A method for making an admixture in liquid form for concrete includes the step of providing a nanocarbon mixture containing at least two different types of nanocarbon particles, with each type of nanocarbon particle having a predetermined percentage range by mass of the admixture, crushing or grinding the nanocarbon mixture into a carbon powder, and wetting and mixing the carbon powder in a water/surfactant mixture using high energy mixing apparatus. The method can also include blending the nanocarbon mixture with a nano-silica based compound, either before or after the wetting and mixing step. An admixture for concrete includes at least two different types of nanocarbon particles in a water/surfactant mixture having a predetermined percentage range by mass of the admixture. The admixture also includes surfactant and can include a nano-silica based suspension stabilizer having a predetermined percentage range by mass of the admixture.