Jan Trejbal

Influence of Recycled Concrete Composition on its Elastic Stiffness

The presented study was focused on the influence of the micronized recycled a concrete sleeper on the mechanical properties of fine-grained concrete, specifically on dynamic modulus of elasticity. The tested material consisted of cement CEM I 42.5R (Radotín), crushed bricks fraction 2-5 mm, 0-4 mm sand and micronized recycled concrete fraction 0-0.125 mm. Individual material samples differed in the quantity of cement and finely ground recycled concrete (milled at Ltd. Lavaris, Libčice – Czech Republic). Cement was replaced by the recycled concrete in an amount of 30, 50 and 70 weight percent. Testing was performed on beams of dimensions 40 × 40 × 160 mm by using non-destructive testing – pulse method. The article compares the values obtained between 2-nd and 28-th day.

Contact Angle Measurement Tool Based on Image Analysis

We present a cheap solution for assessment of contact angles on sessile drops or menisci formed around partially submerged fibers. The proposed system consists of a simple optical set combined with an open-source software CAMTIA. The fully automatic assessment of contact angles is based on image binarization, identification of regions of interest, boundary smoothing, and contour differentiation. After initial setting of calculation parameters, there is no need for further interaction with the user. This eliminates the need for expensive commercial solutions or tedious manual placement of tangents, guarantees consistency in the assessment procedure, and allows fast bulk processing of images.

Using 2D Digital Image Analysis to Locate Position of Micro Fibers in Cross-Sections of Fiber-Reinforced Concrete

This work deals with determination of location of micro fibers positions in fiber-reinforced concrete. The digital images of sectioned cement-paste samples with dimension equal to 40 × 40 mm were used as an information source about the monofilaments positions. Properly acquired digital image of high resolution allows to determinate the number of fibers in samples cross sections and relate theirs coordinates to any point. Optical microscope Carl Zeiss Axio Zoom.V16 with camera and software allowing individual shots composition of examined samples surface was used to obtain these parameters. Cement pastes reinforced with PET (polyethylene terephthalate) micro fibers having diameter equal to 0.4 mm were studied. The total number and the fibers distribution along the height and width of the sample cross section were examined.

Cement Composite Reinforced with Synthetic Fibers: Comparison of Three-Point and Four-Point Bending Test Results

Presented article deals with the influence of PET fiber production on the bending strength of cement-based composite when incorporated into the fresh mortar, and comparison of results of 3-point and 4-point bending test. Cement paste samples were reinforced with 2 wt. % of primary or recycled PET fibers. The bending test was performed on prismatic samples with dimension of 40 × 40 × 160 mm. It was found that samples with recycled PET fibers, compared to primary ones, exhibit a decrease in bending strength. In the case of 4-point bending tests, the samples with recycled PET fibers exhibited higher bending strength than reference samples without any fibers. However, in the case of 3-point bending tests, the samples with recycled PET fibers had lower bending strength than the reference ones. The results suggest that recycled PET fibers could be used as an alternative to reinforce cement-based composites.

Utilization of Surface-Modified Polymer and Glass Micro-Fibers as Reinforcement in Cement Composites

The presented work focuses on plasma modifications of polymer and glass micro-fibers (having 32 and 14 μm in the diameter, respectively) used as randomly distributed and oriented reinforcement of concrete composites. Fiber surfaces were modified by means of the low-pressure coupled cold oxygen plasma in order to attain a strong adhesion with the cement matrix. From the perspective of micro scale, an impact of modifications on both the physical and the chemical surface changes of treated fibers was examined using: (i) a wettability measurements – an evaluation of an interphase interaction between demineralized water and fibers and (ii) the SEM microscopy – an assessment of a surface morphology. From the perspective of macro scale, the interaction between the two materials was examined by destructive four-point bending tests of the cement paste containing both the reference and treated fibers (specimens having dimensions equal to 40×40×160 mm, water to cement ration 0.4) were done. It was shown that the wettability of modified fibers was increased by approx. 10 % and 70 % in the case of glass and polymer fibers, respectively. The SEM morphology analysis revealed fine roughening of treated fibers, if compared to the reference ones. The mechanical testing pointed out on a toughness increase in the post-cracking response of loaded specimens.

