based cements represent a promising, low-CO2 alternative to traditional Portland cement. In magnesium silicate cements, M-S-H is the main phase. Although the thermodynamic properties and hydration mechanisms of this phase have been investigated, studies on its mechanical behaviour remain limited. This study aimed to determine the factors influencing the micro-mechanical properties of the MgO-SiO2 pastes. Detailed chemical (X-ray diffraction, Thermogravimetric analysis, Energy-dispersive spectrometry analysis), microstructural (water porosity), and mechanical (indentation) analyses were conducted. The source of raw materials and the production protocol (mortar mixer, ball mill, pressing) influence the mineralogy of pastes and silicon distribution. Additives have a moderate impact on the mineralogy of pastes. Samples with the lowest porosity exhibit the highest elastic properties. Once the effect of porosity is accounted for, a higher brucite content correlates with increased elastic properties.

Magnesium-based cements offer a sustainable alternative to traditional Portland cement, particularly due to their lower carbon footprint. Despite this advantage, these cements often exhibit lower mechanical properties, including reduced compressive strength. A promising approach to address these limitations is the incorporation of hydrated magnesium carbonates (HMC), such as hydromagnesite (Mg₅(CO₃)₄(OH)₂·4H₂O) and nesquehonite (MgCO₃·3H₂O) that densify the cement matrix and contribute to early-age strength. This study investigates the evolution of water-filled porosity in MgO-based cements with the addition of nesquehonite (Nq) or hydromagnesite (HY). Proton nuclear magnetic resonance (1H NMR) relaxometry measurements were carried out on sealed samples over the initial 28 days of hydration. The 1H NMR methods allowed to resolve the evolution of different water populations in the hydrating pastes: water in crystalline phases, intra-hydrate water (interlayer water), inter-hydrate water (water in between the hydrates) and free water in large capillaries. Although these populations are similar to those of Portland cements, their evolution kinetics and amplitudes are distinct. These results shed new light on the microstructure and hydration kinetics of the new MgO-based cements.

MgO-based cements offer a low-carbon alternative to traditional Portland cement as they can be produced from Mg-silicate minerals with renewable energy. As interest in these alternative binders develops, understanding of their composition and mechanical properties becomes crucial. During hydration, magnesium silicate hydrates (M-S-H) form. While the thermodynamic properties and hydration mechanisms of these phases have been studied, research on their mechanical behavior remains limited. This study investigates the microstructural and elastic properties of M-S-H pastes at the micrometer-scale after undergoing various production protocols. The impact of using carbonates as accelerators on mechanical properties was studied. Detailed chemical (XRD, TGA, SEM/EDS), microstructural (water saturation), and mechanical (indentation) analyses were conducted. When carbonates are used in the curing water, brucite is eventually consumed, but porosity increases. Overall, the plain strain indentation modulus of the M-S-H pastes is highly dependent from the porosity and the curing with carbonates doesn’t change the elastic properties.

Externally bonded CFRP strips have seen exponential growth in their use since their introduction in structural strengthening and are nowadays a mainstream material for extending the service life of deficient structures. The first CFRP-strengthened structures have already exceeded three decades of extended lifespan but –inevitably– some of them will have to be decommissioned in the not-too-distant future. Since the number of strengthened buildings reaching their end of life will increase over the years, it is crucial to consider strategies for managing CFRP waste from decommissioned structures. This is particularly due to stringent legislation regarding construction demolition waste, as well as the urgency for reducing the environmental footprint of construction materials in general. Considering that recycling of thermoset CFRPs is very challenging and the emerging technologies for carbon fibre recovery are complex, energy demanding, and practically downcycling processes, CFRP reuse seems to be currently the most viable pathway to improve the sustainability and circularity of these highly durable structural materials. This paper proposes the reclamation of EBR strips from end-of-life strengthened structures and their reuse as internal reinforcement in new precast concrete elements. Proof-of-concept trials of reclaiming bonded CFRP strips from concrete substrates are presented, along with a study of the bonding performance of reclaimed strips when embedded in cast concrete. Furthermore, the development of sustainable railway sleepers made of recycled aggregate concrete and internally prestressed with reused CFRP strip tendons is presented, as suitable candidate precast elements for the circular use of concrete demolition waste and reclaimed CFRP reinforcement.

