Thermal Control of Large Concrete Elements – Queensland Experience

James Aldred1 and Claire Miller

2

1Technical Director, AECOM

2Structural Engineer, AECOM

Abstract: During the design and construction of large concrete elements it is important to consider how the temperature of the element will be controlled. If the peak temperature is excessive, there is a risk of delayed ettringite formation and external restraint cracking depending on the restraint condition. If the temperature differential is excessive, there is a risk of internal restraint cracking. These can lead to reduced section capacity and durability degradation. The mismanagement of concrete temperature can lead to costly and time consuming repairs on projects.

In Queensland, the monitoring and control of temperatures in large concrete pours is of significant concern due to the hot weather that is experienced especially during the summer months. Hot ambient temperatures can limit the reduction of the concrete placing temperature. It may also increase the peak concrete temperature.

This paper compares the thermal control measures used on three major Queensland projects: the Yeppen Lagoon Bridge in Rockhampton; the Ross River Bridge in Townsville; and thermal cracking in Tee-roff Girders. The relevant standards and criteria, testing methodology, temperature results, and lessons learnt on site will be presented for each of the projects. Based on the outcomes of these three projects, recommendations on effective control methods will be discussed.

Keywords: concrete, peak temperature, delayed ettringite formation, temperature differential.

1. Introduction

When designing and constructing large concrete elements, it is imperative to consider the temperature increase in the concrete during the hydration process. Hydration is an exothermic chemical process that occurs as the concrete hardens. In large concrete elements, the heat from the hydration process cannot escape as quickly as it is generated causing the temperature at the center of the element to rise. This is of particular concern in the Queensland environment due to the hot weather that is experienced, especially in the summer months. If the peak temperature is excessive, there is risk of delayed ettringite formation, strength reduction and thermal restraint cracking. These can lead to reduced section capacity and durability. Failure to control concrete temperature in massive or heavily restrained elements can lead to costly and time consuming remedial measures on projects.

2. Problems with high early temperatures in concrete

High temperature caused by hydration or steam curing can cause defects in concrete ranging from cracking due to delayed ettringite formation and thermal restraint to the reduction in strength and other hardened properties.

2.1. Delayed Ettringite Formation

High temperatures during hydration or accelerated curing can cause expansion and cracking of concrete due to Delayed Ettringite Formation (DEF). There is debate about the exact mechanism of DEF. The generally accepted view is that Portland cement concrete which attains a temperature of approximately 70°C or more during hydration appears to inhibit the formation of preliminary non-expansive ettringite. The unreacted sulfate within the concrete is then available to react with tricalcium aluminate in the cement similar to traditional sulfate attack. Most examples of disruptive DEF have occurred in moist environments such as ports, dams, railway sleepers, etc.

There are many factors which have been found to influence the susceptibility to DEF: sulfate/aluminate ratio, cementitious replacements, cement fineness and reactivity. A peak temperature limit of approximately 70°C regardless of other considerations is probably the best way to help ensure that DEF is not a problem. While not a common deterioration mechanism, it can cause significant problems when it does occur as shown in Figure 1. It is hard to detect if DEF has occurred during the hydration process. In any case, it may already be too late and costly repairs or even replacement may be required.

Figure 1. Cracks up to 10 mm wide caused by DEF [Eriksen (1)]

2.2. Effect on Strength and Other Properties

High early curing temperature tends to increase early strength but reduce longer term development strength as shown in Figure 2 [Nanayakkara (2)]. High early curing temperature also tends to have an adverse effect on chloride penetration and other transport properties [Hooton et al (3), Khatri et al (4)]. Therefore a higher peak temperature during curing could reduce both structural strength and the durability.

Figure 2. Effect of high temperature at early age on compressive strength development

[Nanayakkara (2)]

2.3. Thermal Restraint Cracking

During hydration, the concrete will expand as it heats up and contract as it cools down. If the concrete is restrained, stress will develop in the concrete. When the tensile stress in the concrete exceeds the tensile capacity of the concrete, a crack will be expected to form. There are two types of restraint: internal and external. Internal restraint occurs in large concrete elements because the surfaces of the element cool more rapidly than the core. The resultant temperature differential generates stresses in the concrete [Reynolds et al (5)]. External restraint occurs when one element is cast against another, for instance a ballast wall onto an abutment headstock. Thermal restraint cracks are primarily a durability issue as they provide a pathway for air and water to reach the reinforcing steel. However, if the thermal stresses are significant, the cracking may affect the structural capacity of the concrete.

