MODELLING HEAT OF HYDRATION FOR NEW ZEALAND CONCRETE:

LESSONS LEARNT FROM THE TRANSMISSION GULLY PROJECT

TOM WATSON1, BLAIR OLYNSMA2, JONATHON SWIFT3

1 Robert Bird Group (NZ) Ltd, Wellington, NZ 2 Allied Concrete, Levin, NZ 3 CPB-HEB JV, Porirua, NZ

SUMMARY

The Transmission Gully project is one of the largest civil construction projects currently under construction in New Zealand. The 27 km long four-lane motorway project includes 24 new bridges constructed primarily from reinforced concrete. The large size of the concrete elements requires special consideration of the heat of hydration generated during concrete curing. Such volumes of concrete generate significant heat as they cure due to the hydration reaction between the cement and water.

Robert Bird Group (NZ) Ltd was engaged by CPB-HEB JV to complete heat of hydration analyses for large concrete elements on the project. The aim of these analyses was to demonstrate that concrete temperatures remained within specification limits set by the bridge designers (AECOM-URS Design Joint Venture). This paper outlines the analysis process and presents lessons learnt that could be applied to future projects.

INTRODUCTION

The Transmission Gully PPP project (TG) is currently under construction by CPB-HEB Joint Venture (CPB) in Wellington, New Zealand. The project comprises a 27 km four-lane motorway with 24 bridges through challenging terrain. Some of these bridges have reinforced concrete components of a size rarely seen in local construction. For example, Pier 1 of Takapu Bridge (BR20) has a concrete foundation with a total volume of over 3500 m3 (Figure 1).

Figure 1: Left: Takapu Bridge (BR20) under construction. Adapted and published with permission from original photo copyright Mark Coote (NZTA, 2019). Right: Pier 1 showing the

scale of the foundation relative to a 100 t mobile crane.

Large volumes of concrete create significant heat during curing due to the hydration reaction between the cement and water. If not controlled, this heat can alter the development of normal hydration products within the concrete and cause excessive thermal strains. Both effects can cause cracking which can affect the durability of the concrete. In extreme cases, structural performance can be affected.

Robert Bird Group (NZ) Ltd (RBG) was engaged by CPB to complete heat of hydration analysis for large concrete elements on the project. Strand7® software was used for transient heat analysis using finite elements with properties calibrated to test results from concrete mix designs developed by Allied Concrete (Allied). The aim of this analysis was to demonstrate that concrete temperatures remained within specification limits set by the bridge designers, AECOM-URS Design Joint Venture (DJV).

This paper outlines the analysis process and presents lessons learnt that could be applied to future projects.

BACKGROUND

Heat of hydration is the heat generated by the chemical reaction between cement and water. In large concrete elements this can raise the temperature of the concrete by over 50 °C, causing two main issues:

• Thermal strain-induced cracking in the new concrete, and

• Long-term cracking in the hardened concrete due to changes in the curing process (i.e. Delayed Ettringite Formation (DEF)).

Thermal Strains

The thermal expansion of concrete is linearly proportional to the change in temperature. If restrained, this expansion (and the subsequent contraction during cooling) causes stresses to develop in the concrete. If the tensile stresses are high enough, cracking occurs. New concrete is susceptible to cracking as the heat of hydration stresses are acting on the concrete as its tensile strength is still developing.

Thermal cracking can only occur in conditions where thermal movements are restrained. The restraints prevent free movement, imposing forces onto the concrete. Restraints to thermal movement are normally split into two main types:

• External restraint is when the concrete is expanding or contracting relative to something it is fixed to. Previously hardened concrete (i.e. at a construction cold-joint) or rock/soil that the concrete is cast against are two significant sources of external restraint.

• Internal restraint is when concrete expands or contracts differentially through its thickness. For example, if the inside of the concrete has a higher temperature than the outside, then the cooler outside surface develops tension stresses which can crack the concrete. In extreme cases these stresses cause yielding of the reinforcement.

