CREEP AND SHRINKAGE OF HIGH PERFORMANCE BRIDGE CONCRETE IN ... · Creep & Shrinkage of Bridge...

Creep & Shrinkage of Bridge Concrete in NZ Peter Lipscombe of 14

Creep and Shrinkage of High Performance Bridge Concrete in New Zealand

Peter Lipscombe – Structures Team Leader, Northern Gateway Alliance and Principal, URS New Zealand Limited, Auckland. Andrew Dickson – Structures Team Leader, Manukau Harbour Crossing Alliance and Technical Director, Beca Infrastructure Ltd, Auckland.

Synopsis This paper presents creep and shrinkage data from various major bridge projects constructed recently in New Zealand and compares the performance to that predicted by current design practice. A method of modifying current design practice to account for the use of modern high performance concretes is proposed.


1. Introduction Creep and shrinkage effects have a significant influence on the performance of bridge structures. These effects manifest into unwanted deflections, redistribution of design actions and overall shortening effects. The shortening effects ultimately dictate prestress losses, substructure design actions in continuous bridges and bearing and expansion joint movement ranges. Predicting the effects of creep and shrinkage with greater accuracy will help to optimise bridge design. Current bridge design practice in New Zealand relies on creep and shrinkage predictions based on National Roads Board publication Road Research Bulletin 70 (RRU 70). This document was published in 1984 and is based on concrete specimens constructed between 1973 and 1984. The concrete used in these tests had a specified strength of 37.9 MPa (550psi), a water-cement (w/c) ratio of 0.45-0.50, utilised basalt coarse aggregate and did not contain any admixtures. Concrete technology has changed significantly since these test were done; modern high-performance bridge concretes in New Zealand typically have specified strengths in the range of 50-60 MPa, w/c ratios down to 0.35 and modern concretes make extensive use of super-plasticisers and water reducing admixtures to achieve the low w/c ratios. These high performance concretes exhibit low shrinkage and creep behaviour however there is currently no recognised way of accommodating this better performance in current design practice. The primary design recommendations given in RRU 70 are;

• Creep effects can be modelled using the method given in CEB-FIP 78

• Shrinkage effects can be modelled by doubling the shrinkage given by the method in CEB-FIP 78

This raises the questions;

1. Are the recommendations given in RRU 70 still valid for modern bridge concretes?

2. If not, how can the method be adjusted to account for better performance? 3. Is there a better method for predicting creep and shrinkage? 4. Are the recommendations valid for aggregate types other than basalt?

The following sections of this paper set out to answer these questions through experimental testing of a number of high performance concretes used on major bridge projects. Section 2 of this paper describes the experimental testing and results from Waiwera Viaducts, located approximately 40km north of Auckland. Section 3 describes the specification and testing associated with the Duplicate Mangere Bridge, in Auckland. Section 4 describes the testing associated with Chapel Street Viaduct in Tauranga. Conclusions from these series of tests are given in Section 5.


2. Experimental Testing – Waiwera Viaducts (Waiwera) Waiwera Viaducts are twin precast segmental box girders forming part of the Northern Gateway toll road. The prestressed concrete box girders are each 570m long and have a maximum span length of 76m. Concrete for the box girders was required to have the following characteristics;

• 28 day strength and durability to meet the design requirements

• Early age strength to minimise time in the precast moulds

• Low creep and shrinkage performance. Creep and shrinkage effects on the box girders where analysed using the design recommendations given in RRU 70. These recommendations were believed to be conservative for the concrete mix used and this was to be verified by experimental testing as construction proceeded. Experiment Details The opportunity arose in 2004 to reproduce the initial RRU 70 experiments (outlined in Chapter 3 of RRU 70) to compare the performance of the concrete proposed for Waiwera Viaducts with the original tests. The original test programme was copied, as far as practical, to replicate the original testing. This comprised making identical concrete blocks, 150mmx150mmx 600mm long for shrinkage and creep testing. Each block has a 36mm outside diameter PVC tube cast centrally along its length. The hole created by the tube was then used for a prestressing rod to load the creep specimens. The PVC tube was also used in the shrinkage specimens for consistency. Figure 1 and Photo 1 show details of the test specimens.

600 150

150

EPOXY MORTAR

PRESTRESSING BAR

STEEL BEARING PLATE

a ) Creep or shrinkage specimen

b ) Arrangement for stressing creep specimens

Photo 1. Specimens being cast Figure 1. Specimen details


The following are key parameters used for the tests.