While many designers focus on temperature differentials and internal restraint, most thermal cracking in warmer climates like Queensland is due to external restraint which is dependent on peak temperature.

3. Current Guidelines

By restricting peak and differential concrete temperature during construction, the risk of developing DEF and thermal restraint cracking can be reduced. The Australian Standards currently do not give specific

guidance on the peak and differential temperatures that should be adopted. In Queensland, the Transport and Main Roads (TMR) Standard Specification MRTS 70 (6) gives advice on the temperature limits that should be adopted to minimise the risk of the formation of DEF and internal thermal restraint cracking. The specification states that “where the cross section of a concrete member exceeds 1m x 1m thick or the total

cementitous content of the mix exceeds 500kg/m3”, temperature limits of 75⁰C peak temperature and

25⁰C differential temperature should be adopted. It also states in clause 19.5, that the temperature of the

element should be recorded for a minimum of 120 hours after placement [Queensland Government - TMR (6)].The concrete specification by the Roads and Maritime Service in NSW B80 [NSW Government (7)] limits peak temperature to 70°C but does not limit differential temperature.

The American Concrete Institute defines mass concrete as “any volume of concrete with dimension large enough to require that measures be taken to cope with generation of heat from hydration of cement and attendant volume change, to minimise cracking”. Riding et al (8) reports that seven state departments

specify 35⁰F (20⁰C) as the maximum differential temperature between the core and the surface and two

states limit maximum in-place temperature to 160⁰F (71⁰C) as the maximum concrete temperature. These temperatures are difficult to achieve unless specific temperature control measures are used during the hydration process Gajda and Alsamasam (9) state that specifications typically limit the maximum

temperature to 160⁰F (71⁰C) and the maximum temperature difference to 35⁰F (20⁰C). The value of 20°C relates to the estimated internal restraint for a smooth gravel as shown in Table 1 from BS 8110.2-1985 (10) whereas most coarse aggregate used would be crushed and a limit of 27.7°C would be more realistic. CIRIA C660 (11), which is arguably the most comprehensive assessment of thermal crack control, gives similar values.

Table 1. Estimated temperature differential limits for different aggregates BS 8110.2-1985 (10)

It is commonly accepted that for General Portland (GP) cement, a peak concrete temperature limit of 70⁰C

and a differential temperature limit of 25⁰C will reduce the risk of DEF and internal thermal restraint

cracking to an acceptable level [Follardet al (12)] [Nanayakkara (2)]. If fly ash, slag or silica fume is added to the concrete slightly higher peak temperature limits are acceptable for the control of DEF.

4. Control Measures

If considered during the design process and there is early involvement by the contractor and premix company, control of the concrete temperature in large concrete pours does not need to become a costly and time consuming exercise. There are a variety of methods that can be used to control concrete temperatures on site and the optimum solution will depend on site conditions, construction program and product availability. Below are a variety of control measures that can be used.

4.1. Thermal modeling

Due to the costly repair measures, it is too late to attempt to control temperature rise and mitigate thermal cracking after the concrete has been poured. It is good practice to conduct thermal modeling on all larger elements to determine if specific control measures need to be taken and to assess the most practical options. Thermal modeling can range from relatively simple estimates of peak and differential temperatures to complex finite element analyses to determine areas of stress concentration and to compare control measures.

4.2. Concrete Mix Design

Generally the most practical method of thermal control is modifying the mix composition. Careful consideration should be given to the concrete mix design rather than using a standard mix design or simply following guidelines from regulatory authorities. The cementitious content directly relates to heat of hydration of the mix. When trying to minimize temperature rise in concrete, the target cementitious content

should be the minimum required to achieve the fresh and hardened properties. When determining concrete mix properties, it is recommended to specify a maximum cementitious content to ensure peak temperature will not be exceeded in addition to any minimum cementitious content required for strength and durability properties of the concrete. In addition, to minimise temperature rise for mass concrete pours, it is recommended that the optimum maximum replacement with supplementary cementitious materials, such as fly ash or Ground Granulated Blast-furnace Slag (GGBS), should be used. GGBS is a byproduct of iron smelting producing a latent hydraulic binder with similar total heat of hydration to GP cement but significantly reduced rate of heat generation. Fly ash is a byproduct from the burning of pulverised coal in power station furnaces. The fly ash in Queensland is generally of high quality with low calcium content which reduces both the total heat of hydration as well as the rate of heat generation. While fly ash has a lower heat of hydration than GGBS, it is usually used at a lower replacement level and therefore in most circumstances either product can be used to provide significant reductions in temperature rise.