Delayed Ettringite Formation (DEF)

Ettringite is one of the mineral products created when Portland cement reacts with water. At high curing temperatures (> 70 °C), ettringite does not form. Instead, if water is present, the ettringite forms after the concrete has hardened. This ‘secondary’ ettringite expands within the hardened concrete causing cracking (The Concrete Society UK, 2019). Godart and Divet (2013) presented several examples of cracking attributed to DEF in large concrete structures. This type of cracking has the potential to affect the aesthetics and durability of the concrete.

Performance Specification

It is important to control the peak and differential temperatures in new concrete to avoid thermal strain and DEF induced cracking. However, concrete elements of common dimensions are less likely to have these types of problems as smaller quantities of cement do not generate high temperatures when curing. Additionally, heat is rapidly dissipated in small or thin concrete members, avoiding high differential temperatures. Therefore, the performance specification for heat of hydration control should focus on large concrete elements.

The TG specification required low-heat concrete and heat of hydration analysis for all concrete elements with the smallest dimension exceeding 1600 mm. For these elements the following maximum and differential temperature limits were specified:

• Maximum heat of hydration < 70 °C, at any location within the element.

• Maximum differential temperature < 20 °C.

Demonstrating Compliance

The compliance process adopted for TG can be split into four main parts which are discussed in the remainder of this paper:

• Part 1 covers development of low-heat mix designs, including testing and calibration analysis to match the theoretical thermal properties to the test data.

• Part 2 covers detailed heat of hydration analysis for each element to demonstrate that the specification limits were unlikely to be exceeded on site.

• Part 3 covers revision of the specification for differential temperature limits, where necessary. For some elements this required crack prediction studies and early-age tensile testing of the concrete.

• Part 4 covers temperature monitoring during construction and compares the heat of hydration predictions with reality.

PART 1: TESTING AND CALIBRATION OF LOW-HEAT MIX DESIGNS

Concrete Mix Testing

CPB engaged Allied to develop and test low-heat mix designs as the first step in demonstrating heat of hydration compliance. Allied trialled three different GP cements and four different fly-ash sources before deciding on three low-heat mix designs. Two 40 MPa mix designs were prepared for pump and tremie mixes; both included 30% fly-ash for heat of hydration control. A third 25 MPa mix design was developed specifically for the BR20 foundations shown in Figure 1. To date, at least seven low-heat mix designs have been developed to handle various construction and delivery challenges. All concrete mixes contained imported OPC and fly-ash, and Wellington greywacke aggregates.

The test setup was designed to predict the peak temperatures of the in-situ concrete. Continuous insulation was used to reduce heat loss from the test blocks as much as possible, to simulate the conditions at the centre of large concrete pours. A 1 m3 mould was constructed for the trials with 100 mm thick polystyrene panels lining the sides. A layer of black polythene was placed inside the mould to enable its re-use in subsequent trials. Once filled with concrete, another 100 mm thick polystyrene panel was placed on the top. The general arrangement can be seen in Figure 2.

Figure 2: Left: Test setup for heat of hydration tests. Right: Diagram showing the insulated formwork arrangement.

Two cubic meters of test concrete was batched and compacted into the mould. Three thermocouple wires were tied to a piece of reinforcing bar that was inserted vertically into the centre of the concrete. The thermocouple wires were positioned near the bottom, middle, and 50 mm below the top surface. A fourth thermocouple wire was used to measure the ambient air temperature outside the mould. Testo 176 T3/T4 data loggers were found to provide the most reliable results. These took measurements at 15-minute intervals for at least four days.

Calibration Modelling

Strand7® finite element analysis software was used to model the test blocks using the nonlinear transient heat solver.

The purpose of the calibration modelling was to:

• Validate the analysis method,

• Calibrate the initial ‘best estimate’ thermal properties to locally sourced concretes, and

• Provide refined thermal properties to reduce uncertainty in the final analyses.

The calibration modelling process was as follows:

1. Complete the first-run analysis using ‘best-estimate’ properties derived from recognised theory and relationships as outlined in CIRIA C660 (Bamforth, 2007). This analysis used mix proportions provided by Allied and ambient temperature test data, but the analysis was otherwise independent of the testing.

