ASTM C39 Compressive Strength
Not publishedof exam
Loading RateFracture TypesL/D CorrectionCylinder Tolerances
ASTM C78 Flexural Strength
Not publishedof exam
Third-Point LoadingModulus of RuptureSpan GeometryFracture Location
ASTM C617 Sulfur Capping
Not publishedof exam
Cap ThicknessHardening TimeCapping PlateSulfur Safety
ASTM C1231 Unbonded Caps
Not publishedof exam
Shore A DurometerRetainer RingPad Reuse LimitsPad Inspection
Lab Practice & Program Info
Not publishedof exam
C1077 AccreditationACI 318 AcceptanceMachine VerificationExam Logistics
Quick Facts
- Exam
- CSTT (ACI)
- Credential
- Concrete Strength Testing Tech
- Written Qs
- ~40 MC, min 8/standard
- Time
- 1 hr written + performance
- Pass
- 70% overall, 60% each
- Format
- Written + hands-on demo
- Level
- Lab technician
- Workbook
- CP-19, 25th Edition
- Validity
- 5 years
C39 Fracture Types 1-6
Cone, Cone+Crack, Column, Diagonal, Side, Point
1: double cone2: one cone3: columnar4: diagonal5/6: side/point
C39 vs C78 Testing
C39 Compressive
- Cylinders
- P/A formula
- 10 psi report
C78 Flexural
- Beams
- PL/bd² formula
- 5 psi report
Cylinder crush vs beam bend
C39 Fracture Type Diagnosis
- Cones on both ends→Type 1(Normal, well-capped)
- One cone plus cap cracks→Type 2(Minor cap issue)
- Columnar cracks both ends→Type 3(Check capping quality)
- Diagonal, no end cracks→Type 4(Tap to confirm)
- Side break at top/bottom→Type 5(Common with unbonded)
- Pointed side break→Type 6(Check pad and alignment)
C39 Loading Rate & Reporting
- Loading rate
- 35 ± 7 psi/s
- Rate range
- 28 to 42 psi/s
- Strength report
- Nearest 10 psi
- Specimen age
- From water/cement add
- 28-day tolerance
- ± 20 hours
- 7-day tolerance
- ± 6 hours
- 3-day tolerance
- ± 2 hours
- 24-hour tolerance
- ± 0.5 hour
Fracture Type 1 vs Type 3
Type 1
- Cones both ends
- Normal failure
Type 3
- Columnar cracks
- Capping problem
Good cap vs bad cap
C39 Specimen Tolerances
- End planeness
- 0.002 in maximum
- Perpendicularity
- 0.5°, ~1/8 in./12 in.
- Diameter measure
- Nearest 0.01 in.
- Length measure
- Nearest 0.05 diameter
- Minimum L/D
- 1.00 ratio
- L/D above 1.75
- No correction applied
- Aggregate rule
- Diameter ≥ 3× max agg
C39 L/D Correction Factors
- L/D 1.75
- Factor 0.98
- L/D 1.50
- Factor 0.96
- L/D 1.25
- Factor 0.93
- L/D 1.00
- Factor 0.87
- L/D > 1.75
- No factor needed
C39 Fracture Types
- Type 1
- Cones both endsNormal
- Type 2
- One cone plus cracks
- Type 3
- Columnar cracks, no cone
- Type 4
- Diagonal shear fracture
- Type 5
- Side break, top/bottom
- Type 6
- Pointed side break
C39 Machine Requirements
- Load accuracy
- ± 1.0% of load
- Verify frequency
- Annual + after repair
- Bearing block size
- ≥ 3% over diameter
- Platen planeness
- 0.001 in. in 6 in.
- Verified range floor
- ≥ 10% of capacity
- Spherical seat lube
- Petroleum-type oil only
Middle-Third vs Outside-Third Formula
Middle third
- R = PL/bd²
- Most common case
Outside third, ≤5%
- R = 3Pa/bd²
- Use distance a
Fracture location sets formula
Which C78 Formula To Apply
- Fracture in middle third→R = PL / bd²(Standard case)
- Fracture under 5% outside→R = 3Pa / bd²(a = to nearest support)
- Fracture over 5% outside→Discard the result(Invalid test)
- Before loading a beam→Verify span 3× depth(Within 2% tolerance)
C78 Modulus of Rupture Formulas
- Middle-third fracture
- R = PL / bd²
- Outside-third fracture
- R = 3Pa / bd²
- Discard rule
- > 5% outside middle third
- Rupture report
- Nearest 5 psi
C78 Geometry & Loading Rate
- Span-to-depth
- 3× depth, ± 2%
- Stress rate
- 125-175 psi/min
- Standard beam
- 6 x 6 x 21 in.
- Dimension measurement
- At fractured section
- Measurement precision
- Nearest 0.05 in.