Concrete made with blended cements with high clinker replacement ratios may be at higher risk of plastic shrinkage cracking when experiencing high evaporation rates immediately after casting. This paper investigates the plastic shrinkage behavior of concretes made with a cement with clinker replacement by a blend of calcined clay and limestone, which was compared to a conventional Portland cement and a Portland-limestone cement. In order to assess the risk of cracking, we studied early deformations and accompanying processes in concretes exposed to fast evaporation in a wind tunnel. As could be expected from previous studies, concretes made with both blended cements experienced higher shrinkage and cracking compared to ordinary Portland cement, mainly due to their slower hydration caused by a lower clinker amount and higher dosage of superplasticizer. However, the extent of plastic shrinkage cracking was similar with calcined-clay limestone cement and Portland-limestone cement.

This study investigates the impact of varying steel fiber (SF) content (0

We studied the mechanical properties of MgO/hydromagnesite mortars cured at 20 degrees C both in humid (98 %RH) and dry (57 %RH) environments. The linear storage Young's modulus was determined with quasi-static loading and dynamically by SIngle MOde Resonance Ultrasound Spectroscopy (SIMORUS) measurements. We measured the corresponding loss modulus and the nonlinear counterpart of the storage modulus. Humid environment adversely affected the evolution of the elastic properties, i.e. it caused a reduction of the linear storage Young's modulus and an increase of its nonlinear counterpart, the latter being a proxy of microstructural heterogeneity and potential damage. On the other hand, mortars cured at 57 %RH experienced a monotonous growth of the linear storage Young's modulus and decrease of the loss modulus. Similar trends were observed for compressive strength. We postulate that the lowering of mechanical properties upon moisture uptake is due to the intrinsic effect of adsorbed water on the microstructure.

An emerging strategy to compensate for the greenhouse gas emissions of products is to incorporate carbonaceous materials obtained from removed atmospheric carbon dioxide, mainly obtained through biomass conversion. This approach can turn asphalt pavements into a functional carbon sink. In particular, biochar has been used as a bitumen modifier. However, due to performance limitations, carbonaceous materials were only added in small quantities to asphalt mixtures. An alternative approach is to produce lightweight aggregates to substitute a part of the mineral aggregates of the asphalt mixture. To this end, biochar is pelletised with a hydraulic binder and water in a cold-bonding process, forming spherical pellets labelled as carbon-rich lightweight aggregates (C-LWA). Like other lightweight aggregates, C-LWA showed a reduced mechanical strength compared to conventional mineral aggregates, adversely affecting the asphalt mixture performance. Cracking and rutting resistance almost linearly decreased with C-LWA content. The direct addition of biochar had a similar adverse influence on the mixture performance. Despite a reduced performance, adding biochar and C-LWA reduces the greenhouse gas emissions of asphalt mixtures. Net-zero emissions were estimated for the produced asphalt mixture by adding 5.5 ± 0.4

In the present study, sand-coated carbon fiber reinforced polymer (CFRP) tendons were instrumented with integrated optical fibers. The direct integration during manufacturing did not affect the bond between tendon and concrete and resulted in an excellent measurement quality. The tendons were cast into concrete cubes with different embedment lengths of 5, 10 and 15 times the diameter. The bond between tendon and concrete was tested using direct pull-out tests. Not only the force and the end-slip were measured but also the strain distribution in the tendon. This allowed to derive a position-dependent local bond-slip curve. Significant deviations from the commonly measured average bond-stress vs. end-slip relationship were observed for long and short embedment lengths. A complementary finite element study confirmed that average bond-stress vs. end-slip curves cannot properly predict the strain distribution in pull-out experiments, especially for longer embedment lengths. This allows to conclude that strain distributions are highly affected by local effects, e.g. partial pull-out failure of the concrete. In addition, shear lag was identified as additional parameter affecting the measurements of optical fibers when integrated in materials with low shear stiffness. The shear lag is highly dependent on the radial and axial position and further affected by the tendon material, the diameter of the tendon and the magnitude of the shear stress.

Alkali-silica-reaction (ASR) exposure sites allow studying the behavior of concrete with reactive aggregates in natural exposure on the long term. As such, the exposed specimens can show both the kinetics and magnitude of concrete expansion and the effectiveness of ASR-suppressing measures. Moreover, the combination with accelerated laboratory tests makes it possible to validate the latter. Recently, two exposure sites have been established in Switzerland, one at low elevation in the Midlands and the other one in the Alps at an altitude of 2200 m above sea level. The goal of the present project is to validate the concrete prism test (CPT) and the residual expansion test (RET) used to assess the durability of a structural concrete. Concrete mix designs mirroring the current Swiss market for engineering structures and dam concrete are used. 40 concrete mixtures are produced in total. So far, the CPT has been conducted and the exposure of the cubes at the two sites for an intended duration of 15 years has started. After three years cores will be taken from selected cubes to perform the RET.

