Volumen 60 (2019): Heft 1 (June 2019)

In the most service life models of reinforced concrete structures the initiation phase is the most crucial, because according to models, service life of the structure will end underestimation on conservative side when carbonation achieves the reinforcement for the first time. The square root model is widely used in predicting carbonation depth of reinforced concrete. The model is based on diffusion laws and thereby arguable for inhomogeneous concrete. The model was evaluated by field measurements from one existing concrete building by conducting condition investigation twice at a time interval of 20 years. Samples were taken from exposed aggregate concrete sandwich panels and balcony side panels. Compared to the data collected from large number of buildings, the measured carbonation rates were very common for Finnish concrete buildings made during the 1960s and 1970s. According to this study, in solid concrete the progress of carbonation of concrete can be predicted reliably with Fick’s second law. This model, however, gives too pessimistic predictions for concrete suffering from freeze-thaw damage. Therefore, a new model has been presented for damaged concrete.

The impact of substitution of cement paste with uncalcined clay (bentonite and kaolinite) in the range of 5% by volume of paste on the development of hydration and properties of mortar was investigated. Two issues were addressed, the expected filler effect of the dispersed sub-micron clay particles, and the possible chemical reactivity of the clay.

The study indicated that Portland cement paste may be modified by addition of well dispersed clay and that the impact includes accelerated cement hydration as well as altered distribution of products. Compressive strength development was accelerated, but later age strength was reduced, especially for the bentonite mixes. In contrast, microscopic porosity measurements indicated no detrimental impact on the coarse capillary porosity.

The investigation indicates that for durability related engineering properties, the application of uncalcined clay might be a potential means for reduction of the clinker factor in concrete in support of sustainability.

This paper aims at identifying the direction for more sustainable development of the use of concrete in road infrastructure in an industrialised context.

The increase in the global mean temperature is one of the most severe challenges today. The concrete industry is responsible for significant emissions of greenhouse gases, most attributable to cement production. However, concrete is one of the most important building materials in the world and indispensable for the societal development in countries at all development stages. Thus, the concrete industry needs to take measures for reducing emissions.

This paper investigates possible directions for the development of the concrete industry, to reduce climatic impact and accommodate positive societal growth. The investigation is carried out as a SWOT analysis, focusing on three terms dominating the present discussion on any development within the construction industry; sustainability, industrialisation and digitalisation. The result is a thorough discussion and a set of recommendations for the direction of future research and innovation on sustainable use of concrete in the construction of road infrastructure. The major opportunities and threats are summarised in the conclusions, and future research to be carried out in two of the authors’ PhD-projects are described.

The objective of this experiment is to investigate the behaviour of lightweight aggregate concrete (LWAC) under compression and with stress gradients. Experimental program contained three sets of LWAC which were used for production of 21 prisms. Lightweight aggregate argillite slate, called Stalite, from North Carolina had been used. The sets differed in using dry (0.10% moisture content) or saturated (7.9% moisture content) aggregate. The third set included a small amount of polyvinyl alcohol fibres (PVA). The geometry of the prisms were 100 × 140 × 480 mm (width × length × height). Prismatic samples were loaded centrically and eccentrically in compression.

From the achieved experimental results, it is visible that the lateral deformation of the most stressed fibre is counteracted by the less stressed fibres that confine compressive stress and increase strains. The obtained strain level was much higher than expected, especially for the third set of concrete samples with PVA fibres. Recorded strains in prisms test was in range from 3.08‰ to 6.82‰). In general, LWAC with Stalite showed ductile behaviour followed with very high strains. The third set of samples included a small amount of polyvinyl alcohol fibres (0.5% of volume fractions) was even more ductile and non-brittle.

This research investigates the effect of capillary pressure and the length of the hydration dormant period on the plastic shrinkage cracking tendency of SCC by studying specimens produced with different w/c ratios, cement types and SP dosages. A relationship between the capillary pressure rate and the length of the hydration dormant period is defined, which can explain the cracking severity of the concrete when the volumetric deformation is unknown.

The results show, that the cracking tendency of SCC was the lowest in case of w/c ratio between 0.45 and 0.55, finer and more rapid hardening cement, and lower dosage of SP. The dormant period was prolonged by increasing the w/c ratio, using coarser cement, and higher SP dosage. It was concluded that the cracking tendency of concrete is a function of the capillary pressure buildup rate and the length of the dormant period.

In concrete beam bridges, the end diaphragm at the end of the bridge is a common structural component that connects the main beams and transfers the beam loads to the bridge bearings. In integral bridges the end diaphragm also retains the soil of embankments due to the absence of abutments. Cracking of the front surface on the end diaphragm has been detected in post-tensioned beam bridges in Finland and Sweden. Presumably the post-tensioning of the bridge and the shaping and detailing of the connection of the end diaphragm and main beam have an effect on cracking tendency. The aim of this study is to examine the structural behaviour and the cracking potential of end diaphragms using linear analysis of the post-tensioned bridge and to find measures to prevent the cracking.

The observations collected through field surveys are compared to results of linear FE analysis to clarify the cause of the cracking. The verification of model is performed by comparison of patterns of cracking observed in field surveys and the distribution of maximum tensile stresses in the FE model. With model variations, the effectiveness of measures for the prevention of cracking are observed.

Current theoretical models for predicting the concrete cone breakout capacity of tension loaded headed anchors do not consider the influence of member thickness, size of anchor head, and orthogonal surface reinforcement. In the present study, the influence of the aforementioned parameters was studied both numerically and experimentally. Both the numerical and experimental results showed that the tensile resistance of headed anchors increases by increasing the member thickness or if orthogonal surface reinforcement is present. In addition, the anchorage capacity further increases with increase of the anchor head size.

The current model for predicting the concrete cone failure load of tension loaded headed anchors were refined and extended by incorporating three modification factors to account for the influence of the member thickness, size of anchor head, and orthogonal surface reinforcement. The accuracy of the proposed model was verified based on the results of 124 tests on single headed anchors from literature.

In the beginning of 2017 the design method for punching shear in Finland was changed. The method presented in Eurocode 2 was adopted with some nationally determined parameters and rules. During 2016 and 2017 computational analyses were conducted to compare the previous national design method and the new one. Comparison setups were created in which different parameters were varied. The results were presented the predicted punching resistances differ differences are significant and can be almost 60% in some cases.

Air contents of concrete are necessary for concrete durability in freeze-thaw exposure. According to the Finnish concrete code, the target value for air content varies between 4% and 5.5% for XF - exposure classes. Lately in Finland, some cases showed an elevation of air contents up to 15% in fresh air-entrained concrete at construction site and in drilled concrete samples.

The objectives of this study were to investigate the stability of air entrainment by measuring the air content elevation 30 minutes and 60 minutes after concrete mixing and investigating the concrete sensitivity to segregation. Composition of concretes used in this study include 7 different combination of PCE based superplasticizer and air-entraining agent admixtures, cement content of 425 kg/m³, two consistency classes S3 with water to cement ration of 0.33 and F5 with water to cement ration of 0.38. One cement type was used for all concrete mixes. The concretes were mixed for 2 minutes and 5 minutes mixing times.

The results show that the elevation of the air content of fresh concrete depends on the consistency of the concrete and on the used combination of superplasticizer and air-entraining agents. The higher consistency classes concretes have more risk of air elevation with some combinations of PCE-based superplasticizers and air-entraining agents. The results also indicate that short mixing time would not be enough to achieve total effectivity of some air-entraining agents, especially for higher consistency classes concrete.