¹Technical Center, Okumura Engineering Corporation

Minatoku, Osaka, JAPAN

munehisa.yoshida@okumuradbk.co.jp

　

²Department of Civil Engineering, Kinki University

Higashi Osaka, Osaka, JAPAN

tamai@civileng.kindai.ac.jp

　

　

Porous concrete contains many cavities and it is effective as an adherent basis for marine organisms. This paper presents the results of an investigation undertaken to develop additional data on the properties of porous concrete mixed with granulated fertilizer. Granulated fertilizer is composed of dissolved nutrients, which are useful for the growth of algae in the long term. Test plates were installed in the SETO Island Sea, Japan, in order to evaluate the effectiveness of porous concrete with fertilizer. The size of the test plates for experiment was 2m in height and 1m in width. Properties of three types of mixture proportion were evaluated experimentally, namely, porous concrete, porous concrete with fertilizer and normal concrete. The experimental term was two years from September 1999 to March 2002, and adherence of algae on the test plates was observed. As a result, compressive strength was stabilized over 18 MPa. The algae succession was observed on all test plates, and biota of circumference was formed on test plates. The quantity of large algae on porous concrete was more abundant than on normal concrete. However, the greatest abundance of large algae was found on porous concrete with fertilizer. Seaweed adhesion to porous concrete was found to be greater than with normal concrete. As a result of using a mixture of granulated fertilizer and converter slag, there tended to be a high degree of seaweed cover. It is concluded that porous concrete with fertilizer is effective as an adherent basis of algae.

　

　

In Japan, the harvest of seafood has gradually decreased in sea coastal areas. The main cause of this is the disappearance of kelp forests. Based on recently published public statistical data, reclamation and the construction of artificial revetments constitute the greatest cause of this disappearance. Another reason is the emergence of coralline flats in sea coastal areas brought about by changes in hydrographic conditions. Also, there are some reports that the mineral supply through rivers from inland areas has decreased due to artificial development.

　

The Fisheries Agency in Japan has advocated the recovery of diverse organisms through advances in environmental preservation and improvements to nearby sea areas. This is a change from the previous policy of gaining fisheries simply to forming fishery. According to the guidelines of the Fisheries Agency, regulations for fishing port construction should consider not only the effects of ocean waves and weather, but also the revitalization and the mitigation of aquatic resources. In this way, the environment of organisms which inhabit port facilities can be restored, and aquatic resources can be recovered and increased.

　

Although much research has been done on kelp forest creation, the goal here is to increase the production of aquatic resources. There is a considerable body of research work showing that seaweed can be made to effectively adhere to concrete surfaces through the addition of acute-angled roughness (Hasegawa et al., 1992). In addition, there have been some positive tests in which an effective nutrient composition was dissolved within the ions of a hard materials for the purpose of seaweed growth. However, the elution of the nutrient composition comes only from the surface of a hardened material, and there are some problems with the amount of elution and the elution persistence. Porous concrete is advantageous for the adhesion of algae (Tamai and Nishiwaki 1992), and it has been proven to function as a habitat for microorganisms and small animals (Tamai et al., 1997).

　

The purpose of this study is to establish a method for the urgent recovery of biodiversity in coastal areas. The basis for this biodiversity of the ocean is kelp forest creation. Therefore, enriched porous concrete was developed as a base material into which seaweed was efficiently inserted. This paper describes the physical properties of enriched porous concrete and the effect of adherent seaweed.

　

　

The composition of enriched porous concrete is shown in Figure 1. Porous concrete is a construction material which is less damaging to the environment than normal concrete. The continuous voids of porous concrete become spaces which can be inhabited by organisms, and the coarse surface shape is effective for seaweed adherence. Granulated fertilizer and minerals employed for the growth of seaweed, are mixed into the porous concrete. In sea areas where the oligtrophy or nutrient balance has collapsed, it is possible to supplement a moderate nutrient composition by installing this basic material. Also, it is possible to adjust the fertilizer components and the amount of elution according to the hydrographic conditions of the different sea areas. Enriched porous concrete is a type of technology which can assist in the rapid and natural revegetation of coralline flat sea areas. This technology restores a natural environment to sea areas which have been artificially destroyed.

　

　

　

The only stipulation is that the crushed stone contain particles of equal size. Cement paste is used as the binding material. However, when aggregate No.6 (Size 5-13 mm) crushed stone is chosen, mortar is used for the binding material in order to reduce material separation and contraction from drying. Blast furnace slag cement is used for the cement.

　

It is possible for blast furnace slag cement to reduce adverse effects on flora and fauna caused by the elution of free lime, and to counteract the decrease in durability of the concrete. This is because it has a small, silicic acid-free lime content. High-range water-reducing admixtures are used in an attempt to attain high viscosity and the densification of the binding materials.

　

The granulated fertilizer consists of urea (a nitrogen component) and phosphoric acid (a phosphorus component), which are sealed in a special polymer in two layers. The reason for the seal is to prevent the destruction of the granulated fertilizer when it is mixed with concrete, and to control the amount of elution of the fertilizer component to sea areas. Using a polymer is very safe for the environment.

　

The mixture proportions of test plates are shown in Table 1. The voids of the porous concrete are filled up to 40% with an aggregate containing a binding material. In such a case, the free volume of the porous concrete constitutes 24 percent. The granulated fertilizer makes up 20% of the total cement content by mass. This is considered the ideal mixture for strength and permeability. By changing the mixture, it is also possible to alter the strength. When iron ore and converter slag (a heavy material containing Fe) are used, 20% of the aggregate is replaced with this mixture. Converter slag is used to supply Fe supplement.