Waiwera Tests Original RRU70 Tests

Specimen dimensions 150x150x600mm 150x150x600mm

Number of test specimens

12 30

Pour date 22nd Feb 2005 2nd Oct 1974 & later

Test location Waiwera Grafton.

Age at stressing 50 days 14 days

Prestress 7.06 MPa 7.00 MPa

Location of exposed specimens

Grass slope adjacent to Waiwera Viaducts

Deck surface of Grafton Gully Bridge No.1

Location of sheltered specimens

Under existing State Highway 1 bridge at Waiwera.

Under Grafton Gully Bridge No.1

Table 1. Comparison of test parameters The distance between the Waiwera test site and the original test site is some 40km but the environment, in particular the average annual relative humidity, is essentially the same. Six test specimens were stored at the exposed site; three for creep and three for shrinkage. Similarly six test specimens were stored at the sheltered site; three for creep and three for shrinkage (refer Photo 2). The creep specimens were stressed together longitudinally in a group of three, as shown in Figure 1 and Photo 2. A thin layer of epoxy was used on the block ends to ensure even bearing between the blocks. The shrinkage specimens were chained loosely together and stored adjacent to the creep specimens.

Photo 2. Test specimens on site at sheltered location.


Creep and shrinkage strains were measured with a demountable mechanical strain gauge. (Mayes Instruments Limited) Strains were measured over a gauge length of 200mm. The finest graduation on the strain gauge represents 8 microstrain however the accuracy of strain measurement was less accurate than this due to the sensitivity of the gauge. To minimise error the average of several strain readings was used; the resulting error in the readings (due to gauge limitations) is estimated to be about 24 microstrain. The main differences between the Waiwera tests and the original tests are;

• The prestressing force on the Waiwera creep specimens was not adjusted over time to compensate for losses. In the original tests it was.

• Grass was left to grow around the exposed specimens. This is likely to have made the micro-climate around the test blocks more humid than the original tests.

Whilst the test conditions at Waiwera are essentially the same as the original tests, the concrete mixes are quite different. A comparison of the concrete mix details follows;

Waiwera Tests Original RRU 70 Tests

Concrete specified strength

60 MPa 37.9 MPa

Concrete strength measured (28 days)

47 MPa* 52 MPa

Cement content 350 kg/m3 385 kg/m3

Water/cement ratio 0.40 0.45

Coarse aggregate type Basalt Basalt

Fine aggregate type Kaipara sand Mixture 3 sands

Additives Yes No

Table 2. Comparison of concrete mix parameters

* Note the test specimens at Waiwera were incorrectly batched, they were supposed to have

380kg/m3 of cement and achieve a target strength of 71 MPa. See later discussion. Strain readings were taken on all specimens at intervals starting at 48 hours and continuing over a period of three years. Block temperature, particularly at early ages, had a significant influence on the readings. The temperature strains swamp the shrinkage strains at an early age. To partially mitigate the effects of this, readings were taken early in the morning when the blocks were a uniform temperature. The block temperature was assumed equal to the atmospheric temperature. Measured strains were adjusted for thermal effects (normalised to the temperature during the first reading). The coefficient of thermal expansion was measured as 10.0x10-6 mm/mm/°C Shrinkage Results A graph showing the average shrinkage strain for three test blocks at the sheltered location and three test blocks at the exposed location is given in Figure 2. Superimposed on the graph is the shrinkage strain predicted by the method


recommended in RRU 70, based on an average relative humidity of 78%. The humidity parameter has been determined from the data recorded at the nearest meteorological station, in this case at Whangaparoa Peninsula. The humidity is very similar to the Auckland annual average and the humidity assumed for the RRU 70 experiments (80%).

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TIME (DAYS)

SHRINKAGE (MICROSTRAIN)

RRU 70

Test block - sheltered site

Test block - exposed site

Average all test blocks

Figure 2. Measured shrinkage results versus RRU 70 prediction It can be seen that measured shrinkage relative to that predicted by RRU 70 is 41% for the exposed specimens and 71% for the sheltered specimens. The difference in performance between the sheltered and exposed specimens is attributed to the more humid environment at the exposed site resulting in lower shrinkage. This is consistent with the findings in the original tests; the most exposed specimens – those on top of the bridge – also shrank the least. In those tests the shrinkage strains on the exposed specimens were about two-thirds of sheltered specimen values. Shrinkage tests, in accordance with AS1012.13, were undertaken on the concrete used to make the Waiwera test blocks. These are relatively simple tests conducted on a 75x75x300mm concrete prism. The prism is moist cured for 7 days and subsequently cured at 50% relative humidity and 23 °C. Shrinkage strain is measured until the specimen is 56 days old. Applying the rules of RRU 70 to this (75x75x300mm) shrinkage specimen (with some minor extrapolation due to the small scale specimen used in the AS1012.13 test) the 56 day shrinkage strain is predicted to be 750 microstrain; however it was measured as 480 microstrain. A conclusion from this is that the concrete used in the original RRU tests would have had a measured shrinkage strain of 750 microstrain if it had been tested in accordance with AS1012.13. The Waiwera concrete performed considerably better with only 480 microstrain shrinkage. It is clear that the recommendations for


