Report Implementation of concrete maturity meters

The contents of this report reflect the views of the authors, who are responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the New Jersey Department of Transportation or the Federal Highway Administration. This report does not constitute a standard, specification, or regulation.

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/239572713

IMPLEMENTATION OF CONCRETE MATURITY METERS

Some of the authors of this publication are also working on these related projects:

independent research View project

screws View project

FHWA-NJ-2002-003

IMPLEMENTATION OF CONCRETE

MATURITY METERS

December 2002 Submitted by

Allyn Luke C.T Thomas Hsu Sun Punurai New Jersey Institute of Technology Civil & Environmental Engineering

NJDOT Research Project Manager

Tony Chmiel

In cooperation with

New Jersey Department of Transportation Division of Research and Technology

and U.S Department of Transportation Federal Highway Administration

DISCLAIMER STATEMENT

The contents of this report reflect the views of the authors, who are responsible for the facts and the accuracy of the data presented herein The contents do not necessarily reflect the official views or policies of the New Jersey Department of Transportation or the Federal Highway Administration This report does not

constitute a standard, specification, or regulation

TECHNICAL REPORT STANDARD TITLE PAGE

State of New Jersey

Implementation of Maturity Meters

FHWA-NJ-2002-003

Maturity Method, Concrete Strength

Prediction, General Tolerance Factor

Practical considerations for implementing the Maturity Method to estimate the in-place

strength of concrete are presented Several recommendations are made for adopting ASTM C1074

to NJDOT highway projects Confirmation of strength potential required by C1074 is achieved

using early maturity testing of companion concrete samples The procedure assures applicability of

the predictive equation to the specific structure Tolerance factor analysis is used to calculate the

10th percentile of strengths to determine statistical adjustment to maturity results Confidence is

increased by be adding valid predictions to the data set from which the strength maturity

relationship is derived and from which statistical adjustments are made A method for establishing

initial strength-maturity relationships for quality assurance purposes and then refining the

predictions with a data base are described Field trials in the use of the maturity method were held

in all three NJDOT regions to verify the method and train personnel

A draft specification for the Maturity Method is presented Also presented is a draft manual

describing how the method should be applied to NJDOT projects A step-by-step example is

included A second manual details instrumentation and methods for making temperature

measurements A three part course to train field personnel in the use of the maturity method was

developed and is described in the report

Federal Highway Administration

U.S Department of Transportation

12 Sponsoring Agency Name and Address

14 Sponsoring Agency Code

13 Type of Report and Period Covered

9 Performing Organization Name and Address

7 Author(s) Allyn Luke, C.T Thomas

Hsu

4 Title and Subtitle

8 Performing Organization Report No

3 Recipient’s Catalog No

2 Government

1 Report No

6 Performing Organization NJIT

10 Work Unit No

5 Report Date Jan 2003

997943

11 Contract or Grant No

Table of Contents

PROJECT SUMMARY 1

SYNOPSIS 7

DISCUSSION OF MODIFICATIONS 15

MANUAL FOR PREDICTION OF IN-PLACE CONCRETE STRENGTH USING THE MATURITY METHOD 31

TRAINING OF NJDOT PERSONNEL TO USE THE MATURITY METHOD 48

FIELD VS STANDARD CURED CYLINDERS AND STRUCTURAL STRENGTH 64

OBSERVATIONS ON TEMPERATURE EXTREMES 66

CONCLUSIONS 70

RECOMMENDATIONS 71

APPENDIX A: LITERATURE REVIEW 72

APPENDIX B: LABORATORY TESTING 83

APPENDIX C: FIELD TRIALS 95

BIBLIOGRAPHY 108

Table of Figures

Figure 1 Chart of Kleiger’s data, from Maltrolta, showing “cross-over effect” 10, 75 Figure 2 Temperature and maturity curves for Falls Creek Bridge drilled shaft 13, 80

Figure 3 Alternative cylinder predictions for Scotch Road concrete 18

Figure 4 Alternative predictions for Scotch Road abutment two 20

Figure 5 Strength-maturity curves for studied mixes 22

Figure 6 Strength-maturity approximation curves 23,.37, 60 Figure 7 Visualization of characteristic strength 27

Figure 8 The compressive strength-maturity relation 44

Figure 9 Strength prediction within acceptable tolerance vs maturity 45

Figure 10 Strength-maturity relation after refinement process 47

Figure 11 Scotch Road temperatures and strength prediction over time 64

Figure 12 Secaucus Rd Bridge 11-day temperature history 66

Figure 13 Route 33 temperatures and strength prediction over time 68

Figure 14 Maturity is adjusted area under temperature-time curve 73

Figure 15 Step one to determining the datum temp or Q 83

Figure 16 Datum temperature determined from K vs curing temperature 84

Figure 17 Q value from natural log of K over inverse of absolute temperature 85

Figure 18 Strength-maturity relationship for Secaucus Road mixes 87

Figure 19 Inverse of strength over inverse of age 88

Figure 20 Datum temperature for Scotch Road abutment concrete 89

Figure 21 Q for Scotch Road abutment concrete 89

Figure 22 Strength-maturity curve for Scotch Rd abutments with cylinder strengths 90

Figure 23 Step 1 towards maturity parameters for Rt 33 concrete 91

Figure 24 Determination of datum temperature for Rt.33 concrete 91

Figure 25 Q for Rt 33 concrete 92

Figure 26 Strength-maturity relationship for Rt 33 concrete 92

Figure 27 VES maturity parameters step 1 93

Figure 28 VES datum temperature 6.3°C 94

Figure 29 VES Q value 94

Figure 30 Maturity and strength prediction using custom equation function 96

Figure 31 Secaucus Road Bridge test 1 location 1 98

Figure 32 Secaucus Road Bridge test 1 location 2 98

Figure 33 Route 1 & 9 VES logger temperature curves 106

Figure 34 Strength-maturity relationship for VES concrete 107

PROJECT SUMMARY

The major intent of this study is to explain how the maturity method can be used to estimate the strength of in-place concrete for highway construction NJIT started studying the maturity method for NJDOT in 1995 to verify the strength of very early strength concrete patches Since then, several other studies on the maturity method have been conducted for NJDOT Collectively, these studies presented a convincing case for utilizing the maturity method to predict the strength of early age concrete in highway structures The purpose of this project was to move the method from an experimental to a practical setting