2. Compare the results of the first-run analysis to the Allied test results to understand how accurately the analysis can predict the heat of hydration temperatures. Calibrate the thermal properties (heat generation, thermal conductance, specific heat, insulation properties, activation time) to better match the Allied test results.

3. Re-run the analysis with the calibrated thermal properties and compare the results. Decide if further refinement is required or accept the results and adopt the calibrated parameters.

Figure 3 shows the Allied test results against the RBG modelling predictions for 25 MPa concrete mix 2519PTGL, before and after calibration. The charts compare temperature at the top and middle of the test block, recorded over four days. The smoother solid lines are results from the Strand7® model, and the dashed lines are from the Allied test results. The other test mixes showed similar results, albeit with higher peak temperatures due to their higher cement contents.

Figure 3: Temperature curves for the 25 MPa low-heat mix design. Left: Uncalibrated results showing differences in CIRIA C660 default values and test results. Right: Calibrated results

with modifications to the heat generation parameters and activation time.

PART 2: DETAILED HEAT OF HYDRATION MODELLING

Finite Element Analysis for Heat of Hydration Modelling

Heat of hydration estimates in NZ have typically been based on a simplified method using CIRIA C660 (Bamforth, 2007). Edwards and Donoghue (2016) describe the limitations of this method in their 2016 NZCIC paper that presented six case studies of large NZ concrete pours. The CIRIA C660 method is adequate for heat of hydration estimates for prismatic sections (2D analysis) but is limited for 3D problems involving complex geometry, heat transfer to ground, or multi-stage concrete constructions. TG had all three of these challenges across six of the largest bridges on the project.

For these reasons, it was decided to adopt a finite element analysis (FEA) modelling method for the project. Strand7® was used for the analysis due to the capability of its transient heat solver and RBG’s previous experience with the software. The three different model types, as shown in Figure 4, were:

• 2D axisymmetric plate analysis for piles and circular columns

• 2D plate analysis for prismatic sections (abutment beams and pier diaphragms)

• 3D brick analysis for non-prismatic sections (pier crossheads, BR20 foundations)

Figure 4: Different types of FEA model use for the heat of hydration analysis on the project: Left to right: 2D axisymmetric plate pile analysis; 2D plate abutment beam (second stage pour); 3D brick quarter-model of BR20 crosshead. Different colours represent materials

(concrete, soil) with different thermal properties.

The use of each model type was based on its ability to model the member geometry accurately with the fewest finite elements (to reduce computational time). While each of the model types used different modelling techniques, the material properties and inputs were similar.

Concrete Properties: Heat Generation of Cement

For TG, the CIRIA C660 ‘two-part’ adiabatic curve was adopted for the heat generated by the cement. This curve considered the addition of supplementary cementitious materials (SCMs) such as fly-ash. The curve was calibrated, as outlined in Part 1, to ensure the testing and modelling agreed within reasonable margins. For modelling, the adiabatic curve (units J/kg vs time) was converted to a heat of hydration rate (unit J/s/m3 vs time).

The following main parameters were calibrated:

• Q41: 41-hour heat generation per kilogram of cement (unit J/kg). This can be modified by scaling it according to the difference in heat gain between the analysis and test.

• t2: Activation time. This parameter controls the onset of the second part of the two-part curve. For concrete mixes with set retarders, it can be modified to directly match the onset of heat development evident from the test results.

Figure 5 below shows the adiabatic curves and rate of heat generation curves for two 40 MPa concrete mixes. These mixes are identical except for the addition of set retarder and the substitution of 30% fly ash in one of the mixes.

Figure 5: Charts showing the adiabatic curve (left) and rate of heat generation (right) for two 40 MPa mixes. They are identical mixes except the mix represented by the blue line has 30%

fly ash with set retarder.

Concrete Properties: Thermal Conductivity

Thermal conductivity describes how heat flows through a material. In hardened concrete, the thermal conductivity is mainly affected by the type of aggregate used. In young concrete, the thermal conductivity is affected by the amount of free water within the concrete mix and stabilises to a constant value when the free water is consumed by the hydration reaction. The relationship proposed by Ruiz (2003) was used to capture the change in thermal conductivity with time.