New methods are proposed for the verification of the presence of superabsorbent polymers (SAP) in freshly mixed concrete and estimation of SAP quantity. The methods are in general based on flushing concrete with excess water. They allow separating the light, water-sorbed hydrogel particles from the mineral components in the fresh concrete and making these particles available for further tests. Two types of tests are proposed: Test 1 serves for a visual verification of the presence of SAP (qualitative test), while Test 2 enables quantifying the mass of the collected SAP as a proxy of their concentration in concrete (quantitative test). Different procedures are proposed for these two test methods and their performance is evaluated. The testing procedures were scrutinized in an interlaboratory study carried out by 14 participants from 12 countries. All participating groups detected the presence of SAP in the mix using the qualitative procedures (Test 1). Based on this outcome, we suggest that this method should be applied in the field. In contrast, while most participants obtained reasonably reliable results with the quantification procedure of Test 2, some participants reported large errors. Therefore, the quantification method needs to be further refined, starting from the experience gained in this interlaboratory study.

An emerging strategy to remove CO2 from the atmosphere and compensate for the greenhouse-gas emissions of cement and concrete is based on incorporating biochar into concrete. With this approach, concrete can be turned into a functional carbon sink (C-sink). Until now, biochar has been used without modification to replace part of the cement or of the aggregates in concrete. However, this technology comes with a number of practical problems, which include the high water absorption of the biochar (due to its high specific surface) and hazards (dust, risk of combustion). In this paper we present an alternative approach in which biochar is first processed into lightweight aggregates in a cold-bonding process. To this end, biochar is pelletized together with a small amount of hydraulic binder and with water and forms round pellets that further harden with hydration time. In this way, carbon-rich lightweight aggregates (C-LWA) are obtained that are easier to handle than the pure biochar. The C-LWA pellets have similar porosity and strength as conventional LWA and can be used for similar applications. Yet, the CO2 emissions from sintering traditional LWA are avoided and the C-LWA are instead an effective C-sink. We demonstrate that it is possible to incorporate in the pellets and eventually in the concrete a sufficient amount of carbon to compensate for the original emissions of concrete. The net-zero emissions concrete obtained with this approach possesses mechanical performance sufficient for typical structural applications in buildings.

Cement production is linked to a substantial CO2 emission contributing to about 5-8% of the man-made emissions. However, hardened cementitious materials can absorb CO2 in the process called carbonation, both during the service life of the structures and during their demolition and recycling phase. As experimental data of CO2 absorption during the recycling phase are scarce, the goal of the study is to experimentally determine the CO2 absorption of recycled concrete aggregates (RA) from the point of crushing until reuse in recycled aggregate concrete (RC). Samples from three plants producing both RA and RC were collected and stored both in the plants and in the laboratory for several months. The change in the amount of uncarbonated cement paste of the RA with time was determined by using a novel method: image analysis of samples embedded in epoxy, ground and sprayed with phenolphthalein. This allowed to calculate the CO2 absorption during the storage of RA. The amount of absorbed CO2 corresponds to about 5.4-12.6% of total CO2 emission originally stemming from the production of cement.

Abstract New methods are proposed for the verification of the presence of superabsorbent polymers (SAP) in freshly mixed concrete and estimation of SAP quantity. The methods are in general based on flushing concrete with excess water. They allow separating the light, water-sorbed hydrogel particles from the mineral components in the fresh concrete and making these particles available for further tests. Two types of tests are proposed: Test 1 serves for a visual verification of the presence of SAP (qualitative test), while Test 2 enables to quantify the mass of the collected SAP as a proxy of their concentration in concrete (quantitative test). Different procedures are proposed for these two test methods and their performance is evaluated. The testing procedures were scrutinized in an interlaboratory study carried out by 14 participants from 12 countries. All participating groups detected the presence of SAP in the mix using the qualitative procedures (Test 1). Based on this outcome, we suggest that this method should be applied in the field. In contrast, while most participants obtained reasonably reliable results with the quantification procedure of Test 2, some participants reported large errors. Therefore, the quantification method needs to be further refined, starting from the experience gained in this interlaboratory study.