During the mix design process there can often be a tradeoff between the different requirements of the concrete mix. For example, whilst fly ash and GGBS reduce temperature rise, they can also reduce the rate of strength gain which may be important when considering the construction program. In massive concrete elements, this is rarely a problem as the effective maturity within the concrete can be several times that of the standard cured cylinders. It can be an issue in precast applications with significant variation in section dimensions as discussed in the section on Tee-roff girders later.

There is a common misperception that it is necessary to have a particular minimum cementitious content for durability requirements. Buenfeld and Okundi (13) showed that, at a given w/cm ratio, the higher binder content actually increased chloride ion ingress in concrete and all other transport processes. An unnecessarily high cementitious content may lead to increased cracking due to thermal stresses and shrinkage which could reduce durability.

There can also be conflicting requirements between the local authority standards and project specific requirements. For example, whilst an appropriate low heat concrete mix may require more than 60% GGBS, the current MRTS 70 requires that a minimum of 55% of the total mass of cementitious content be GP cement. MRTS 70 also requires a minimum of 20% fly ash to neutralize mildly reactive aggregates that can be found in Queensland to avoid aggregate testing. The use of GGBS may achieve the same benefit in ASR control but it is not given as a deemed to comply option. For heavily restrained elements, it can be helpful to use more than 45% fly ash to reduce the peak temperature. Through the projects discussed in the presented case studies, it was found that the authorities in Queensland have proven to be receptive to technically supported proposals to deviate from specific requirements of their specifications. Indeed, they are keen to learn how to design and construct better infrastructure. This is the reason Queensland leads the nation in ASR mitigation for example.

Where fly ash or slag is not available, it is possible to use silica fume to reduce the total cementitious quantity. While the presence of silica fume may result in more rapid heat generation (by speeding up the reaction), it permits significant cement reduction at the same strength to reduce total heat generation.

The use of GGBS calls for careful consideration. It actually generates similar, or sometimes greater, heat than normal cement but it does so more slowly. So in a typical situation the heat is able to escape and the peak temperature is reduced provided there is not excessive insulation. In massive sections, such as raft slabs more than three metres thick, the heat cannot escape quickly enough and the peak temperature may be similar to a pure Portland cement concrete (Bamforth 14).

4.3. Concrete Placing Temperature

It is accepted that for every degree that the concrete placing temperature is decreased the peak temperature in turn is reduced. There are numerous ways to reduce the initial concrete placing temperature on site, including:

Adding flake ice to the concrete mixture during batching. The practical upper limit of ice that can be added to concrete will depend on the free water content of the mix.

Cooling of raw materials. This can include shading or spraying cool water on the aggregates.

Reducing the ambient temperature by placing at night/early morning or during the cooler months.

Injecting liquid nitrogen into the concrete mix.

Reducing time between batching and placement.

4.4. Insulation

Formwork or extra insulation can be used to slow the escape of heat, and reduce the temperature differential. There is a balancing act between peak and differential temperatures to consider when using insulation. Whilst insulation reduces the differential temperature, it can also increase the peak temperature detrimentally. However, it is considered that for concrete elements where the minimum dimension is greater than 1.5m the use of insulation has little effect on the maximum concrete temperature in most concrete mixes [Gajda and Alsamasam (9)]. However, insulation does increase the cross-sectional area which achieves elevated temperature and may be subject to external restraint cracking. Premature removal of insulation can result in thermal shock as the concrete surface cools rapidly to ambient temperature. Insulation should be kept in place until the hottest portion of the concrete cools to within the temperature differential limit of the average ambient air temperature.

