NB/T 35092-2017Design code for temperature control of concrete dam (English PDF)

Foreword

This document was issued on 28 March 2017 by the National Energy Administration of the PRC and takes effect on 1 August 2017.

It is a NB/T standard: recommended rather than compulsory, but it is the text a Chinese reviewer applies when assessing a submission.

It is classified under ICS 27.140, Chinese classification P 59.

NB/T 35092-2017, Design code for temperature control of concrete dam, is an energy industry standard of the People's Republic of China. It was issued on 28 March 2017 by the National Energy Administration and took effect on 1 August 2017.

The Chinese title of the document is Hunningtu Ba Wendu Kongzhi Sheji Guifan (混凝土坝温度控制设计规范). The English title printed on the cover of the standard is Design code for temperature control of concrete dam, and it matches the Chinese title clause by clause: it is a design code (设计规范) for the temperature control (温度控制) of concrete dams (混凝土坝).

The cover carries the classification data of the document: ICS 27.140, which is the international class for hydraulic energy engineering, and the Chinese Standard Classification code P 59, which places the document among hydraulic and hydropower engineering standards. The record number of the standard is J2346-2017.

The document is a NB/T standard: recommended rather than compulsory in the strict legal sense, but it is the text a Chinese design reviewer applies when assessing the temperature control design of a concrete dam, and contract specifications routinely make it binding.

The department in charge of drafting is the China Renewable Energy Engineering Institute (水电水利规划设计总院). The approving department is the National Energy Administration (国家能源局). The date of implementation printed on the title page is 1 August 2017. The standard is published by China Electric Power Press, Beijing, 2017.

The cover carries no replacement note, so the document does not supersede an earlier edition. It is the first unified industry code addressing temperature control design for concrete dams: before it, the principles, contents, methods and technical requirements of that design were not covered by a single standard in the hydropower sector.

The document is organised in fifteen chapters. Chapters 1 and 2 give the general provisions and the terms; chapters 3 and 4 cover raw materials and concrete mix design; chapter 5 collects the design data and calculating parameters; chapter 6 covers the calculation of the temperature field; chapter 7 covers the jointspacing of the dam and the joint grouting temperature.

The later chapters carry the design of the temperature control measures proper, the temperature control of concrete in particular parts of the dam, the requirements placed on construction, and the monitoring and analysis of temperature during the construction period, which is chapter 15.

The standard is accompanied by an Explanation of provisions (条文说明), a substantial companion section that restates each clause and gives the engineering reasoning, the test data and the project experience behind it. Several of the passages reproduced in this listing come from that section, and they are identified as such.

The document is written for concrete dams in hydropower projects. The dam height classes used throughout follow the two design codes cited in the explanation of clause 1.0.2: a dam lower than 50 m is a low dam, a dam between 50 m and 100 m is a medium dam, and a dam higher than 100 m is a high dam.

1 Scope

NB/T 35092-2017 is the Chinese design code for temperature control of concrete dams. It exists because of a problem peculiar to mass concrete: cement generates heat as it hydrates, and in a structure tens of metres thick that heat cannot escape. The interior of a freshly placed block rises tens of degrees above ambient over the following days, then cools slowly over months and years towards the stable temperature of the site. As it cools it wants to contract, and the surrounding rock and the already-hardened concrete below will not let it. The result is tension, and if the tension exceeds the tensile strength the dam cracks - not superficially but through the section, giving water a path through the structure and compromising the very thing the dam is for. Temperature control is therefore not a construction convenience but a design requirement, and it is designed before the first pour. This code sets out how. It covers the thermal properties of the concrete and the boundary conditions, the calculation of temperature fields and of the resulting thermal stresses, and then the control criteria themselves: the allowable foundation temperature difference within the restraint zone, the allowable internal and surface temperature differences, and the permissible rate of cooling. Around those it sets the measures used to meet them - the choice of cement and of mineral admixture to reduce the heat of hydration, precooling of the aggregate and of the mix water, the height and timing of lifts and the interval between them, embedded pipe cooling with its water temperature and flow rules, and surface insulation against cold waves - together with the joint spacing of the dam, the monitoring of temperature during construction, and the treatment of the situation when the criteria are exceeded. It was issued on 28 March 2017 by the National Energy Administration and took effect on 1 August 2017.

