Life cycle assessment of a column supported isostatic beam in high-volume fly ash concrete (HVFA concrete)

Nowadays, a lot of research is being conducted on high-volume fly ash (HVFA) concrete. However, a precise quantification of the environmental benefit is almost never provided. To do this correctly, we adopted a life cycle (LCA) approach. By considering a simple structure and an environment for the material, differences between traditional and HVFA concrete regarding durability and strength were taken into account. This paper presents the LCA results for a column supported isostatic beam made of reinforced HVFA concrete located in a dry environment exposed to carbonation induced corrosion. With a binder content of 425 kg/m3 and a water-to-binder ratio of 0.375, the estimated carbonation depth after 50 years for a 50 % fly ash mixture does not exceed the nominal concrete cover of 20 mm. As a consequence, no additional concrete manufacturing for structure repair needs to be included in the study. Moreover, structure dimensions can be reduced significantly due to a higher strength compared to the reference concrete used in the same environment. In total, about 32 % of cement can be saved this way. The reduction in environmental impact equals 25.8 %, while this is only 11.4 % if the higher material strength is not considered

Factors affecting the monitoring of the early setting of concrete by ultrasonic P-waves

Ultrasonic P-wave measurements are widely used to monitor concrete setting. Although the largest wave velocity increase occurs during setting, the earliest increase is rather caused by other factors. Air bubble migration, internal settling, formation of ettringite and early C-S-H, workability loss and thixotropy might affect the velocity change in time. Tests on mortar in which cement was replaced by bentonite, confirmed the possible influence of thixotropy on the measurements. The effect of air bubble migration, internal settling and workability loss was proven to be restricted by testing a mixture in which the cement was replaced by inert material. In a cement mixture, the precipitation of hydration products might however accelerate settling and workability loss. During cement hydration simulations, the change in porosity due to the formation of early C-S-H and ettringite was considered for the calculation of the elastic properties of the granular framework. Nevertheless, the calculated velocity hardly increased before percolation and thus could not confirm that the first velocity increase is attributed to formation of early hydration products. Thus, apart from thixotropy, none of the other factors could unarguably be indicated as the cause of early velocity increase

Relating ultrasonic velocity measurements on fresh concrete to CEMHYD3D microstructure simulations

Ultrasonic measurements can be used to monitor concrete setting. To relate these measurements with fundamental structural changes in the cement paste, the results are compared with the microstructure development as simulated with an adjusted version of the pixel model Cemhyd3D. Mixtures in which the Portland cement was replaced by different dosages of blast-furnace slag and fly ash were tested. The fraction of percolated solid particles is het most important microstructure parameter determining the change of the wave velocity during setting. However, the measured wave velocity starts to increase earlier than the solid percolation, which is by other researchers assumed to be caused by the formation of early-age- C-S-H and AFt. The simulations could not confirm the latter. After percolation, the formation of additional hydration products increases the elastic moduli and thus the wave velocity. This becomes especially visible for the high-volume slag mixture. After approximately 24h, the presence of slag causes a second acceleration of both the hydration product formation and the wave velocity increase. The pore space depercolation seems unimportant in the study of the setting of concrete or mortar

Effect of superplasticizers on hydration and setting behaviour of cements

Cement-superplasticizer compatibility has become a major area of interest due to the increasing use of high performance and self-compacting concrete mixtures. The interaction between superplasticizer and cement can cause significant retardation effects on the hydration and setting properties of cement. However, not much data have been published to quantify this. In this study, mortar mixtures with three cement types and two different superplasticizers were tested by calorimetric and ultrasonic measurements to investigate the effects on both hydration and setting. Also the strength of the mortar samples was tested. The types of superplasticizers used, were polycondensates of naphtalene sulphonate (PNS) and polycarboxylic ethers (PCE). Based on the ultrasonic measurements, the addition of PCE and PNS caused an initial setting retardation of approximately 1.2 and 3.4 h on the portland cement, 1.9 and 4.8 h on the portland-fly ash cement and 0.5 and 1.6 h on the blast-furnace cement mixtures. This difference decreased after 16 to 20 h when the stiffness development of the samples with PNS or PCE approached the mixtures without superplasticizers. Similar to the setting, the release of the heat of hydration was more postponed by the addition of PNS than by PCE. Contrary to PNS, PCE benefited the strength of the mortar mixtures. Generally, the effect of the superplasticizers clearly differed according to the cement type in the mixture

Ultrasonic wave energy and frequency spectrum as an alternative to wave velocity to monitor concrete or mortar setting

The setting of mortar or concrete can be monitored by measuring the velocity of ultrasonic p-waves sent through a fresh sample at regular intervals. However, the wave energy or the frequency spectrum might contain even more information. To investigate their use in the characterization of setting behaviour, ultrasonic measurements were performed on concrete and mortar mixtures with an increasing percentage blast-furnace slag or fly ash and on mixtures with several types of blast-furnace slag cement. The change of the received energy in time was compared to the result of the penetration resistance test and the frequency spectra were collected in a three-way array (sample x frequency x concrete age) and analysed with multi-way techniques (PARAFAC and PARAFAC2) to quantify mutual differences. The change of the energy during setting was similar to that of the ultrasonic velocity. Replacement of Portland cement by blast-furnace slag or fly ash causes retardation in the increase of both quantities. For the energy measurements, the thresholds E/Eref = 0.02 and 0.13 and proposed to easily determine respectively initial and final setting. Regarding the analysis of the received frequency spectrum, the samples could be sorted by the multi-way models in agreement with the expected setting behaviour