1. Maturity Method Strength Tests As Good As Concrete Cylinders
Statistical evaluation of early-age strength measurements of steam cured prestressed box beams
Allyn Luke
Presentation to ACI Fall Convention 2006

2. As Good As Concrete Cylinders
How good are concrete cylinders?
ACI 214 answers that question.
Must comply with ACI 318
Treat maturity method strength tests in essentially the same way as cylinder strength tests.
Analyze predictive error in the same way strength deviations are analyzed by ACI 214.
Part of a maturity method verification procedure
Instead of using a grand average as the compressive strength, equation represents the strength at some time and the deviation of a strength test from the predicted strength is analyzed.

3. Concrete Strength Testing Using the Maturity Method
With enough tests, as many as needed for ACI 214, the maturity method can make strength measurements as good as standard cylinder tests
Statistical adjustment can be made to express the required average strength
Frequently used mixes
Consistent mixes
Precast/Prestressed

4. Concrete Strength
Average, at 28-days or other time, of at least two cylinders of the same batch tested following procedures of ASTM C 39
Following ASTM C 31 for field cured cylinders, the strength of the concrete in the structure at some point in time
Cylinders represent structure’s strength for removal of forms and shores
With an adequate sample and care one can be reasonably confident of the concrete strength from testing cylinders. With an adequate sample and care one can make confident measurements of the concrete strength using the Concrete Maturity Method

5. Prestressing Transfer Strength
NJDOT/FHWA study investigating using the maturity method to measure prestressing transfer strength
Currently, after 12 hrs. of steam curing, if companion ASTM C 31 cylinders have reach the specified release strength, then the prestressing can be released
Cylinders now used to predict release strength would be replaced by maturity method
Reduce cylinder testing for this purpose – Still required to prove service strength with cylinder testing

6. The Maturity Method ASTM C 1074, Temperature-Time Factor option
Strength predicted from temperature history
Strength comes from hydration of cement, hydration of cement creates heat, if the heat can be accounted then the strength can be known
Measurement accomplished through strength-maturity relationship

7. ASTM C 1074 Restrictions Adequate water Early-age temperature effects
Additional confirmation of strength potential
Continuous Verification of Strength-Maturity relationship helps assure these

8. Strength-Maturity Relationship
Strength-Maturity relationship is the predictive element, measure maturity: read strength
Developed from prior experience, similar strength gain mixes
Experiment, test strengths at increasing ages, record maturities, method of C 1074
Plot maturity on x and strength on y, fitting a curve using Excel’s logarithmic Trendline produces predictive equation: fc = b ln(ttf) + a

9. Verification Principle
? fc = b ln(ttf) + a ?
VP is a confidence building measure
The strength-maturity equation must predict the strength of companion test cylinders before it can be applied to the structure.
If the companion strength can be predicted, then the prediction can be applied to the structure.
C 31 cylinders account for early-age effects
Cylinder tests satisfy ASTM C 1074 requirement for supplementary indications of strength .

10. Verifying Verification CIPLOC
Cured in-place lift-out cylinder
New nondestructive test method
Compare prediction to actual strength
Preformed cores
ASTM C 873,Strength of Concrete Cast In-Place In Cylinders
Match-cured
ASTM C 31 cylinder is placed into a heat well cast into the structure
CIPLOC is of the same concrete, undergoes the same temperature and curing history, therefore it has the same strength as the structure.
Lift-out when ready to test
Ciplocs are developed to confirm the prediction of the strength-maturity relationship. Perhaps other uses can be found.
CIPLOC in heatwell

11. Experimental Plan Instrument and cast structure and companions
Use strength-maturity equation to predict companion strength
If companion strength is predicted within standard deviation the equation is validated for use on structure
Use equation to predict structural strength in neighborhood of CIPLOC
Compare structural prediction to CIPLOC concrete strength

12. Experimental Setup

13. Early Development of Strength

14. Early Development of Strength

15. Verification of Predictive Results
Table 1 Predictive verification tests with structural strength prediction and measurement

16. As Good as Concrete Cylinders Statistical Assumptions
Predictive error is normally distributed about the predictive curve
The standard deviation of the error is applicable for correcting the prediction to whatever the level of confidence needed

17. Required Average Strength
Required average strength from ACI 318 , f’cr, is an overdesign such that the specified strength, f’c, will be exceeded for a given %, usually 90%, of tests
Correction factor added to specified strength
Using Tables 4.1 & 4.3 and eq. 4.1b

18. Required Factors for Eq. 4-1a

19. Computation of Required Average Strength
Maturity must indicate 5260 psi for a 90% certainty that 4960 psi has been achieved.

20. 90% Prediction

21. Other Statistical Methods
Implemented easily using spreadsheets
Tolerance Factor Method
Rigorous Stone and Reeve Method
Carino’s Alternative Method
Applicable to non-destructive testing found in ACI 228

22. Conclusions
The maturity method can be used to predict the release strength for prestressed concrete elements.
With enough tests to establish the standard deviation of the predictive equation, the maturity method can give measures of in-place strength that are as good as cylinder tests. The advantage is that the measure is in the structure, where it is important to know.

23. Thank You
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