Chapter 1, General provisions, opens the standard at page 1 and sets out the purpose, the field of application and the relationship of this code with the other standards of the sector.

The explanation of clause 1.0.1 states the reason the code was written: the temperature control design of concrete is an important component of the design of a concrete dam, and at the time of drafting the hydropower industry had no unified standard laying down the principles, the contents, the methods and the technical requirements of that design. Preparing this code was therefore held to be of significant guiding value for the construction of hydropower projects.

The explanation of clause 1.0.2 defines the field of application through the dam height classes already fixed by two companion codes. The Design code for concrete gravity dams NB/T 35026 and the Design code for concrete arch dams DL/T 5346 provide that a dam height below 50 m is a low dam, a dam height from 50 m to 100 m is a medium dam, and a dam height above 100 m is a high dam.

That classification matters because the temperature control requirements of the code are graded: the taller the dam and the stronger the foundation restraint, the tighter the allowable temperature difference and the more elaborate the cooling arrangement that the design has to provide.

The general provisions establish that temperature control design is not an accessory to the structural design of the dam but part of it, to be carried through from the selection of the raw materials to the monitoring of the concrete during construction.

The chapter also positions the code within the hierarchy of Chinese hydraulic standards: where a subject is already covered by a specific code, such as concrete construction or the quality control of concrete, that code continues to apply and this one adds the temperature control requirements.

The general provisions are the shortest chapter of the standard, occupying page 1, but they govern the reading of everything that follows, in particular the distinction between requirements that apply to all dams and requirements that apply only above a stated dam height.

Chapter 2, Terms, follows at page 2 and gives the vocabulary used in the rest of the document.

2 Terms

Chapter 2 collects the terms of the code and their definitions, and runs from page 2 to page 4 of the standard.

The vocabulary of the chapter is the vocabulary used consistently in the following chapters: the steady, quasi-steady and transient temperature fields, the border temperature, the jointspacing of the dam, the joint grouting temperature, and the classes of concrete distinguished by placing method and by position in the dam body.

Because the code is applied together with the concrete construction standards of the sector, the terms of chapter 2 are drafted to be consistent with those used in the Hydraulic concrete construction specification DL/T 5144 and in the Roller compacted concrete construction specification for hydraulic structures DL/T 5112.

3 Selection of concrete raw materials

Chapter 3 runs from page 4 to page 6 and covers the selection of the raw materials of the concrete, in four clauses.

Clause 3.1, Cement, at page 4, covers the choice of the cement type and the properties that matter for temperature control, in particular the heat of hydration.

Clause 3.2, Admixture, at page 4, covers the mineral admixtures, of which fly ash is the one that recurs throughout the worked examples of the standard.

Clause 3.3, Aggregate, at page 5, covers the aggregate. The statistics reproduced in the explanation of chapter 4 show which aggregates the reference projects used: artificial limestone aggregate in most of them, artificial granite at Mianhuatan, artificial basalt at Dachaoshan, natural gravel sand at Shimenzi and at Longshou, and natural aggregate at Jinghong.

Clause 3.4, Additive and mixing water, at page 5, covers the chemical additives and the mixing water.

The selection of the raw materials is treated as the first step of temperature control design, because the heat generated by the binder and the thermal properties of the aggregate set the temperature rise that the later chapters then have to control.

4 Design of concrete and mix proportion

Chapter 4 runs from page 6 to page 8 and has two clauses: 4.1, Design of concrete, and 4.2, Design of concrete mix proportion.

The explanation of clause 4.1.1 states that the concrete shall be designed according to the working conditions of the dam and the climatic characteristics of the project area, and that the zoning design of the dam concrete shall consider a reasonable match between the concrete strength grade and the properties of impermeability, frost resistance, resistance to abrasion and erosion, corrosion resistance and carbonation resistance.

The explanation of clause 4.1.2 stresses that the design of the concrete shall consider whether the temperature control is actually achievable, and that a high concrete strength grade shall not be pursued for its own sake. A richer mix raises the heat of hydration and makes the temperature control harder, so strength and temperature control have to be balanced.

The explanation of clause 4.2.2 sets the lower limits on the cementitious material content by reference to the companion codes. The Hydraulic concrete construction specification DL/T 5144 provides that the cementitious material content of mass concrete in the interior of a structure should not be lower than 140 kg per cubic metre.