For TG, a thermal conductivity based on the recommendations from CIRIA C660 (for sandstone aggregates with crushed non-siliceous sand) was adopted. This gives an average early-age thermal conductivity of 1.61 W/mK, stabilising to about 1.4 W/mK after 60 hours. It is possible that these values underestimate the thermal conductivity for Wellington Greywacke sandstone which is high in quartz and feldspar; however, the thermal analysis results were not sensitive to this value.

Concrete Properties: Specific Heat

The specific heat capacity of a material is the amount of energy required to raise its temperature. The specific heat of new concrete changes as the free water is consumed by the hydration reaction due to the high specific heat capacity of water. Strand7® does not allow specific heat to be changed as a function of time, and so a constant value was adopted to reflect the degree of hydration over the period of interest. CIRIA C660 recommends that the specific heat capacity is calculated from the average degree of hydration over the first 30 hours. The adiabatic heat curve can be used for this calculation as the degree of hydration is proportional to the heat generated at a given point in time. For TG, the concretes specific heat capacity was about 1.0 kJ/kgK across all concrete types.

Concrete Properties: Mix Temperature

High concrete mix temperatures will typically lead to higher peak and differential temperatures. Allied provided typical temperature data for concrete from their Kiwi Point plant (Figure 6). This data spanned approximately 18 months between 2016 and 2017. The general trend followed a sine curve with peaks and troughs aligned to the summer and winter seasons. The temperature spread on a given date is about 5 – 10 °C. The sine curve trendline was used to approximate the mix temperature for the analyses.

Figure 6: Typical concrete temperature for concretes delivered by Allied from their Kiwi

Point plant.

Temperature limits were imposed on the concrete supply as follows:

• Expected mix temperature as per the best-fit sine curve shown in Figure 6.

• Maximum average mix temperature of 2 °C above the expected temperature.

• Maximum allowable temperature for a single load of 4 °C above expected.

Estimating Ambient Temperature

The ambient temperature was derived from CliFlo database records for Porirua (NIWA, 2016). Maximum, minimum, and average temperatures for each season were fit to a sine curve with a 24-hour period. Comparison between these estimates and the actual ambient temperatures recorded on-site showed reasonable agreement. Site controls were required in the case of unseasonably warm weather.

Estimating Formwork Insulation Properties (Convection Coefficient)

The convection coefficient is used at the boundaries of the finite element model to describe how heat flows through the face or edge of the elements into the atmosphere. CIRIA C660 provides some data for common formwork materials but it does not offer a method to calculate the convection coefficient for typical formwork build-ups (such as plywood, polystyrene, and joists). As the convection coefficient is the inverse of the R-values, an online R-value calculator, provided by Design Navigator, was used for this calculation with minor modification (Stoeklein, 2016). This tool calculates the R-values using the NZS4214:2006 method and includes a

database of common NZ building materials and insulation. For plain 17 mm form-ply the convection coefficient is around 5.2 W/m2K, and drops to 0.7 W/m2K when insulated with 50 mm of polystyrene.

Modelling Formwork Removal

An important consideration and output for heat of hydration modelling is the stripping time for formwork. When formwork is stripped, the concrete surface rapidly cools. This produces a high differential temperature that can cause cracking. This ‘thermal shock’ effect is made worse when insulated formwork is used. Strand7® allows the analyst to define a convection coefficient as a function of time to account for this effect. Based on recommendations in CIRIA C660, the convection coefficient of the bare concrete surface is estimated at around 30 W/m2K with a nominal wind allowance of 6 m/s (the year-round average for Wellington).

Special Case: Modelling Piles

The heat of hydration assessment for large diameter piles proved to be a difficult problem, and it’s a topic that could have a dedicated practice paper. The main challenges were as follows:

• There is little information for thermal properties of soils (NZ or otherwise) in the context of the heat of hydration analysis. For the thermal properties of soils, we referred to GNS data published for ground-source heat pump design (GNS Science, 2013).

• The analysis showed high differential temperatures at depth, which were outside the project specification and difficult to verify by monitoring or testing.

• There are few documented precedents in the literature. We were able to find one paper by Al-Mosawe (2002) that presented comparable results to our analysis.

