Early strength performance
� of modern weak rock mass shotcrete mixes
�

by C.C. Clark, M.A. Stepan, J.B. Seymour and L.A. Martin

 C.C. Clark, M.A. Stepan, J.B. 

Seymour, members SME, and L.M. 

Martin are mechanical engineer, 

engineering technician, mining 

engineer and research mechanical 

engineer, respectively, at the National 

Institute for Occupational Safety and 

Health, Spokane, WA. Paper number 

TP-09-063. Original manuscript 

submitted December 2009. Revised 

manuscript accepted for publication 

August 2010. Discussion of this 

peer-reviewed and approved paper 

is invited and must be submitted to 

SME Publications by April 30, 2011. 

Abstract n The National Institute for Occupational Safety and Health (NIOSH), Office of Mine 

Safety and Health, Ground Control Engineering Branch is investigating the use of shotcrete in 

weak rock mass mines with the objective of reducing fatalities and injuries resulting from rock 

fall accidents. When shotcrete is used as part of a multi-element ground support system, it is 

necessary to know when the material has developed a compressive strength of approximately 

1 MPa (145 psi), the early threshold for safe re-entry of miner and machine into shotcreted 

mine workings. At this value, the material has developed enough strength to be self-supporting 

and to allow for installation of other ground support components that require drilling of the 

shotcrete layer without degradation. NIOSH researchers have developed methods and portable 

test equipment to measure shotcrete strength on site in the first six hours after application using 

a partial beam test standard, ASTM C 116-90 (1990). These advances were demonstrated in 

tests with five commercially available shotcrete mixes, sprayed as dry shotcrete using fieldexpedient methods and equipment. The strength values from these tests allowed for real-time 

identification of the early strength threshold and were consistent with strengths reported using 

laboratory-type equipment. Measuring the early strength properties of shotcrete directly at the 

mine site can improve mine safety by identifying appropriate re-entry times and providing a 

convenient means of conducting on site quality control during shotcrete applications. 

Introduction 

Rock fall is a major hazard in underground mining, with 36% of all 

underground metal/nonmetal mine fatalities (1996-2004) being attributed to 

this cause. U.S. Mine Safety and Health 

Administration (MSHA) statistics show 

that of these unplanned falls of rock, 

95% weighed less than 1 t (1.1 st), with 

the majority, 59%, weighing 11 kg (25 

lbs) or less (Fig. 1). These small rocks 

can develop 106 N (23.8 lbf) of force 

over a 3-m (10-ft) fall (Lacerda, 2004). 

These falls often occur between traditional ground support elements (e.g., 

bolts, trusses, timber, etc.) and can only 

be prevented by surface or skin control 

such as shotcrete and wire mesh. 

For mining operations in weak rock 

mass conditions (rock mass rating 20 

to 45, very poor to poor), such as the 

Nevada underground gold mines, rock 

falls are a constant threat. This is due in 

part to the unraveling of the poor quality rock. 

The mining method commonly 

used in these mines is a modified form 

of mechanized cut-and-fill amenable 

to high ore recovery rates in irregular 

deposit geometries. Shotcrete is a key 

ground support element in this mining method and, as a result, the time 

required for the shotcrete to set sufficiently for safe re-entry is a key factor 

in the mining cycle. Because the early 

strength properties of shotcrete mixes 

used in these mining applications are 

not well defined and difficult to measure in situ, there is a risk that miners 

will prematurely enter workings before the shotcrete has gained sufficient 

strength to provide adequate ground 

support.

To address these concerns, the National Institute for Occupational Safety 

and Health (NIOSH) developed an inmine test protocol and applied it to the 

study of early-age shotcrete strength for 

five dry shotcrete mixes currently used 

in weak rock mass mines. 

The ultimate goal of this work is 

to enable a mining engineer to characterize and define the strength properties of a particular shotcrete mix and 

thus determine when the mining cycle 

could safely restart with miners and 

machinery. This safe re-entry time 

is determined by knowing when the 

shotcrete applied to the surface of an 

underground opening has gained the 

minimum strength required to resist 

the normal ground pressure (Iwaki et 

al., 2001) and has developed sufficient 

bond strength or initial skin control in-
terlock such that the shotcrete-rock 

matrix does not fall apart within the 

first 20 minutes after application 

(Rispin et al., 2003). 

Figure 1 

U.S. metal mine rockfalls, 1996 - 2004 (after Zipf, 2002 and Lacerda, 2004). 

Background 

Mechanized cut-and-fill stopes in 

Nevada require extensive ground support. Typical support includes a shotcrete flash coat 19 to 25 mm (0.75 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. 