The same explanation records that the Roller compacted concrete construction specification for hydraulic structures DL/T 5112 provides that the cementitious material content of roller compacted concrete in permanent structures should not be lower than 130 kg per cubic metre, while the Design code for concrete gravity dams NB/T 35026 provides that the total cementitious material content of roller compacted concrete should not be lower than 140 kg per cubic metre.

Considering that most present projects adopt a long design age and that the concrete strength grade is not high, this code controls the cementitious material content at not lower than 140 kg per cubic metre.

The explanation of clause 4.2.3 addresses the air content: the air content of the concrete has a large influence on its strength and on its frost resistance, so an appropriate air content standard shall be adopted during mix proportion design, and the loss of air content during transport and placing shall be taken into account.

The explanation of clause 4.2.4 states that the method and the names used to classify concrete by slump follow the Standard for quality control of concrete GB 50164, whose grading is reproduced in Table 4-1 of the explanation.

Table 4-1, Classification of concrete slump, gives four classes: T1, low-plasticity concrete, slump 1 cm to 4 cm; T2, plastic concrete, slump 5 cm to 9 cm; T3, flowing concrete, slump 10 cm to 15 cm; and T4, high-flowing concrete, slump not less than 16 cm.

The same explanation records that dam concrete is mainly placed as low-plasticity and plastic concrete, and that for reinforced concrete with a reinforcement ratio not exceeding 1 percent the slump of 3 cm to 6 cm is obtained by interpolating between the slump values of classes T1 and T2 in Table 4-1.

For roller compacted concrete the explanation states that, on the strength of the large body of project practice of recent years, a field VC value of 3 s to 8 s is appropriate. Table 4-2 of the explanation collects the VC value statistics of a set of domestic roller compacted concrete projects.

Table 4-2 lists, for each project, the strength grade, the water to binder ratio, the type and proportion of admixture, the sand ratio, the aggregate type and the VC value. The projects listed are Yantan, Puding, Fenhe Second Reservoir, Jiangya, Mianhuatan, Shimenzi, Dachaoshan, Longshou, Puhekou, Suofengying, Jinghong, Zhaolaihe, Longtan, Guangzhao, Guanyinyan and Huangdeng.

The water to binder ratios in that table run from 0.41 to 0.6, the admixture proportions from 34 percent to 65.4 percent, and the sand ratios from 30 percent to 35.5 percent. The admixture codes used are F for fly ash, PT for phosphorus slag and tuff, MH for manganese iron slag and limestone powder, and H for power plant ash and slag powder.

The explanation of clause 4.2.5 reports the test research carried out on full-graded concrete by the United States Bureau of Reclamation and, in China, by the China Institute of Water Resources and Hydropower Research, the Changjiang River Scientific Research Institute, Hohai University, the Chengdu Engineering Corporation, Sinohydro Bureau 8 and the Kunming Engineering Corporation. The existing results show that the compressive strength of full-graded concrete is about 70 percent of that of wet-screened concrete tested on standard specimens.

Table 4-3 of the explanation, continued on page 88 of the standard, gives the ratio of the properties of full-graded concrete to those of wet-screened concrete measured at three projects: Xiluodu (Chengdu Institute), Maerdang (Nanjing Institute) and Goupitan (Changjiang Institute). The properties compared are compressive strength, splitting tensile strength, flexural strength, axial tensile strength, ultimate tensile value, creep, compressive elastic modulus and tensile elastic modulus.

At Xiluodu the ratios run from 0.82 to 1.05 for compressive strength, 0.54 to 0.73 for splitting tensile strength, 0.57 to 0.62 for flexural strength, 0.60 to 0.84 for axial tensile strength, 0.54 to 0.74 for the ultimate tensile value, 0.61 to 0.75 for creep, 1.08 to 1.20 for the compressive elastic modulus and 1.03 to 1.14 for the tensile elastic modulus.

At Maerdang the corresponding ranges are 0.66 to 0.78, 0.80 to 0.82, 0.43 to 0.55 for axial tensile strength, 0.59 to 0.62 for the ultimate tensile value, 1.00 to 1.14 for the compressive elastic modulus and 0.76 to 0.84 for the tensile elastic modulus. At Goupitan they are 0.96 to 1.05 for compressive strength, 0.60 to 0.75 for axial tensile strength, 0.47 to 0.65 for the ultimate tensile value, 0.69 to 0.75 for creep and 1.08 to 1.20 for the compressive elastic modulus.

