Improving Concrete Properties With Fiber Addition

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Carbon Fibers

Chopped carbon fibers were added to the fresh concrete. The fibers were 6.1mm in length and with 4.6GPa tensile strength, 243GPa tensile modulus, and specific gravity of 1.8.

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PET Fibers

The PET fibers for the research were hand cut from PET bottles with an average of 50mm length and around 1.5mm width. The density of the material was found to be 1.45g/cm³. Figure 1 shows a photo of the four fibers used in the study.

[pic 1]

Fig. 1 – Fibers used in this study

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Procedure

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Preparing the Mix

Each of the four fibers in the study was tested at four different percentages (volume of fibers/ volume of concrete) along with tests on plain concrete as a control. For each percentage, compressive, flexural, and tensile strengths were tested on concrete specimens that were moist-cured for 28 days.

The concrete mix used in this study was a rich mix with small size aggregates, as described by the Cement Association of Canada [10]. The plain concrete was made with proportions shown in TABLE I.

TABLE I

Plain Concrete Mix

Material

Mass (kg/m³)

Water

189

Cement

377

Coarse aggregate

897

Fine aggregate

864

The concrete-fibers mixtures were prepared by gradually adding the fibers to the fresh concrete while mixing adding enough fibers on plain concrete admixture until the desirable percentage of fibers by volume was reached. The best way found to mix the fibers with the concrete was thoroughly mix the regular ingredients of the concrete (cement, water, coarse and fine aggregates) and then slowly add the fibers to the concrete while the mechanical mixer was rotating. The homogeneity of the mix was visually evaluated.

The workability of the concrete was one of the properties compared in this study, and effort was made to keep the workability, in all mixtures, between 75mm and 100mm. After the fibers were totally mixed with the fresh concrete, a slump test was performed in accordance with CSA A23.2-5C [11]. For samples with slump outside the desirable range, plasticizer was added and another slump test was performed. This was repeated until the slump of the mixture reached the specified range or until the maximum allowed volume of plasticizer was used.

The concrete cylinders used to tensile (150mm dia. and 300mm long) and compressive strength (100mm dia. and 200mm long) tests were consolidated by rodding, whereas the beams (150mmX150mmX500mm) used for flexural strength tests were consolidated by vibration. The cylinders and beams were casted in accordance with CSAA23.2-3C [12].

After casting the specimens were left to set, and were demolded on the following day. The specimens were then immersed in water to cure for 28-days, according to CSAA23.2-3C [12].

Capping of all samples for compressive strength tests was made with sulfur-based capping compound.

All tests were carried according to CSA Standard, for compressive [13], tensile [14], and flexural [15] strengths.

The addition method for cellulose fibers was a little different from the others. It had to be dispersed in water with the aid of a small mechanical mixer and then added to the concrete. During the whole process, all the water used to disperse the fibers was deduced from the water added to the mix, keeping the same water cement ratio in all mixes.

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Results

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Cellulose Fibers

In comparison with the four fibers investigated in this study, adding cellulose fibers resulted in the least reduction of workability, even though it was necessary to use plasticizer at all percentages.

Mixture prepared with the first two percentages of cellulose fibers (0.2% and 0.3%) achieved the desired slump range and the mixtures were workable and could easily be rodded and vibrated. However, with 0.4% and 0.5% fibers the mixtures had slump lower than specified values, although they were able to be rodded and vibrated with the 0.5% mixture being slightly harder to work with.

The results showed that the decrease in the compressive strength is proportional to the percentage of the cellulose fibers. At 0.5% of cellulose, concrete had its compressive strength reduced by 17%. Performance of concrete on compressive strength with addition of cellulose fibers is shown in Fig. 2.

[pic 2]

Fig. 2. Compressive strength for concrete with cellulose fibers and its standard deviation.

Addition of cellulose fibers also worsened concrete flexural and tensile strength as Shown in TABLE III.

Although the addition of cellulose fibers slightly increase the cost of the concrete mixture (see TABLE II) and decrease the compressive, tensile strength of the Considering concrete price, the addition of cellulose fibers to concrete is not an advantage for the mechanical properties evaluated, however for other parameters, such as the ones cited by Jerry H. Morton [2], it may be advantageous. A summary of results for the performance of cellulose fibers added to the concrete matrix can be seen at

TABLE III and the economical comparative can be found at TABLE II.

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Steel fibers

The workability of the steel fibers was remarkably impaired by the ability of the steel to build on itself. Often times the mix did not have the desired slump but remained workable, and was found to segregate with more than 1200ml/m3 plasticizer was added.

At the