ASTM C1550-20

Standard Test Method for Flexural Toughness of Fiber Reinforced Concrete (Using Centrally Loaded Round Panel)

SIGNIFICANCE AND USE
5.1 The post-crack behavior of plate-like, fiber-reinforced concrete structural members is well represented by a centrally loaded round panel test specimen that is simply supported on three pivots symmetrically arranged around its circumference. Such a test panel experiences bi-axial bending in response to a central point load and exhibits a mode of failure related to the in situ behavior of structures. The post-crack performance of round panels subject to a central point load can be represented by the energy absorbed by the panel up to a specified central deflection. In this test method, the energy absorbed up to a specified central deflection is taken to represent the ability of a fiber-reinforced concrete to redistribute stress following cracking.
Note 1: The use of three pivoted point supports in the test configuration results in determinate out-of-plane reactions prior to cracking, however the support reactions are indeterminate after cracking due to the unknown distribution of flexural resistance along each crack. There is also a change in the load resistance mechanism in the specimen as the test proceeds, starting with predominantly flexural resistance and progressing to tensile membrane action around the center as the imposed deflection is increased. The energy absorbed up to a specified central deflection is related to the toughness of the material but is specific to this specimen configuration because it is also determined by the support conditions and size of the specimen. Selection of the most appropriate central deflection to specify depends on the intended application for the material. The energy absorbed up to 5 mm central deflection is applicable to situations in which the material is required to hold cracks tightly closed at low levels of deformation. Examples include final linings in underground civil structures such as railway tunnels that may be required to remain water-tight. The energy absorbed up to 40 mm is more applicable to situations in ...
SCOPE
1.1 This test method covers the determination of flexural toughness of fiber-reinforced concrete expressed as energy absorption in the post-crack range using a round panel supported on three symmetrically arranged pivots and subjected to a central point load. The performance of specimens tested by this method is quantified in terms of the energy absorbed between the onset of loading and selected values of central deflection.  
1.2 This test method provides for the scaling of results whenever specimens do not comply with the target thickness and diameter, as long as dimensions do not fall outside of given limits.  
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.  
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.5 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 30-Sep-2020
• ICS: 91.100.40 - Products in fibre-reinforced cement
• Technical Committee: C09 - Concrete and Concrete Aggregates
• Drafting Committee: C09.42 - Fiber-Reinforced Concrete

Relations

• Refers: ASTM C31/C31M-24a - Standard Practice for Making and Curing Concrete Test Specimens in the Field
• Effective Date: 01-Apr-2024
• Refers: ASTM C670-24a - Standard Practice for Preparing Precision and Bias Statements for Test Methods for Construction Materials
• Effective Date: 01-Feb-2024
• Refers: ASTM C670-24 - Standard Practice for Preparing Precision and Bias Statements for Test Methods for Construction Materials
• Effective Date: 01-Jan-2024
• Refers: ASTM C31/C31M-24 - Standard Practice for Making and Curing Concrete Test Specimens in the Field
• Effective Date: 01-Jan-2024
• Refers: ASTM C125-19a - Standard Terminology Relating to Concrete and Concrete Aggregates
• Effective Date: 15-Dec-2019
• Refers: ASTM C125-19 - Standard Terminology Relating to Concrete and Concrete Aggregates
• Effective Date: 01-Jan-2019
• Refers: ASTM C125-18b - Standard Terminology Relating to Concrete and Concrete Aggregates
• Effective Date: 01-Oct-2018
• Refers: ASTM C125-18a - Standard Terminology Relating to Concrete and Concrete Aggregates
• Effective Date: 01-Jul-2018
• Refers: ASTM C31/C31M-18 - Standard Practice for Making and Curing Concrete Test Specimens in the Field
• Effective Date: 01-Jan-2018
• Refers: ASTM C125-18 - Standard Terminology Relating to Concrete and Concrete Aggregates
• Effective Date: 01-Jan-2018
• Refers: ASTM C125-16 - Standard Terminology Relating to Concrete and Concrete Aggregates
• Effective Date: 15-Dec-2016
• Refers: ASTM C125-15b - Standard Terminology Relating to Concrete and Concrete Aggregates
• Effective Date: 15-Dec-2015
• Refers: ASTM C31/C31M-15ae1 - Standard Practice for Making and Curing Concrete Test Specimens in the Field
• Effective Date: 15-Nov-2015
• Refers: ASTM C125-15a - Standard Terminology Relating to Concrete and Concrete Aggregates
• Effective Date: 01-Jul-2015
• Refers: ASTM C125-15 - Standard Terminology Relating to Concrete and Concrete Aggregates
• Effective Date: 01-Feb-2015

Overview

ASTM C1550-20: Standard Test Method for Flexural Toughness of Fiber Reinforced Concrete (Using Centrally Loaded Round Panel) is a widely recognized test standard developed by ASTM International. This method measures the flexural toughness and energy absorption of fiber-reinforced concrete (FRC) by evaluating round panels subjected to a central point load. The test quantifies post-crack performance, offering key insights into the material’s ability to redistribute stresses after cracking – a critical property for both cast and shotcrete fiber-reinforced concrete applications.

ASTM C1550-20 enables engineers, specifiers, and manufacturers to assess the ductility, durability, and resilience of FRC in challenging structural environments, ensuring that materials meet performance specifications required in infrastructure, civil, and mining projects.