5 Design data and calculating parameters of temperature control

Chapter 5 runs from page 8 to page 10 and collects the data and the parameters on which the whole temperature control calculation rests. It has three clauses.

Clause 5.1, Air temperature, water temperature and ground temperature, at page 8, covers the environmental data of the project site: the air temperature series, the water temperature of the reservoir and of the river, and the ground temperature.

Clause 5.2, Parameters of bedrock, at page 8, covers the thermal and mechanical parameters of the foundation rock, which govern the restraint the foundation exerts on the concrete placed against it.

Clause 5.3, Parameters of concrete performance, at page 9, covers the parameters of the concrete itself: the adiabatic temperature rise, the thermal conductivity, the coefficient of linear expansion, the elastic modulus, the creep and the ultimate tensile value.

The ratios of Table 4-3 reproduced in the previous chapter belong to this part of the design work as well: the parameters measured on wet-screened specimens have to be converted before they are used for full-graded dam concrete, and the compressive and tensile elastic moduli of full-graded concrete are higher than those of the wet-screened material, while its tensile strength and ultimate tensile value are lower.

6 Calculation of temperature field

Chapter 6 runs from page 10 to page 14 and covers the calculation of the temperature field of the dam. It has four clauses.

Clause 6.1, Determination of steady and quasi-steady temperature field border temperature, at page 10, fixes the boundary temperatures on which the two long term fields are computed.

Clause 6.2, Steady temperature field, at page 11, covers the field the dam reaches in the long run, which determines the final temperature of the body and therefore the closure temperature the design has to reach through cooling.

Clause 6.3, Quasi-steady temperature field, at page 12, covers the field that oscillates with the annual cycle of air and water temperature once the heat of hydration has dissipated.

Clause 6.4, Transient temperature field, at page 12, covers the field during construction, when the heat of hydration, the placing temperature, the lift thickness, the interval between lifts and the pipe cooling all act together.

The three fields are the basis of the temperature control criteria of the later chapters: the difference between the transient field during construction and the steady field fixes the amount of artificial cooling required, and the quasi-steady field fixes the surface protection requirements.

7 Jointspacing of dam and joint grouting temperature

Chapter 7 begins at page 14 and covers the spacing of the joints of the dam and the temperature at which the joints are grouted.

Clause 7.1, Jointspacing of dam, at page 14, covers the spacing of the transverse joints and, where they are used, of the longitudinal joints, which is the primary means of limiting the length and the width of a placing block and therefore the temperature stress it develops.

The explanation of the standard returns to this point when it discusses the concrete around openings in the dam body: the concrete on either side of an opening generally has a large length to width ratio, an unfavourable shape that tends to produce large temperature stresses along the length, and setting construction joints to reduce the length to width ratio of the placing block effectively reduces those stresses and improves the distribution of stress.

The joint grouting temperature is the design temperature to which the dam body has to be cooled before the joints are grouted; the pedestal concrete provisions of chapter 12 show how it is applied to a structure placed between the foundation and the arch dam proper.

12 Temperature control of concrete in particular parts of the dam

Chapter 12 of the standard covers the temperature control of the concrete placed in particular parts of the dam. The explanation of the chapter, at pages 125 to 127 of the standard, covers clause 12.5, concrete around openings; clause 12.6, concrete of the plugging bodies in the dam body; clause 12.7, pedestal concrete; clause 12.8, abrasion resistant concrete; and clause 12.9, concrete at closure joint positions.

The explanation of clause 12.5.1 states that the openings in the dam body are mainly the outlets provided for flood discharge, sediment flushing, water supply, water release and diversion, together with the intakes provided for power generation. In the shape design and the layout of those openings the designer shall consider not only the degree to which they affect the stress in the dam body, but also the stress state of the concrete around them under external temperature change. Where necessary, temperature reinforcement for the construction period shall be provided in the concrete around the openings.

The explanation of clause 12.5.2 states that the concrete around the openings is sensitive to temperature change, and that a long interruption of placing leaves the surface of the lift exposed for a long period, so that cracks appear relatively easily under the action of ambient air temperature change.