calculating shrinkage, as given in RRU 70, for the small specimens of Waiwera concrete, over-estimate the actual shrinkage by a factor of 65% ie. 480/750. It is noted that this ratio is very similar to the average shrinkage strain measured on the Waiwera test blocks divided by those predicted by RRU 70, refer Figure 2. ie. 58%, based on 1000 day strains (250 microstrain/435microstrain). Based on the 75x75x300mm blocks tested in the laboratory and the 150x150x600mm blocks tested on site, it can be stated that the shrinkage strain of this high performance concrete can be predicted by the method proposed in RRU 70 scaled by the following ratio. scale factor = shrinkage strain measured in accordance with AS1012.13 750 microstrain Figure 3 below shows how the scaled RRU 70 shrinkage prediction compares to the observed experimental shrinkage.

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RRU 70 scaled by AS1012.13 shrinkage

750


Figure 3. Measured shrinkage strains of the Waiwera test blocks compared to the RRU 70 method scaled by 64% (480/750) An alternative to using the scaled RRU 70 is to use the method given in the Australian Standard for Bridge Design, AS5100.5. The charts given in AS5100.5 only go up to a relative humidity of 70%, many areas in New Zealand exceed this annual average. For instance Waiwera has an annual relative humidity of 78%. The calculations for shrinkage are not particularly sensitive to this parameter when the humidity is in this range. For instance the ultimate shrinkage strain calculated at 70% humidity is 4% more than that based on 78% humidity, when evaluated in accordance with CEB-FIP


78. Applying the method of AS5100.5 to the Waiwera test blocks, using a basic shrinkage strain of 480 microstrain (as per AS1012.13 test) then the predicted shrinkage over-estimates the observed average shrinkage strain, refer Figure 4. The ratio between observed and predicted strain is 73% (values taken at 1000 days).

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AS 5100.5 predicted shrinkage based on

tests in accordance with AS 1012.13


Figure 4. Measured shrinkage strains of the Waiwera test blocks compared to the AS5100.5 method An error was made in the concrete mix constituents when batching the test specimens, as noted in Table 2. This resulted in the measured concrete strength being less than the specified strength. Further test blocks were made with the correct concrete mix. These blocks were intended to replace the original blocks but at 300 days they were damaged beyond repair. The creep and shrinkage strains up until 300 days matched closely the results of the first set of test blocks. The error made in the mix constituents is considered not to affect the conclusions drawn. Creep Results The anticipated shortening behaviour of the prestressed (creep) test specimens is expected to follow the graph shown in Figure 5. To examine the effect of creep shortening, without the influence of shrinkage, the average experimental shrinkage strains have been subtracted from the total shortening strain measured on the creep test specimens. Thus the graph of shortening effects due to load effects only (creep + elastic shortening) is obtained and is given in Figure 6. Superimposed on the graph is the shortening strain, induced by load effects, as predicted by the method recommended in RRU 70.


Figure 5. Shortening effects on concrete loaded at time T0.

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TIME (DAYS)


RRU 70

Test block - sheltered site

Test block - exposed site

Figure 6. Measured creep & elastic shortening versus RRU 70 prediction Similar to the finding for shrinkage, RRU 70 significantly over-predicts creep. The ratio between measured and predicted strains is approximately 60% (at 1000 days) for both the exposed and sheltered specimens. The effect of site exposure on creep appears to be less pronounced than for shrinkage. This finding is consistent with the original tests that formed the basis of RRU 70.