Key Topics

• Flexural Toughness Measurement: This standard defines methods for determining the energy absorbed by FRC panels from the onset of loading to defined levels of central deflection, ensuring accurate characterization of post-crack behavior.
• Centrally Loaded Round Panel Test: Specimens are supported on three symmetrically arranged pivots and loaded at the center. This setup replicates bi-axial bending and is representative of real in situ structural behavior.
• Energy Absorption and Toughness: Toughness is expressed as the area under the load-deflection curve up to specified deflections (such as 5 mm, 10 mm, 20 mm, or 40 mm). The amount of energy absorbed correlates directly to the material’s post-crack performance.
• Panel Preparation and Dimensioning: The standard maintains strict requirements for panel thickness (75 mm nominal) and diameter (800 mm nominal), with correction factors provided for minor deviations to ensure uniformity and reproducibility.
• Applicability for Cast Concrete and Shotcrete: The procedure accommodates both cast-in-place and sprayed (shotcrete) methods, recognizing the importance of aligning test specimens with real-world construction techniques.
• Precision and Repeatability: The round panel method is valued for its within-batch repeatability, improving quality control in both laboratory and field environments.

Applications

ASTM C1550-20 is crucial for a variety of sectors requiring high-performance, fiber-reinforced concrete, particularly where post-crack toughness and energy absorption are critical for structural performance. Key applications include:

• Tunnels and Underground Structures: Ensures the durability and crack resistance of final linings, with energy absorption at lower deflection values (e.g., 5 mm) supporting watertightness demands.
• Mining Infrastructure: Evaluates shotcrete linings under severe deformation scenarios, where higher deflection (e.g., 40 mm) measurements are relevant for safety and service life.
• Industrial and Civil Concrete Flooring: Assesses suitability for flooring systems subjected to heavy loading and frequent impact, helping to specify FRC that maintains integrity after cracking.
• Durability and Quality Assurance: Enables comparison of alternative fiber types and dosages, supporting product development, procurement, and compliance with engineering specifications.

Related Standards

ASTM C1550-20 references several key standards for terminology, specimen preparation, and statistical analysis:

• ASTM C125 – Terminology Relating to Concrete and Concrete Aggregates
• ASTM C31/C31M – Practice for Making and Curing Concrete Test Specimens in the Field
• ASTM C670 – Practice for Preparing Precision and Bias Statements for Test Methods for Construction Materials

In addition, the standard was developed following internationally recognized principles for standardization as laid out by the World Trade Organization (WTO) Technical Barriers to Trade (TBT) Committee.

Summary

ASTM C1550-20 is a best-practice standard for measuring the flexural toughness and energy absorption of fiber reinforced concrete using a centrally loaded round panel test. By quantifying post-crack performance under controlled and repeatable conditions, the standard supports the design, specification, and quality assurance of FRC in a wide range of demanding civil, industrial, and underground applications.

