INFLUENCE OF POLYPROPYLENE FIBER LENGTH AND CONTENT ON CONCRETE PROPERTIES: A TWO-FACTOR ANOVA APPROACH

ABSTRACT

This paper explores the effect of polypropylene fiber length and polypropylene fiber content on compressive strength of concrete. Experiments were carried out by changing the fiber lengths (10÷50 mm) and fiber contents (0.1÷0.5%), which were compared by two-factor analysis of variance (ANOVA). The results showed that incorporation of polypropylene fibers increases the compressive strength of the concrete significantly though only under optimal limits. The maximum strength was obtained under 30 mm long and 0,2% high content fibers, and then the performance steadily decreased with decreased homogeneity and increased dispersion. The statistical analysis proved that the fiber content is an important factor as well as the fiber length, but the latter has a greater effect.

АННОТАЦИЯ

В данной статье исследуется влияние длины и содержания полипропиленовых волокон на прочность бетона при сжатии. Экспериментальные исследования проведены с изменением длины волокон (от 10 до 50 мм) и их содержания (от 0,1 до 0,5%), результаты которых были проанализированы с использованием двухфакторного дисперсионного анализа (ANOVA). Полученные данные показали, что введение полипропиленовых волокон существенно повышает прочность бетона при сжатии, однако только в пределах оптимальных значений. Максимальная прочность была достигнута при длине волокон 30 мм и содержании 0,2%, после чего наблюдалось постепенное снижение показателей вследствие уменьшения однородности и увеличения дисперсии материала. Статистический анализ подтвердил, что как содержание, так и длина волокон являются значимыми факторами, при этом длина оказывает более выраженное влияние на прочностные характеристики бетона.

Keywords: polypropylene fiber, fiber-reinforced concrete, compressive strength, ANOVA, optimization.

Ключевые слова: полипропиленовое волокно, фибробетон, прочность при сжатии, дисперсионный анализ (ANOVA), оптимизация.

1. Introduction

Concrete has become one of the most common construction materials in the world because of its versatility, durability and relatively low cost [1]. Although common concrete is extensively used, concrete has a series of underlying disadvantages, the first of which is a brittle nature and a lack of tensile strength, which exposes it to crack propagation due to mechanical forces and environmental factors [2]. In an attempt to eliminate these shortcomings, scientists and designers have been developing the use of different forms of fibers in concrete leading to the fiber-reinforced concrete (FRC) [3, 4]. Of the various types of fiber materials studied, polypropylene fibers have received a lot of attention due to their low density, chemical stability, corrosion resistance, and cost effectiveness. The incorporation of polypropylene fibers enhances a number of important mechanical and durability characteristics of concrete, such as crack resistance, impact toughness and post-cracking behavior [5]. The performance of fiber-reinforced concrete is however not only dependent on the type of fibre, but also on factors such as the length of the fibre, the fibre content and the distribution of the fibre in the cementitious matrix [6]. These parameters are important to optimize because a high percentage of fiber may cause adverse effects, including low workability, clustering of fibers, and lower compressive strength [7].

Here, it is important to conduct systematic experimental assessment and statistical testing to identify the best fiber properties that would increase concrete performance [5]. Two-factor analysis of variance (ANOVA) is a powerful means to determine the relative contribution of fiber content and fiber length on the mechanical behavior of polypropylene fiber-reinforced concrete [9, 10]. The simultaneous analysis of the two variables will allow defining the most important variables and their interactions so that the resulting mix designs will be as efficient and reliable as possible. The current research looks at the influence of the length of polypropylene fiber and the content of concrete compressive strengths. This study, based on a set of systematic experiments and statistical analysis using two-way ANOVA, determines the best fiber content and sizes that contribute to mechanical performance.

2. Method

The results of the experiment primarily helped us to establish the influence of various amounts (%) and lengths (mm) of polypropylene fibers incorporated into the concrete make up on the compressive strength of concrete by applying two-way analysis of variance. On the basis of descriptive statistics, the mean compressive strength of the control sample containing no added fibers is 39.95 MPa. After the change of the fiber length, the following results were obtained: the average strength changed to 43.606 MPa using the fibers of the length 10 mm, 43.84 MPa using the fibers of the length 20 mm, 44.29 Mpa using the fibers of the length 30mm, 42.92 MPa using the fibers of the length 40 mm and 41.78 MPa using the fibers of the length 50 mm. Therefore, the highest results were obtained in fiber length of approximately 30 mm, and beyond this length the strength of the concrete decreased.

Interesting are also the results of the analysis on the fiber content: with 0.1, the average strength of the concrete raised to 43.87 MPa and with 0.2 the maximum result is 45.192. It was 44.812 MPa at 0.3 and this too is regarded as a good outcome. But at 0.4 per cent, there was a drop to 42.37 Mpa and at 0.5 per cent to 40.2 Mpa. This means that 0.2-0.3% is the optimum content of polypropylene fibers and beyond this, the strength reduces.

