SYNTHETIC MACRO-FIBERS REINFORCED SELF COMPACTING CONCRETE FOR LIGHTWEIGHT PRECAST ELEMENTS. A CASE STUDY

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1 SYNTHETIC MACRO-FIBERS REINFORCED SELF COMPACTING CONCRETE FOR LIGHTWEIGHT PRECAST ELEMENTS. A CASE STUDY Jean Philippe Bigas (1), Bruno Pellerin (1), Fabien Deschryver (1), Pietro Massinari (2) and Giovanni Plizzari (3) (1) CHRYSO, Sermaises, France (2) CHRYSO Italia S.P.A., Lallio, Italy (3) Università degli Studi di Brescia, Italy Abstract In the last decade Self Compacting Concrete has demonstrated an overwhelming development. More recently synthetic macrofibers have been introduced on the market. Those fibers constitute an optimal solution to reinforce concrete elements. Combination of both SCC and synthetic macrofibers allows to increase benefits of using SCC, especially in terms of labor work savings while at the same time achieving equal or even better mechanical performances. This study was performed in order to clearly established the benefits of such a combination in the fabrication of lightweight precast wall elements. Those lightweight wall elements are constituted of polystyrene insulation blocks embedded between two concrete layers. The paper describes the four steps process being used in order to replace existing reinforcement in the current panels by synthetic macrofibers: Step 1: Selection and characterization of the different macrofibers. Optimization of reference SCC mix design and characterization of mechanical performances. Step 2: At the lab scale, characterization of the mechanical performances (tensile strength, elastic modulus) of the different SCC macrofibers combinations in order to select the optimal macrofiber in terms of nature and dosage. Step 3: Dimensioning calculation of the lightweight precast elements. Step 4: Test of the best combination at real scale (11 m x 2.5 m x 0.2 m) lightweight panels and complete characterization of their mechanical performances. 1. INTRODUCTION Since their introduction in the precast industry, Self Compacting Concrete have been widely used. Such a success can be explained by increased productivity resulting from time savings at the pouring stage. Increased productivity is also coming from the improved facing aspects of concrete elements thus reducing the need for aesthetic repair. 1013

2 Fiber Reinforced Concrete is also gaining more and more interest[1] in structural type of applications, because of enhanced properties in terms of durability[2]. Precast industry has been rapidly interested in the Fiber Reinforced Concrete technology in order to optimize manufacture process and enhance at the same time concrete elements overall properties. Recently synthetic macrofibers have been introduced on the market. Such fibers can improve properties of hardened concrete. From a dosage of 2 kg/m 3 those fibers enhance both tensile and flexural strength of concrete. Shear strength at constant load and ductility of the material can thus be improved. An especially interesting application is the manufacture of concrete precast panels. Aim of this work was to investigate the possibility of replacing in such elements the traditional surface reinforcement with synthetic fibers incorporated into the concrete matrix in order to combine properties of both Self Compacting Concrete and Fiber Reinforced Concrete. After a complete laboratory study in order to optimize the concrete mix design and to assess the structural overall properties of the sample, a full scale trial was conducted. 2. RAW MATERIAL CHARACTERIZATION Two C30/37 class SCC concrete were tested in this study. First one is a typical SCC mix, while second one is a lightweight SCC mix. Concrete mix proportioning are displayed in table 1. Table 1: Mix proportions of SCC Mix design C30/37 SCC Gravel 6-12 mm 920 kg/m 3 Sand 0-6 mm 700 kg/m 3 Sand 0-3 mm 220 kg/m 3 CEM II A/LL 42.5R 320 kg/m 3 Limestone Filler 20 kg/m 3 HRWRA CHRYSO Fluid Premia l/m 3 Water 180 litre Density 2364 kg/m 3 Slump Flow 700 mm Mix design C30/37 lightweight SCC Gravel 6-12 mm 160 kg/m 3 Expanded Clay 0-8 mm 260 kg/m 3 Sand 0-6 mm 380 kg/m 3 Sand 0-3 mm 260 kg/m 3 CEM II A/LL 42.5R 400 kg/m 3 Limestone Filler 150 kg/m 3 HRWRA CHRYSO Fluid Premia l/m 3 Water 220 litre Density 1835 kg/m 3 Slump Flow 650 mm Three different fibers are commercially available, CHRYSO Fibre S50, S40 and S25, they mainly differ by their length. Geometrical and mechanical properties of the studied fibers are presented in table

