Proceedings of the International rilem conference Materials, Systems and Structures in Civil Engineering 2016

41

International RILEM Conference on Materials, Systems and Structures in Civil Engineering 

Conference segment on Service Life of Cement-Based Materials and Structures 

22-24 August 2016, Technical University of Denmark, Lyngby, Denmark 

 

addition of PP-fibres leads to a higher water demand and increases the risk of bleeding. Thus, 

a special mix design is required. A high content of fly ash is added to ensure the stability of 

the paste. A low heat slag cement (CEM III/A 32.5 N-LH/NA) is chosen to keep the heat 

evolution as low as possible and thus to reduce restraint stresses at early ages. 

 

Table 1: Concrete mix 

Cement CEM III/A 32,5 N-LH/NA  [kg/m³] 

340 

Fly ash 

[kg/m³] 

135 

Gravel 0/16 

[kg/m³] 

1617 

Water [kg/m³] 

182 

Superplasticizer [kg/m³] 

2,7 

PP-fibres [kg/m³] 

2,0 

w/c-ratio (k

FA

 = 0,4) 

[-] 

0,46 

 

2.2  Heat of hydration 

The heat of hydration of the concrete was measured using an adiabatic calorimeter. The 

results are shown in fig. 2. The low heat slag cement in combination with the high amount of 

fly ash leads to a moderate heat release. 

 

 

Fig. 2: Heat of hydration 

 

To take into account the effect of temperature on the reaction rate, a maturity approach based 

on the Arrhenius formula is used [12]. The real concrete age t is transformed into the 

equivalent age 

 using the relation 

 

(1)

in which   is the activation energy and   is the ideal gas constant (  = 8,314 J/(mol K)). The 

activation energy mainly depends on the cement type. A constant value of   = 40 kJ/mol is 

assumed for the calculations. The equivalent age 

 defines the time that is needed at 20°C to 

reach the same hydration degree as under the given temperature history 

. 

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42

International RILEM Conference on Materials, Systems and Structures in Civil Engineering 

Conference segment on Service Life of Cement-Based Materials and Structures 

22-24 August 2016, Technical University of Denmark, Lyngby, Denmark 

 

 

2.3  Mechanical short term properties 

The compressive strength, the tensile strength and the Young’s modulus in tension have been 

tested at ages of 1, 2, 3, 4, 7, 14 and 28 days. The compressive tests were performed on cubes 

with an edge length of 150 mm. The tensile strength and the Young’s modulus in tension have 

been tested on cylindrical specimens (d = 80 mm, h = 300 mm) in direct tensile tests. Fig. 3 

shows the test results.  

The combination of slag cement and fly ash leads to a relatively slow evolution of the 

compressive strength which still shows a significant increase for ages greater than 14 d. The 

tensile strength and the Young’s modulus evolve quicker and reach their final value earlier. 

 

 

 

 

Fig 3: Evolution of compressive strength, tensile strength and Young’s modulus 

 

The calculation of restraint stresses requires a continuous description of the evolution of the 

mechanical properties. Therefore the exponential function 

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43

International RILEM Conference on Materials, Systems and Structures in Civil Engineering 

Conference segment on Service Life of Cement-Based Materials and Structures 

22-24 August 2016, Technical University of Denmark, Lyngby, Denmark 

 

 

(2)

is fitted to the experimental results, see fig. 2. The corresponding model parameters are listed 

in table 2. 

 

Table 2: Model parameters for the description of mechanical short term properties 

 

f  [N/mm²] 

A [-] 

B [-] 

t

k

 [d] 

compressive 

strength 

64.9 -3.1 -0.65 1.0 

tensile 

strength 

2.25 -6.7 -2.64 1.0 

Young’s modulus 

31500 

-2.0 

-1.42 

1.0 

 

2.4 Autogenous shrinkage 

The autogenous shrinkage has been tested on horizontal specimens with a cross section of 

100 mm x 60 mm and a length of 1000 mm. The specimens can move totally free on a layer 

of neoprene and are sealed with PE-foil and a metallic mould against moisture loss. The 

shrinkage strain is measured at the ends of the specimens with a highly sensitive LVDT. 

The strain evolution shows an intense swelling in the first 1.5 days followed by a continuous 

shrinkage, see figure 4. The swelling at the beginning compensates a part of the subsequent 

shrinkage, but for the evolution of stresses the shrinkage becomes more important, because 

the Young’s modulus is significantly higher in this phase. 

 

 

Figure 4: Autogenous shrinkage 

 

2.5 Viscoelasticity 

Early age concrete shows an intense viscoelastic behaviour that decreases continuously with 

the progression of the hydration process [2,13,14]. The viscoelastic behaviour greatly 

influences the evolution of stresses in restrained construction parts. Because no experimental 

results are available for the concrete used here, assumptions were made to take into account 

the viscoelastic behaviour for the calculation of stresses. The data from [14] was used as a 

reference because it describes the creep behaviour of a concrete with the same cement type 

and a similar strength evolution. The double power law (DPL) [15] is used to define the creep 

coefficient: 

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