There were five interrelated objectives for this study:

1 To develop a procedures manual for implementing the maturity method

2 To recommend a NJDOT specification for implementing the maturity method

3 To evaluate field vs standard cured concrete cylinders and compare these to the in-place estimates of concrete strength

4 To monitor the maximum temperatures inside large concrete pours

5 To train NJDOT Materials and Construction personnel, contractors and

concrete suppliers in the use of the maturity method using three ongoing projects These pilot projects also allowed further study of the temperature behavior of concrete

The study began with a review of the literature on the maturity method

Conversations on the maturity method were held with other State’s Departments of Transportation, contractors, equipment suppliers and other users of the maturity method were also held This information, coupled with the field experiences of the research team, led to the general conclusion that the maturity method is an effective tool to monitor concrete strength Two manuals were developed to aid field

application of the maturity method by NJDOT The first is entitled, “Manual For

Prediction Of In-Place Concrete Strength Using the Maturity Method,” which

summarizes the overall procedure for applying the maturity method to highway

construction The second manual is a how-to-do-it guide, entitled, “Detailed NJDOT

Manual for the Maturity Method,” describes the instrumentation and methods for making temperature measurements, performing maturity computations and predicting concrete strength

The procedures manual is based on ASTM C 1074 “Standard Practice for Estimating Concrete Strength by the Maturity Method.” Nine important modifications of ASTM C

1074 were made in order to practically implement the maturity method on NJDOT projects These modifications are incorporated into the procedures manual They are:

1 Either the temperature-time or the equivalent age maturity methods are

suitable for highway projects provided that the strength-maturity relationship has been verified The temperature-time method is preferred on account of its simplicity

2 When implementing the temperature-time method, three possible values for the datum temperature are specified The choice is dependent on how quickly the concrete gains strength

3 Prior laboratory testing of the design mix is important for reliable strength prediction on strength critical applications For quality assurance means for starting a testing program without prior laboratory testing are suggested

4 Preparation and testing of field cured cylinders is important for reliable in-place strength prediction

5 The strength-maturity relationship should be verified in order to be sure that it

is applicable to the structure

6 In order to increase confidence in the prediction, the results of verification tests should be added to the maturity data base and the maturity predictive model for the concrete mix should be recomputed

7 When expressing the strength-maturity relationship as an equation, the use of Excel’s logarithmic Trendline is recommended

8 An adjustment of the prediction is required to yield the 10th percentile of

strengths recommended by ACI

9 Regular training sessions in the use of the maturity method should be given to NJDOT Materials and Construction personnel, contractors and concrete

suppliers

Three innovations are presented among these modifications The first is the

requirement that the strength-maturity relationship be verified For strength critical applications, companion cylinders are cast and match cured along with the structure When the strength-maturity relationship is able to reasonably predict the strength of those cylinders at three different early ages, the relationship is considered validated and can be applied to the structure The second innovation is that the results of verification testing should be added to the data set from which the predictions are made and then the predictive equation is recomputed By this procedure confidence

in the prediction grows as more test results become available The third innovation is

a method for starting a quality assurance program with no prior laboratory testing An assumed prediction is checked and refined through the verification process

A specification that places the responsibility for implementing the maturity method is presented The decision-making responsibilities will still reside with the engineer The specification requires that the contractor to submit a plan for the engineer’s approval prior to beginning the work detailing how the maturity method will be

applied The specification also requires certification of persons making the maturity measurements and calculations Currently, only one commercial company provides any certification for the maturity method However, this limitation does not eliminate the need for this provision For this approach to be successful, both NJDOT and the contractor must be prepared to accept and act on the predictions

When applying the maturity method for quality assurance there will still be a need for standard cured concrete test cylinders, although use of fewer cylinders than currently utilized is envisioned However, when the maturity method is used for strength

critical operations, prior preparations and field cured cylinders are essential

The field trials also revealed that elevated curing temperatures, approaching that of mass concrete, frequently occur in highway structures The reason is thought to be the increased use of more active cements at higher cement factors and lower water-cement ratios Such behavior can occur where it is least expected, like on bridge decks, which have large surface to volume ratios that should readily dissipate heat Temperature monitoring is useful for identifying these situations and avoiding the detrimental effects of high temperatures on concrete durability

To familiarize NJDOT personnel with the use of the maturity method a three part course was developed and is described in the report The first part is a general classroom session that introduces maturity concepts and methods The second part

is a laboratory session that provides practice in making temperature measurements and performing maturity computations The third part is a field session where the new users actually apply method under actual field conditions on an active project Periodic offerings of this three part course are recommended

During the course of this study, field trials using the maturity method were held in all three NJDOT regions These trials revealed two significant findings First, it was found that current winter concreting procedures may overheat concrete pours,

creating high temperature conditions that exceed even the worst of summer

concreting operations The second finding has to do with the effects of a typical chloride inhibitor on the temperature behavior of concrete It was found that calcium nitrite modified the rapid temperature rise normally associated with the early strength gain of concrete Chloride inhibited mixes followed ambient temperature changes quite closely, with positive effects on the rate of strength gain and the ultimate

strength

INTRODUCTION

The maturity method is a nondestructive testing approach for predicting the in-place strength of concrete It is detailed in the American Society for Testing and Materials (ASTM) standard C 1074, “Standard Practice for Estimating Concrete Strength by the Maturity Method”(1) The basic premises of the maturity method are: (1) concrete derives strength from the hydration of cement; (2) the hydration of cement produces heat; and (3) if the amount of heat can be measured, then an estimation of the extent

of the hydration reaction can be made and, from that, the strength of the concrete can be predicted The temperature-time approach to the maturity method, which this report recommends, approximates the heat production as the area under the

temperature over time curve From the measured maturity, the strength of a concrete mass is determined by reference to the previously determined relationship between maturity and strength

The NJDOT has been moving towards adopting the maturity method on highway projects for some time The reason is that the method can greatly simplify field

verification of the concrete strength Before the potential benefits of the maturity method can be realized several important questions need to be answered:

• How good are the maturity method’s strength predictions? What kind of effort

is needed to make the method work?