The same explanation adds that the concrete on the two sides of an opening generally has a large length to width ratio and an unfavourable shape, so that large temperature stresses arise along the length; setting construction joints to reduce the length to width ratio of the placing block effectively reduces the temperature stress and improves the distribution of stress.

To reduce the risk of cracking of the top concrete caused by the stress concentration when an opening is closed at the crown, the thickness of the placing lift that closes the opening is required to be not less than 1.5 m.

The explanation of clause 12.6.1 states that, when the allowable maximum temperature of the concrete of a plugging body is determined, the position of the plugging body in the dam body, the placing conditions, the method of delivery into the block, the concrete strength grade and the aggregate grading all have to be considered.

The explanation of clause 12.6.2 states that the concrete of a plugging body should be cooled in stages, and that the cooling process shall likewise follow the principle of small temperature differences, early cooling and slow cooling. At the Jinping I hydropower station the cooling rate of the plugging body of the diversion bottom outlet in the dam body was 0.6 degrees C per day in the early stage, 0.5 degrees C per day in the middle stage and 0.3 degrees C per day in the late stage.

The explanation of clause 12.7.1 describes the pedestal. For an arch dam, where the geological conditions of the bedrock are relatively poor, excavation and an enlarged foundation are adopted to raise the bearing capacity of the foundation. The pedestal is a concrete load transferring structure set between the dam body and the foundation, wider than the thickness of the dam body at the corresponding position. Whether the pedestal concrete is to be considered as foundation or as part of the dam body is not at present a settled question.

The same explanation notes that there are two common ways of jointing a pedestal. In the first the pedestal is treated as a large mass of backfill concrete and its joints are designed separately. In the second, the transverse joints of the pedestal are set in line with those of the dam body to satisfy the load transfer requirements of the arch dam structure, and whether longitudinal joints are provided is decided according to the concrete placing capacity and the temperature control conditions. Where the joints of the pedestal concrete and those of the dam body do not coincide, appropriate measures shall be taken at the offset positions to avoid the splitting caused by stress concentration at the joint ends.

The left bank concrete pedestal of the Jinping I hydropower station has a maximum length of nearly 100 m in the river direction and a dimension of about 60 m across the river, lies in the upper middle part of the left bank, and has a maximum unjointed block area of 4400 square metres. After comparing plumb vertical joints, offset joints and inclined joints, the scheme finally selected was a single inclined joint within the pedestal.

The explanation continues that the pedestal has a large contact area with the bedrock and a relatively complex shape, and that the foundation restraint is strong, so a three dimensional finite element method shall be used for the temperature stress analysis. The calculation model shall include at the same time the foundation bedrock, the pedestal concrete and the dam body concrete above it, so that the interaction of the three is accurately reflected.

The explanation of clause 12.7.2 states that, as the pedestal is the load transferring structure of the arch dam, a grouting system shall be provided at the contact surface between the pedestal and the arch dam, at the contact surface between the pedestal and the bedrock and at the joint surfaces of the pedestal, and grouting shall be carried out after cooling to the design temperature.

The same explanation adds that, when the pedestal concrete is treated as foundation, it shall be cooled to the design temperature before the dam concrete is placed, and that design temperature may be taken as the ground temperature at a depth of 3 m below the surface in the dam site area. When the pedestal concrete is treated as part of the dam body, the dam concrete and the pedestal concrete may be placed continuously, the stable temperature of the pedestal shall be taken as the stable temperature of the dam body at the corresponding position, and the joint surfaces of the pedestal concrete may be grouted at the same time as the joints of the dam body.

The explanation of clause 12.8.1 states that abrasion resistant concrete shall, besides meeting the requirement of the design strength grade, also have good resistance to abrasion, good volume stability and good constructability; in view of that special character, the clause emphasises the special requirements placed on the selection of the raw materials, that is the cement, the aggregate, the admixture and the chemical additives.

According to the relevant test research, under the same cement type and water to binder ratio, concrete into which polypropylene fibre, polyvinyl alcohol fibre, that is PVA fibre, or cellulose fibre has been mixed uses 2 kg to 3 kg more water per cubic metre than concrete without fibre, but its ultimate tensile value is higher than that of concrete without fibre. With PVA fibre in particular the ultimate tensile value is 10 percent to 34 percent higher than without, and the increase is more marked at early age. The abrasion resistance strength of fibre reinforced concrete is also improved to a certain degree.