Keywords: ASTM C1550-20, flexural toughness, fiber reinforced concrete, post-crack behavior, energy absorption, round panel test, shotcrete, structural concrete testing.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: C1550 − 20
Standard Test Method for
Flexural Toughness of Fiber Reinforced Concrete (Using
Centrally Loaded Round Panel)
This standard is issued under the fixed designation C1550; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope* C125Terminology Relating to Concrete and Concrete Ag-
gregates
1.1 This test method covers the determination of flexural
C670Practice for Preparing Precision and Bias Statements
toughness of fiber-reinforced concrete expressed as energy
for Test Methods for Construction Materials
absorption in the post-crack range using a round panel sup-
portedonthreesymmetricallyarrangedpivotsandsubjectedto
3. Terminology
a central point load. The performance of specimens tested by
3.1 Definitions—For definitions of terms used in this test
this method is quantified in terms of the energy absorbed
method, refer to Terminology C125.
between the onset of loading and selected values of central
deflection.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 central deflection—the net deflection at the center of
1.2 This test method provides for the scaling of results
the panel measured relative to a plane defined by the three
whenever specimens do not comply with the target thickness
pivotsusedtosupportthepanel;thisisaconditioneddeflection
anddiameter,aslongasdimensionsdonotfalloutsideofgiven
that excludes extraneous deformations of the load train and
limits.
local crushing of the panel at the point of load application and
1.3 The values stated in SI units are to be regarded as
points of support.
standard. No other units of measurement are included in this
3.2.2 compliance—a measure of the tendency of a structure
standard.
to deflect under load, found as the inverse of stiffness or
1.4 This standard does not purport to address all of the
deflection divided by the corresponding load.
safety concerns, if any, associated with its use. It is the
3.2.3 load train—those parts of a testing machine that
responsibility of the user of this standard to establish appro-
experience load and undergo straining during a mechanical
priate safety, health, and environmental practices and deter-
test, including the actuator, frame, support fixtures, load cell,
mine the applicability of regulatory limitations prior to use.
and specimen.
1.5 This international standard was developed in accor-
dance with internationally recognized principles on standard-
3.2.4 toughness—the energy absorbed by the specimen
ization established in the Decision on Principles for the
equivalent to the area under the load-deflection curve between
Development of International Standards, Guides and Recom-
the onset of loading and a specified central deflection.
mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee. 4. Summary of Test Method
4.1 Moldedroundpanelsofcastfiber-reinforcedconcreteor
2. Referenced Documents
fiber-reinforced shotcrete are subjected to a central point load
2.1 ASTM Standards:
while supported on three symmetrically arranged pivots. The
C31/C31MPractice for Making and Curing Concrete Test
load is applied through a hemispherical-ended steel piston
Specimens in the Field
advanced at a prescribed rate of displacement. Load and
deflectionarerecordedsimultaneouslyuptoaspecifiedcentral
1 deflection. The energy absorbed by the panel up to a specified
This test method is under the jurisdiction of ASTM Committee C09 on
Concrete and ConcreteAggregates and is the direct responsibility of Subcommittee central deflection is representative of the flexural toughness of
C09.42 on Fiber-Reinforced Concrete.
the fiber-reinforced concrete panel.
Current edition approved Oct. 1, 2020. Published November 2020. Originally
approved in 2002. Last previous edition approved in 2019 as C1550–19. DOI:
5. Significance and Use
10.1520/C1550-20.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
5.1 The post-crack behavior of plate-like, fiber-reinforced
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
concrete structural members is well represented by a centrally
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. loaded round panel test specimen that is simply supported on
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1550 − 20
suitably model structures of all sizes. Differences in post-crack behavior
three pivots symmetrically arranged around its circumference.
exhibited in this test method can be expected relative to cast fiber-
Such a test panel experiences bi-axial bending in response to a
reinforced concrete members thicker than 100 mm. Because fiber align-
central point load and exhibits a mode of failure related to the
ment is pronounced in structures produced by shotcreting, and the
in situ behavior of structures. The post-crack performance of
maximum aggregate size in shotcrete mixtures is typically 10 mm,
round panels subject to a central point load can be represented
post-crack behavior in specimens tested by this method are more
representative of in situ behavior when they are produced by spraying
by the energy absorbed by the panel up to a specified central
rather than casting concrete.
deflection. In this test method, the energy absorbed up to a
specified central deflection is taken to represent the ability of a
6. Apparatus
fiber-reinforced concrete to redistribute stress following crack-
6.1 Testing Machine—A servo-controlled testing machine
ing.
incorporating an electronic feed-back loop that uses the mea-
NOTE 1—The use of three pivoted point supports in the test configura-
tion results in determinate out-of-plane reactions prior to cracking, sured deflection of either the specimen or the loading actuator
however the support reactions are indeterminate after cracking due to the
to control the motion of the actuator shall be used to produce
unknowndistributionofflexuralresistancealongeachcrack.Thereisalso
a controlled and constant rate of increase of deflection of the
a change in the load resistance mechanism in the specimen as the test
specimen without the intervention of an operator. To avoid
proceeds, starting with predominantly flexural resistance and progressing
unstable behavior after cracking, the system stiffness of the
to tensile membrane action around the center as the imposed deflection is
increased. The energy absorbed up to a specified central deflection is
testingmachineinclusiveofloadframe,loadcell(ifused),and
related to the toughness of the material but is specific to this specimen
support fixture shall exceed that of the specimen. The system
configuration because it is also determined by the support conditions and
stiffness of the testing machine can be determined in accor-
size of the specimen. Selection of the most appropriate central deflection
dance with the procedure described in Annex A1. Load-
tospecifydependsontheintendedapplicationforthematerial.Theenergy
controlled test machines incorporating one-way hydraulic
absorbedupto5mmcentraldeflectionisapplicabletosituationsinwhich
the material is required to hold cracks tightly closed at low levels of
valves or screw mechanisms lacking an electronic feed-back
deformation. Examples include final linings in underground civil struc-
loop for automatically controlling the rate of increase in
tures such as railway tunnels that may be required to remain water-tight.
displacement shall not be used. The load-sensing device shall