• Is it really necessary to determine the maturity properties for each mix, or can some reasonable assumptions be made about concrete behavior to simplify the application of the maturity method?

• Are there ways to check the method’s prediction in practice to assure safety and quality?

• What kind of training is needed for construction personnel in the use the

method?

The overall objective of this research project was to address these questions and to chart a plan for the NJDOT to implement the maturity method Implementation of the

maturity method involves both theoretical and practical considerations On the

theoretical side, ASTM C 1074 presents two alternative ways to formulate maturity, the temperature-time factor and the equivalent age methods The numerical value that results from either of these methods is called the maturity Each has distinct advantages and disadvantages

On the practical side, a decision must be made regarding the extent of the laboratory work needed, which in turn depends on the desired confidence level When high levels of confidence are required, as when strength-critical operations are being timed, significant prior laboratory testing and field cured cylinders are essential However, when maturity is used for quality assurance purposes only, elimination of lab testing and reduction in the number of field cylinders can be considered Another important issue is choosing the measurement method and defining how the maturity results should be analyzed

This report begins with a synopsis of the literature review and the field trials that were conducted during the course of the project (the complete literature review and field trial details are included in appendix A and B) These studies formed the basis of the nine modifications developed to adapt ASTM C 1074 to NJDOT projects Each modification, intended to be incorporated into a manual for the maturity method, is described along with its rationale Next, two manuals are presented that were

developed to facilitate field application of the maturity method The first is a “Manual for the Prediction of In-place Concrete Strength Using the Maturity Method,” which summarizes the overall procedure for applying the maturity method to highway

construction The second manual, entitled, “Detailed NJDOT Manual for the Maturity Method,” describes the instrumentation and methods for making temperature

measurements using equipment immediately available to NJDOT personnel The report concludes with a discussion of some significant observations on thermal

behavior of concrete made during the field trials

SYNOPSIS

Literature Review

The most important document to study when considering the maturity method is

ASTM C1074 “Standard Practice for Estimating Concrete Strength by the Maturity

Method.” (1) This standard defines maturity as “the extent of cement hydration in a

concrete.” “Maturity is evaluated from the recorded temperature history of the

concrete by computing either the temperature-time factor or the equivalent age

mdthod.” Its use is suggested for timing critical construction activities such as: (1)

removal of formwork and reshoring; (2) post-tensioning of tendons; and (3)

termination of cold weather protection According to the standard, it is also useful “to

estimate strength of laboratory specimens cured under non-standard temperatures.”

Two different methods are described in ASTM C1074 for analyzing concrete maturity

The first is the temperature time factor method (TTF), which utilizes the Nurse-Saul

equation:

Where: TTF is the temperature time factor,

Ta is the average temperature over a time increment,

T0 is the datum temperature, and

∆t is the time increment

An examination of equation 1 shows that it is an integration of the temperature-time

curve utilizing the trapezoidal method Basically, in this formulation, the maturity is

the area under the temperature-time curve

The second approach for analyzing maturity is the equivalent age (EqA) method,

which is computed using the Arrhenius Equation:

t e

(Eq.2) where: te is the equivalent age

Ta is the average absolute temperature over the time increment, ∆t

Ts is the absolute reference temperature

∆t is the time increment, and

Q is the apparent activation energy divided by the universal gas

constant

Like the TTF method, the equivalent age method is an integration of time and

temperature, except the temperature difference is embedded an exponential function

The major conclusion drawn from the literature review is that both methods rest on

sound scientific principles, and both yield good predictions when properly applied in

the field However, the Temperature-Time factor method is somewhat easier to

understand and apply, so it is recommended for NJDOT projects ASTM C 1074

summarizes the procedure for applying the maturity method in four steps:

1) a strength-maturity relationship is developed in the lab on the mixture to be

used

2) 2) the temperature history of the concrete being tested is recorded from

placement to the time the strength estimate is needed; 3)

3) the maturity index is calculated; and

4) the strength at that maturity is estimated from the strength-maturity

relationship

An appendix to ASTM C 1074 describes the methods for determining the datum

temperature Sample calculations are also included

Some important limitations on the maturity method are expressed in ASTM C 1074

First, it assumes that the concrete is maintained in a condition that permits cement

hydration If there is insufficient water, hydration will cease, and the this method will

produce erroneous predictions Second, the method does not take into account the effects of early-age concrete temperature on the long-term ultimate strength It has been shown that there is a significant effect on the ultimate concrete strength

asserted by the temperature conditions of the first day or so Finally, the maturity method needs to be supplemented by other indicators of the strength of the concrete mixture The maturity method does not actually test a strength quality, like a Schmidt Hammer tests surface hardness or the Windsor Probe tests penetration resistance It

is possible to erroneously predict that a pool of water could support a truck if the maturity method were carelessly used Therefore, it is required that other measures

to confirm strength be made It is crucial that any practical applications account for these limitations The verification method proposed from this study will do this

It is interesting to note that the maturity method developed out of work on electrical and steam curing of concrete by Nurse and Saul Saul coining the term “maturity” and stated the Maturity Law as: “Concrete of the same mix at the same maturity (reckoned in temperature-time) has approximately the same strength whatever

combination of temperature and time go to make up that maturity.”(2) Later

investigators found good correlation between the TTF maturity, computed utilizing equation 1, and concrete strengths However, experiments done in the 1950s by Kleiger discovered a “crossover effect,” which is depicted in figure 1(3)

Figure 1 Chart of Kleiger’s data, from Maltrolta, showing “cross-over effect”

This figure shows that for the same concrete, a sample cured at a higher temperature will gain strength faster than one cured at a lower temperature However, at some point the strength of the sample cured at a lower temperature “crosses over” the one cured at a higher temperature to achieve a higher ultimate strength In other words, there is no unique strength-maturity relationship This problem can be overcome through the use of cylinders cured under conditions similar to those of the structure whose strength is to be predicted