TIME

Elastic shortening

Shrinkage shortening

Creep shortening

T0 =time of load application

SHORTENING STRAIN


3. Experimental Testing – Duplicate Mangere Bridge (Auckland) Historically basalt has been the concrete aggregate used in Auckland bridge structures, and the RRU 70 tests were principally carried out on concrete made with basalt aggregate. However, with the reduction in availability of basalt, greywacke aggregate is now widely used, and some concrete suppliers essentially supply only greywacke aggregate based concrete. The RRU 70 recommendations were used to determine design creep and shrinkage values for the Duplicate Mangere Bridge, currently under construction. Creep and shrinkage are important design considerations for this 644m long continuous bridge, and so careful consideration was given to the question of how to specify the concrete to achieve creep and shrinkage consistent with the design values without being unduly conservative and limiting the concrete supply options. Based on reference to TR11 (Properties of New Zealand Concrete Aggregates – CCANZ) and discussion with experts in the field, it was estimated that the RRU 70 concrete would be unlikely to give a test shrinkage strain any lower than 750 microstrain if tested in accordance with the AS1012.13 standard shrinkage test. Coincidentally, it was found that the RRU70 recommendations, if applied to the standard test specimen, environment and age, would give a shrinkage strain estimate very close to this value. The project specification thus required that the superstructure concrete achieved a shrinkage strain of less than 750 microstrain when tested in accordance with AS1012.13. Compliance had to be demonstrated at the beginning of production and then at intervals throughout the superstructure construction. Sampling for the shrinkage testing is from production mixes and not small laboratory trial mixes. It is noted that for modern day bridge concretes a shrinkage strain limit of 750 microstrain is not particularly onerous. The RTA (NSW) standard concrete specification requires that all concrete used in the B1 and B2 environments (i.e. most superstructures) has a standard test shrinkage strain less than 700 microstrain. The VicRoads specification places a limit of 750 microstrain on all concrete Because the factors affecting shrinkage and creep are similar and because creep data is much less readily obtainable, a specification limit was not placed on the creep characteristics of the concrete. However creep testing in accordance with AS1012.16 is being carried out on the production concrete and the results being supplied to the designers to enable them to confirm that performance is consistent with the design assumptions. The concrete being supplied for the superstructure utilises a greywacke (Hunua) coarse aggregate, Kaipara sand fine aggregate, GP cement and a water reducing admixture, with a specified strength of 50 MPa and a w/c ratio of 0.37.


The shrinkage testing carried out to date has shown a standard test shrinkage (AS1012.13, 56 day) of around 650 microstrain, comfortably within the specified 750 microstrain maximum and confirming the suitability of greywacke aggregate concrete for structures designed using the RRU 70 recommendations. As expected, the shrinkage is higher, however, than the shrinkage experienced with the higher strength basalt aggregate concrete used in the Waiwera Viaduct. Creep test results are not yet available. 5. Experimental Testing – Chapel Street Viaduct (Tauranga) Chapel Street Viaduct is a 560m long bridge on the western approach to Tauranga Harbour Bridge. The Viaduct superstructure is constructed from prestressed tee-roff beams manufactured from self-compacting concrete (SCC). Further details of the tee-roff beams are given in a companion paper at this conference titled “Post-tensioned tee-roff beams made with self compacting concrete”. Shortening effects resulting from creep and shrinkage were taken into account in design by following the recommendations of RRU 70. During detailed design there was concern that SCC might be prone to greater creep and shrinkage effects than predicted by RRU 70 because of the high fines content and the extensive use of additives in the concrete mix. Two SCC mixes were tested; details of the mixes are given below.

Mix 1. Mix 2.

Concrete specified strength

60 MPa 60 MPa

Concrete strength measured (28 days)

83.5 MPa 70.0 MPa

Cement content 365 kg/m3 + SCM 445 kg/m3

Water/binder ratio 0.32 0.40

Coarse aggregate type Greywacke Greywacke

Table 3. Self compacting concrete mixes used on the Tauranga Project Creep and shrinkage tests were undertaken on the two mixes to see how the concretes compared in performance to RRU 70. Shrinkage tests were undertaken in accordance with AS1012.13 and creep tests were undertaken in accordance with AS1012.16. Table 4 summaries the test parameters and test results.

Mix 1. Mix 2.

