MEASUREMENT OF TEMPERATURE PROFILE IN MASS CONCRETE Yousif Hummaida Ahmed 1 and Aya Abdalla Khogali Ahmed 2 1
Civil Eng. Dept, Faculty of Engineering, University of Khartoum,
[email protected] 2
MSc. In Structural Engineering (University of Khartoum),
[email protected] ُمـسْــت َْخـلـَص
,( نتٌجة لتمٌه األسمنت وكذلك الحرارة المحٌطة00C˚ ٌمكن لدرجة الحرارة داخل الخرسانة ان تصل الى مستوٌات عالٌة (اعلى من وٌكون هذا التأثٌر أكبر خاصة فً حالة الخرسانة الكتلٌة والمناخات الحارة حٌث معدل.األمر الذي له تأتٌر ضار على خواص الخرسانة هذه الورلة البحثٌة تمثل تجربة لٌاس لدرجات الحرارة فً ستة اعماق مختلفة داخل خرسانة كتلٌة ذات محتوي.تبدد الحرارة منخفض فً االعمار األولى للخرسانة (بعد الصب مباشرة ولمدة سبع اٌام) اثناء فصل الشتاء وذلك باستخدام اسالك مزدوجة3م/ كجم303 ًاسمنت من الخرسانة داخل لالب معزول حرارٌا من3 م0.3 ولمحاكاة عنصر داخل خرسانة كتلٌة تم صب.)Thermo-couple( حرارٌة ً بلغت ألص. سجلت درجات الحرارة وتم عرضها بشكل مخطط بٌانً بالنسبة للزمن.)م1.2*0.3*0.3( الخارج وله شكل مستطٌل بأبعاد وبلغ الصى فرق فً درجات الحرارة بٌن. ساعة من الصب21 ً فً منتصف العمك بعد حوالC˚63.3 درجة حرارة داخل الخرسانة . ساعة من الصب12 بعدC˚21 الخرسانة الداخلٌة والخارجٌة
ABSTRACT Temperature inside concrete resulting from cement hydration and the ambient temperature can reach high level (above 70 oC) which has a detrimental effect on concrete properties. This effect is manifested especially in mass concrete and hot climate that has low rate of heat dissipation. This paper presents experimental measuring of temperature profile of mass concrete with cement content (375 kg/m 3) at early age during winter season, i.e. right after placement of concrete and up to 7 days using thermocouple wires at six different depths. To simulate an element inside a mass concrete, 0.3 m 3 of concrete was cast into a rectangular mould with dimension of (0.5*0.5*1.2)m that was thermally insulated from outside. Temperature readings were presented in graphical form with respect to time. The maximum peak temperature (63.5oC) reached at the centre of the mass concrete after almost 21 hours after casting. Temperature gradient between exterior and interior concrete reached max value of 21oC after 12 hours from casting. It recommended repeating this experiment during summer time. Keywords: Temperature profile, Mass concrete, Delayed ettringite formation (DEF), Thermal gradients in concrete.
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1. Introduction In common with many chemical reactions, the hydration of cement compounds is exothermatic. The temperature at which hydration occurs greatly affect the rate of heat development. For Portland cements, about one-half of the total heat is liberated between one and three days, about three-quarters in 7 days, and nearly 90 per cent in six months. In fact, the heat of hydration depends on the chemical composition of the cement [1]. In normal concrete construction, the heat developed by the hydrating cementations materials is quickly dissipated to the environment and this usually poses no serious problems. However, in large or mass concrete elements, thermally induced early-age cracking is a problem requiring special attention at the design and construction stages. During the early stages of hydration, when the internal temperature of the immature concrete is increasing, the cooler surface zone is subjected to tensile stresses and surface cracks, usually fairly shallow, may occur within a few days after casting. At later ages, after the peak temperature has been reached and the internal concrete enters the cooling phase, the increased stiffness of the surface zone now acts as a restraint to the thermal shrinkage of the internal concrete. Internal sections are, therefore, subjected to tensile stresses and significant internal cracking is possible [2]. So, an accurate and thorough understanding of the temperature differences occurring in the concrete at different times (as the physical properties change and evolve) is essential for a proper analysis of cracking potential. The ambient weather has a great effect on concrete temperature beside heat of hydration, it necessary to limit the temperature differential or gradient within concrete. If cracking to be avoided. The maximum temperature differential between the interior and exterior concrete should not exceed 20°C to avoid crack development [1]. Also, At higher temperatures, the solubility of gypsum is decreased so that some of it might not react with C3A and do so only later, causing an expansive reaction of the type known as sulfate 20