In a two factor ANOVA, two variables were considered independent. The initial was the length of the fiber and the second was the number of fibers. The average values are presented in table 1 below, along with the statistical indicators realized with the addition of fibers of various lengths to the compressive strength of concrete. When we observe the values of dispersion and standard deviation, we find that the variability is also increasing with the increase in the fiber length. Take, as an example, the dispersion of 10 mm fibers being 4.72, we know that, at 20 mm, the dispersion is less by a factor of four, and at 30 mm by a factor of three. It implies that the obtained results with 30 mm long fibers are consistent and constant, and this choice can be taken as the most optimal one regarding the composition of concrete. But at 40 and 50 mm fibers, the dispersion was 5.17 and 7.21, respectively, so the results are not identical, and different strength increases are possible. This again proves the adverse impact of fibers that are longer than desirable.

Table 1.

Statistical indicators by fiber length

The standard deviation is used to measure the distribution of the data around the mean. When the standard deviation is small, then all values are close to the mean. Having a large standard deviation, the data is highly scattered i.e. they are very far out of the mean. As an illustration, the compressive strength of concrete is 30, 31, 30.5 Mpa, the standard deviation is low. But when measured at 25, 30, 35 Mpa the standard deviation is high. The standard error is the margin according to which the mean of the chosen samples may vary with the true mean. The standard deviation is always bigger than the standard error, since it is the standard deviation/√n (n being the number of experiments).

The more experiments one has the less the standard error, that is to say that the higher the reliability of the mean. The conclusion on the fiber length is that, with the addition of fibers, the compressive strength of concrete increases dramatically, but only to some optimal length. The best value is 30 mm, as it maximizes the average strength of concrete and, at the same time, gives consistent results. The loss of strength at 40 and 50 mm fibers, and the rise of dispersion, implies that excess fiber length adversely affects the concrete matrix at the mechanical level.

Table 2.

 Statistical indicators on fiber content

Based on the statistical measures of the quantity of fibers (Table 2), the analysis of the effect of increasing the quantity of fibers on the compressive strength of concrete, or the extent of trustworthiness of the results in this process, was performed. The compressive strength of an ordinary concrete (0%) was 39.95 Mpa. This is considered to be the first value and on comparison with this the superiority of fiber concrete can be seen. The mean strength of the 0.1 percent fiber concrete was 43.87 MPa, and this result was considerably higher than that of ordinary concrete. To prove this indicator using the statistical parameters, the standard deviation is insignificant - 0.44, and the average error is 0.19, which means that the value was obtained with a high degree of accuracy and the measurements were almost equal. Therefore, fiber concrete 0.1% gave a firmer strength.

3. Result and discussion

The average compressive strength of concrete at the following stage (0.2% was added) was 45.19 Mpa. This is the highest result. The standard deviation in statistical indicators is 0.66, the mean error is 0.29, and the dispersion is 0.44 which, even though these values indicated a small dispersion compared to 0.1% fibers, is still very low. This ensures the consistency of the outcome. Therefore, the best quantity of fibers is approximately 0.2% that can give you the maximum strength of concrete. The average concrete strength was 44.81 Mpa when the percentage of fibers was 0.3. This is a little less than the maximum of 0.2 but far more than a normal concrete. The standard deviation - 1.14, and the mean error - 0.51 have bigger statistical values than the earlier ones. This means that measurement results have some form of dispersion. The average value is however high, this implies that even 0.3 percent fibers reinforce the concrete though the best level is around 0.2 percent. Table 3 contains the two-factor variance indicators.

Table 3.

Two-factor variance indices

The sum of squares is one of the key indicators in the table of results of the two-factor variance provided that allocates all of the variance on each factor. The total squares of fiber quantity was 19.19 but fiber length was considerably higher - 83.33. This implies that the length of the fiber influences compression strength of concrete more than the number of fibers. Mean square result indicated that the value of the fiber quantity is 3.83, and the fiber length is 16.66. These indicators are used to calculate the F - statistic of the two-factor variance. Based on the results, the fiber quantity F-statistic was found to be 6.86 and a very high 29.83 on the fiber length. The effect of the factors is said to be reliable because these values are larger than the nearest possible threshold F value.

The fiber content probability value is P=0.00161, which is smaller than 0.05, and therefore statistically significant. The more significant effect was on fiber length, P=0.000000275. It is extremely high degree of reliability, and this means that fiber length is the primary determinant of concrete strength.

Conclusion

1. The experimental and statistical procedures carried out show clearly that both length and content of polypropylene fibers are a decisive factor in the compressive strength of concrete. Although the performance was significantly enhanced with the incorporation of fibers, relative to plain concrete, the increments were very sensitive to the optimization of fiber parameters.

2. It can be seen that the best fiber length is around 30 mm, at which the compressive strength was the highest, and the results were consistent and reliable with no high variability. Fibers that were shorter than this value are also productive, but the gains were less significant. On the other hand, longer fibers (40-50 mm) not only decreased compressive strength, but also increased the variability of results, meaning that overly long fibers have a detrimental impact on the homogeneity of the concrete matrix.

3. The two-factor ANOVA further highlighted that both the fiber content and fiber length are statistically significant but it is the fiber length that affects compressive strength more.

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