PSI - Issue 74

Małgorzata Lenart et al. / Procedia Structural Integrity 74 (2025) 44 – 49 Małgorzata Lenart / Structural Integrity Procedia 00 (2025) 000–000

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appropriate composition of the composite, such as through the use of polymer additives and, above all, the use of proper care of the composite embedded in the structure, Pichler (2017). The purpose of this research was to compare the effect of the cement composite modification by using a polymer additive (dosed on two levels) and the simultaneous impact of different types of test specimen care on the course of total shrinkage recorded over 90 days. 2. Types of volumetric changes in cement composites The volumetric changes of unloaded concrete can be divided into autogenous changes, which include chemical shrinkage (resulting from a decrease in the volume of hydration reaction products relative to the volume of substrates) and shrinkage of self - consuming (involving the uptake of water from capillary pores by hydrating cement grains), Zhang (2025), Shen (2016). Autogenous changes also include autogenous swelling caused, for example, by too high sulfate content, free CaO, MgO in the cement or alkali-silica reaction. On the other hand, volumetric changes caused by external factors include shrinkage from drying possibly swelling of the composite in an aqueous environment due to water absorption, shrinkage due to extreme temperature drop as well as expansion due to extreme temperature rise, Li (2022). Volumetric changes caused by external factors also include changes due to chemical reactions between the aggressive external environment and the chemical compounds in the cementitious composite, such as shrinkage from carbonation or swelling due to sulfate corrosion, Neville (2012), Song (2024). The impact of an aggressive external environment on the cement composite is included in the exposure classes listed in the EN 206 standard "Concrete. Specification, performance, production and conformity”. Among the volumetric changes of practical importance, we should also mention plastic shrinkage, which occurs in concrete that has been placed in place and is in a plastic state. This shrinkage is partly related to autogenous changes and partly to external factors such as the evaporation of water from the surface of freshly placed concrete and/or the suction of water by an absorbent ground, Nasir (2023). 3. Factors affecting shrinkage deformations As mentioned earlier, the size of shrinkage deformations depends on both the materia l composition of the composite and the external environment in which the given cement composite is used. Among the material factors, it should be mentioned : the size of the w/c ratio (the higher the value, the greater the shrinkage), the amount and size o f aggregate contained in the composite (the more aggregate - both in terms of quantity and size in the composite - the lower the shrinkage), the amount and type of cement (the more cement and the finer it is, the higher the shrinkage), the type and amount of admixtures and/or additives used in the composite (e.g. anti-shrinkage admixtures, polymer additives, fibers), Wang (2025). Polymer additives cause beneficial changes in the structure of the modified composite, Łuk owski (2008). The use of polymer additives causes: • the formation of a continuous film penetrating the cement matrix, • changes in the size and type of crystals in the cement binder, • reduction of the porosity of the composite due to the reduction of the amount of mixing water. In addition, the presence of the polymer has a significant impact on improving the structure of the contact layer of the cement paste with the aggregate. In polymer-cement composites, the space between the cement grains is filled with directed polymer molecules, which increases the contact surface of the matrix with the aggregate and, as a result, causes the densification of the interphase zone , Liu (2022). The polymer membrane formed also surrounds the cement grains. Thus, it slows down the process of its hydration. This property causes a delay in the development of the composite's compressive strength, but on the other hand allows to reduce the shrinkage value. Another beneficial effect of introducing a polymer additive into a cementitious composite is its ability to bridge microcracks, which is beneficial due to the reduction of shrinkage and improvement of durability. Total shrinkage also depends on external atmospheric conditions, i.e., relative humidity, ambient temperature, wind speed, and the associated concrete care provided. It also depends on the age of the concrete, as it decreases significantly