attack [3] which is known as Delayed Ettringite Formation (DEF). It has been proposed that, ‘‘primary’’ ettringite, formed at early age is destroyed by high-temperatures (>70°C)[4-6]. Therefore, ettringite develops again at later ages in concrete structures if exposed to water (either intermittently or permanently) will cause expansion or cracking. Higher temperature during and following the initial contact between cement and water reduce the length of the dormant period so that the overall structure of the hydrated cement paste becomes established very early. Although a higher temperature during placing and setting increases the very early strength, it may adversely affect the strength from about 7 days onwards. The explanation is that a rapid initial hydration appears to form products of a poorer physical structure, probably more porous, so that a proportion of the pores will always remain unfilled. It follows from the gel/space ratio rule, this will lead to a lower strength compared with a less porous, though slowly hydrating, cement paste in which a high gel/space ratio will eventually be reached [7]. More temperature effects in concrete are mentioned in state of the art section. In concrete members with high cement content, as is the case with high performance concrete, there is a considerable temperature rise even in ordinary structural elements such as beams and columns. The temperature and moisture content in concrete are major factors influencing concrete carbonation, chloride diffusion and steel corrosion rate in the concrete [8]. The behavior of structures is often dependent on concrete modulus of elasticity, which is strongly affected by temperature that is reduced at temperatures in excess of 121 °C [9]. As a result, there is a need to measure temperature inside concrete in the climate of Sudan and determine to what extend temperature will increase and whether it exceeds the 70 °C for thick section at high cement content of 375 kg/m3 or higher. The objective of this
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paper is to satisfy this need especially investigating whether a typical grade 40 concrete mix used by Sudanese construction industry will exceed the 70oC. Therefore we simulated an inner core of mass concrete by designing and fabricating an insulated timber mould and measured mass concrete temperature profile for seven days. This was carried out at six different depths spaced 20 cm using thermo-couple wires (TCW) and data logger program. This paper presents graphical presentation of temperature readings with respect to time and depths. 2. Materials and methods 2.1. Apparatus fabrication 2.1.1. Mould fabrication To measure the temperature inside an early age mass concrete two rectangular moulds of 80mm thickness timber boards were fabricated (Figure 1). The smallest one has dimensions of (0.5*0.5*1.2)m and the larger has dimensions of (0.7*0.7*1.2)m, the smallest one was strengthened by thick timber strips wrapped outside at four equal intervals of height to prevent the sides of the box from opening under concrete pressure during casting. Internal faces were covered with water proof membrane to prevent the box sides from absorbing the mixing water from concrete.
The smallest box was placed inside the largest box leaving about 10cm gap between the two moulds. This gap was partially filled with white plastic polystyrene as shown in Figure1. However, at the top four cm of the mould the entire gap was completely filled with polystyrene and all that space was insulated by plastic sheets all around the mould top to prevent any air entrance. All these measures were carried out to provide thermal insulation to simulate an element inside mass concrete. The mould top face was left open and the bottom side rested on a 1.6cm thick timber plate. The entire assembly of mould was rested on concrete floor of laboratory..
Figure2: Locations of thermo-couple wires
2.1.2. Preparation (TCW)
of
thermo-couple
wires
Thermo-couple wires (TCW) (type K) were used to measure the temperature deep inside the concrete at six different depths spaced 20 cm at the central axes (Figure 2). An additional wire was used to determine the ambient temperature.
Figure1: Box plan illustrating thermal insulation
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TCWs were connected to a multi-meter device that was connected to a data logger computer software program (LAB VIEW PROGRAM version 8.5 supplied by Finn Haugen Tech Teach Company) to record and save continuous temperature readings inside concrete as shown in Figure 3.
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Table 1: Mix Proportion of Concrete Grade 40.
Figure 3: Final setup of experimental apparatus showing data logger. The TCWs were calibrated using hot and cold water with predetermined temperatures. In order to measure the temperature exactly in the central axes of the mass concrete, a thin timber pole was fixed between the bottom and top faces of the box. The TWCs were fixed at different depths along the pole as shown in Figure 4 Illustrations
Figure 4: Fixing TCWs by timber pole at the centre of box with 20 cm space between each adjacent ones.
2.2.
