The Effects of Admixtures Addition on Slump Decrease and Concrete Compressive Strength at B-1 Concrete Works

Semarang - Solo Toll Road is part of the Trans Java highway, which has a strategic meaning in the development of road networks, specifically in Central Java and also on a regional scale. The construction project generally consists of excavation work, stockpiles and bridge work. The number of work in the project therefore allows the emergence of a problem that can hinder the development process and one of which was related to concrete casting in the construction of the Kali Putih Bridge 1, where there was a slump decrease in columns P1 and P3 concrete pre-casting (30 Mpa). However, a slump test was carried out on column P1 in STA. 61 + 368,500, and P3 in STA. 61 + 293,500, which resulted in 3.5 cm and 4 cm collapse respectively. Hence, they did not meet specification on B-1 concrete, which was 7.5 ± 2.5 cm. In addition, Superplasticizer was used to overcome this problem, as it was capable of restoring freshness/workability. This study, therefore, aimed to determine the effect of the incorporating superplasticizer and retarder into concrete, on its compressive strength, slump decrease, and time, utilizing 24 test cylindrical specimens (7-days and 14-days), through pressure testing. Furthermore, the slump testing was conducted every 20 minutes for 120 minutes, to determine the reduction time, and the findings further indicated that adding a retarder to class B-1 concrete was capable of increasing the functional period (67 minutes on a slump of 7 ± 2.5 cm). However, the addition of Superplasticizer was also able to repair the slumps, which did not meet the requirements, including the specimen that involved the incorporation of a retarder (82.5 minutes, which resulted in a 1.9 cm slump that was further increased to 17.5 cm after augmentation. Furthermore, these effects were due to its ability to reduce the compressive strength by 3.13% and 6.14% on the 7-days and 14-day specimens, respectively. However, the SPSS v.23.0 test results showed the difference in compressive strength between the normal concrete + retarder and normal concrete + retarder + SP, with a sig value of 0.021 <a = 0.05 (Turkey HSD Test) and 0.0009 <a 0.05 (LSD Test).