Shotcrete is an integral and vital 

component of this ground support 

system. In the initial application, a 

remote controlled shotcrete machine 

sprays shotcrete to form a skin or 

shell. This thin skin prevents very 

small rock debris from falling and 

fouling the machinery that is subsequently used to remotely install the 

mesh and bolts. The second application of shotcrete ties together the 

plated and bolted mesh and prevents 

unraveling of the small-sized rock.

If this material is not confined, the 

small-sized rock can loosen and fall 

out, allowing additional material to 

fail and eventually causing substantial slabbing and ultimately massive 

failure. 

Most observed ground control 

failures involve broken rock within 

0.5 m (1 to 2 ft) of the excavation perimeter (Bauer and Donaldson, 1992). 

The loading and failure mode is illustrated in Fig. 2. 

Figure 2 

Flexural resistance model for a loosened block representing a distributed load (after 

Diamantidis and Bernard, 2004). 

With this support method, the curing time or setting of 

the shotcrete is critical to the mining cycle. Re-entry time 

is the minimum curing time required for shotcrete to develop enough strength to protect miners. In other words 

(quoting Rispin, 2005),“the re-entry time really defines when 

work can resume in an advancing underground heading.” 

What this means in practical terms is that the shotcrete can 

be drilled and other ground support components installed 

without damaging the long-term strength properties of the 

shotcrete (O’Toole and Pope, 2006; Clements, 2009). 

Development of early strength is a characteristic of 

sprayed shotcrete and occurs considerably faster than cast 

shotcrete and ordinary concrete (Fig. 3). In addition, fiber 

additives have been developed that can greatly increase the 

toughness and tensile strength of shotcrete. This greatly benefits the structural properties of the shotcrete in terms of 

failure strength after onset of initial cracking. 

Early strength typically refers to the strength values that 

the shotcrete material achieves shortly after being sprayed 

(zero to six hours). Early strength values from unconfined 

compression tests as low as 0.5 MPa (73 psi) have been used 

as a benchmark to identify conditions under which re-entry 

can safely and acceptably be permitted on international mining properties (Rispin, 2005). However, for safe re-entry 

practices in North American mines, the shotcrete should 

typically develop a compressive strength of 1 to 1.6 MPa 

(145 to 233 psi) to be competent enough for drilling operations required for the installation of other ground support 

(O’Toole and Pope, 2006). The time reported for the safe 

re-entry has been as soon as two hours (Knight et al., 2006), 

with four hours and compressive strength equivalent to 1 

MPa (145 psi) being the norm (Rispin et al., 2003; Rispin, 

2005; O’Toole and Pope, 2006). 

Test protocol 

The early strength testing of shotcrete introduces two issues not normally faced when conducting tests with concrete. 

First, samples of the shotcrete must be obtained as the material is applied or sprayed, rather than poured or cast into test 

cylinders. Moreover, it is not realistic to extract samples of 
green or partially cured shotcrete, via a coring process, from a 

shotcrete test panel (Heere et al., 2002; Clements, 2004). Second, because the shotcrete gains strength at an accelerated 

rate, samples must be tested very quickly after collection. 

Figure 3 

Typical early strength partial beam test values for dry-mix 

shotcrete and concrete, n=54 samples. 

Researchers trying to determine the early age strength 

(zero to six hours) of green shotcrete have had to resort to 

indirect test methods due to the difficulty in handling the 

material. Methods have been demonstrated using a Meyco 

penetrometer, ASTM C 1117-89 (1989) (Jolin et al., 1999; 

Heere et al., 2002) and ASTM C 403-99 (1999) (Rispin et al., 

2003; Clements, 2004; Knight et al., 2006; O’Toole and Pope, 

2006; Bernard, 2008), pneumatic pin (Iwaki et al., 2001), long 

partial beam, ASTM C 116-90 (1990) (Morgan, 1991; Heere 

et al., 2002) and partial beam, ASTM C 116-90 (O’Toole and 

Pope, 2006; Bernard, 2008). 

An example of early compressive strengths values obtained for five shotcrete mixes using penetrometer and partial beam methods as reported by O’Toole and Pope (2006) 

are shown in Fig. 4. 

Figure 4 

Summary of early-age penetrometer and sprayed beam 

compression tests from surface trials of shotcrete mixes 

(after O’Toole and Pope, 2006). 