Saturated soil and rock are good conductors of heat, which draws heat away from the pile surface. In addition, soil below about 6 m depth has a year-round temperature of less than 14 °C. These factors make it difficult to control differential temperatures at depth without resorting to active cooling measures. To avoid this, we carried out crack prediction analysis to demonstrate that the differential temperatures in the piles were unlikely to cause significant cracks. This analysis is discussed in more detail under Part 3.

Special Case: Modelling Multi-stage Pours

Most elements can be considered as standalone elements without the need to specifically consider construction staging; however, the following staging effects were considered for this project:

• Heat interaction between adjacent elements separated only by a construction joint. For example, the assessment of a pier headstock with consideration of the connected column. In this case, adjacent concrete elements were modelled assuming they had reached a constant ambient temperature. In all but one case, the size of the elements and the timing between pours validated this approach.

• For the BR20 foundations, the size of the pours (11 pours around 350 m3 each) and the timing between them (about two weeks) required staging to be considered as an integral part of the analysis.

Figure 7 shows the heat of hydration model for the BR20 Pier 1 foundation. This foundation contains about 3500 m3 of concrete poured over 11 stages.

To model staged construction, the thermal properties for each concrete pour were turned off and on at the right moment in the sequence. In Strand7®, property vs time tables were used to control the timing throughout the analysis. A half-model reduced the number of elements and

made use of the foundation’s (near) symmetry. The Strand7® documentation has a simple staged heat of hydration analysis example that outlines the process in detail (Strand7, 2014).

Figure 7: BR20 multi-stage foundation construction. This half-model simulates the heat of hydration for about 3500 m3 concrete poured over 11 stages, two weeks apart. The contour

on the right shows the internal temperature shortly after the final stage is poured.

PART 3: DIFFERENTIAL TEMPERATURE THRESHOLDS AND CRACK PREDICTION

STUDIES

Exceedance of the Differential Temperature Limit

The analysis showed that the 20 °C differential temperature limit could not be achieved (without additional measures) for several elements:

• Differential temperatures for large diameter piles in saturated soil and rock were typically 27 – 29 °C across the width of the piles.

• Differential temperatures were exceeded for many above-ground elements when the formwork was stripped (7 to 14-day stripping time). Refer to Figure 8. This ‘thermal shock’ was worse with the use of insulated formwork as the differential temperature would rise by up to 10 °C upon formwork stripping.

Figure 8: Temperature vs time predictions for the BR20 abutments showing 20 °C temperature limit exceeded following formwork stripping.

Early-age Tensile Testing of Concrete

The DJV requested cracking analysis for the relevant members to understand the risks associated with non-conformance with the 20 °C differential temperature limit. Knowledge of the tensile strength gain of the concrete was required to assess early-age cracking. While there is good information on the hardened (28-day) properties of NZ concretes from studies by McKechnie (2003), it was difficult to find published data for younger concretes. References such as CIRIA C660 (Bamforth, 2007) and the CEB-FIP Model Code (CEB-FIP, 1993) provided

Differential temperature rises by 5 – 10 °C

Formwork

stripping at 14

days (336 hours)

relationships for tensile strength gain of concrete, but there remained uncertainty in the applicability to NZ sourced low-heat mix designs incorporating set retarders.

For these reasons, Allied organised for early-age tensile tests to be carried out on the proposed concrete mixes. Splitting tests were completed by Opus for the 40 MPa low-heat mix designs on 3-day old cylinders. Mean tensile splitting strengths were 2.0 MPa and 2.6 MPa for tremie and pump mixes respectively.

Calibrating Mechanical Properties

The assessment of thermal cracking risk vs time requires information on the development of tensile strength, Young’s modulus, and the coefficient of thermal expansion for the concrete.

The tensile strength vs time relationship for the concrete was based on the CEB-FIP (1993) model calibrated to two data points:

• Three-day splitting test results provided by Allied, and

• Hardened properties of NZ concretes (28-day properties) provided in CCANZ TR11 report and (Mackechnie, 2003).

