5 TECHNOLOGICAL METHODS TO

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1 5 TECHNOLOGICAL METHODS TO INCREASE THE IMPERMEABILITY AND STRENGTH OF CONCRETE H. PASCHMANN Forschungsinstitut der Zementindustrie Diisseldorf, Germany 1 Introduction When dealing with liquids which pose a threat to the environment, the ground water and soil can be protected temporarily against contamination by catchment structures in the form of sumps and impermeable production and storage surfaces. The impermeability of the construction material is particularly important where such catchment structures are made of uncoated concrete. An extensive trial programme was carried out at the Research Institute of the Cement Industry in Diisseldorf to investigate means of using concrete technology to increase the impermeability of concrete to penetrating liquids. The investigations were specifically aimed at the action of mineral and organic additions in increasing impermeability as well as at the effects of cement type, cement content and waterlcement ratio. This paper only contains results referring to the effect of silica fume on the impermeability and strength of concrete. The paper is an extract of the article "The effect of mineral and organic additions on the impermeability to organic liquids and on other properties of concrete", published in number 01/94 and 02/94 of the German journal "Beton". 2 Materials 2.1 Basic concrete materials The cements used were Portland cements PZ 45 F with specific surface areas (Blaine values) between 3900 and 4100 cmz/g and blast furnace slag cement HOZ 45 L with a Blaine value of 4000 cmz/g. The aggregates used 38

2 were gravel and sand from river Rhine. The grading curve AB 16 was made up from the 012, 112, 218 and 8/16 size fractions. Silica fume in the form of an aqueous suspension containing 50 wt.% solids was used as mineral concrete addition with pozzolanic properties. 2.2 Concretes The investigations were carried out on a total of 11 basic mixes (table 1). The type of cement, the cement content and the wlc ratio were varied while keeping the same type of aggregate and grading curve. Silica fume was added in quantities between 5 and 30 wt.% of the cement. The samples remained for 1 day in the mould, 6 days in a cloud chamber at 20 "C and 100 % relative humidity, and then, until tested, in a climatic chamber at 20 'C and 65 % relative humidity. In the penetration tests at concrete age of 3 month drill cores with heigths of 150 mm and diameters of 80 mm were used. The compressive strengths of the concretes were measured on cubes with edge lengths of 150 mm at concrete ages of 2, 7, 28 and 91 days. 2.3 Liquids The organic solvents methylene dichloride (CH,CI,), from the group of halogenated aliphatic hydrocarbons, and n-heptane (C,H,,), from the group of aliphatic hydrocarbons, were used as the test liquids for the investigations, as being substances which experience shows penetrate readily into concrete. According to previous findings, methylene dichloride is the liquid which penetrates best into concrete. 3 Results concerning the impermeability 3.1 General Table 2 shows the results of the penetration tests. It contains the penetra- tion volumes of methylene dichloride and n-heptane after 72 h in l/m2 at concrete age of 3 months.

3 3.2 Influence of w/c ratio The penetration of liquids into concrete is determined primarily by the impermeability of the hardened cement paste matrix. The most effective measure for increasing the impermeability of the concrete is therefore to lower the w/c ratio and reduce the capillary porosity (fig. 1). For the same cement content, the penetration volume could be halved by lowering the wlc ratio from 0.60 to For the same w/c ratio, the penetration increased significantly with increasing cement content and thus increasing matrix volume. 3.3 Influence of silica fume The penetration of liquids into concrete can be reduced significantly by the addition of silica fume due both to the filling action and to the pozzolanic properties. With increasing quantities of silica fume up to 15 wt.% of the cement, the penetration volume could be reduced to about 60 % (fig. 2). Larger added quantities did not bring any further advantages. With the addition of 10 wt.%, the penetration into different concretes could be reduced to 70 O/O on an average (fig. 3). 4 Results concerning the strength Table 3 shows the results of the compressive strength tests. With the addition of silica fume in quantities of 5 to 10 wt.% of the cement, the compressive strength could be improved for 20 to 30 %. Larger added quantities did not bring any further advantages. With the addition of 10 wt.% silica fume, the strength of different concretes could be increased for 25 % on an average (fig. 4). 5 Relation between strength and impermeability Both lowering the w/c ratio and the addition of silica fume result in increasing strength as well as increasing impermeability. Therefore the relation between strength and impermeability is independent of the fact, wether the concrete contains silica fume or not (fig. 5).

4 Table 1: Mixture of concretes

5 Table 2: Penetration volumes of methylene dichloride and n-heptane after 72 h at concrete age of 3 months

6 Table 3: Compressive strength of concretes

7 Fig. 1 : Influence of w/c ratio on penetration methylene dichloride 0% 5% 10% 15% 20% 30% silica fume in wt.-% of cement Fig. 2 : Influence of different quantities of silica fume on penetration (Portland cement PZ 45 F, c = 360 kg/m3, w/c = 0.45)

8 10 wt.% silica fume Fig. 3 : Influence of 10 wt.% silica fume on penetration of methylene dichloride 10 wt.% silica fume without silica fume Fig. 4 : Influence of 10 wt.% silica fume on compressive strength

9 compressive strength in ~ /mm~ Fig. 5 : Relation between compressive strength and penetration of methylene dichloride at conrete age of 3 months