Concrete mix proportion
A grade 40 concrete mix was prepared with proportions listed in Table 1. This mix which contains a high range water reducer & setretarder admixture (SP901), i.e. Type G, is considered a typical mix used by the Sudanese concrete suppliers for grade 40. 22
Constituents (units)
Amount (kg) per m3
Cement (Mass brand, Alshamal cement company) ( kg)
375
Water (kg)
180
Coarse aggregate (kg)
1105
Fine aggregate (kg)
740
SP901 (Type G) (litre/100 kg of cement) of 1.1specific gravity
2.813
Total density (kg)
2402.813
The concrete constituent temperature (except cement and SP) was raised to about 46oC for sand and 42oC for gravel by exposure to sun rays for six hours. The water was heated up to 31oC by an electrical heater to simulate hot weather concreting. The constituents were mixed in four equal batches due to capacity of the lab mixer. After five minutes mixing each batch, the initial wet concrete temperature was measured and recorded. Then the concrete was poured into the mould and compacted using a wooden rod. The rod has a square x-section of 5cm*5cm. After the 3rd batch was poured, the mould sides stared to widen due to high lateral concrete pressure, this was immediately rectified by strapping wooden bars around the mould using steel wires as ties as shown in Figure 5.Therefore, in the last batch, the SP901 admixture dosage was increased to provide self-compactability to avoid vibration by the wooden rod. The time intervals of casting and initial temperature at time of pouring parts of each batch were recorded in Table 2. After 21hrs from concrete casting, curing compound Antisol-WB (Sika Company) was applied to the top surface to prevent evaporation of mixing water, subsequently, preventing concrete plastic shrinkage cracking, but not thermal cracking. The temperature at each depth was recorded immediately after concrete casting at intervals of 45 seconds until 7 days. Temperature readings at each wire depths yielded huge data, so the hourly
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average readings were calculated and presented in graphs (Figures 6-13). Table 2: Time of casting and initial temperature of wet concrete for each batch Pouring Mix Batch Part 1st batch 2nd batch 3rd batch 4th batch
1st part 2nd part 1st part 2nd part 1st part 2nd part 1st part 2nd part
Time Of Casting 12:28 PM 12:32 PM 12:39 PM 12:41 PM 12:48 PM 12:50 PM 1:15PM 1:17 PM
Initial wet concrete Temperature ˚ ( C) 29 30 30 29 30 32 33 31
it reached its peak temperature due exothermal reaction of cement hydration. While, for wire located at T4 (Figure 9), the temperature did not increase until six hours after casting, this may be due to exposure of this depth to the ambient temperature caused by delaying cast of the successive batch to stop the box widening. This is consider the main reason for delaying hydration of cement, after that the temperature increased up to its peak value after 21hrs from casting. Figures 10 and 11 show that TCWs depths T5 and T6 decreased right after the concrete placement and lasted for about 8 hours. This decrease in temperature was expected due to the increased dosage of the Type G super-plasticizer (SP 901) that delayed the cement hydration and consequently dropped its temperature. After that the temperature increased until it reached its peak after 22 hrs for T5 and 26hrs for T6. It is noted that the reading in T6 exhibited a lower rate of temperature increase. This may be resulted from ambient temperature effect at this depth which is close to the top surface that permitted heat dissipation.
Figure 5: Restraining the movement of formwork by tying the outside box.
3.
Results and discussion
Hourly average temperature readings at all TCWs depths with respect to time were presented in in Figures 6-12. They increased at a higher rate until reaching their peak values after one day approximately. Then after that they decreased at a slower rate until reaching thermal equilibrium at all wires. Both values and times of occurrence of the peak temperature at each location of TCWs are illustrated in Figure 13. For TCWs depths T1, T2 and T3 (Figures6-8), one can notice that, the heating period begins right after the concrete placement with high incremental rate, and lasts for about 21hrs when Sudan Engineering Society Journal
As shown in Figure 12, the heating period at all locations ended by reaching their peak temperatures after about 21hrs, and then after cooled slowly. After 170 hrs tentative thermal equilibrium was reached between all locations inside the concrete and the ambient temperature (4oC different). Also it is worth noting that to predict temperature at any certain time, the profile in Figure 12 can be used. TCWs temperature readings agreed well with Ref.[10], despite they used concrete with lower cement content (200 kg/m3) and smaller section (0.60*0.30*0.60) m3. In their experimental setup all depths took about one day to reach the peak temperature of 64oC that occurred at the bottom of mould because of thermal insulation at the bottom face. Also, after 160h thermal equilibrium was reached between the concrete element and the climatic chamber environment 20oC. In our experiment all depths took more time to reach thermal equilibrium due to higher temperature in our laboratory and the smaller area of the 2016, Volume 62; No.2
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exposed surface. It is obvious from Figures 6-11, that the curves of temperature versus time are not smooth due to some missing data occurred during mid night when the data logger program crashed. Figure 12 shows that the maximum temperature gradient (difference) reached was 21oC between
internal location T3 and exterior location T6 after 12 h from casting, this is more than 20oC at which thermal cracks can be happen [1]. Indeed hair cracks appeared at that time and due to the short duration of that critical gradient the no deep thermal cracks were observed.
