2.4 Summary
3.1.3 Production scheme
The concretes were produced in the laboratory in accordance with the mixing and pro- cessing regime described in Table 3.4.
Table 3.4: Procedures followed in the production of the concretes
Procedure Time
1st Addition of aggregates Mixing 45 s
2nd Addition of cement Mixing 45 s
3rd Adittion of water Mixing 90 s
4th Addition of additive Mixing 60 s
5th Casting of concrete in wall formworks Compacting
6th Casting of concrete in cylindric molds Compacting 2 x 10 s 7th Wrapping of the concrete elements
with jute sacking Curing 90 d
Production step
3.1 Concrete samples
The preparation of the concretes was performed using a pan mixer with a nominal ca- pacity of 250 dm3. After mixing the aggregates, cement was added followed by water and finally the additive. The concretes were gradually placed in layers in formworks and molds and compacted with hydraulically driven vibrators. Concrete walls were cast using 1200 mm x 1000 mm x 200 mm formworks and concrete cylinders by means of standardized molds (Diameter/Height = 150/300 mm).
During 90 days following the production of the concrete elements, the concrete walls and cylinders were stored at 20 °C, covered with jute sacks and kept wet to avoid drying of the edges. This storage condition allowed the concrete elements to reach high levels of hydration before the production of the samples could continue. In total, three concrete walls and 30 concrete cylinders were made with the mixture MRC and one wall and 15 cylinders were made with each one of the mixtures MLC and MHC. After curing, up to 99 cylindrical cores with a diameter of 75 mm were drilled out of each concrete wall and 1 cylindrical core with the same diameter was taken from each cylinder. Then, sections from the top and bottom of each core were cut off to set a desired sample height of 150 mm, and subsequently, the top and bottom faces of the specimens were sanded to ensure parallel alignment between the surfaces. Based on these cylindrical specimens (D/L = 75/150 mm), the concrete samples were prepared according to the geometries illustrated in Fig. 3.2. The samples were classified in 4 types meeting different purposes as follows:
• Type 1: For the studies of compressive strength, modulus of elasticity and defor- mation behaviour of the concretes, samples with a diameter of 75 mm and a height of 150 mm were used.
• Type 2: Tensile strength of the concrete was measured in samples with a 10 mm deep notch. These samples were notched because of two reasons: in first place, reducing the effective area of the samples facilitates the execution of the tensile strength tests as it ensures the failure of the concrete samples in the notched section and not in the vicinity of the top and bottom surfaces glued to the testing machine, which according to DAfStb booklet 422 [N13], would invalidate the measured value. In second place, as reported by Bonzel and Kadle˘cek [26], micro cracks formation due to drying influences the results of the tensile strength tests signifi- cantly. This influence can be attenuated if the moisture gradient within the notch section is reduced, which was accomplished by sealing the notch (see Fig. 3.2).
• Type 3: The development in time of the relative humidity and temperature within the concrete pores was registered in the samples taken from the cylindrical molds by means of relative humidity sensors. In order to place the sensors in the desired positions within the samples, a positioning device was developed to fix 4 stainless steel tubes in the cylindrical molds. During casting, rounded steel bars were intro- duced through the steel tubes and pushed 10 mm into the concrete to prevent the cement paste from entering the tubes. Afterwards, the bars were removed leaving
an uncovered part of the hole and the steel tubes empty, which allowed the fur- ther ingress of the sensors (see Sample type 3 in Fig. 3.2). With the help of this positioning device, one steel tube was placed in the central axis of the sample to a depth of 75 mm and three more were placed 10, 20 and 30 mm separated from the central axis in a 3 x 120° arrangement (see also Fig. 3.6). The depths of the latter were set to 30 mm to avoid any interference on the measurements between the holes.
• Type 4: The water content of the concretes was periodically measured by weight- ing small segments with a height of approximately 35 mm. At the end of the test phase, all these samples were dried at 105 °C and the dry weights of reference of the concrete mixtures were determined.
Finally, the upper and bottom faces of all samples were sealed with aluminium cored polypropylene films forcing the moisture exchange between the cylindrical samples and the environment to take place only through the lateral surfaces. In this way, the moisture distribution within a given sample can be assumed to be equal in every position along the axis of symmetry.
150
75 2
10
75
150
75
35
75 150
3x30 75
Type 1:
Compressive strength Modulus of elasticity Creep and shrinkage
Type 2:
Tensile strength Type 3:
Relative humidity Temperature
Type 4:
Water content 3x10
10
uncovered hole steel tube sealed
sealed
Figure 3.2: Geometry of the prepared samples (dimensions in mm)
After preparation, the samples were stored above water in sealed tubs at 20 °C, which guaranties an environment with relative humidity close to 100 % and constitutes a stan- dardized alternative for storing concrete samples according to DIN EN 12390-2 [N7].
The samples were kept in the tubs for over 150 days before the conditioning by 20 °C started (see Chapter 3.2.1). A time scheme of the samples production and further storage is presented in Fig. 3.3.