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2013FallShotcreteEMag

Shotcrete Corner Table 1: Set time results Date Jan-12 Jun-13 Dec-13 Feb-13 Final set time 4 minutes 4 minutes 6 minutes n/a Table 2: Flexural strength results Date Jan-12 Initial material temperature 5°C (41°F) 7 days 5.6 MPa (810 psi) 28 days 6 MPa (870 psi) Fig. 3: End beams in a steel mold after Table 3: Volume of permeable voids and boiled absorption results shooting underground Date Jan-12 Jun-12 Feb-13 the different parameters were typical to normal Initial mix 5°C (41°F) 27°C (81°F) 23°C (73°F) shotcrete operations. The control mixture results temperature met the usual standard; therefore, these results Volume of permeable 15.8 15.0 15.9 are not presented in the article as they are not voids, % relevant to the topic. Setting time was deter- Boiled absorption, % 7.1 7.0 7.1 mined using a hand-held penetrometer in accor- dance with ASTM C1117, “Standard Test Graph 1: Early-age compressive strength of end beam specimens Method for Time of Setting of Shotcrete Mix- tures by Penetration Resistance (Withdrawn 2003).” Early-age compressive strength was determined using the end-beam test method, adapted from ASTM C116, “Test Method for Compressive Strength of Concrete Using Por- tions of Beams Broken in Flexure (Withdrawn 1999)”2 (refer to Fig. 3). Later, age compressive strength was determined in accordance with ASTM C1604, “Standard Test Method for Obtaining and Testing Drilled Cores of Shotcrete.” The boiled absorption and volume of permeable voids was determined in accor- dance with ASTM C642, “Standard Test Method for Density, Absorption, and Voids in Hardened Concrete.” Also, the shotcrete nozzleman was asked to provide comments regarding the evaluation of the material (including rebound) Graph 2: Later-age compressive strength of shotcrete cores based on his experience. The rapid-strength-gain dry-mix shotcrete was tested for all of the prop- erties listed above at different material and ambient temperatures in order to observe the effects of temperature on the shotcrete mixture. Setting-time results are provided in Table 1. Early-age compressive strength and later-age compressive strength development curves with relation to material temperature are shown respectively in Graphs 1 and 2. The target com- pressive strengths in both graphs are typical for a mining shotcrete specification. The flexural strength results are presented in Table 2. The volume of permeable voids and boiled absorp- tion results are given in Table 3. Shotcrete • Fall 2013 27


2013FallShotcreteEMag
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