Problems have been reported with the penetrometer type 

devices, particularly a difficulty in obtaining consistent results. This is due to the penetrometer pin contacting materials of varying density within the matrix of the material being 

sampled. Penetrometers were not used in this study because 

of the difficulties reported in regard to their use, nonlinearity of penetrometer test results with those obtained from 

beam tests (Fig. 4) and the need for developing a second test 

protocol. 

Beam molds have been used successfully for creating 

shotcrete test specimens in the U.S. and Canada (Heere et 

al., 2002). NIOSH researchers selected the partial beam standard, ASTM C 116-90 (1990) to measure the early strength 

of shotcrete, one to six hours after the material was sprayed. 

The molds can be sprayed by hand or manipulator arm, using either wet and dry shotcrete, and provide suitable results. 

The early-age shotcrete specimen can be sprayed, demolded, 

and placed in the test machine without degradation to the 

sample as a result of its low strength. During the partial 

beam strength test, a compressive load induces a complex 

shear failure in the shotcrete specimen. This type of loading 

and failure more closely replicates the shotcrete failures that 

are typically observed in the mines. 

An Aliva 252.1 series shotcrete machine was used to 

spray the fibered and nonfibered dry-mix shotcrete. The 

Aliva’s design employs a hopper and predampener and is 

commonly used to apply dry-mix shotcrete in underground 

mines in Nevada and Alaska. The use of a hopper and predampener allows for a consistent mix of the shotcrete with 

an even distribution of material at the nozzle. The dry mix 

was sprayed at an average water-to-cement ratio of 35%. 

The water content of the mix as it left the pre-dampener 

was 5%, with the nozzleman adding an additional 2.75% for 

a final water content of 7.75% for the spray. The same task 

trained nozzleman was used throughout the testing program 

to insure application consistency. 

A mold containment system consisting of two frames 

was developed to restrain the partial beam boxes and orient 

them at a 45° angle to reduce the amount of rebound that 

is trapped within the boxes as they are sprayed. A smaller 

frame contains three box molds that can be hand-carried 

separately to a testing location to conduct the one-hour tests 

(Fig. 5 upper left). A larger frame was constructed with forklift pockets to allow the entire unit to be moved to the test 

location (Fig. 5). 

Figure 5 

Partial beam box molds and spraying frame. 

This two-frame system allows the two-hour 
 

through six-hour samples to cure undisturbed while tests are 

conducted with the one-hour samples. 

A partial beam testing machine was designed to automatically load the samples at a fixed rate while measuring 

load and displacement (Fig. 6). 

Figure 6 

Early strength shotcrete test machine. 

This automated load cycle 

greatly reduces the chance of human error and enhances 

the consistency of the test results. This field-worthy design 

incorporates a small footprint, is self-contained and has a stiff 

frame press configuration with advanced load rate and loadmeasurement capability. Test parameter measurements are 

presented to the operator in proper scale and resolution on 

a graphical and numeric output display. The machine’s load 

resolution is 4.5 N (1 lbf) over a 22.2 kN (5,000 lbf) operating 

range. A servo loop-controlled head applies a load to the test 

specimen at a displacement rate of 1.278 mm/min (0.050 in./ 

min) in accordance with ASTM C 116-90 and automatically 

returns to its starting position after 6.35 mm (0.25 in.) of displacement. Load and displacement values are collected and 

stored on a thumb drive. 

Figure 8 

Vertical crack in a shotcrete test specimen. 

Figure 7 

Partial beam box mold and test specimen. 

Figure 9 

Shotcrete specimen showing post-test failure plane. 

As shown in Figs. 5 and 7, partial beam test samples are 

obtained by spraying shotcrete into 102 x 102 x 152 mm (4 x 

4 x 6 in.) mold boxes. After the samples have been sprayed, 

tests are conducted at one-hour intervals over the next six 

hours (one-hour through six-hour tests). The shotcrete samples are carefully de-molded 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 (Fig. 6). 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 (Fig. 

8) along the platen-to-sample contact edges, which are indicative of the failure plane shown in Fig. 9. 

The vertical crack is a complex failure that exhibits elements of shear, tensile and compressive failure modes. The 

test machine exerts a compressive load on the shotcrete sample through the test fixture that initiates primarily a shear 

failure in the sample. Figure 10 depicts examples of shotcrete 

failure modes. 

Figure 10 

Shotcrete failure modes (after Rose, 1985). 

This early strength test process is repeated 

for three specimens so that an average test result can be determined. 

Figure 11 shows the shotcrete early strength development 

as a function of time for the five commercially available weak 

rock mass, dry shotcrete mixes tested by NIOSH researchers. 

Figure 11 

Summary of early-age compressive strength tests with 

sprayed partial beam samples of five commercially 

available dry shotcrete mixes, n=72 samples. 