The energy absorbed up to 40 mm is more applicable to situations in that
have a resolution sufficient to record load to 650 N.
the material is expected to suffer severe deformation in situ (for example,
NOTE 3—Although it is commonly believed that servo-controlled
shotcrete linings in mine tunnels and temporary linings in swelling
systems, incorporating a feed-back loop in which the measured central
ground).Energyabsorptionuptointermediatevaluesofcentraldeflection
displacementofthespecimenisusedtocontrolthemotionoftheactuator,
can be specified in situations requiring performance at intermediate levels
are capable of overcoming the disadvantages of a structurally compliant
of deformation.
testing machine, this will depend on the speed and sensitivity of the
5.2 The motivation for use of a round panel with three
feed-backloopandthemechanicalresponserateoftheloadingapparatus.
supports is based on the within-batch repeatability found in
A more reliable configuration comprises a servo-controlled actuator in
3 4
which the measured displacement of the actuator is used in the feed-back
laboratory and field experience. The consistency of the
loop to control the motion of the actuator combined with a high load train
failure mode that arises through the use of three symmetrically
stiffness. Experience has indicated that the redistribution of stress that
arranged support pivots results in low within-batch variability
occurs in fiber-reinforced concrete panels following cracking of the
in the energy absorbed by a set of panels up to a specified
concrete matrix generally results in stable post-crack behavior provided a
central deflection. The use of round panels also eliminates the
testing machine complying with the requirements of this section is used.
sawing that is required to prepare shotcrete beam specimens.
6.2 Support Fixture—The fixture supporting the panel dur-
5.3 The nominal dimensions of the panel are 75 mm in ingtestingshallconsistofanyconfigurationthatincludesthree
thickness and 800 mm in diameter. Thickness has been shown symmetrically arranged pivot points on a pitch circle diameter
to strongly influence panel performance in this test, while of 750 mm (see Fig.A1.4 in AnnexA1).The supports shall be
variations in diameter have been shown to exert a minor capableofsupportingaloadofupto100kNappliedvertically
influence on performance. Correction factors are provided to atthecenterofthespecimen.Thesupportsshallbesufficiently
rigidsothattheydonotdisplaceintheradialdirectionbymore
account for actual measured dimensions.
NOTE 2—The target dimensions of the panel specimen used in this test
than 0.5 mm between the onset of loading and 40 mm central
are held constant regardless of the characteristics of aggregate and fibers
deflection for a test involving a specimen displaying a peak
used in the concrete comprising the specimen. Post-crack performance
load capacity of 100 kN. The three supports must also not
maybeinfluencedbysizeandboundaryeffectsiflargeaggregateparticles
translate by more than 0.5 mm in the circumferential direction
orlongfibersareusedintheconcrete.Theseinfluencesareacknowledged
during a test. The pivots shall not restrict rotation of the panel
and accepted in this test method because issues of size effect and fiber
alignment arise in actual structures and no single test specimen can
fragments after cracking. The support fixture shall be config-
ured so that the specimen does not come into contact with any
portionofthesupportfixtureapartfromthethreepivotsduring
Bernard, E. S. “Correlations in the Behaviour of Fibre Reinforced Shotcrete
a test.Aphotograph of a suggested design is shown in Fig. 1.
Beam and Panel Specimens,” Materials and Structures, RILEM, Vol 35, pp.
The contact between the specimen and each pivot shall
156–164,April 2002.
4 compriseasteeltransferplatewithplandimensionsofapproxi-
Hanke,S.A.,Collis,A.,andBernard,E.S.,“TheM5Motorway:AnEducation
mately 40 × 50 mm with a spherical seat of about 4 mm depth
in Quality Assurance for Fibre Reinforced Shotcrete,” Shotcrete: Engineering
Developments, Bernard (ed.), Swets & Zeitlinger, Lisse, pp. 145-156, 2001.
machined into one surface to accept a ball pivot (see Fig. 2).
Bernard, E. S. and Pircher, M., 2001, “The Influence of Thickness on
Thedistancebetweenthesurfaceofthepanelandthecenterof
Performance of Fiber-Reinforced Concrete in a Round Determinate Panel Test,”
the pivot shall be 20 6 2 mm. The diameter of the pivot ball
Cement, Concrete, and Aggregates,CCAGDP,Vol23,No.1,pp.27–33,June2001
. shallbe16 62mm.Theradiustothecenterofeachpivotball
C1550 − 20
radius of the hemispherical portion of the head shall be 80 6
5 mm, and that of the piston shaft 50 6 5 mm.
7. Specimen Preparation and Sampling
7.1 Produce specimens with an overall diameter of 800 6
10 mm and a thickness of 75 -5/+15 mm. Panels shall not be
tested if dimensions are outside of the specified limits. The
standard deviation in 10 measures of thickness taken in
accordance with the instructions in the procedure section must
be less than 3.0 mm. Maintain these dimensions regardless of
the size of aggregate or length of fiber used in the concrete or
shotcrete.Makethesideofthespecimenperpendicularrelative
FIG. 1 Photograph of a Suggested Support Fixture
to the faces.
7.2 Prepare specimens in such a way as to approximate the
shallbe375 62mmmeasuredfromaverticalaxisthroughthe
mode of placement in situ. Specimens representing cast con-
central load point. The subtended angle between each pair of
crete shall therefore be cast, while those representing shotcrete
pivotsshallbe120 60.5°measuredattheverticalaxisthrough
shall be sprayed. Specimens shall be screeded to the required
the central load point. Grease is permitted to reduce friction in
thickness before the concrete has hardened (see Appendix X1
theseatofeachpivot,butrollersorgreasearenotpermittedto
for further recommendations regarding specimen production).
reduce friction between the transfer plates and specimen.
Remove molds when the concrete has attained sufficient
6.3 Deflection Measuring Equipment—Determine the cen-
strength so that the specimen can be placed into the testing
tral deflection of the specimen relative to the support points in
position without being damaged.
a manner that excludes extraneous deformations of the testing
NOTE 7—Grinding or sawing of the surface to reduce an overly thick
machine and support fixture. This is achieved by one of two
panel to the required thickness is possible, but may influence the
methods. If the displacement of the tensile surface of the panel
performance of the as-cast or sprayed surface concrete.
at the center is measured relative to the pivot supports, then no
7.3 Moldsfortheproductionofspecimensshallconsistofa
correction for extraneous deformations of the testing machine
base and side made of either non-reactive metal or coated
andsupportfixtureneedbemadetotherecordeddeflections.If
plywood.The base and side shall be sufficiently rigid so as not
themovementoftheloadingpistonrelativetothecrossheadof
to vibrate or permanently distort during casting or spraying.
the testing machine is used to measure deflection, the deflec-
The interior face of the mold shall be 75 mm deep so that a
tion record must be adjusted to discount extraneous deforma-
screed may be run directly across the surface to produce a
tions. A method of adjusting the deflection record to account
specimen of correct thickness. See Appendix X1 for recom-
forextraneousdeformationsisgiveninthecalculationsection.
mendations regarding mold design.