The ‘probable’ Young’s modulus was estimated for the mix designs based on their water/cement ratios and information from CCANZ TR11 (2003). The CEB-FIP (1993) relationship for Young’s modulus was calibrated to fit this value at 28 days. For values prior to 28 days, Young’s modulus values are related to the strength gain of the concrete which was calibrated to the tensile test results.

The coefficient of thermal expansion was derived from the CCANZ TR11 data, which showed a near-constant coefficient of thermal expansion of 9.5 µε/°C across the range of water/cement ratios considered. CIRIA C660 and Azenha (2009) note that the coefficient of thermal expansion stabilises to its final value within one day and so a constant value of 9.5 µε/°C was adopted for the analysis.

Developing Differential Temperature Limit vs Time Curves

With the parameters described above, the thermal cracking vs time relationship was plotted from the equations given in CIRIA C660. Figure 9 presents the differential temperatures expected vs the theoretical cracking threshold for TG piles. This was used as a tool for assessing the age at which cracking is most likely to occur.

Figure 9: Differential temperature vs time plot showing the theoretical cracking threshold against predicted differential temperatures in large-diameter piles from 1650 mm to 2100 mm

diameter.

Risk of cracking over this time period (1 -2 days following pour)

Predicting Crack Widths

For TG, the exceedance of the cracking threshold was only a problem for the large diameter piles. As previously discussed, this was predicted to occur deep in the piles where low-temperature, high conductance soil and rock is in contact with the curing concrete. The methods outlined in CIRIA C660 were used estimate the crack widths that could develop. The maximum crack width that could develop in the outside of the pile was calculated for four different concrete ages. The predicted crack widths in the piles were negligible at less than 0.1 mm.

PART 4: SITE RESULTS VS PREDICTIONS

Most large concrete elements were monitored on-site as required by the project specification. The monitoring results allowed for direct comparison between the heat of hydration predictions and reality. The predictions were within +/-5 °C of recorded temperatures and typically higher than observed values. Comparisons for two separate elements at BR20 are shown below. Abutment 1, BR20

Figure 10: Comparison of predicted vs actual temperatures for BR20 Abutment 1. Note that the data logger malfunctioned between 50 – 100 hours.

The abutment beams for BR20 were approximately 25 m long, 5 m wide, and 2 m deep. They were cast against the ground with insulated formwork. A 40 MPa low-heat pump mix with 30% fly ash was used. The results of the thermal monitoring are shown in Figure 10 with the following observations:

• The predictions were about 5 degrees higher than measured on-site.

• The peak temperatures were low for this element (both in predictions and in reality). This was due to its relatively low thickness, the low ambient temperatures, and heat loss to the ground.

• During, and for two days following the pour, the ambient temperatures were significantly lower than expected. This most likely contributed to the lower measured temperatures.

• The data loggers malfunctioned between 50 – 100 hours and had to be reset. Reliability of data loggers under site conditions was a common issue observed in the records.

• While differential temperatures remained beneath the 20 °C threshold throughout the monitoring, during the first 24 hours the differential temperature exceeded the theoretical cracking threshold over a short period. No cracking was observed on-site,

and it is not expected that cracking occurred since there is known conservatism in the theoretical cracking threshold.

Pier 2 Crosshead, BR20

Figure 11: Comparison of predicted vs actual temperatures for the BR20 Pier 2 crosshead structure.

The piers for BR20 were the largest suspended concrete elements on the project. They were approximately 25 m long, 4 m wide, and taper along their length from 4 m at the centre to 2.5 m at the ends. A large void over the column provided access into the hollow square column that supported the crosshead. The results of the thermal monitoring are shown in Figure 11 with the following observations:

• The measured temperatures were about 5 degrees higher than predicted. This was unusual for this project, as site results were typically lower than predictions.

• Ambient temperatures were significantly higher on-site than expected. This may have contributed to the differences observed.

• The cooling rate for the centre of the pour was higher than predicted, implying the insulation was not as effective on-site compared to analysis assumptions.

• Differential temperatures closely followed the predictions up to 100 hours. This implied an upward ‘shift’ in the temperature results that could be attributed to the mix or ambient temperatures, normal differences in cement supplied, or effects associated with long-duration pour.

