# Dimensional Stability of SBR-Modified Cementitious Mixtures for Use in 3D Additive Construction
## 5 Results and Discussion
### Drying Shrinkage  
Drying shrinkage is a major cause of cracks. When cementitious mixtures come into contact with ambient air, drying begins and the dried exterior contracts. Thus, shrinkage stress occurs on the exterior surface and cracks ensue when this stress exceeds the tensile strength of the cementitious mixture. Important parameters that impact drying shrinkage include the aggregate content, stiffness, water content, cement content, time of exposure, relative humidity, and size and shape of the concrete member [19]. Also, among the admixtures, pozzolanic materials (except for the AE agent, hardening accelerator, and fly ash) also increase the drying shrinkage. The thin cracks that initially occur on the surface due to drying shrinkage become deeper as time passes. This drying shrinkage can be reduced by increasing the amount of coarse aggregate in the mix of concrete and decreasing the unit water content [21]. Figure 8 shows the results of the shrinkage test obtained from this study. Figure 8a shows the hourly variation over 24 hours and Figure 8b indicates the daily variation over 28 days. It can be observed that the shrinkage dramatically increased between 5 and 10 hours of curing. Figure 9 shows the drying shrinkage developments for the first of 28 days, according to the SBR/cement ratio. These results show that the drying shrinkage for the SBR/cement ratios of 0, 0.05, 0.10, 0.15, and 0.20 were 331 \(\times\) 10\({}^{-6}\), 282 \(\times\) 10\({}^{-6}\), 225 \(\times\) 10\({}^{-6}\), 184 \(\times\) 10\({}^{-6}\), and 144 \(\times\) 10\({}^{-6}\), respectively. A decreasing trend was observed in drying shrinkage as the SBR/cement ratio increased. Generally, the drying shrinkage of cementitious mixtures varies depending on the aggregate/cement and water/cement ratios, but generally ranges between 200 \(\times\) 10\({}^{-6}\) to 1200 \(\times\) 10\({}^{-6}\)[22]. Figure 8: Drying shrinkage strain comparisons for different SBR/cement ratios at (**a**) curing age: 24 hours and at (**b**) curing age: 28 days. Previous studies related to this research include the report by [PERSON] et al. [23], which revealed that the drying shrinkages at 28 days were \(510\times 10^{-6}\), \(788\times 10^{-6}\), and \(430\times 10^{-6}\) for the SBR/cement ratios of 0, 0.15, and 0.25, respectively. [PERSON] et al. [24] reported that the drying shrinkage sharply decreased as the polymer latex content increased, and the degree of shrinkage varied depending on the latex type and dosage. [PERSON] [25] showed that the drying shrinkage decreased as the SBR/cement ratio (i.e., 0, 0.10, and 0.20) increased. [PERSON] et al. [10] stated that the drying shrinkage of polymer-modified mortar may be either larger or smaller than that of ordinary cement mortar, and generally tends to decrease with an increase in the polymer-cement ratio. The cause of decrease in drying shrinkage due to the increase in SBR/cement ratio was explained by [PERSON] et al. [26] and [PERSON] et al. [27] as the result of polymer film formation in latex-modified cement mortar. The film acts as a microfilament, controlling the contraction phenomenon in the matrix and preventing moisture evaporation by blocking pores. Moreover, [PERSON] [28] reported that compared to ordinary cement mortar, the reduction in the drying shrinkage of latex-modified mortar is mainly due to the effects of the surfactants and antifoamers contained in latex. [PERSON] [29] explained that the polymer-modified concrete exhibits lower drying shrinkage compared to that of ordinary Portland concrete because of the high-water retention of polymer-modified concrete, and its high tensile strength improves extensibility, reducing cracks caused by drying shrinkage. Similarly, it was found that the drying shrinkage of polymer-modified cementitious mixtures can be reduced through various factors. Active methods of further reducing drying shrinkage include adding expansive additives during the mixing of cementitious mixtures and by adding ethylene during polymer formulation [30].