Regardless of the method of deflection measurement selected,
7.4 Control the diameter of the mold through careful atten-
use a displacement transducer with a resolution sufficient to
tion to manufacture. Maintenance of the correct thickness is
record deflection to 60.05 mm.
subject to the skill of personnel charged with finishing the
NOTE 4—All components of the load train in a test system experience
deformation when the specimen is placed under load. If the deflection of
specimens. For normal setting concrete, sufficient time is
the specimen is measured relative to the machine crosshead, then the
normally available to screed the surface to obtain a uniform
deformationoftheloadtrainisincludedasextraneousdeformationsinthe
thickness.Accelerated shotcrete may, however, stiffen quickly
deflection record. Additional extraneous deformations may arise from
leaving insufficient time to adequately screed the surface. In
local crushing of concrete under the load point (especially debris on the
such cases, it is necessary to produce several specimens and
surface), or from crushing of any debris between the specimen and
transferplates.Thissecondformofextraneousdeformationusuallyresults
only retain those that are uniform in thickness.
in curvature in the initial portion of the load-deflection curve.
7.5 Sampling—Prepare at least three specimens for each
NOTE 5—If the deflection of the center of the tensile surface of the
specimen is measured directly with a transducer, an incomplete or batch of concrete or shotcrete tested.Asample shall consist of
erroneous deflection record may occur if a crack opens at the point of
atleasttwosuccessfultests.Asuccessfultestinvolvesafailure
measurement. It may be possible to alleviate this problem through the use
that includes at least three radial cracks. Specimens occasion-
ofatransducerwithaprobeapproximately20mmwide.Theprobeshould
ally fail in a beam-like mode involving a single crack across
not exceed this width because off-center cracks may induce exaggerated
the specimen that is characterized by low energy absorption.
apparent deflections if they occur adjacent to a wide probe.
The result of such a test shall be discarded. Only two
6.4 Data Logging System—Record the deflection imposed
specimensneedbetestedifbothspecimensfailbytherequired
on the panel and corresponding applied load simultaneously at
modeandhaveastandarddeviationinthicknessnotexceeding
a rate sufficient to record deflection in increments of no more
3.0 mm.
than 0.02 mm using a digital recording system.
NOTE 6—As a guide, the majority of specimens having standard
8. Conditioning
dimensions and exhibiting normal strength fail at a load of less than 40
kN.
8.1 The purchaser shall specify the curing and moisture
6.5 Loading Piston—The load point shall consist of a steel conditioning requirements to be used prior to testing, and the
hemisphericalpistonwiththedimensionsshowninFig.3.The test age. If the specimens are continuously moist cured and are
C1550 − 20
FIG. 2 Detail of Transfer Plate and Pivot Support
9.2 Measure the diameter of the panel to the nearest 2 mm
at three places coincident with the intended support locations
and calculate the average diameter. If the average diameter of
the specimen is less than 790 mm or greater than 810 mm,
discard the specimen.
9.3 Operate the testing machine so that the piston advances
at a constant rate of 4.0 6 1.0 mm/min up to a central
displacement of at least 45.0 mm.
NOTE 9—The test can be extended to an end-point deflection greater
than 45 mm if it is desired to examine behavior at higher levels of
deformation.
NOTE 10—The central deflection at which cracking of the concrete
FIG. 3 Hemispherical End of Loading Piston
matrix first occurs is approximately 0.50 mm for a 75 mm thick concrete
specimen of normal strength and composition, exclusive of extraneous
displacements. A rate of displacement equal to 4.0 mm/min therefore
to be tested in a moist condition, complete testing within 15
causes cracking of the concrete matrix in about 8 s. However, if a
displacement-controlled testing machine is used and the surface of the
min after removal from the moist curing conditions, or apply a
specimen is rough, as is often the case with shotcrete specimens, the
curingmembraneorwetburlaptocontroldryingfromthetime
effective displacement rate of the center of the specimen may be less than
of removal until testing is completed.
4.0mm/minatthestartofatest.Experiencehasshownthatlocalcrushing
NOTE 8—Drying shrinkage strains occur in a specimen that is allowed
of concrete under the load point usually occurs within the first few
to dry. These strains may result in micro-cracks and may reduce the
millimetres of movement. Research has also shown that small changes in
flexural strength and post-crack energy absorption of the panel.
the effective rate of central displacement have only a minor influence on
energy absorption for displacement rates within the range of 0.5 to 10
9. Procedure
mm/min.
9.1 Mountthetestspecimeninthetestapparatusbyplacing
the molded face onto the three transfer plates resting on the
Bernard, E. S., “The Influence of Strain Rate on Performance of Fiber-
pivots. Center the panel with respect to both the supports and
Reinforced Concrete Loaded in Flexure,” Cement, Concrete, and Aggregates,
loading piston. CCAGDP, Vol 23, No. 1, pp. 11–18, June 2001.
C1550 − 20
NOTE 12—The result of a test is a load-deflection record indicating
9.4 Count the number of radial cracks occurring between
resistance to load between the onset of loading and a central deflection of
the center and the perimeter. Any flexural crack occurring on
atleast40mm.Dependingonthemethodofdeflectionmeasurementused,
the tensile face of the panel is counted as a full crack provided
the load-deflection record may include extraneous deformations associ-
its average width exceeds 0.5 mm upon completion of the test
ated with load train compliance or crushing of the concrete around the
and removal of the load.
load point or supports, or both. Extraneous deformations associated with
NOTE 11—Energy is absorbed by fiber-reinforced concrete in this test load train compliance are regarded as systematic errors. These are
through a number of processes. Minor amounts of energy are absorbed
discounted by computing the deformation of the load train (which is
eitherthroughelasticdeformationofthespecimenorasaresultoffriction
proportional to the load imposed on the specimen) and subtracting this
between the underside of the specimen and the transfer plates at the three
from the recorded deflection at each deflection increment. The adjusted
supports. The majority of energy is absorbed through the process of fiber
deflection δ of the specimen at a given load P exclusive of extraneous
pull-out and deformation that takes place as each crack opens in response
deformations associated with load train compliance can be calculated as:
toimposeddeformation.Cracksthatsufferminimalopeningdonotabsorb
δ 5δ 2 PC (1)
significant amounts of energy and thus can be ignored. Given that the
m LT
averagemaximumcrackopeningforeachofthethreeradialcracksinthis
where:
test is 10 mm at 40 mm central deflection, a crack of less than 0.5 mm
δ = the measured deflection including extraneous deformation due
width is regarded as insignificant. Laboratory experience has also dem- m
to compliance of the load train, and
onstrated that small cracks appear to have little effect on total energy
C = the compliance of the load train (see Annex A1).
absorption.
...