Rastrup (4) and other investigators, (5,14,21,22,23) studied ways to address the crossover problem It was proposed the Arrhenius function, used to describe the relationship between temperature and the rate of chemical reactions, might be used These considerations ultimately led to the equivalent age maturity method expressed in equation 2, by Freisleben and Peterson.(5)

Several published surveys regarding use of the maturity method were found during the literature review The first was a two part survey conducted by Rens (35) at the University of Colorado in 1998 with a follow-up survey in 2000 The initial 1998 survey, which received an 88 percent return, found that 57 percent of the responding states were already utilizing the maturity method Of those 35 percent were doing so

in strict accordance with ASTM C1074, while 17 percent had modified the method to their particular uses Interestingly, only 50 percent of the respondents were aware of limitations of C1074 mentioned previously Continued research was favored by

69 percent of the respondents, and several commented that they felt the procedures for developing the strength-maturity relationship were not practical

The follow-up survey by Rens in 2000 sought information about the difficulties and limitations of the method, changes in usage, and suggestions for further study Of the respondents, 40 percent were actively engaged in research, and the same

percentage had incorporated the maturity method within the previous two years It was also found that 77 percent of respondents agreed that cylinders need to be cured under field conditions Indiana, Nevada, and Colorado required that cylinders

be cured under conditions similar to those in the field Of those responding 84

percent agreed that concretes with water cement ratios of 0.53 to 0.28 cured at 5º, 25º, and 50º C are reasonable values at which to study early age temperature

effects

Another survey of the maturity method usage was done by Tikalsky, Scheetz and Tepke (36) of Penn State University for the Pennsylvania DOT in 2000 They too garnered an 88 percent return The survey found that 32 states had conducted or were conducting research on the maturity method Thirteen states had established protocols for the method, while 7 reported that the method was too cumbersome for practical use Quality control was the intended use of the method in 8 states, and 5 utilized it for structural acceptance Saw cutting was the objective for 2 states,

formwork removal in 3 states, and opening of pavement to traffic in 4 states

Twenty-one states are using or researching using the method for highways, 5 for columns, 11 for bridge superstructures and 7 for substructures Twenty-nine states are using or are considering using the Nurse-Saul equation, while only 2 are considering using the equivalent age method Isothermal curing is used in six states, while in two states mortar cubes are tested to determine maturity constants Thirteen states use

a single accepted value for those parameters The performance of the maturity method was rated “excellent” in 5 states, “good” in 14, “fair” in 4, and “poor” in 3 The method was required of contractors in 4 of the responding states, and 17 states reported positive feedback from contractors, with only 1 negative

Recently, a very high profile application of the maturity method involving the

reconstruction of the Webbers Falls Bridge in Oklahoma was reported. 29 The

Webbers Falls Bridge, which carries I 40 over the Arkansas River, collapsed on May

26, 2002 after being struck by a barge The bridge was reconstructed and reopened

to traffic on July, 29, just 47 days later The maturity method played a critical role in timing the removal of formwork for supporting columns and other substructure

elements In some cases, formwork removal was possible in as little as 13 hours! A

data collection system known as intelliRockTM, developed by Nomadics of Oklahoma, was used to collect the temperature data and compute the maturity Charts like figure 2 were used for this work

Figure 2 Temperature and maturity curves for Falls Creek Bridge drilled shaft

Contact was made with Mr Pete Byers, project manager for the Webbers Falls

project, and he explained how the maturity method was used so effectively on this project They utilized a modification of a procedure currently in use by the State of Texas (36) The temperature behavior was monitored on a real time basis with

continuous computation of the maturity number When the method indicated that sufficient strength was present in the structure, match cured field companion

cylinders, made of the same concrete as the structure, were tested by two

independent agencies If the maturity method predicted the cylinder strength within

10 percent, then the predictive curve was accepted as valid, applicable to the

structure, and construction proceeded Mr Byers, with near forty years of

construction experience, admitted he was skeptical when the method was first

introduced to him However, he is now an advocate of the maturity method and will use it again if the occasion arises

Field trials

Field trials of were arranged in all three NJDOT regions The main purpose of these trials was to put maturity measuring equipment into the hands of NJDOT personnel and let them try it out for themselves The secondary purpose was to see what could

be learned about the temperature behavior of different concretes in various structures

at different times of the year During the first outing in each region, the research team was on hand to instruct and assist During the follow-up monitoring and

evaluation period, regular contact was maintained with the field personnel doing the work

The first set of field trials was conducted in Region North during the winter of 2001–

2002 This project involved pouring deck slabs at the Secaucus Road overpass of Route 1 & 9 During these trials the effects of cold-weather concreting methods on the temperature behavior and probable strength development was studied

Observations of deck pour suggested that current DOT requirements for wintertime concreting procedures may overheat the concrete This observation has prompted

us to recommend a change in NJDOT specifications for winter curing, which will be described in a subsequent section Another, somewhat surprising, observation was made at the Secaucus Road project: very different temperature profiles occurred at different areas of the same deck, contrary to expectations

In the Central Region, during early June 2002, a second set of field trials was

conducted on the integral abutments for the Scotch Road Bridge over Route 295 Tests of early cylinders showed good correlation with the predictive equation

developed in the lab for these structures This observation led to the most significant finding of this two-year research project The ability of the predictive equation to predict the strength of early age concrete test cylinders, of the same concrete as the

structure, confirms the applicability of the predictive equation to the structure

Eventually, we found that the State of Texas uses this very technique for verifying the strength-maturity relationship This simple procedure solves a significant limitation placed on the maturity method by ASTM C 1074 Namely, it provides confirmation of the strength potential of the concrete while it confirms that the equation being used to predict structural strength is appropriate for that particular concrete

In Region South, field trials were conducted under summertime conditions of 2002, with air temperatures over 100° F for several sunny days At the Brooks Creek

overpass extending Route 33, thermal behavior of a set-retarded, chloride-inhibited mix was monitored The effects of the chloride inhibitor on the thermal behavior of the mix during the early curing were particularly noteworthy The mix followed the ambient temperatures right from the start, never displaying any heat generation of its own Maturity measurements were also made of VES concrete cast under

summertime conditions on Route 1 & 9 in Trenton Using previous predictions, it was estimated that these patches could have been opened to traffic after just 4½ hours

DISCUSSION OF MODIFICATIONS

The general procedure for applying the maturity method comes from ASTM C1074

“Standard Practice for Estimating Concrete Strength by the Maturity Method.”