3 Table 2: Physical characteristics of macrofibers CHRYSO Fibre S50 CHRYSO Fibre S40 CHRYSO Fibre S25 Length (L f ) 50 mm 40 mm 25 mm Equivalent diameter (φ f ) 1.0 mm 1.0 mm 1.0 mm Aspect ratio (L f /φ f ) Tensile strength 650 MPa Density 920 kg/m 3 Melting point 160 C Elastic modulus 5.0 GPa 3. LABORATORY OPTIMIZATION The two SCC concrete were reinforced by various combinations of CHRYSO Fibre S40 and CHRYSO Fibre S25 fibers between 3 and 5 kg/m 3. Their length being too large for the specific application, CHRYSO Fibre S50 were not used. Seven concrete mixes were designed, four standard fiber reinforced SCC, two lightweight fiber reinforced SCC were compared to the plain Self Compacting Concrete (Mix-1). The relative fiber dosage and nature is presented in table 3. Table 3 : Fiber composition of concrete mixes and relative mechanical performance Mix Concrete Fiber (kg/m 3 ) Ec fct fc S25 S40 GPa MPa MPa Mix-1 SCC Mix-2 SCC Mix-3 SCC Mix-4 SCC Mix-5 SCC Mix-6 Lightweight SCC Mix-7 Lightweight SCC In order to evaluate the structural performance of the different fiber reinforced mixes, mechanical behavior was tested. Mechanical properties of the hardened concrete are assessed by means of measurements of modulus of elasticity (Ec), tensile strength (fct) and compressive strength (fc on cubic samples). Results are displayed in Table 3. Toughness tests were performed according to the standard UNI [3], and compared to the performance of plain SCC with no fiber addition. For this purpose, 4 points bending tests were performed on 600x150x150 mm 3 (Fig. 1) notched samples using an INSTRON 1274 press. In this study Crack Mouth Opening Displacement (CMOD) is measured using an extensometer while Crack Tip Opening Displacement (CTOD) is also measured using an LVDT. 1015

4 Nominal Stress [MPa] 5,0 4,5 4,0 3,5 3,0 2,5 2,0 1,5 1,0 0,5 MIX-1-Plain Concrete MIX-2-S40_3kg/m 3 MIX-3-S40_4kg/m 3 MIX-4-S40_5kg/m 3 MIX-5-S25_4kg/m 3 0,0 0,00 0,05 0,10 0,15 0,20 0,25 0,30 CTOD m [mm] Figure 1: nominal stress - CTOD curves - of the concrete MIX-1 to MIX-5. As displayed on figure 1, the maximal strength (stress at peak) increase with increasing CHRYSO Fibre S40 fiber content. Also, the synthetic fibers seem to increase the strain capacity of materials after damage (stiffening behavior). The highest strength is recorded for the CHRYSO Fibre S25 fibers, which are shorter than the CHRYSO Fibre S40 fibers. This result will be confirmed by complementary tests, however, it may be explained by larger number of fibers at constant weight dosage. At the opposite of the standard SCC, lightweight SCC is demonstrating a maximal strength decrease with increasing CHRYSO Fibre S40 fiber content. The slope of the linear part of the nominal stress CTOD curve being higher when the fiber content is less (figure 2). 3,5 Nominal Stress [MPa] 3,0 2,5 2,0 1,5 1,0 MIX-6 3 kg/m3 MIX-7 4 kg/m3 0,5 0,0 0,00 0,05 0,10 0,15 0,20 0,25 0,30 CTOD m [mm] Figure 2: nominal stress - CTOD curves - of the concrete MIX-6 to MIX