Time Depend Wettability Deterioration of Plasma Treated Polymeric Macro-Fibers

Presented work is focused on the time depend wettability deterioration of plasma treated polymeric macro-fibers. The commercial fibers designed especially for reinforcement of concrete composites – Concrix ES (made from polyolefin) and BeneSteel (polyethylene and polypropylene mixture) were surface modified by oxygen cool low-pressure coupled plasma to attain their water wettability enhancement. The wettability development of thus treated fibers was observed through contact angle sizes between fiber surfaces and distilled water using a direct horizontal optical static method. Contac angle measurements were realized (i) immediately, after (ii) 1 day, (iii) 7 days and (iv) 30 days over the treatment execution, while fibers were stored on the air on standard laboratory condition in the meantime (temperature ~22 °C, moisture ~50 %). Both, the treated Concrix ES and BeneSteel fibers exhibited significant wettability increase. The enhanced wettability of modified fibers stayed approximately constant even after 30 days over the treatment execution in the case of BeneSteel, while in the case of Concrix ES the wettability decreased almost to the reference (no treated) samples, respectively.

Utilization of the waste from the marble industry for application in transport infrastructure: mechanical properties of cement pastes

This article is focused on the mechanical testing of cement-based samples containing a micronized waste marble powder used as replacement of standard binders. Tested materials consisted of cement CEM I 42.5 R (Radotin, Czech Republic) and three different amounts of the marbles (25, 50 and 70 wt. %). Standard bending and compressive tests of the prismatic samples having dimensions equal to 40 × 40 × 160 mm were done in order to reveal an influence of marble amount on flexural and compressive strength, respectively. Moreover, the dynamic modulus of elasticity and dynamic shear modulus were examined and compared after 7 and 28 days of mixture curing.

PERFORMANCE OF CEMENT COMPOSITES REINFORCED WITH SURFACE-MODIFIED POLYPROPYLENE MICRO- AND MACRO-FIBERS

This paper focuses on the mechanical properties investigation of cement pastes reinforced with surface treated polymer fibers. The cement matrix was composed of Portland cement (CEM I 42.5 R, w/c ratio equal to 0.4). Two polypropylene fiber types (micro- and macro-fibers) were used as randomly distributed and oriented reinforcement in volume amount of 2 %. The fibers were modified in the low-pressure inductively coupled cold oxygen plasma in order to enhance their surface interaction with the cement matrix. The investigated composite mechanical properties (load bearing capacity and response during loading) were examined indirectly by means of four-point bending mechanical destructive tests. A response of loaded samples containing treated fibers were compared to samples with reference fibers. Moreover, cracking behavior development was monitored using digital image correlation (DIC). This method enabled to record the micro-cracks system evaluation of both fiber reinforced samples.

PULLOUT BEHAVIOR OF OXYGEN PLASMA TREATED POLYMER FIBERS FROM CEMENT MATRIX

The aim of this work is to describe bonding properties between surface treated polymer fibers and a cement matrix. In order to increase an interaction between the matrix and fiber surfaces, two fiber types having approx. 0.5 mm in diameter were modified by mean of oxygen plasma treatment. Surface physical changes of treated fibers were examined using SEM morphology observation and interfacial adhesion mechanical tests. The principle of mechanical tests rested on a single fiber pulling out from the matrix (cement paste, CEM I 42.5 R, w/c 0.4). The embedded length was equal to 50 % of original fiber length (50 mm), where the fiber free-end displacement and force resisting to the displacement were monitored. It was pointed out that interfacial shear stress needed to break the bond between the modified fibers and the matrix increased almost by 15–65 % if compared to reference fibers. When the fiber free-end displacement reached to 3.5 mm, the shear strength increased almost twice.

PLASMA TREATMENT IMPACT ON PHYSICAL AND CHEMICAL PROPERTIES OF POLYMERIC FIBERS

Presented work focuses on chemical and physical properties of plasma modified polymeric macro-fibers. Polyethylene terephthalate (PET) and polypropylene (PP) fibers having approx. 300 μm in diameter were modified using cold oxygen plasma in order to achieve their surface changes needed for durable bond and adhesion with cement matrixes. A duration of plasma modification differed between 5 to 480 seconds, where an effect of the treatment was examined. Fiber surfaces chemical changes were researched via wettability measurement with demineralized water (the measurement was repeated immediately and after 1, 7 and 30 days to find out the changes stability). Physical changes were studied by means of weight balance (determination of weight loss) and tensile strength tests. It was found that wettability was enhanced significantly – up to two times, while mechanical properties of treated fibers decreased only slightly.