CONCLUSIONS AND RECOMMENDATIONS

The analysis and testing completed for the Transmission Gully project have provided many lessons learnt regarding how the heat of hydration can be managed for NZ concrete structures.

Testing and Calibration

• The ‘best-estimate’ thermal parameters recommended by CIRIA C660 provided a fair approximation for heat development in insulated test blocks. The temperature predictions were between 5 – 10 °C higher, which is appropriately conservative in the absence of relevant test data.

• Allied used a set retarding admixture in their mix designs, which is reflected in the early temperature results. This is easily adjusted in the concrete heat generation models by changing the activation time, ‘t2’ for the cement.

• The CIRIA C660 models overestimated the total heat generated compared to the Allied tests. This is most likely due to different properties of the OPC and/or fly-ash sourced in the UK where CIRIA C660 was written. This difference was calibrated by adjusting the Q41 parameter (heat generation per kilogram of cement at 41 hours). While this was a consistent conclusion for the Transmission Gully test mixes, it is impossible to draw more general conclusions about NZ concretes versus the CIRIA C660 models without similar testing across a broader range of locations and cement origins.

• The negative slope in the temperature curves beyond 40 hours provided a good indication of the insulation efficiency.

Detailed Heat of Hydration Modelling

• Detailed finite element modelling appeared to offer better accuracy in the heat of hydration estimates compared to the simplified methods provided in CIRIA C660.

• The type of FEA model (2D, 3D, axisymmetric etc.) should be chosen to provide the most accurate results for the fewest finite elements to reduce computational time.

• The CIRIA C660 two-part adiabatic curve appeared to be a suitable model for NZ concretes. In the absence of test data, the ‘default’ CIRIA C660 parameters are expected to give conservative results when used with FEA.

• Design Navigator (or equivalent calculation to NZS4214:2006) was a useful tool for estimating the insulation properties of building materials. It can also be used for developing alternative insulation options should it be requested by the Contractor.

• Thermal modelling of piles is challenging due to the interaction of the concrete with soil and rock. Additionally, it is difficult to measure temperatures at significant depth where the highest differential temperatures are expected. As such, there is potential for more research on this topic. Our analysis showed that differential temperature limits were difficult to achieve for large diameter piles cast against saturated soil and rock.

• Multi-stage analysis of very large concrete elements is possible using Strand7®. The Strand7® documentation has a worked example for staged heat of hydration analysis.

Differential Temperatures and Crack Prediction

• The use of 50 mm of expanded polystyrene (EPS) insulation and its subsequent removal created the potential for ‘thermal shock’ issues. The solution was either to leave the formwork in place for longer (which was impractical) or consider uninsulated forms with shorter stripping times.

• Over insulating the concrete should be avoided. ‘Just enough’ should be used to keep temperatures within the differential temperature limits. In several cases, it was demonstrated that uninsulated plywood forms with shorter stripping times produced a better outcome than insulated forms with longer stripping times.

• It may be difficult to avoid thermal cracking in certain cases (e.g. large diameter bored piles). The analyst should be prepared to complete crack calculations to demonstrate that cracks will not create durability issues for the structure. Guidance on acceptable crack widths can be found in CIRIA C660.

• 20 °C is a good starting point for differential temperature limits, and it is appropriate for the avoidance of cracking in the first 5 days. However, once the concrete mix designs are known, a differential temperature limit vs time relationship should be developed to understand the risk of thermal cracking as the concrete develops strength.

• Active cooling measures for mixes are not favoured by suppliers or contractors due to the additional complication it adds to the supply and concrete placement process. Provided that low-heat mix designs are adopted, active cooling measures should not be required.

• Future research should be undertaken to improve the early-age tension strength data for NZ concretes.

Site Results vs Predictions

• Heat of hydration analysis can only provide temperature estimates within +/-5 °C of on-site measurements despite sophisticated modelling and calibration. This is to be expected as there are many assumptions and uncertainties that cannot be accounted for during the analysis. Comparison of the results with Edwards and Donoghue (2016) showed that the FEA method can provide more accurate estimates than the simplified CIRIA C660 methods.

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