The explanation of clause 12.8.2 states that abrasion resistant concrete placed over a large area generally has a high strength grade, a small thickness and a large area, is strongly restrained by the bedrock or by the underlying concrete, and is a typical embedded slab structure. The ratio of height to length has a marked influence on the stress distribution of the placing block: for thin abrasion resistant concrete over a large area, the temperature stress produced by a uniform temperature drop puts the whole central section of the placing block in tension. When the height to length ratio is 1 to 8, the entire central section is close to uniform tension and the foundation restraint coefficient is 0.962 to 0.997.

According to the relevant three dimensional finite element analyses, within the foundation restraint zone the longer the concrete placing block, the greater the temperature tensile stress produced; as the length increases the rate of increase of the stress gradually falls, but the extent of the high tensile stress zone expands and the risk of defects and cracks in the concrete grows. From the point of view of temperature stress control, therefore, controlling the size of the placing block is the most effective measure for reducing the foundation restraint on the concrete, controlling the temperature stress and reducing the risk of cracking. The long side of an ordinary concrete placing block should generally be controlled below 20 m.

The explanation of clause 12.8.3 states that where the strength grade of the abrasion resistant concrete differs greatly from that of the underlying concrete, the two should be placed in separate zones within the same lift and vibrated at the same time, so that the interface between the different concretes bonds well. In some projects the abrasion resistant concrete was not placed and vibrated together with the underlying concrete and the lift surface treatment was poor, and under the action of flowing water pressure the abrasion resistant concrete was lifted and destroyed over a large area.

The explanation of clause 12.8.4 states that surface running water is one of the effective measures for controlling the maximum temperature of abrasion resistant concrete, and that uninterrupted running water curing should be applied after final set. When the air temperature drops sharply, however, comprehensive protective measures shall be taken to prevent surface cracks, and the surface protection material used shall provide both moisture retaining curing and thermal insulation; a sealing hardener may be considered.

The explanation of clauses 12.9.1 and 12.9.2 states that the stress concentration at the end of a joint easily produces cracks, and that to prevent the joint surface from extending upward, appropriate temperature control measures shall be added on top of the structural measures and the closure of the joint end.

14 Requirements for construction

Chapter 14 of the standard covers the temperature control requirements placed on construction. Its clauses include 14.2, Raw materials and mixing, at page 132 of the standard, and 14.3, Transport and placing, also at page 132.

Clause 14.2 covers the handling of the raw materials and the mixing operation: the precooling of aggregate and mixing water, the use of ice, and the control of the temperature of the mix at the outlet of the mixing plant.

Clause 14.3 covers transport and placing: the loss of temperature and of air content between the mixing plant and the block, the temperature of the concrete on delivery into the block, and the placing temperature.

These construction clauses are the point at which the temperature control design becomes an instruction to the contractor, and they are the counterpart of the monitoring requirements of chapter 15.

15 Temperature monitoring and analysis during the construction period

Chapter 15, Temperature monitoring and analysis during the construction period, begins at page 134 of the standard and closes the document. It has seven clauses.

Clause 15.1, General requirements, at page 134, sets out the general monitoring requirements for the construction period.

Clause 15.2, Monitoring of raw material temperature, at page 134, covers the measurement of the temperature of the aggregate, the cement, the admixture and the mixing water.

Clause 15.3, at page 134, covers the monitoring of the concrete temperature at the outlet of the mixing plant, of the temperature on delivery into the block and of the placing temperature, which are the three checkpoints of the delivery chain.

Clause 15.4, Monitoring of the internal temperature of the concrete, at page 134, covers the instruments embedded in the dam body and the readings taken from them.

Clause 15.5, Monitoring of pipe cooling, at page 135, covers the monitoring of the water cooling system: the water temperature, the flow rate and the cooling rate achieved.

Clause 15.6, Monitoring of the air temperature in the placing block and of the temperature of the insulation layer, at page 135, covers the ambient conditions of the block and the performance of the surface protection.

Clause 15.7, Data analysis and feedback, at page 135, closes the standard: the measured data are analysed and fed back into the temperature control design, so that the cooling schedule and the surface protection can be adjusted while construction is still under way.