5 This behavior cannot be explained yet and will need further investigations. However, higher fibers CHRYSO Fibre S40 content in MIX-7 seems to lead to a better post-peak behavior of the concrete element showing a softer reduction of its strength after peak. This is not the case of the MIX-6 containing a lower amount of S40 fibers, which shows, after peak, a sharp and severe drop of its strength. From that respect effect of fiber content is following what is expected. 4. CASE STUDY ON PRECAST PANELS AT INDUSTRIAL SCALE In order to evaluate the possibility of substituting the network of traditional surface reinforcement by synthetic fibers, several manufactured panels were made. Regarding Fiber Reinforced SCC panel, the concrete were made at a 3 kg/m 3 of CHRYSO Fibre S40 content. As a reference, a SCC panel with no fibers but reinforced by usual reinforcement (5mm x 20cm x 20cm wire mesh) was also poured. The two panels, were meter length, 2,5 meter width and have a 20 centimeter thickness. The total weight of each panel is about 7500 kg. Pouring is made in three steps. A first 5 cm sub-layer using either reinforced SCC or Fiber Reinforced SCC is poured. Then a 10 cm insulating heart made of polystyrene blocks is placed in between reinforcement. Finally a 2 cm top-layer of SCC or Fiber Reinforced SCC is poured and surface finish is achieve in both cases using a 3 cm concrete layer. Process is displayed in the following pictures. Figure 3: Pouring of standard SCC (left) and Fiber reinforced SCC (middle) sub-layer and top-layer (right). 5. MECHANICAL TESTING OF THE WALL PANELS Each panel was tested at 80 days of age. In order to reproduce realistic conditions the experimental program was designed to study the behavior of panels placed horizontally in a vertical plan in order to reproduce the combined bending effects of the panel weight and of the wind in the most critical conditions (Figure 4). Panel behavior was analyzed in two different phases, first under service conditions where crack development and crack pattern is especially important and under ultimate loading conditions where rupture mechanisms are studied. The residual deformations are thus analyzed after different cycles of loading and unloading at increased levels of load. 1017

6 Figure 4:Panel testing (left) and cracking pattern of Fiber Reinforced SCC (right). Details of the mechanical analysis will be published elsewhere[4], however the main results are the following. The panel in Fiber Reinforced SCC demonstrates a mechanical behavior similar to that of traditional panel, even if rigidity after cracking is lower in the case of Fiber Reinforced SCC. In both cases, bending involves the formation of longitudinal cracking along the whole height of the panels. The crack pattern is somewhat different. Cracking is localized in the case of standard SCC, with a rather large opening. In the case of Fiber Reinforced SCC, cracking pattern is more diffuse with less opening. 6. CONCLUSION Full scale study was performed in order to compare the ability of synthetic macrofibers to replace surface reinforcement of panel walls. Both laboratory tests on specimens and full scale tests on panel walls demonstrate that synthetic macrofibers can replace surface reinforcement in that type of concrete elements. Furthermore it is possible to benefit from macrofibers advantages in terms of mechanical performances. Better repartition in the concrete matrix gives a more diffuse cracking pattern and leads to less opened cracks. This case study highlights the possibility to combine both usage qualities of Self Compacting Concrete and Fiber Reinforced Concrete in order to improve overall quality of concrete while saving time labor time. ACKNOWLEDGEMENTS The authors would like to acknowledge Mozzo Prefabbricati S.r.l. at Zevio for full scale testing and the laboratory P. Pisa from the University of Brescia for the development of the experimental testing. REFERENCES [1] ACI SP 182 Structural Applications of Fiber Reinforced Concrete, American Concrete Institute, Farmington Hills, 1988 [2] Di Prisco, M., Felicetti R., Plizzari G.A., BEFIB 2004, Proceedings of the 6 th RILEM Symposium on Fiber Reinforced Concrete, RILEM PRO, 39, 2004, [3] UNI 11039, Calcestruzzo rinforzato con fibre di acciaio, parte I/II, [4] Cominoli L., Massinari P., Plizzari G.A., Submitted for publication 1018