Ponded water can be used as an insulation layer on the top surface of elements. The benefit of ponded water is that it greatly reduces autogenous shrinkage of the concrete, facilitates heat loss due to latent heat of evaporation, and it tends to dissipate the heat away from the hotter areas. It also has the added advantage of limiting any detrimental effects of early chloride penetration due to overtopping when elements are placed in splash zones.

4.5. Cooling pipes

Permanent cooling pipes can be used to remove heat from the interior of the concrete. Whilst this can be a costly control measure to implement, they give a large degree of control over the maximum concrete temperature and greatly reduce the time that insulation is required. This solution is ideal when an economical source of water is available such as a lake or river. If cooling pipes are considered on a project, they need to be considered during the design phase to ensure that they integrate with the structural design of the element [Gajda and Alsamasam (9)]. In regards to heat transfer, steel pipes are ideal for a cooling system but may cause long term durability concerns if the sealing of the cooling pipe system is not considered carefully.

5. Case Studies

Recently, three major projects in Queensland, Yeppen Lagoon Bridge, Ross River Bridge and precast prestressed girders have all had to consider the temperature effects due to large concrete elements. All of the projects considered different control measures to meet the temperature limits specified. The following case studies investigated the key factors dictating possible control measures and comparing their relative effectiveness.

5.1. Yeppen Lagoon Bridge – Rockhampton

The 420m long Yeppen Lagoon Bridge has a combination of deck units with deck slab and transversely stressed deck unit spans. The pile caps were 1.5m deep and therefore the thermal effects on the concrete needed to be considered.

Initial thermal modeling was undertaken to assess the effect of cementitious content and fly ash replacement level on peak and differential temperature in a 50MPa concrete mix. The structural design required 50MPa due to the anticipated presence of acid sulfate soils. Table 2 shows that the concrete mixes which included 40% fly ash in the mix design were expected to comply with the specified limits. This initial thermal modeling also suggested that no further control measure would be required if a mix with 40% fly ash was used.

Table 2. Estimated Peak and Differential Concrete Temperatures for Pier Pile Caps [Sager (15)]

Mix Designation S50 S50-FA S50-HC S50-HC-FA

Concrete Grade S50 S50 S50 S50

Cementitious Content 450 kg/m3 450 kg/m

3 500 kg/m

3 500 kg/m

3

Fly Ash 25% 40% 25% 40%

Summer Concrete

Placement Temperature

25°C 25°C 25°C 25°C

Mix Designation S50 S50-FA S50-HC S50-HC-FA

Summer Peak Temperature 76°C (NC) 68°C 82°C (NC) 73°C

Summer Differential

Temperature

23 °C 19 °C 25°C (NC) 21°C

Winter Concrete Placement

Temperature

20°C 20°C 20°C 20°C

Winter Peak Temperature 69°C 61°C 74°C 65°C

Winter Differential

Temperature

25°C (NC) 22°C 27°C (NC) 24°C

Note that 18 mm thick plywood insulation has been assumed. NC = Non-conformance

During the construction of the pile caps, the contractor initially used a 50MPa mix with 25% fly ash. On

Pier 9 this resulted in a peak concrete temperature of over 80⁰C as had been estimated. This was

recorded as a non-conformance. As a result, the concrete mix for future pile caps was changed to 40MPa 25% fly ash which resulted in a peak temperature just below 70°C in Pier 11 (see Figure 3). This was allowable as further investigations had revealed that there were no acid sulfate soils in the vicinity of the proposed pile caps and the structural design had original been based on a 40MPa compressive strength.

Figure 3. Thermal data from Pier 11 Pile Cap Yeppen Lagoon Bridge

5.2. Ross River Bridge – Townsville

The Ross River Bridge is a six span concrete Tee-roff bridge that is a part of the Townsville Port Access Road Project. The river pile caps on the project were 2.34 metres thick and 7.2 metres wide as they were required to resist the impact of a ship.

The concrete mix design for the substructure required a 50MPa compressive strength and a maximum chloride diffusion coefficient of 2.8 x 10

-12 m²/s as the pilecaps are located in the tidal zone. Due to the

lack of GGBS in the region at the time of construction, GGBS was not specified. A concrete mix with a relatively high cementitious content and with 25% fly ash replacement was selected.

To reduce the concrete placement temperature, the pile cap was poured overnight during November to avoid the high ambient daytime temperatures. The construction schedule did not allow for pouring during the winter months. The maximum allowable amount of ice was also added to the concrete to reduce the placing temperature. It should be noted that adding ice to a concrete mixture is often done by hand and can add considerable time to the batching process.