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it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
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Designation: C1550 − 19 C1550 − 20
Standard Test Method for
Flexural Toughness of Fiber Reinforced Concrete (Using
Centrally Loaded Round Panel)
This standard is issued under the fixed designation C1550; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope*
1.1 This test method covers the determination of flexural toughness of fiber-reinforced concrete expressed as energy absorption
in the post-crack range using a round panel supported on three symmetrically arranged pivots and subjected to a central point load.
The performance of specimens tested by this method is quantified in terms of the energy absorbed between the onset of loading
and selected values of central deflection.
1.2 This test method provides for the scaling of results whenever specimens do not comply with the target thickness and diameter,
as long as dimensions do not fall outside of given limits.
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.5 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
C31/C31M Practice for Making and Curing Concrete Test Specimens in the Field
C125 Terminology Relating to Concrete and Concrete Aggregates
C670 Practice for Preparing Precision and Bias Statements for Test Methods for Construction Materials
3. Terminology
3.1 Definitions—For definitions of terms used in this test method, refer to Terminology C125.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 central deflection—the net deflection at the center of the panel measured relative to a plane defined by the three pivots used
This test method is under the jurisdiction of ASTM Committee C09 on Concrete and Concrete Aggregates and is the direct responsibility of Subcommittee C09.42 on
Fiber-Reinforced Concrete.
Current edition approved Oct. 1, 2019Oct. 1, 2020. Published November 2019November 2020. Originally approved in 2002. Last previous edition approved in 20122019
as C1550C1550 – 19.–12a. DOI: 10.1520/C1550-19.10.1520/C1550-20.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1550 − 20
to support the panel; this is a conditioned deflection that excludes extraneous deformations of the load train and local crushing of
the panel at the point of load application and points of support.
3.2.2 compliance—a measure of the tendency of a structure to deflect under load, found as the inverse of stiffness or deflection
divided by the corresponding load.
3.2.3 load train—those parts of a testing machine that experience load and undergo straining during a mechanical test, including
the actuator, frame, support fixtures, load cell, and specimen.
3.2.4 toughness—the energy absorbed by the specimen equivalent to the area under the load-deflection curve between the onset
of loading and a specified central deflection.
4. Summary of Test Method
4.1 Molded round panels of cast fiber-reinforced concrete or fiber-reinforced shotcrete are subjected to a central point load while
supported on three symmetrically arranged pivots. The load is applied through a hemispherical-ended steel piston advanced at a
prescribed rate of displacement. Load and deflection are recorded simultaneously up to a specified central deflection. The energy
absorbed by the panel up to a specified central deflection is representative of the flexural toughness of the fiber-reinforced concrete
panel.
5. Significance and Use
5.1 The post-crack behavior of plate-like, fiber-reinforced concrete structural members is well represented by a centrally loaded
round panel test specimen that is simply supported on three pivots symmetrically arranged around its circumference. Such a test
panel experiences bi-axial bending in response to a central point load and exhibits a mode of failure related to the in situ behavior
of structures. The post-crack performance of round panels subject to a central point load can be represented by the energy absorbed
by the panel up to a specified central deflection. In this test method, the energy absorbed up to a specified central deflection is taken
to represent the ability of a fiber-reinforced concrete to redistribute stress following cracking.
NOTE 1—The use of three pivoted point supports in the test configuration results in determinate out-of-plane reactions prior to cracking, however the
support reactions are indeterminate after cracking due to the unknown distribution of flexural resistance along each crack. There is also a change in the
load resistance mechanism in the specimen as the test proceeds, starting with predominantly flexural resistance and progressing to tensile membrane action
around the center as the imposed deflection is increased. The energy absorbed up to a specified central deflection is related to the toughness of the material
but is specific to this specimen configuration because it is also determined by the support conditions and size of the specimen. Selection of the most
appropriate central deflection to specify depends on the intended application for the material. The energy absorbed up to 5 mm central deflection is
applicable to situations in which the material is required to hold cracks tightly closed at low levels of deformation. Examples include final linings in
underground civil structures such as railway tunnels that may be required to remain water-tight. The energy absorbed up to 40 mm is more applicable
to situations in that the material is expected to suffer severe deformation in situ (for example, shotcrete linings in mine tunnels and temporary linings in
swelling ground). Energy absorption up to intermediate values of central deflection can be specified in situations requiring performance at intermediate
levels of deformation.
5.2 The motivation for use of a round panel with three supports is based on the within-batch repeatability found in laboratory
and field experience. The consistency of the failure mode that arises through the use of three symmetrically arranged support
pivots results in low within-batch variability in the energy absorbed by a set of panels up to a specified central deflection. The use
of round panels also eliminates the sawing that is required to prepare shotcrete beam specimens.
5.3 The nominal dimensions of the panel are 75 mm in thickness and 800 mm in diameter. Thickness has been shown to strongly
influence panel performance in this test, while variations in diameter have been shown to exert a minor influence on performance.
Correction factors are provided to account for actual measured dimensions.
NOTE 2—The target dimensions of the panel specimen used in this test are held constant regardless of the characteristics of aggregate and fibers used in
the concrete comprising the specimen. Post-crack performance may be influenced by size and boundary effects if large aggregate particles or long fibers
are used in the concrete. These influences are acknowledged and accepted in this test method because issues of size effect and fiber alignment arise in
Bernard, E. S. “Correlations in the Behaviour of Fibre Reinforced Shotcrete Beam and Panel Specimens,” Materials and Structures, RILEM, Vol 35, pp. 156–164, April
2002.
Hanke, S. A., Collis, A., and Bernard, E. S., “The M5 Motorway: An Education in Quality Assurance for Fibre Reinforced Shotcrete,” Shotcrete: Engineering
Developments, Bernard (ed.), Swets & Zeitlinger, Lisse, pp. 145-156, 2001.
