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

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.

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

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

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

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

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

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

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 .

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

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

Experimental Setup

Early Development of Strength

Early Development of Strength

Verification of Predictive Results 5824 6068 Table 1 Predictive verification tests with structural strength prediction and measurement

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

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

Required Factors for Eq. 4-1a

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

90% Prediction

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

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.

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