karst, kuat tekan, berat jenis, batako ABSTRACK The purpose of this study is to determine, (a) Effect of partial replacement of fine aggregate (sand) with hard limestone powder (karst) with different variations - different to the compressive strength of brick. (2) Effect of partial replacement of fine aggregate (sand) with a powder of hard limestone (karst) with different variations - different to the weight of brick. (3) What percentage of the optimal partial replacement of fine aggregate (sand) with hard limestone (karst) to achieve the maximum compressive strength of brick. (4) What is the optimal percentage of partial replacement of fine aggregate (sand) with hard limestone (karst) to achieve a minimum density on the brick. The method used in this study is the experimental method, which is conducted an experiment to obtain a result which confirms the relationship between variables was investigated. The variables that influence directly in this study were (1) variable: the compressive strength and density of brick with holes due to the variation of partial replacement of fine aggregate (sand) with a powder of hard limestone (karst). (2) independent variables: the replacement of some variation of the fine aggregate (sand) with a powder of hard limestone (karst) with a dose ratio of Pc: 7 Ps: 0 Ks: 0.5 W; 1 Pc: 6 Ps: 1 Ks: 0, 5 W; 1 Pc: five Ps: 2 Ks: 0.5 W; 1 Pc: 4 Ps: 3 Ks: 0.5 W; 1 Pc: three Ps; four Ks: 0.5 W. Based on the results of this study concluded: (1) There is a negative influence of fine aggregate replacement of some of the variation with powder sand hard limestone (karst) on the compressive strength of hollow concrete brick. Which can be seen on the results of regression analysis with the Curve estimation Qubic model obtained probability value 0.025) at 5% significance level. (2) The specific gravity test results showed that the more the use of powder karst, resulting in more severe types of small value. From a variety of fine aggregate replacement part with hard limestone powder (karst) used in this study have qualified gravity Concrete with low gravity (low-density concretes) by Dobrowolski (1998), ie 240-800 gr/cm3. It is proved where the weight of bricks is a variation of at least V (1 Pc: 3 Ps: 4 Ks: 0.5 W with a value of 687 kg/m3 density. (3) The maximum compressive strength obtained from Curve estimation regression analysis Qubic model of 5.79 MPa with a variation of 2 (1 Pc: 6 Ps: 1 Ks: 0.5 W). Of the many variations of partial replacement of fine aggregate with a hard limestone powder (karst) used in this study has fulfilled the compressive strength Concrete conditions of low gravity (low-density concretes) by Dobrowolski (1998), ie 0.35 to 6.9 MPa, karst, compressive strength, density, adobe

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