Thermal modeling indicated that with the limited control measures proposed to the concrete mix design and initial placing temperature, the specified peak and differential temperature limits would be exceeded. It was concluded that cooling pipes were required to keep the temperatures below the required maxima. Due to a change in the construction team over the lifespan of the project, this decision was made during the construction phase after the design of the pile cap had been completed. This added complexities into the placement of the cooling pipes around the existing pilecap reinforcement which could have been avoided if considered earlier in the design phase. Figure 4 shows the predicted temperature profile 35

hours after placement using 3 layers of 8 cooling pipes. Figure 5 shows the predicted and recorded temperature profiles for the Hottest Spot (HS), concrete adjacent to the pipe (P) and bottom surface (B) of the pilecap. The solid lines indicate the peak temperature whilst the dashed indicate the differential

temperature. The thermal modeling predicted that the peak temperature would be approximately 73⁰C and

the differential 24⁰C: both below their respective limits. The recorded maximum temperature was just less

than 70°C and the differential 15°C. The model indicated that the water flowing through the cooling pipes could be turned off 72 hours after the concrete has been placed. This is reflected in both graphs as a sharp increase in the temperature adjacent to the pipe after 72 hours in Figure 5. Earlier initial modeling had suggested that 4 layers of 15 pipes might be required which would have caused even more disruption to construction.

Figure 4. Temperature profile (ºC) at concrete peak temperature, 35 hours after concrete

placement.

Figure 5a). Predicted temperature profiles for the Ross River

Figure 5 b). Recorded temperature profiles for the Ross River Bridge

5.3. Thermal Cracking in Precast Girders

A thermal-stress analysis was performed for a typical girder on a bridge project to investigate the cause of the concrete cracking observed during construction. The analysis focused on the development of thermal stress at early age and comprised a thermal model and a stress model in series. It is understood that there may be several contributing factors during the construction process that may cause cracking in the concrete and the information presented focuses purely on thermal stresses developed during the curing process and does not address other possible contributing factors.

The influence of various particulars of the production methodology on the thermal stresses was assessed. These included the effect of the steam curing and steel formwork, the effect of the casting duration, the effect of water blasting the girder’s end-block and the thermal effect of using compressed air to remove the formwork. While simplified to facilitate the modelling process, the assumptions made regarding the various production parameters are broadly consistent with the information received regarding the manufacturing process. Figure 6 shows the temperature profile estimated at 20 hours after casting with a maximum temperature of nearly 90°C in the end block.

Figure 6. Temperature (°C) in the girder at 20 hours - Longitudinal section through the girder at mid-height.

The stress model ignores the presence of reinforcing or prestressing and the other effects which were considered to have a negligible influence on the thermal model. Based on this analysis, the peak tensile stresses created by early age temperature differentials within the modelled girder appear generally consistent with the cracking observed in the production girders. However there were observed cracks in the girders which did not coincide with areas of high thermal stress indicating that other factors also had contributed to cracking during the construction process. It appeared that the main factors contributing to these early age temperature differentials after the end of the steam curing are the heat of hydration of the concrete mix and the section geometry which influenced the thermal inertia as well as continued temperature rise in the thicker end block elements. The water blasting of the end-block was shown to cause significant thermal shock to the external surface of the girder but otherwise have a limited effect on the thermal stresses within the girder.

The stress patterns generated by the temperature differentials were compared with the cracking patterns observed onsite. The stress model showed that the cracking patterns were likely to have developed due to thermal stresses between 12 hours and 25 hours after concrete casting.

The stress analysis was performed using the original mix which had a peak temperature of 90°C and a mix with the recommended modifications to cementitious content, fly ash replacement and placement temperature to reduce the peak temperature to 70°C. As the w/c ratio is kept constant and the steam curing procedure was unchanged, the mechanical properties were considered to be the same.

The estimated tensile stresses in the longitudinal direction at 22 hours for each concrete mix are shown in Figure 7 and 8 for comparison. There is a significant reduction in both the magnitude and extent of tensile stresses by using the recommended mix.