70 T1 ambient
Temperature (˚C)
60 50 40 30 20 10 0 0
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 Time after casting (hr)
Figure 6: Hourly temperature at bottom (T1). 70
Temperature (˚C)
60
T2 ambient
50 40 30 20 10 0
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 Time after casting in (hr)
Figure 7: Hourly temperature at 20 cm above bottom (T2).
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2016, Volume 62; No.2
Yousif Hummaida Ahmed and Aya Abdalla Khogali Ahmed
70
Temperature (˚C)
60
T3 ambient
50 40
30 20 10
0 0
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 Time after casting in (hr)
Figure 8: Hourly temperature at 40 cm above bottom (T3). 70
Temperature (˚C)
60 50
T4
40
ambient
30 20 10 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 Time after casting in (hr)
Figure 9: Hourly temperature at 60 cm above bottom (at centre) (T4). 70
Temperature (˚C)
60
T5 ambient
50 40 30 20 10 0 0
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 Time after casting in (hr)
Figure 10: Hourly temperature at 80 cm above bottom (T5). Sudan Engineering Society Journal
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70
Temperature (˚C)
60 T6 ambient
50 40 30 20 10 0
0 10 20 30 40 50 60 70 80 90 100 110 120130 140 150 160 170 Time after casting in (hr)
Figure 11: Hourly temperature at 100 cm above bottom (T6 70 T1 T3 T5 Ambient
50 40
T2 T4 T6
30
20 10 0 0
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 Time after casting in (hr)
Figure 12: Hourly temperature profile at T1 to T6 locations inside concrete after 26 hrs
T 6
53.5˚C after22hrs
T 5 TCWs location
Temperature (˚C)
60
60.9˚ C
after 21 hrs 63.5 ˚C
T 4
62.4˚C
T 3
after21hrs 58.9˚C
T 2
after21hrs 54.6˚C
T 1 52
54
after 21hrs 56 58 60 Peak temperature (˚C)
62
64
Figure 13: Values and times of occurrence of peak temperature . 26
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4.
temperature profiles in mass concrete, Cement & Concrete Composites, vol. 26 no. 6,pp. 695–703.
Conclusion
Temperature inside the cast concrete (grade 40) reached the maximum value of 63.5oC at the centre of concrete after 21 hours from casting. This value is below the critical 70oC that will destroy the primary ettringite and cause delayed ettringite formation (DEF).
[3]. Dodson, V 1990,Concrete Admixtures, Van Nostrand Reinhold 211 pp, New York. [4]. Taylor, H.W.F. 1997, Cement chemistry. 2nd ed. Thomas Telford Publishing, London.
This paper presented an experimental work that was performed during winter season and using high range water reducing and retarding admixture (Type G). The peak temperature is below the critical 70oC, this may be due to the presence of Type G admixture. We recommend repeating this experiment during summer season when ambient temperature can reach more than 47oC in Khartoum, Sudan. Also if a mix contains higher cement content, i.e. > 375kg/m3 and without adding a retarding admixture, the outcome may be worse. At such climatic condition we expect that temperature inside concrete can reach 70oC and could increase the hazard of DEF.
[5]. Lawrence, C. 1998. Physiochemical and mechanical properties of Portland cements In: Hewlett PC, editor. Leas chemistry of cement and concrete, 4th ed. Arnold Publisher. [6]. Heinz, D & Ludwig, U, 1987, Mechanism of secondary ettringite formation in mortars and concretes subjected to heat treatment. Concrete Durability––Katharine and Bryant Mather International Symposium SP-100, ACI, Farmington Hills, p. 59–71. [7]. Neville, A M 2011, Properties of concrete, 5thedn, Pearson PLC, Harlow. [8]. Yuan, Y& Jiang, J2011, Prediction of temperature response in concrete in a natural climate environment, Construction and Building Materials, vol. 25, no. 8 3159– 3167
References: [1]. Neville, A & Brooks, J 2010,Concrete technology, 2ndedition, Pearson PLC, Harlow. [2]. YunusB, 2004, A numerical model and associated calorimeter for predicting
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