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2016SpringShotcreteEMag

ultimate strength as a result of their pozzolanic activity.3 However, the percent of these contributions may vary depending on the SCM type and replacement level. Slag cement is widely used in conventional concrete due to its advantages on long-term strength and durability. As a waste material of the iron refining industry, the contribution of slag cement as an economical and sustainable material to be used in shotcrete is well known.4 However, as a result of its lower hydration rate, which decreases the early-age strength development, the application of shotcrete mixtures containing slag cement in tunnel projects is limited.5 To offset this detrimental impact on early-age performance that differs from other shotcrete applications containing SCMs, mixtures tested in this case study included a pozzolanic based rheology control agent (TYTRO  RC 430), which contributes and accelerates the C-S-H gel formation leading to increased early-age strength development. The early-age strength development is measured with beam end testing (Fig. 1). The beam end test involves the crushing of sprayed beams, which are 3 x 3 x 16 in. (75 x 75 x 400 mm) in size, by the use of a small hydraulic pump that applies direct compression until failure occurs (Fig. 2). Although the device is similar in design to other compressive testing machines, its small size makes it portable, which provides an advantage in field conditions.6 Figure 3 shows the comparison of early-age strength development of three different binder systems containing a plain mixture with 100% OPC (Mix 3), a binary system with 25% fly ash (Mix 1), and a binary system with 40% slag cement (Mix 2), which were tested with the beam end test. All the mixtures met or exceeded the specification requirement to reach 145 psi (1 MPa) within 3 to 4 hours. However, it should be noted that mixtures containing fly ash and slag cement had slightly higher strength than the OPC mixture up to 4 hours followed by a slower strength development for slag cement mixture between 5 and 8 hours. Considering the very early-age strength development is mainly influenced by the accelerator, these results are expected because all three mixtures have the same type and dosage rate of accelerator (TYTRO SA 530 at 10% of total cementitious weight). On the other hand, equivalent strength for fly ash and OPC mixtures, even after 5 hours, is most likely due to the synergetic impact of the SCM and pozzolanic-based rheology control agent (TYTRO RC 430 at 0.89% of total cementitious weight), which balances the early-age strength reduction associated with fly ash with strength Fig. 1: Setup for beam end test Fig. 2: Beams prepared to test early-age strength development Fig. 3: Impact of cementitious materials on early-age strength development improvement of TYTRO® RC 430. 24 Shotcrete • Spring 2016


2016SpringShotcreteEMag
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