The estimated longitudinal stresses were modelled to reach a peak of 22 MPa in the end block with the original mix but were reduced to nearly half at 12 MPa with the recommended mix. The areas where estimated tensile stresses exceeded 3.5 MPa and the risk of cracking would be high in the original mix extended beyond the end-block and along the interface between the web and the lower flange either side of the diaphragm. However areas where estimated stresses exceeded 3.5 MPa were limited to the end-block in the recommended mix.

Figure 7. Stress in the Z-direction for the Original mix at 22 hours

Figure 8. Stress in the Z-direction for the proposed reduced temperature mix at 22 hours

6. Recommendations

From comparing the above projects an appreciation is developed that a customized solution to large concrete pours is required on a project by project basis due to the differing competing project constraints. However, although the optimal solution for each project will be different, the following should always be considered early on in the projects life cycle to minimize temperatures:

Thermal modeling is imperative in determining how the concrete mix will behave and whether further controls are required and can be very accurate

Concrete mix design is the most effective way to control the heat of hydration temperature and careful consideration should be given to concrete mix design in large concrete pours rather than using an off the shelf mix design. Specifying maximum and minimum cementitious contents for the concrete mix design should be considered to limit peak temperature. A change in the industry attitude is required for this to be implemented.

Controlling concrete placement temperature is another important thermal control method but the practical limit on concrete placement temperature at the project location needs to be considered.

Cooling pipes are an effective but expensive way to control concrete temperature but consideration needs to be given to installation, operation and decommissioning of the system.

7. Acknowledgement

The authors would like to thank the people involved in the projects discussed for their cooperation and

help during construction. Special thanks go to Dr Peter Shaw and David Dubois who conducted the FE

Modelling as well as Dr Wayne Roberts from Queensland TMR.

8. References

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2. Nanayakkara, A. “Importance of Controlling Temperature Rise Due to Heat of Hydration in Massive Concrete Elements”, University of Moratuwa, Srik Lanka.

3. Hooton, R., Pun, P., et al., “Influence of Silica Fume on Chloride Resistance of Concrete”, University of Toronto, 1997, Toronto, Canada.

4. Khatri, R.P., Sirivivatnanon,V., Marsh, P and Heeley, P., ‘Effect of Curing on the Resistance to Chloride Penetration of Concretes’, Proceedings 5th CANMET/ACI Int. Conf. on Durability of Concrete, edited by V.M. Malhotra, Barcelona, Spain, June 2000, pp. 487-505.

5. Reynolds, C., Steedman, J. et al., “Reynold’s Reinforced Concrete Designers Handbook – 11th

Edition”, Taylor and Francis, 2008, London, United Kingdom.

6. Queensland Government – Department of Transport and Main Roads, “Concrete (MRTS 70)”, Queensland Government, 2011, Queensland, Australia.

7. NSW Government – Transport Roads & Maritime Services, “ Guide to QA Specification B80 – Concrete Work For Bridges Edition 2”, NSW Government, 2012, New South Wales, Australia.

8. Riding, K., Poole, J., et al., “Evaluation of Temperature Prediction Methods for Mass Concrete Members”, ACI Materials Journal September-October 2006.

9. Gajda, J., Alsamasam, E. “Engineering Mass Concrete Structures – Professional Development Series”, Portland Cement Association, 2006, Portland, United States of America.

10. British Standard, “Eurodocde2: Design of Concrete Strucutres (BS 8110.2-1985)” 1985, United Kingdom.

11. Bamforth, P B, “Early-age Thermal Crack Control in Concrete” CIRIA C660, 2007

12. Folliard, K., Barborak, R. “Preventing Alkali-Silica Reaction and Delayed Ettringite Formation in New Concrete” Centre for Transportation Research, 2006, Texas, Unites States of America.

13. Buenfeld, N.R. and Okundi, E., "Effect of cement content on transport in concrete", Mag. Concr. Res., 50, (1998), 339-351.

14. Bamforth. P., 1980 “I-situ measurements of the effect of partial Portland cement replacement using either fly ash or ground granulated blast furnace slag on the performance of mass concrete”. Proc. Of Instn. Of Civ. Engrs., Part 2 No. 69., September 1980. pp777-800. London.

15. Sager, D., Shaw, P., et al., “Yeppen Lagoon Bridge – Supplementary Specification”, AECOM, 2012, Brisbane, Australia.