Improving Concrete Properties With Fiber Addition

E. Mello, C. Ribellato, E. Mohamedelhassan^[1]

Improving concrete properties with fibers addition

Abstract—This study investigated the improvement in concrete properties with addition of cellulose, steel, carbon and PET fibers. Each fiber was added at four percentages to the fresh concrete, which was moist-cured for 28-days and then tested for compressive, flexural and tensile strengths. Changes in strength and increases in cost were analyzed. Results showed that addition of cellulose caused a decrease between 9.8% and 16.4% in compressive strength. This range may be acceptable as cellulose fibers can significantly increase the concrete resistance to fire, and freezing and thawing cycles. Addition of steel fibers to concrete increased the compressive strength by up to 20%. Increases 121.5% and 80.7% were reported in tensile and flexural strengths respectively. Carbon fibers increased flexural and tensile strengths by up to 11% and 45%, respectively. Concrete strength properties decreased after the addition of PET fibers. Results showed that improvement in strength after addition of steel and carbon fibers may justify the extra cost of fibers.

Keywords—Concrete, compressive strength, fibers, flexural, strength, tensile strength.

1. Introduction

Concrete is the second most consumed substance on earth; on average, each person uses nearly three tonnes a year. Through time, different materials have been added to concrete in order to improve or alter its properties. The addition of fibers, such as steel, glass, polymeric materials, carbon, cellulose, and nylon to fresh concrete in order to improve specific characteristic(s) such as compressive strength, toughness, flexural strength, flexural toughness, and/or abrasion, has received more attention from researchers and the concrete industry lately [1].

As concluded by Morton [2], adding cellulose fiber to concrete resulted in low shrinkage cracking, excellent freeze and thaw performance, high toughness, fire resistance, and reduced rate of water absorption. Moreover, it causes reduction in crack generation/propagation, helps to protect embedded rebars, and may result in small increase on flexural and compressive strength. . The aforementioned study found that addition of cellulose fiber does not decrease the workability of the fresh concrete as other fibers usually do. According to Cement and Concrete association of New Zealand [1], cellulose fibers can improve frost and impact resistance and reduce permeability of concrete.

Addition of steel fibers can increase compressive, tensile, and flexural strengths of concretes along with the post-cracking ductility. Furthermore, the steel fibers raise the resistance of concrete to cracking. The use of steel fiber increases impact resistance and provides ductile failure under compression, flexure and torsion, besides increase in fatigue resistance, as noted by Ramzi et al [3].

Carbon fibers have low density, high thermal conductivity, good chemical stability and excellent abrasion resistance, and can be used to reduce or eliminate cracking and shrinkage [4]. These fibers increase some structural properties such as tensile and flexural strengths, flexural toughness and impact resistance, as reported by Chung [5]. Carbon fibers also increase freeze-thaw durability and dry shrinkage. However, the addition of carbon fibers decreases the electrical resistance, according to Chen and Chung [6].

Although PET fibers resistance in the alkali environment provided by Portland cement is poor, according to T. Ochi et al [7] and D.A. Silva et al [8], PET fiber addition increased ductility and reduced shrinkage cracking, as noticed by Foti [9]. T. Ochi et al [7] reported that PET fibers addition improved the bending strength and toughness.

1. Materials

1. Aggregates

The coarse aggregate used in this research was crushed stone with relative density of 2.68, and nominal maximum size of 16mm. The grain size of the fine aggregates ranges between 0.075 and 4.75 mm and has relative density of 2.64.

1. Cellulose Fibers

The pulp grade used on this research was the Northern Bleached Softwood Kraft (NBSK), which is commonly used as reinforcement in produced papers. The material applied in the experiment had a high moister content (more than 500%) and was oven dried prior to being added to the fresh concrete. The fibers have a density of 1.1g/cm³ and can be dispersed in water.

1. Steel Fibers

        The steel fibers used in this research were 33mm long and 0.55mm diameter with a hooked end and a tensile strength of 1200MPa and a density of 7.85g/cm³

1. 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.

1. 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

1. Procedure

1. 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.