No. 541 June 2011 

Field-Use Early-Strength Shotcrete Test System 

Objective 

To develop a practical method of measuring shotcrete early 

strength onsite in the first six hours after application using a 

partial beam test standard, ASTM C 116-90. Determining the 

early-strength development of as-sprayed shotcrete can improve 

mine safety by identifying appropriate reentry times and 

providing a convenient means of quality control during 

application (Figures 1–3). A more thorough understanding of 

shotcrete early strength will lead to improvements in ground 

support practices, thereby preventing groundfalls and reducing 

mine roof-fall accidents. 

Background 

When shotcrete is used as part of a multicomponent ground 

support system, it is important to know when the material has 

developed enough strength to be self-supporting and allow for 

reentry and emplacement of the remaining support elements that 

require drilling of the shotcrete layer without degradation. 

Typical reported values for this threshold strength for North 

American mines range from 1 to 1.6 MPa (145 to 233 psi) 

[O’Toole and Pope 2006], with a compressive strength 

equivalent to 1 MPa (145 psi) being the norm [Rispin et al. 

2003; O’Toole and Pope 2006]. Typical ground control support 

includes a shotcrete flash coat 19- to 25-mm (3/4- to 1-in) thick, 

followed by screen, plates, bolts, and a second layer of shotcrete 

bringing the combined thickness to 75 to 100 mm (3 to 4 in). In 

areas requiring rehabilitation, the second layer of shotcrete is 

plated and bolted as well. While testing cored samples after 24 

hours is standard, this is not practical for the one- to six-hour 

period following application when shotcrete unconfined 

compressive strengths are less than 10 MPa (1,450 psi). Indirect 

methods are typically used to determine shotcrete strength 

during the early stages of curing because the partially cured or 

green material is difficult to sample and test. While problems 

with inconsistent test results have been reported with the 

penetrometer-type devices, beam molds have been used 

successfully for creating shotcrete test specimens in the United 

States and Canada. 

Operating the Partial Beam Test System

The test unit is a self-contained, servo-controlled, stiff- frame 

press (Figure 1). 

Figure 1. Early-strength shotcrete test machine.

Partial beam test samples are obtained by 

spraying shotcrete into 102- x 102- x 152-mm (4- x 4- x 6-in) 

mold boxes as shown in Figure 2. After the samples have been 

sprayed, tests are conducted at one-hour intervals over the next 

six hours (one- through six-hour tests). The shotcrete samples 

are carefully demolded by disassembling the mold fixtures and 

removing the enclosed sample. Next, a shotcrete sample is 

placed in a specialized testing fixture and centered under the 

loading head of the test machine (Figure 1). When the test 

sequence is initiated, a programmable-logic-controller- (PLC) 

driven press applies a fixed-rate load to the sample. The load 

profile is shown on a graphical output display, and the measured 

test parameters (time, displacement, and load) are stored on a 

thumb drive. Once the operator observes a well-defined peak in 

the load profile curve, the test is completed and the test 

machine’s loading platen can be returned to its initial starting 

position. Peak load is typically reflected by the development of 

large vertically oriented cracks (Figure 3) along the platen-tosample contact edges, which are indicative of the failure plane. 
 

 Figure 2. Partial beam box mold and test specimen.

 Figure 3. Vertical crack in shotcrete test specimen.

Shotcrete early-strength values obtained from cast and shot 

samples using the test system are shown in Figure 4. There is a 

marked difference in early-strength gain between sprayed and 

cast shotcrete. The cast shotcrete samples have a similar 

strength gain profile to that of cast concrete samples. 

 Figure 4. Typical early-strength partial-beam test values for dry-mix 

shotcrete and concrete, n=54 samples. 

Design Features 

Mobile stiff-frame press 

Compact size for transport and storage 

Graphical user interface with PLC-servo control 

Visual load display of test status 

USB thumb drive for data collection and storage 

Power choices: 110-V AC through UPS

Specifications

Press

Capacity 22 kN 5,000 lbf

Height 71 cm 28 in

Width 31 cm 12 in 

Depth 26 cm 10 in 

Test speed 1.27 mm/min 0.05 in/min

Positioning speed 91.4 mm/min 3.6 in/min

Drive system ball screw

Control PLC servo

Load display, 

collection, and 

storage system

graphical user interface with

USB thumb drive

Control Box

Height 38 cm 15 in 

Width 18 cm 7 in 

Depth 36 cm 14 in 

Power 110-V AC through UPS

Operating system Eaton Controls

For More Information 

For more information on the early-strength test machine, 

contact Lewis Martin, LMartin@cdc.gov, (509) 354-8077, or 

the Health Communications Coordinator (OMSHR@cdc.gov), 

NIOSH Office of Mine Safety and Health Research, P.O. Box 

18070, Pittsburgh, PA 15236-0070. 

To receive NIOSH documents or for more information 

about occupational safety and health topics, contact: 1-800-

CDC-INFO (1-800-232-4636), 1-888-232-6348 (TTY), email: cdcinfo@cdc.gov, or visit the NIOSH Web site at 

http://www.cdc.gov/niosh. 

Mention of any company name or product does not 

constitute endorsement by the National Institute for 

Occupational Safety and Health.

References 

O’Toole D, Pope S [2006]. Design, testing and implementation of in-cycle 

shotcrete in the northern 3500 orebody, Mt. Isa, Australia. Shotcrete for 

Underground Support X (SUS-X). In: Morgan D, Parker H, eds. 

Proceedings of the 10th International Conference, Whistler, BC, Canada: 

American Society of Civil Engineers, pp. 316–327. 

Rispin M, Knight B, Dimmock R [2003]. Early re-entry into working faces in 

mines through modern shotcrete technology—part II, The Canadian 

Institute of Mining – Mines Operations Centre (CIM-MOC), Saskatoon, 

SK, Canada. 

DHHS (NIOSH) Publication No. 2011–186