However, based on the findings previously described in the synopsis, nine important modifications to the standard are recommended to implementing this method on NJDOT projects These nine modifications are incorporated into the procedures manual:

1 Either the temperature-time and equivalent age maturity methods are

acceptable for highway projects provided that the strength-maturity

relationship has been verified The temperature-time method is preferred on account of its simplicity

2 When implementing the temperature-time method, one of three specified values for the datum temperature is assigned, dependent on how quickly the concrete gains strength

3 Prior laboratory testing of the design mix is important for reliable strength prediction on strength critical applications

4 Preparation and testing of field cured cylinders is important for reliable in-place strength prediction

5 The strength-maturity relationship should be verified in order to be sure that it

is applicable to the structure

6 In order to increase confidence in the prediction, the results of verification tests should be added to the maturity data base and the maturity predictive model for the concrete mix should be recomputed

7 When expressing the strength-maturity relationship as an equation, the use of Excel’s logarithmic Trendline is recommended

8 An adjustment of the prediction is required to yield the 10th percentile of

strengths recommended by ACI

9 Regular training sessions in the use of the maturity method should be given to NJDOT Materials and Construction personnel, contractors and concrete

suppliers

A discussion of each of these modifications follows Then a specification and the procedures manual follows those discussions The modifications are:

1 Accept Either Temperature-Time Factor or Equivalent Age Methods

Provided That the Strength-Maturity Relationship Can Be Verified; the Time Temperature-Time Factor Method is Preferred

ASTM C 1074 allows for use of either the temperature-time factor (TTF) or the

equivalent age (EqA) methods The literature indicates that both methods make excellent predictions of concrete strength under laboratory conditions, and both

demonstrate good reliability when properly applied in the field There is no

theoretical reason to exclude either method However, it is recommended that the

temperature-time factor method be adopted for NJDOT projects since it is somewhat easier to understand and apply The majority of the states implementing the maturity method have based their protocols on the temperature-time factor method, while only two have specified the equivalent age method However, since the one company in the country that certifies competence in the maturity method uses the equivalent age method, it was thought unreasonable to prohibit its use Whichever method is

chosen the contractor must verify the strength-maturity relationship as described below

2 When applying the Temperature-Time Factor Method, Use With Three

Accepted Values For Datum Temperature

Theoretically, the datum temperature is the temperature below which hydration of cement ceases When the datum temperature is determined following the

procedures of ASTM C 1074, especially for very early strength concretes, it is found that the datum temperature can be above 10º C It is obvious that concrete gains strength very well at 10º C The datum temperature is more properly regarded as a parameter reflecting the initial rate of strength gain rather than the temperature below which no strength is gained

Therefore, it is recommended that three datum temperatures be standardized for three general rates of initial strength gain: slow, normal, and fast A datum

temperature of -10º C should be applied to concretes that gain strength slowly, like fly ash concrete or concrete curing under cold conditions A datum temperature of 0º C should be applied to concrete in most applications For fast very early strength (VES) concrete, a datum temperature of 6.5º C has been shown to give very good results and should be used

Since the 0° C datum temperature is recommended for most applications, it is

important to examine this assumption closely and confirm its validity The

temperature-time factor method assumes that there is a linear relationship between

the temperature and the reaction rate of cement, and, therefore, the rate of strength gain Some argue that the temperature-time factor method is too simple to capture the real behavior of hardening concrete Others warn that the datum temperature is the only adjustable factor when applying the Nurse-Saul formula, and that to fix a value like zero for most mixes, as is recommended by this project, diminishes any sensitivity the method may possess To investigate these questions the data from the field studies undertaken for this project was analyzed and compared using three approaches: the equivalent age method, the time–temperature factor method using measured datum temperature, and the time–temperature factor method using zero as the datum Agreement within a range of 10 percent of the actual measured cylinder strengths was established as an acceptable prediction If all the methods give

acceptable results, then the simplest one would be preferred

Utilizing Scotch Road data, a strength-maturity relationship was developed and

predictions were made of the cylinder strength tests done in the lab and in the field The result is seen in Figure 3

Scotch Rd Bridge Test 2 Cylinder Strength Prediction Comparing TTF with EqA

Figure 3 Alternative cylinder predictions for Scotch Road concrete

The curves of the field cylinder temperatures are plotted with strength predictions using: (1) a datum temperature of zero; (2) the measured datum temperature of 2.71°

C; and (3) the equivalent age Also plotted as points are the measured cylinder strengths with bars representing a 10% variation

The curve predicting strength based on the derived datum temperature of 2.71 °C predicts six of the nine cylinders within 10% of the actual cylinder strength, the datum temperature of 0°C hits seven, and the equivalent age only five

The purpose of this analysis is to show that the results of various methods produce essentially the same result Coincidentally, choosing a 0° C datum temperature, was the appropriate choice and performed best in this trial It is not anticipated that this would always be the case

Since it is envisioned that computers will be used to compute maturity, little time would be lost computing the maturity using both Nurse-Saul and Arrhenius methods The results could then be consider something like two additional cylinder tests and in

a case where there is a large difference one might want to choose the lower

prediction

It is fortunate that 0° C appears to be appropriate for regular uses The main reason why 0° C datum temperature is recommended for normal concrete is that it simplifies the method and avoids the tedious testing of sets of cubes or cylinders cured at different temperatures to identify datum temperature for each mix This would make the method impractical for all but the most important uses Therefore, it is

recommended that for highly retarded mixes, like those with high fly ash contents, mixes that might take some days to reach substantial strength levels, a datum

temperature value of -10° C is recommended For highly active mixes like New Jersey’s VES concrete mix, a value of +6.5° C should be adopted Whatever datum temperature is chosen, the predictive curve and the maturity of the structure must use the same value

