Dosage of Concrete Fibers for Underwater Self-Compacting Concrete

In the preparation of underwater self-compacting concrete (SCC), it is required to have both high fluidity and strength. Compared with ordinary concrete which is of low tensile strength and poor toughness, SCC has an even lower water/binder ratio, so plastic shrinkage cracks are more likely to occur. Research shows that adding a certain amount of fibers to concrete materials can effectively improve its crack resistance. Currently, the two most used types of fibers are steel fibers and polypropylene fibers. Steel fibers have high elastic modulus and excellent physical properties. And polypropylene fibers have advantages of high strength, low cost, small specific gravity and are not easy to absorb water. The two types of fibers with different dosage rates are tested here to find the optimum dosage range of each of them. And because there are always some shortcomings for adding just one kind of concrete fibers, the two types are also blended, so that the two can complement each other to enhance the performance of concrete. And its suitable dosage range is tested as well.

Preparation of Materials

Cement: ordinary portland cement PO 42.5; Coarse aggregate: crushed stone; Fine aggregate: river sand with a fineness modulus of 2.7; Admixtures: fly ash, polycarboxylate superplasticizer.

During the preparation of the concrete specimen, it is necessary to ensure that the fibers are mixed evenly, so the mixing time should be appropriately extended. First, weigh the materials according to the mix ratio design, then put the aggregate and cementitious material into the mixer and mix for 30s. Then evenly put the fibers in and mix for 60s, then add water and the superplasticizer and mix for 90s. The concrete slump test and 28-day compressive strength test are applied to evaluate the fluidity and the mechanical properties of the concrete.

Concrete adding steel fibers

According to relevant research and engineering experience, the dosage of steel fibers is generally not more than 3%. Therefore, steel fibers with dosages ranging from 0% to 3% (0%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%) have been tested.

The test results show that the slump of underwater self-compacting concrete will decrease with the increase of the dosage of steel fibers. The reason may be that the surface of fibers will absorb part of the water and the superplasticizer, causing the slump to decrease. From the changing trend, with the increase of the steel fiber content, the slump showed a small decrease first (0.5%-2%). When the dosage of steel fibers is more than 2%, the decrease is significantly greater. The compressive strength of concrete will increase with the increase of the steel fibers dosage. The reason is that the distribution of steel fibers in the concrete can form a kind of network structure, which works together with the aggregates in the concrete and significantly improves the compressive stress. From the changing trend, with the increase of steel fibers dosage, the compressive strength of concrete increases rapidly in the beginning (from 0%-2%), and the increase in compressive strength is limited when the content is greater than 2%. Based on the test results of the slump and compressive strength, it is suggested that the dosage of steel fiber is 1%-2%.

Concrete adding polypropylene fibers

Polypropylene fibers with dosages of 0%, 0.5%, 1%, 1.5%, and 2% are tested. From the test results, with the increase of polypropylene fibers dosage, the slump of the underwater self-compacting concrete shows a continuous decrease. The reason is similar to that of the steel fiber reinforced concrete. When the dosage of polypropylene fibers is greater than 1%, the decline of the slump of the concrete is significantly greater. The compressive strength of underwater self-compacting concrete shows a changing trend of a rapid increase in the beginning (a dosage range of 0%-1%), then a steady state (1% and 1.5% dosage rates), and then a slight decrease (from 1.5% to 2%) with the increase of polypropylene fibers dosage. The reason of the decrease may be that larger polypropylene fibers cause certain difficulties in the mixing of concrete, and it is easy to form agglomerates in the concrete, which causes the compactness of the concrete to decrease and is not conducive to the improvement of the compressive strength. So for the preparation of underwater self-compacting concrete, it is suggested that the dosage range of polypropylene fibers is about 0.5%-1%.

Concrete adding both steel fibers and polypropylene fibers

For finding suitable dosages for blending steel fibers with polypropylene fibers, six combinations of steel fibers with dosage rates of 1%, 1.5%, 2% blending with polypropylene fibers with a dosage rate of 0.5% or 1% are tested.

From the test results, 1.5% steel fibers with 0.5 and 1% polypropylene fibers both have relatively high compressive strength and also maintain a good slump. Based on the results and considering cost effectiveness, it is recommended to blend 1.5% steel fibers with 0.5% polypropylene fibers if the project conditions allow.