Bernard, E. S. and Pircher, M., 2001, “The Influence of Thickness on Performance of Fiber-Reinforced Concrete in a Round Determinate Panel Test,” Cement, Concrete,
and Aggregates, CCAGDP, Vol 23, No. 1, pp. 27 –33, June 2001 .
C1550 − 20
actual structures and no single test specimen can suitably model structures of all sizes. Differences in post-crack behavior exhibited in this test method
can be expected relative to cast fiber-reinforced concrete members thicker than 100 mm. Because fiber alignment is pronounced in structures produced
by shotcreting, and the maximum aggregate size in shotcrete mixtures is typically 10 mm, post-crack behavior in specimens tested by this method are
more representative of in situ behavior when they are produced by spraying rather than casting concrete.
6. Apparatus
6.1 Testing Machine—A servo-controlled testing machine incorporating an electronic feed-back loop that uses the measured
deflection of either the specimen or the loading actuator to control the motion of the actuator shall be used to produce a controlled
and constant rate of increase of deflection of the specimen without the intervention of an operator. To avoid unstable behavior after
cracking, the system stiffness of the testing machine inclusive of load frame, load cell (if used), and support fixture shall exceed
that of the specimen. The system stiffness of the testing machine can be determined in accordance with the procedure described
in Annex A1. Load-controlled test machines incorporating one-way hydraulic valves or screw mechanisms lacking an electronic
feed-back loop for automatically controlling the rate of increase in displacement shall not be used. The load-sensing device shall
have a resolution sufficient to record load to 650 N.
NOTE 3—Although it is commonly believed that servo-controlled systems, incorporating a feed-back loop in which the measured central displacement
of the specimen is used to control the motion of the actuator, are capable of overcoming the disadvantages of a structurally compliant testing machine,
this will depend on the speed and sensitivity of the feed-back loop and the mechanical response rate of the loading apparatus. A more reliable configuration
comprises a servo-controlled actuator in which the measured displacement of the actuator is used in the feed-back loop to control the motion of the
actuator combined with a high load train stiffness. Experience has indicated that the redistribution of stress that occurs in fiber-reinforced concrete panels
following cracking of the concrete matrix generally results in stable post-crack behavior provided a testing machine complying with the requirements of
this section is used.
6.2 Support Fixture—The fixture supporting the panel during testing shall consist of any configuration that includes three
symmetrically arranged pivot points on a pitch circle diameter of 750 mm. mm (see Fig. A1.4 in Annex A1). The supports shall
be capable of supporting a load of up to 100 kN applied vertically at the center of the specimen. The supports shall be sufficiently
rigid so that they do not displace in the radial direction by more than 0.5 mm between the onset of loading and 40 mm central
deflection for a test involving a specimen displaying a peak load capacity of 100 kN. The three supports must also not translate
by more than 0.5 mm in the circumferential direction during a test. The pivots shall not restrict rotation of the panel fragments after
cracking. The support fixture shall be configured so that the specimen does not come into contact with any portion of the support
fixture apart from the three pivots during a test. A photograph of a suggested design is shown in Fig. 1. The contact between the
specimen and each pivot shall comprise a steel transfer plate with plan dimensions of approximately 40 × 50 mm with a spherical
seat of about 4 mm depth machined into one surface to accept a ball pivot (see Fig. 2). The distance between the surface of the
panel and the center of the pivot shall be 20 6 2 mm. The diameter of the pivot ball shall be 16 6 2 mm. The radius to the center
of each pivot ball shall be 375 6 2 mm measured from a vertical axis through the central load point. The subtended angle between
each pair of pivots shall be 120 6 0.5° measured at the vertical axis through the central load point. Grease is permitted to reduce
friction in the seat of each pivot, but rollers or grease are not permitted to reduce friction between the transfer plates and specimen.
6.3 Deflection Measuring Equipment—Determine the central deflection of the specimen relative to the support points in a manner
that excludes extraneous deformations of the testing machine and support fixture. This is achieved by one of two methods. If the
displacement of the tensile surface of the panel at the center is measured relative to the pivot supports, then no correction for
extraneous deformations of the testing machine and support fixture need be made to the recorded deflections. If the movement of
the loading piston relative to the crosshead of the testing machine is used to measure deflection, the deflection record must be
adjusted to discount extraneous deformations. A method of adjusting the deflection record to account for extraneous deformations
FIG. 1 Photograph of a Suggested Support Fixture
C1550 − 20
FIG. 2 Detail of Transfer Plate and Pivot Support
is given in the calculation section. Regardless of the method of deflection measurement selected, use a displacement transducer
with a resolution sufficient to record deflection to 60.05 mm.
NOTE 4—All components of the load train in a test system experience deformation when the specimen is placed under load. If the deflection of the
specimen is measured relative to the machine crosshead, then the deformation of the load train is included as extraneous deformations in the deflection
record. Additional extraneous deformations may arise from local crushing of concrete under the load point (especially debris on the surface), or from
crushing of any debris between the specimen and transfer plates. This second form of extraneous deformation usually results in curvature in the initial
portion of the load-deflection curve.
NOTE 5—If the deflection of the center of the tensile surface of the specimen is measured directly with a transducer, an incomplete or erroneous deflection
record may occur if a crack opens at the point of measurement. It may be possible to alleviate this problem through the use of a transducer with a probe
approximately 20 mm wide. The probe should not exceed this width because off-center cracks may induce exaggerated apparent deflections if they occur
adjacent to a wide probe.
6.4 Data Logging System—Record the deflection imposed on the panel and corresponding applied load simultaneously at a rate
sufficient to record deflection in increments of no more than 0.02 mm using a digital recording system.
NOTE 6—As a guide, the majority of specimens having standard dimensions and exhibiting normal strength fail at a load of less than 40 kN.
6.5 Loading Piston—The load point shall consist of a steel hemispherical piston with the dimensions shown in Fig. 3. The radius
of the hemispherical portion of the head shall be 80 6 5 mm, and that of the piston shaft 50 6 5 mm.
7. Specimen Preparation and Sampling
7.1 Produce specimens with an overall diameter of 800 6 10 mm and a thickness of 75 -5/+15 mm. Panels shall not be tested