Figure 4 shows the 6-day temperature history and the strength predicted for the base of abutment 2 at the Scotch Road project

Scotch Rd Bridge Test 2 TTF Matutity and EqA Strength Prediction Bottom Center Using

e^(8.5-2000 / mat^1.12)

Figure 4 Alternative predictions for Scotch Road abutment two

The use of different datum temperatures is seen to produce similar results that

diverge slightly as time passes At an age of one day the equivalent-age strength prediction is almost 20 percent higher than that made with the temperature-time factor However, that difference narrows at both the earlier and later ages All

predictions were above 2000 psi at one day

3 Prior Laboratory Testing is Important for Reliable Strength Prediction

The maturity method finds its most challenging application in the timing of strength critical operations Such projects require a high level of confidence in the assigning the maturity parameters, and then deriving and verifying the strength-maturity

relationship When predicting strength critical operations, two validation tests are recommended: one involving small slabs or blocks and one involving test cylinders These should be cast a few weeks prior to field testing The larger slab/block

specimens more accurately depict the probable structure’s temperature behavior and also help forewarn of the potential for temperature spikes

The use of the maturity method is not limited to predicting concrete strength for the timing of strength critical operations It can also be used for the more routine tasks of quality control and quality assurance When used in this way the required levels of confidence are lower, and the method need not be applied with the same rigor In this mode the objective is simply to assess reasonable progress towards the required 28-day strength If the progress towards the 28-day strength is not as expected, then the causes can be more quickly investigated The maturity method can not replace the standard 28-day compression test for acceptance of a concrete batch However, once the strength-maturity relationship is verified, it can replace most early age

cylinder testing and pick-up problem mixes weeks sooner than traditional methods

The maturity method is particularly useful in assessing the flexural strength of

pavements AASHTO T-97 “Standard Test Method for Flexural Strength of Concrete” uses relatively large specimens and is very delicate Consequently, results often vary widely and are usually lower than actual The flexural strength-maturity

relationship would be evaluated just like the method for cylinders previously

described Instead of testing for the compressive strengths of cylinders, the flexural strength of beams using AASHTO T-97 would be determined As long as the flexural strength-maturity relationship was periodically verified, it would not be necessary to test the flexural strength of each pour In Iowa this method is currently used to

assess the readiness of new pavements to accept construction traffic with excellent results

The rest of this section proposes a procedure by which an assumed strength-maturity relationship can be used as a starting point, thus eliminating the need for preliminary laboratory testing This less rigorous procedure is appropriate for applications that are not strength critical such as quality assurance and quality contol It is based on

an assumed strength-maturity relationship that is iteratively refined with each

subsequent test until it consistently produces accurate results

This study used Excel’s Trendline charting function to plot the strength-maturity

relationship, and fit a predictive equation to the data as a function of the maturity for the particular mixes The correlation coefficient, R2, for these equations have

generally been quite good, ranging between 0.96 and 0.99 The Trendline function produces an equation of the form y = B ln(M) + A This is exactly the form that

Plowman(9) recommended in the 1950s, except it uses the natural logarithm rather than the base 10 logarithm, and it presents the constant term second rather than first For the purposes of the maturity method, y is the predicted strength, B, a coefficient

is related to the initial rate of strength gain, ln(M) is the natural log of the maturity and, A, a coefficient that is related to the water cement ratio, cement factor, and type

of cement and perhaps temperature It was noticed that there was a significant

similarity between all the curves generated for the mixes studied The strength-

maturity curves for the study mixes are shown for comparison in Figure 5

Strength Maturity Relationships For Studied Mixes With Excel "best fit" Equations

Rt 33 Secaucus Rd.

VES

Figure 5 Strength-maturity curves for studied mixes

It is noted that all these equations have B values very close to 1100 This suggests that there is an average rate of initial strength gain for all the mixes This is thought

to reflect the use of restricted cements, a narrow range of cement factors, and water cement ratios If the average value of 1100 is used as the B coefficient, and the A coefficient (which is more sensitive to cement type, content and water cement ratio)

is varied over the range of A values encountered in this study, a family of strength gain curves likely to be encountered on NJDOT projects is produced, which is shown

in Figure 6

Strength Maturity Relationship Aproximation Curves for non-critical strength prediction

Maturity (deg C day)

4 1100 ln (M) + 0

3 1100 ln (M) - 1000

2 1100 ln (M) - 2000

1 1100 ln (M) - 3000

5 1100 ln (M) + 600 for VES

Figure 6 Strength-maturity approximation curves

In order to utilize these curves for a quality assurance program, the first step is to select a curve that ends with the anticipated 28-day strength The predictive curve is verified by checking against samples tested at early ages and at 28 days The

equation can then be adjusted until an acceptable level of correspondence to the actual results is achieved Ultimately, only the 28-day strength would be confirmed These curves themselves need not be the initial starting points A curve from some similar mix developed previously could also start the iterative process and be refined

to produce a final result

4 Use Importance of Strength Prediction to Dictate Need for Field Cured

Cylinders

One of the most significant limitations on the maturity method for predicting concrete strength is the effect the early-age temperature effects have on the ultimate strength This so-called “cross-over” effect, described by Kleiger (14), shown in figure 1,

operates in such a way that for the same batch of concrete, samples cured at a lower temperature will gain strength less quickly than the same mix cured at a higher

temperature However, at some point, the strength of the lower temperature cured mix crosses over the higher curing temperature mix ultimately reaching a higher strength This phenomenon is the reason why ASTM C 1074 has a stated limitation warning that the test method presented does not account for the early age

temperature effects on the ultimate strength It is also the reason that 34 States currently are researching or are using field cured cylinders rather than laboratory cylinders for field application of the maturity method