- Load pattern
- Third-point loading
C617 Hardening Times
Low strength 2 hours, high strength 16 hours
Under 5000 psi: 2 hr5000 psi+: 16 hrNever skip the check
C617 vs C1231 Capping
C617 Sulfur
- Bonded mortar
- 2-16 hr harden
- 1/4 in. thick
C1231 Unbonded
- Neoprene pad
- Reusable instantly
- 1/2 in. pad
Bonded mortar vs reusable pad
Which Capping Method To Use
- Strength 1,500-12,000 psi→Unbonded neoprene cap(ASTM C1231)
- Strength outside that range→Sulfur or ground ends(Not C1231 eligible)
- Ends not plane 0.002 in.→Cap, grind, or saw(Before testing)
- One end already sulfur-capped→Unbonded cap other end(Mixed capping OK)
- High-volume acceptance testing→Unbonded pads(Faster turnaround)
- Need highest cap strength→Sulfur mortar(16 hr for 5000 psi+)
C617 Cap Dimensions
- Cap thickness avg
- 1/4 in. (6 mm)
- Cap thickness max
- 5/16 in. (8 mm)
- Cap planeness
- 0.002 in., same as C39
- Plate oversize
- ≥ 1 in. over diameter
- Plate planeness
- 0.002 in. in 6 in.
C617 Process & Safety
- Sulfur temperature
- About 265°F (130°C)
- Hardening, under 5000 psi
- 2 hours minimum
- Hardening, 5000 psi+
- 16 hours minimum
- Cap strength at 2 hr
- 5,000 psi minimum
- Compound reuse limit
- 5 uses before recharge
- Reuse at 5000 psi+
- Not permitted
- Hazard gas
- Hydrogen sulfide (H2S)
- End condition needed
- Must be dry first
C1231 Durometer Matching
Softer pad for low strength, firmer pad for high strength
50 duro: 1500-6000 psi60 duro: 2500-7000 psi70 duro: highest strength
Shore A 50 vs 60 Pads
Shore A 50
- 1,500-6,000 psi
- Softer pad
Shore A 60
- 2,500-7,000 psi
- Firmer pad
Match durometer to strength
C1231 Pad & Ring Specs
- Pad thickness
- 1/2 in ±1/16 in
- Durometer tolerance
- ± 5 points
- Retainer inside diameter
- 102-107% of specimen
- Retainer cavity depth
- ≥ 2× pad thickness
- Pad reuse limit
- 100 uses (unqualified)
- Crack discard rule
- > 3/8 in. length
C1231 Durometer vs Strength
- Shore A 50
- 1,500-6,000 psi
- Shore A 60
- 2,500-7,000 psi
- Usable strength range
- 1,500-12,000 psi
- Below 1,500 psi
- Unbonded caps barred
- Above 12,000 psi
- Unbonded caps barred
CSTT Pass Rule
Seventy overall, sixty each standard, both exams required
70% overall60% per standardWritten + performance
Written vs Performance Exam
Written exam
- 40 MC questions
- 1 hour, closed-book
Performance exam
- Hands-on demo
- Pass/fail only
Both required to certify
Which ASTM Standard Applies
- Cylinder compressive break→ASTM C39(Load / area)
- Beam flexural break→ASTM C78(Modulus of rupture)
- Sulfur mortar capping→ASTM C617(Bonded caps)
- Neoprene pad capping→ASTM C1231(Unbonded caps)
- Lab accreditation rules→ASTM C1077(Personnel + equipment)
Lab Practice & Program Facts
- Lab accreditation standard
- ASTM C1077
- Personnel requirement
- CSTT certification
- One strength test
- Average of 2 cylinders
- Low-test trigger
- > 500 psi below f'c
- 3-test average rule
- Average must reach f'c
- Certification validity
- 5 years from passing
- Written exam pass
- 70% overall, 60% each
- Written exam length
- 40 MC, 1 hour
Standards Tested (Editions)
- C39/C39M-18
- Compressive strength cylinders
- C78/C78M-18
- Flexural strength beams
- C617/C617M-15
- Sulfur mortar capping
- C1231/C1231M-15
- Unbonded neoprene caps
Common Traps
L/D correction ≠ always applied
Only if L/D ≤ 1.75 ≠ Above 1.75, no correction
C39 age ≠ demold time
Age counts from water add ≠ Not from demolding time
Cap thickness avg ≠ max
Average 1/4 in. allowed ≠ Max 5/16 in. any point
Type 5/6 ≠ automatic fail
Common with unbonded caps ≠ Investigate pad condition first
70% overall ≠ enough alone
Also need 60% per standard ≠ Both criteria must pass
Written exam ≠ whole cert
Performance exam required too ≠ Both parts must pass
Pad hardness ≠ fixed value
± 5 point tolerance allowed ≠ Check hardness annually
Last Minute
- 1.C39 rate: 35±7 psi/s
- 2.C39 report: nearest 10 psi
- 3.C78 report: nearest 5 psi
- 4.C78 span: 3× depth
- 5.C617 caps: 1/4 in. avg
- 6.C617 harden: 2 or 16 hr
- 7.C1231 pad: 1/2 in. thick
- 8.C1231 durometer: ± 5 tolerance
- 9.Pass rule: 70% plus 60% each
- 10.Both exams required to certify
- 11.L/D correction only below 1.75
- 12.End planeness: 0.002 in. max
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