if dimensions are outside of the specified limits. The standard deviation in 10 measures of thickness taken in accordance with the
C1550 − 20
FIG. 3 Hemispherical End of Loading Piston
instructions in the procedure section must be less than 3.0 mm. Maintain these dimensions regardless of the size of aggregate or
length of fiber used in the concrete or shotcrete. Make the side of the specimen perpendicular relative to the faces.
7.2 Prepare specimens in such a way as to approximate the mode of placement in situ. Specimens representing cast concrete shall
therefore be cast, while those representing shotcrete shall be sprayed. Specimens shall be screeded to the required thickness before
the concrete has hardened (see Appendix X1 for further recommendations regarding specimen production). Remove molds when
the concrete has attained sufficient strength so that the specimen can be placed into the testing position without being damaged.
NOTE 7—Grinding or sawing of the surface to reduce an overly thick panel to the required thickness is possible, but may influence the performance of
the as-cast or sprayed surface concrete.
7.3 Molds for the production of specimens shall consist of a base and side made of either non-reactive metal or coated plywood.
The base and side shall be sufficiently rigid so as not to vibrate or permanently distort during casting or spraying. The interior face
of the mold shall be 75 mm deep so that a screed may be run directly across the surface to produce a specimen of correct thickness.
See Appendix X1 for recommendations regarding mold design.
7.4 Control the diameter of the mold through careful attention to manufacture. Maintenance of the correct thickness is subject to
the skill of personnel charged with finishing the specimens. For normal setting concrete, sufficient time is normally available to
screed the surface to obtain a uniform thickness. Accelerated shotcrete may, however, stiffen quickly leaving insufficient time to
adequately screed the surface. In such cases, it is necessary to produce several specimens and only retain those that are uniform
in thickness.
7.5 Sampling—Prepare at least three specimens for each batch of concrete or shotcrete tested. A sample shall consist of at least
two successful tests. A successful test involves a failure that includes at least three radial cracks. Specimens occasionally fail in
a beam-like mode involving a single crack across the specimen that is characterized by low energy absorption. The result of such
a test shall be discarded. Only two specimens need be tested if both specimens fail by the required mode and have a standard
deviation in thickness not exceeding 3.0 mm.
8. Conditioning
8.1 The purchaser shall specify the curing and moisture conditioning requirements to be used prior to testing, and the test age. If
the specimens are continuously moist cured and are to be tested in a moist condition, complete testing within 15 min after removal
from the moist curing conditions, or apply a curing membrane or wet burlap to control drying from the time of removal until testing
is completed.
NOTE 8—Drying shrinkage strains occur in a specimen that is allowed to dry. These strains may result in micro-cracks and may reduce the flexural strength
and post-crack energy absorption of the panel.
9. Procedure
9.1 Mount the test specimen in the test apparatus by placing the molded face onto the three transfer plates resting on the pivots.
Center the panel with respect to both the supports and loading piston.
C1550 − 20
9.2 Measure the diameter of the panel to the nearest 2 mm at three places coincident with the intended support locations and
calculate the average diameter. If the average diameter of the specimen is less than 790 mm or greater than 810 mm, discard the
specimen.
9.3 Operate the testing machine so that the piston advances at a constant rate of 4.0 6 1.0 mm/min up to a central displacement
of at least 45.0 mm.
NOTE 9—The test can be extended to an end-point deflection greater than 45 mm if it is desired to examine behavior at higher levels of deformation.
NOTE 10—The central deflection at which cracking of the concrete matrix first occurs is approximately 0.50 mm for a 75 mm thick concrete specimen
of normal strength and composition, exclusive of extraneous displacements. A rate of displacement equal to 4.0 mm/min therefore causes cracking of the
concrete matrix in about 8 s. However, if a displacement-controlled testing machine is used and the surface of the specimen is rough, as is often the case
with shotcrete specimens, the effective displacement rate of the center of the specimen may be less than 4.0 mm/min at the start of a test. Experience
has shown that local crushing of concrete under the load point usually occurs within the first few millimetres of movement. Research has also shown that
small changes in the effective rate of central displacement have only a minor influence on energy absorption for displacement rates within the range of
0.5 to 10 mm/min.
9.4 Count the number of radial cracks occurring between the center and the perimeter. Any flexural crack occurring on the tensile
face of the panel is counted as a full crack provided its average width exceeds 0.5 mm upon completion of the test and removal
of the load.
NOTE 11—Energy is absorbed by fiber-reinforced concrete in this test through a number of processes. Minor amounts of energy are absorbed either through
elastic deformation of the specimen or as a result of friction between the underside of the specimen and the transfer plates at the three supports. The
majority of energy is absorbed through the process of fiber pull-out and deformation that takes place as each crack opens in response to imposed
deformation. Cracks that suffer minimal opening do not absorb significant amounts of energy and thus can be ignored. Given that the average maximum
crack opening for each of the three radial cracks in this test is 10 mm at 40 mm central deflection, a crack of less than 0.5 mm width is regarded as
insignificant. Laboratory experience has also demonstrated that small cracks appear to have little effect on total energy absorption.
9.5 Remove the failed specimen fragments from the test apparatus, and measure the thickness at three points along each of the
cracked surfaces and at the center so that the resulting 10 values provide a representative estimate of the average thickness of the
specimen. Measure the thickness to the nearest 1 mm and calculate the average thickness to the nearest 1 mm. If the average
thickness is less than 70 mm or greater than 90 mm, discard the specimen. Calculate the standard deviation in thickness. If the
standard deviation in thickness exceeds 3.0 mm, discard the specimen.
10. Calculation
10.1 Adjust the load-deflection record by subtracting extraneous deformations associated with compliance of the load train and
crushing of concrete under the load point and at the supports. If the load-deflection record was obtained using a transducer that
measured the deflection of the tensile surface of the specimen relative to the transfer plates, adjustments need only be made for
crushing of concrete at the transfer plates. If the deflection of the specimen was measured through the loading mechanism of the
testing machine, this record includes extraneous displacements that must be deleted from the deflection record to reveal th
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