Shrinkage test AS1012.13 AS1012.13

Shrinkage test specimen 100 dia x 200mm long 100 dia x 200mm long

Initial curing Moist curing for 7 days Moist curing for 7 days

Subsequent curing RH=50%, T=23°C RH=50%, T=23°C


Test period 119 days 115 days

Shrinkage strain from 28 days to end of test period

221 microstrain 274 microstrain

Creep test AS1012.16 AS1012.16

Creep test specimen As above As above

Curing As above As above

Age at stressing 28 days 28 days

Prestress 23.00 MPa 23.00 MPa

Creep strain at end of test period

520 microstrain 777 microstrain

Table 4. Creep and shrinkage results for concrete mixes used on the Tauranga Project Applying the rules of RRU 70 to the shrinkage specimen, taking account of the curing conditions, the predicted shrinkage strain over the test period (28 days to 115 days mix 2/119 days mix 1) is 258 microstrain. Mix 1 exhibited marginally less shrinkage than predicted by RRU 70 while Mix 2 exhibited marginally greater shrinkage than predicted. Applying the rules of RRU 70 to the creep specimen, in a similar way to above, the predicted creep strain is 1030 microstrain. The experimentally measured creep is 50% and 75% of this value for mixes 1 and 2 respectively. The above test results indicate that RRU 70 gives realistic shrinkage values for the SCC used but conservatively over-estimates creep. Chapel Street Viaduct is currently under construction. To date the creep performance of the prestressed beams, as measured by prestressing camber growth, is performing as calculated although there is considerable scatter in the results. 5. Conclusions The recommendations given in RRU 70 have been the basis for creep and shrinkage design of New Zealand bridges since its publication in 1984. Starting in 2005 the same series of tests, that formed the basis of RRU 70, were repeated using a high performance bridge concrete. This was done to see how the results compared with those predicted by RRU 70. In addition further experimental testing has been undertaken on the concrete used for the Duplicate Mangere Bridge and Chapel Street Viaduct. These results have been presented in this paper. They further justify the following conclusions drawn below.


1. Replicated test blocks made with high performance concrete (60MPa, basalt aggregate) and stored in the open at a bridge site exhibited significantly less creep and shrinkage than those tested for, and forming part of the basis of the RRU Bulletin 70 recommendations. The RRU 70 recommendations for concrete creep and shrinkage give values that can be unnecessarily conservative for high-performance bridge concrete.

2. Shrinkage of the test blocks would be more accurately modelled by scaling the

RRU 70 predicted shrinkage by the ratio; scale factor = shrinkage strain measured in accordance with AS1012.13 750 microstrain

3. The method of calculating shrinkage in AS5100.5 also over-estimates the

shrinkage observed on the Waiwera test blocks. The method given in 2 above more accurately models the average shrinkage observed in the tests.

4. The factors that influence shrinkage ie. mix design and environment, also

influence creep. For the specimens tested, RRU 70 over-estimates creep effects to a similar extent that it over-estimates shrinkage effects.

5. Creep effects observed on the test blocks would be more accurately modelled by

scaling the RRU 70 predicted creep by the shrinkage ratio given above in 2.

6. RRU 70 predictions also appear conservative (based on standard laboratory shrinkage test results) for a 50 MPa bridge concrete utilising greywacke aggregate, although not by the same margin.

7. SCC can be made to perform at least equivalent to the creep and shrinkage performance implicit in the recommendations of RRU 70.

8. Where creep and shrinkage is a significant design issue and design is carried out using the RRU 70 recommendations, concrete should be specified as having a maximum shrinkage when tested in accordance with AS1012.13 of 750 microstrain.

9. Advantage could be taken of the improved creep and shrinkage characteristics of high performance concrete by allowing the RRU 70 predictions to be scaled by the scale factor defined in 2. above.

The above conclusions are based on a small pool of data gained from a limited number of laboratory tests and relatively small specimens tested on site in imprecisely defined environmental conditions. Further testing and investigation is desirable to enable more robust conclusions to be reached, and should be encouraged and facilitated. However in the interim the above conclusions represent a much needed step forward in design and specification practice.


Acknowledgements The authors wish to thank; - Northern Gateway Alliance who funded the testing at Waiwera Viaducts. - Allied Concrete who supplied the concrete and some test results for the Waiwera test blocks. - Fletcher Construction, Firth Industries and Golden Bay Cement for the data related to the Duplicate Mangere Bridge and Chapel Street Viaduct. - New Zealand Transport Agency for permission to publish. References RRU 70 National Roads Board Road Research Unit Bulletin Number 70, 1984. CEB-FIP 78. CEB-FIP model code for concrete structures” Comite European du Beton – Federation Internationale de la Precontrainte. 1978. AS1012.13-1992. Australian Standard. “Methods of testing concrete – Method 13: Determination of the drying shrinkage of concrete for samples prepared in the field or in the laboratory. AS1012.16-1992. Australian Standard. “Methods of testing concrete – Method 16: Determination of the creep of concrete cylinders in compression. TR11 Cement & Concrete Association of New Zealand. “Properties of New Zealand Concrete Aggregates” by J.R.Mackechnie. AS 5100.5-2004 Australian Standard. “Bridge Design – Concrete”.

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