Prof. Dr. Ömer Arıöz – Hasan Kalyoncu Üniversitesi
Araş. Gör. Ç. Özge Özelmacı Durmaz – Hasan Kalyoncu Üniversitesi
Dr. Evren Arıöz – Eskisehir Teknik Üniversitesi
Ready mixed concrete (RMC) is one of the most important construction materials produced by mixing fine and coarse aggregate, cement and water with or without adding chemical and mineral admixtures and in some cases fibers. RMC is initially plastic and it can be placed into desired shape and size of formworks during its fresh state. It solidifies over time due to the hydration of cement and finally becomes entirely hardened material. In fresh state, RMC should be workable that means ease of mixing, transporting, placing, compacting and finishing without segregation, excess bleeding and without extra effort. RMC should have proper setting time to allow suitable consolidation and finishing. Hardened RMC should satisfy required strength and durability. It should be marked that the use of chemical admixtures is widespread mainly because many benefits to improve or modify demanded fresh and hardened properties of RMC.
The main reasons to use chemical admixture in the production of RMC are reducing the costs of concrete structures, retaining RMC quality during mixing, transporting, placing, and curing operations especially in extreme weather conditions, and overcoming some difficult problems faced during concreting operations. Chemical admixtures can be divided into following groups; water-reducers, air entrainers, set controlling admixtures and some special chemicals such as corrosion inhibitors, coloring admixtures, alkali-silica reactivity inhibitors, etc. Classes of chemical admixtures for RMC are demonstrated schematically in Figure 1.
Figure 1. Classes of chemical admixtures for RMC
Water reducing admixtures are mainly used for three purposes; reducing amount of water without changing the workability thus increasing the strength and durability; increasing plasticity to make the RMC flow better without changing water/cement (w/c) ratio in order to improve placing properties; reducing cement quantity to make concrete more environmental and economic. Reasons for the use of plasticizers in RMC production is demonstrated schematically in Figure 2.
Figure 2. Reasons for the use of plasticizers in RMC production
Air entraining admixtures are used to entrain and stabilize air voids in RMC. Water expands 9% in volume during freezing and ice particles in pores of concrete exert pressure. When water is frozen it expands into air bubbles therefore use of air entrainers improves durability of concrete in freeze-thaw environments. Function of air entraining admixtures in freezing and thawing cycle is demonstrated schematically in Figure 3.
Figure 3. Function of air entraining admixtures in extreme weather conditions (cold weather)
Set controlling admixtures are accelerators and retarders. Accelerators affect the rates of reaction between cement and water. It accelerates setting by reducing time for changing from plastic to hardened state. This provides shorter setting times and early strength developments. Set retarding chemical admixtures slow the rate of water penetration to cement and affect hydration process of RMC. Thus, initial setting time can be delayed. The main purpose of using retarders is eliminating cold joints. Figure 3 can be considered as an example that can be observed in structures. When the ambient temperature is 150C, RMC is still in a plastic state until finishing process is completed. Therefore, there may be no difficulty in concreting operations. However, the setting time will be shorter when the ambient temperature is 30 0C. In this example, the setting time coincides with the placing process of RMC. In this case, when the set retarding admixture is used, sufficient time is provided to finish the concrete operations.
Figure 4. Use of set retarding admixtures in RMC production
An experimental study was carried out to investigate effects of plasticizing chemical admixtures on the properties of fresh and hardened ready mixed concrete. Concrete mixtures RMC-A was produced without using plasticizing chemical admixture and RMC-B was produced by using plasticizing chemical admixture, respectively.
In the experimental study, slump values of the fresh concrete mixtures were measured. Amount of water in mixture RMC-A was 200 kg and slump value of mixture was measured as 180 mm. Equal slump value was measured for mixture RMC-B with 156 kg of water. Test results revealed that same plasticity level can be reached by significantly lower amount of water when plasticizing chemical admixture was used.
Compressive strength tests were carried out at the ages of 28 days in order to examine the hardened properties of the concrete mixtures. Water/cement ratio of mixtures RMC-A and RMC-B were 0.67 and 0.52, respectively. Compressive strength test results indicated that 28-days strength value of RMC-A was measured as 34 MPa. However, higher strength value can be obtained for mixture prepared by using chemical plasticizing admixture. 28-days strength value of RMC-B was measured as 53 MPa. This can be attributed the lower w/c ratio of concrete mix RMC-B than that of concrete mixture RMC-A.
Acknowledgements
Authors kindly thank to Polisan Kimya San. A.Ş. for their material supply in order to carry out this experimental study.
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