To overcome this significant problem concrete that has experienced the same age temperature history as the concrete under test is needed to make an accurate prediction Use of companion cylinders from the field that have been cured with the structure can provide that concrete The cylinders need not be kept with the structure for the whole time up to testing It is thought that one or two days under the same conditions as the structure is sufficient to account for the early-age effects

early-When the maturity method is being used to predict the strength for non-strength

critical operations one day of field curing is also recommended However, for quality acceptance testing, the samples need to be cured under conditions as uniform as are possible In the field, quality acceptance cylinders would be cured in a tank of lime saturated water before being moved to the lab to moderate temperature fluctuations while monitoring the maturity Under such conditions the temperature behavior and therefore the maturity characteristics for each mix would be expected to be the same

If the strength was not tracking as expected it would be an indication of a wrong mix

or a drastic change of the mix so an investigation into why this has occurred could begin

5 Verify the Strength-Maturity Relationship

The key recommendation of this report is to establish a procedure by which the

accuracy of the maturity method’s prediction of the concrete’s strength is tested This recommendation is intended to overcome the limitations placed on the method by ASTM C 1074 requiring additional confirmation of strength potential When field cured cylinders are used the early-age temperature effects are accommodated Even the final limitation of ASTM C 1074, requiring sufficient moisture to sustain hydration, is addressed to some extent If insufficient water to sustain hydration of the cement were the case then it can be expected that the concrete will fail to meet its strength targets as well

This recommendation is based on field observations presented above and on

provisions from Texas’s maturity protocol Testing of early maturity companion

cylinders of the structure’s concrete will show that the strength-maturity relationship applies to the concrete in question Once that fact is established then the prediction

is applied to the structure

This recommendation is thought to be sufficiently rigorous to convince a skeptical inquirer, as well as a field practitioner, of the applicability of the method and its

prediction to the concrete in question Depending on the importance of the

prediction, whether it pertains to strength-critical operations or not, determines

amount of verification necessary When crucial assessments are needed it might be necessary to verify each batch For less critical operations the amount of testing required will depend on the amount of confidence the responsible party needs to establish

6 Refine The Prediction

The strength-maturity relationship is often derived based on six, sometimes fewer, tests of a single laboratory batch of the concrete in question Again, figure 2 is an example This lack of rigor does not inspire confidence in the method but it has been successfully used and certainly can serve as a beginning for the refinement process

In this process, results from successful field verification tests would be added to the data set from which the predictive equation and the characteristic strength

adjustment are derived The predictive equation and the characteristic strength

adjustment would then be recomputed as new data was collected The process

would continue until a comfortably significant number of tests, around 30 tests, are present in the data set Periodic checks of the strength-maturity relationship should adjust the prediction for expected seasonal variations in actual field strengths

7 Use Excel’s logarithmic Trendline Function to Express the Strength-Maturity Relationship

Traditionally, the strength-maturity relationship is represented by a simple graph

showing the strength on the y-axis and maturity on the x-axis, like that shown figure 2

It is developed by plotting the concrete strength against the maturity at which the strength test was conducted After several tests at successive doubling time periods,

a curve is fitted through these points This curve is then used by measuring the field maturity and then referencing the same value on the curve to find the predicted

strength

Strength predictions are best made using calculators and computers However, an equation is needed to do so A wide choice of curve-fitting programs is available to analyze maturity data, and several mathematical forms have been suggested A hyperbolic form, equation 8 in the literature review appendix, introduced by Chin (22)and refined by Carino (23) is probably the best representation available Its

complexity, however, limits its use except by the most sophisticated of users One of the earliest forms is from Plowman9, presented in the literature review appendix as

equation 2, y = A + B Ln(M) It turns out that by utilizing Excel’s logarithmic

Trendline function, Plowman’s equation is obtained exactly with an inconsequential transposition of terms These equations have been found to have excellent fits, to the strength-maturity data, as measured by the regression coefficient R2, usually in excess of 0.95 The ready availability of Excel recommends its use as a practical Visualization of characteristic strength matter Any curve-fitting program that

produces good fits would also be acceptable

8 Adjust The Prediction To The Characteristic Strength

The strength of concrete is a statistical measure Due to its non-homogeneous

nature the strength of concrete is expected to vary within some statistical bounds A prediction of strength needs to account for those variations as well Therefore, the characteristic strength method, derived from ACI 228 (34) is recommended The concept is represented graphically in figure 7

Figure 7 Visualization of characteristic strength

It can be seen that the mean strength, which is the usual value taken to be the

concrete strength, is lowered by some amount to the characteristic strength This adjustment is based on the amount of confidence needed, 75 percent is the typically accepted value, and the number of compression tests done The standard error of the prediction, the difference between the predicted value and the value measured by cylinder compression tests, is multiplied by the one-sided tolerance factor to produce the characteristic strength adjustment The table for the one-sided tolerance factor can be found as Tab 6.1 in the report of ACI 228.1, “In-place Methods to Estimate Concrete Strength” A simplified version of that table is presented as Table 1

Table 1 One-sided tolerance factors for 75 % confidence level

expected to be exceeded for 90 percent of the tests This characteristic strength will

be the presumptive strength of the structure

9 Remember The Need For Continued Training

Training in the maturity method, as for most things, requires either immediate

application or frequent little reminders to stay current Only a few NJDOT people have taken the method into the field More problematic is that currently no local contractors have sufficient training to implement the maturity method Until

contractors have personnel trained to undertake the required responsibilities these several NJIT people or contractors would have to perform these tasks To address this problem it could be stipulated that the ACI Field Technician, recently required on NJDOT projects, would accept the outlined responsibilities To get such a

certification ACI would need to establish a training program for the maturity method Currently, the Con-Cure Company, of St Louis, is the only ACI certified source of instruction Their use of the Arrhenius method is a major reason that the Manual for Implementing the Maturity Method does not exclude that approach Temporally , New Jersey colleges or New Jersey ACI could put together a course that could serve

as certification until a national program could be instituted NJDOT must decide what level of certification would be required and be sure that some people in the

department would have been trained to that level

SPECIFICATION FOR THE MATURITY METHOD

The following is a suggested specification for using the maturity method to monitor a structure:

Since [insert whatever kind of strength needs be measured and why (i.e early age

flexural strength as indication of readiness of repairs to accept normal traffic)] needs

to be known, the maturity method for predicting the in place strength of concrete shall

be used The use of the method will be governed by the New Jersey Manual for Prediction of In-Place Concrete Strength Using the Maturity Method

The contractor will present a means and methods plan to the engineer for approval The plan should state whether the temperature-time factor or the equivalent age method is to be used to computed the maturity The appropriate maturity parameters should be indicated It needs to detail the measurement system to be used and show it in conformity with the requirements of ASTM C 1074 Methodology for field curing the cylinders for the required time needs be covered Demonstration or

certification of a contractor’s ability to properly utilize the maturity method shall be provided to the engineer The contractor, under the supervision of the engineer, will

be responsible for determining the strength-maturity relationship of the concrete proposed for use on the project for which maturity testing is to be done The

concrete for developing this relationship must be produced using the same materials, proportions, equipment and procedures expected to be used on the project The contractor is responsible for instrumenting the structures and gathering the data, and computing the results The contractor will supply the results of the maturity testing to the engineer before strength-critical operations are allowed The determination that sufficient strength has been reached will be the decision of the engineer When maturity testing results are used for quality assurance the engineer will determine that the strength is tracking properly towards its target strength

MANUAL FOR PREDICTION OF IN-PLACE CONCRETE STRENGTH USING THE MATURITY METHOD

Scope

The maturity method is used for predicting the in-place compressive strength of Portland cement concrete It can aid in the timing of strength-critical operations It can also be used for quality assurance Implementing the maturity method requires two steps First, prior to field application, a strength-maturity relationship for the proposed mix must be established Second, after instrumenting the structure or, the quality assurance samples, the strength-maturity relationship for the particular

concrete is used to predict the in-place strength of the structure or the quality

assurance samples made of the same mix A detailed discussion of the maturity method, and instructions in the following procedures, using NJDOT equipment as of January 2000, are given in “Implementation of Maturity Meters,” which can be found

in the “Training of NJDOT Personnel” section below

The following procedures adapt the temperature-time factor method of ASTM C1074 for use on highway structures Use of the other method presented in C1074, that of equivalent age, is permitted if proof of ability to employ that method can be furnished and the resulting prediction can be verified using procedures specified below

Terminology

Maturity is a quality of concrete that accounts for the effects of temperature and time

on concrete strength

The maturity method employs a measured temperature history to predict concrete

strength of a specific mix

A specific mix is a specified combination of materials making up a particular

concrete design that varies only within accepted standards Changes of cement

manufacturer or type, aggregates, admixtures, and dosage rates represent different mixes for which predictions need to be altered or verified

The temperature-time factor (TTF) is the maturity computed using the Nurse-Saul

function

The equivalent age (EqA) is an alternative way to compute maturity It is computed

using

the Arrhenius equation

Compressive strength is the average of three concrete test cylinders tested using

AASHTO T22

The strength-maturity relationship is an equation, or a chart, used to predict

concrete strength from its maturity

The predicted strength is the concrete strength for a specific mix predicted by the

strength-maturity relationship at a given maturity

The verification process proves the ability of the strength-maturity relationship to

predict the strength of a given concrete batch

The characteristic strength is the strength of concrete expressed so that it will

exceed the required strength 90 percent of the time, the 10th percentile of strength It

is a conservative expression of the required concrete strength

Strength-critical operations are those construction activities that cannot be safely

undertaken before the concrete has reached a required strength

Apparatus

1 An instrument conforming to the requirements of ASTM C 1074 is required This device must measure temperatures within the range of -20º C to 100º C, with an accuracy of ± 1º C Measurements need to be recorded at least every half hour for the first 48 hours, and hourly thereafter The data file must be available for analysis and archiving It should be rugged enough for field use or be carefully protected

2 Temperature sensors accurate to ± 1º C or better, compatible with the chosen apparatus

3 A computer for programming, recovering, saving, and analyzing the data

Devices that display maturity values directly are permitted only as long as the temperature history upon which the values are computed, and notes on maturity parameters used, are available

4 Microsoft’s Excel, or a computer program to transform temperature data into maturity and strength values

5 A device for measuring the compressive strength of concrete following AASHTO

T 22 (ASTM C 39), if concrete test cylinders are to be tested

6 A device for measuring the flexural strength of concrete beam samples, following AASHTO T 97 (ASTM C 78), if beams are to be tested

Step One Procedures – Determine the Strength-Maturity Relationship

When developing the strength-maturity relationship, the concrete must be made of the same materials, in the same proportions, using the same equipment and

procedures as those expected on the project It is anticipated that a verification batch, within a month of application, would be used for this purpose Laboratory batches can be used, but field-mixing conditions are preferred Changes of materials

or proportions require generation of a new strength-maturity relationship

Establishing a Datum Temperature for a Particular Mix

The datum temperature adjusts the method to the particular mix These values must

be chosen properly to properly predict concrete strength

A datum temperature of zero is recommended for most applications For highly retarded mixes, those that strengthen very slowly due to very low temperatures, admixtures or pozzolanic materials, use -10° C Very Early Strength concrete in New Jersey uses a datum temperature of +6.5°C

Alternatively, the datum temperature for the particular concrete to be used in the project can be determined using the laboratory procedures presented in ASTM C

1074 These determinations must be available to and accepted by the engineer Measure temperatures

1 Record weather conditions, initial temperature, slump, and air content

2 Cast a minimum of 18 cylinders, following AASHTO T23

3 As soon as possible, embed thermocouples into two cylinders so that they are

at least two inches from any surface A stiff wire is sometimes needed to get the thermocouple wire past the large aggregate

4 Keep these two cylinders with the other companions cylinders, one near the center, the other near an edge, of the group The temperature of the cylinders will be the average of these two

5 Other available temperature channels are used to monitor any other object cast with the verification batch A 3x3x3/4 ft slab should be cast for decks and slabs, a 2x2x2 ft block for other structures, to check for temperature spiking potential Minimally, cast a 5-gal pail of the concrete and install two

thermocouples, one into the center and the other at the edge of the specimen