These mixes were applied using a sprayed mold system subjected to ASTM C 116-90 partial beam methods. 

Examination of the strength gain with time for the mixes 

tested shows four of the five mixes plateau at the four-hour 

mark with one of the five plateauing at the two-hour mark. 

This initial set and then phase transfer was observed in all 

shotcrete samples tested in the NIOSH study. This characteristic has been well referenced in the literature on shotcrete (Jolin et al., 1999; Heere et al., 2002; Knight et al., 2006; 

Rispin, 2005; O’Toole and Pope, 2006; Bernard, 2008) and is 

depicted as well in Fig. 4. The initial strength gain provided 

by the accelerant additives appears to progress into a cementitious hydration phase. For the mixes tested, three out 

of the five reached the early re-entry strength threshold of 1 

MPa (145 psi) within three hours and were confirmed again 

at the four-hour mark. All of the shotcrete mixes exceeded 

1 MPa (145 psi) after 24 hours of curing. 

These early compressive strength values are in good 

agreement with O’Toole and Pope (2006), who reported 

trends between 0.25 and 2.25 MPa for tests on 1-to-6-hour 

partial beams (Fig. 4) and Bernard (2008), who reported 

trends between 0.2 and 3 MPa for tests on 1-to-10-hour partial beams. 

The SCA mixes were supplied by a different shotcrete 

manufacturer than the K1 and P2 mixes. SCA is manufactured in Nevada and used in Nevada and Montana mines. K1 

and P2 are manufactured in Washington and used in Alaska 

and Idaho mines. The SCAPF and SCAPT-100 are fibered 

versions of the SCA mix. SCAPF is a poly-fibered mix, and 

SCAPT-100 is a steel-fibered mix. K1 has a greater percentage of accelerants than P2, while P2 has a greater percentage than SCA. After six hours of curing, the fibered mixes 

exceeded the capacity of the test machine (shown as dotted 

lines in Fig. 11), with compressive strengths greater than 2 

MPa (290 psi). For this test method, the addition of fibers 

generally produced higher early strengths. 

Conclusions 

A field-expedient test method and portable on site 

test equipment have been developed to measure shotcrete 

strength in the first six hours after application. Using this 

test method and equipment, underground mine personnel 

can measure the early compressive strength properties of “asplaced” shotcrete and clearly identify the shotcrete’s early 

strength threshold in real time. This information allows more 

informed decisions to be made regarding safe re-entry times 

and can also improve mine safety by supplying pertinent 

site-specific shotcrete information for ground control plans 

and providing a viable means of conducting on site quality 

control during shotcrete applications. 

This information is important because there are a variety of factors that can influence the early development of 

shotcrete strength, including the mine environment, ambient 

temperature of the applied shotcrete and host rock, shotcrete 

mix design, characteristics of the shotcrete additives, application quality and quantity and the water-to-cement ratio at 

which the shotcrete mix was actually sprayed. These factors 

are difficult to replicate in simulated tests conducted elsewhere, and delays in obtaining test results negate their usefulness in controlling shotcrete quality and preserving safety. 

The partial beam test method and portable early strength 

test machine were successfully demonstrated in tests with 

five commercial shotcrete mixes, which are currently used 
for ground support in weak rock conditions. Three out of the 

five shotcrete mixes shown in Fig. 11 developed the accepted 

minimum standard for early compressive strength of 1 MPa 

(145 psi) (Morgan, 1991; Clements, 2004 and 2009; Bernard, 

2008) within four hours of curing. 

Because safe re-entry depends on ensuring that the shotcrete has gained adequate strength, the key consideration 

is the actual strength of the shotcrete applied to the surface 

of the underground opening. Although these tests provide 

a valuable indication of the shotcrete’s potential strength 

gain, they are not an actual measurement of the shotcrete’s 

in-place strength. 

Acknowledgments

The authors thank James Schumacher at Thiessen Team 

USA, Elko, NV; Vince Mendive at Thiessen Team USA, Big 

Timber, MT; and Mark Mudlin, BASF, Master Builders, Elko, 

NV for their technical support with commercially available 

shotcrete mixes. The authors also thank Justin Deemer and 

Rocky Wegland at Westech Automation, Spokane, WA, for 

their technical and mechanical assistance in the development 

of the early strength test machine. 

Disclosure

The findings and conclusions presented in this document 

have not been formally disseminated by the National Institute for Occupational Safety and Health and should not be 

construed to represent any agency determination or policy. 

Mention of any company name or product does not constitute endorsement by NIOSH. 

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