Vesa Penttala

A novel cement-based hybrid material

Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) are known to possess exceptional tensile strength, elastic modulus and electrical and thermal conductivity. They are promising candidates for the next-generation high-performance structural and multi-functional composite materials. However, one of the largest obstacles to creating strong, electrically or thermally conduc- tive CNT/CNF composites is the difficulty of getting a good dispersion of the carbon nanomaterials in a matrix. Typically, time-consuming steps of purifica- tion and functionalization of the carbon nanomaterial are required. We propose a new approach to grow CNTs/CNFs directly on the surface of matrix particles.

Freezing-Induced Strains and Pressures in Wet Porous Materials and Especially in Concrete Mortars

Abstract A theory based on thermodynamics will be presented by which the pressure in the pore structure of wet porous materials can be deduced during freezing. The pore structure is partly filled with liquid and inert gases such as air. The theory is based solely on thermodynamic relationships; no knowledge of the real geometry of the pore system or the degree of liquid filling in the void space is needed. The only inputs needed in the theory are relative humidity and temperature measured in the sample chamber during the freezing. The validity of the theory will be compared with the test results of mortar samples frozen and thawed in a low temperature calorimeter. During the cooling from 20 to −70°C and subsequent heating of the sample, the strains, heat capacity, and ice evolution of the samples were measured simultaneously in the calorimeter. Two of the three mortar samples were produced using an air-entraining admixture.

Stress and strain state of concrete during freezing and thawing cycles

Abstract The objective of this work is to calculate the pressures, stresses, and strains induced into moist concrete during freezing and thawing. The applied theory is based on thermodynamics and the linear theory of elasticity. If no additional salts are dissolved in the pore water the inputs needed in the theory are relative humidity and temperature measured in the sample chamber and inside concrete and evaporable water amount in the pore structure. Theoretical results were compared with the test results made with two concretes cured under water or at 96% relative humidity. One of the concretes was air entrained and in the comparison concrete no air-entraining agents were used. In the test cylinders cured under water the largest tensional stresses in freezing occurred on the surface of the test cylinders both in the axial and tangential direction. The largest tensional stress was 2.2 MPa, both in air-entrained and in non air-entrained concretes. The largest tensional stresses in the warming phase took place at the end of the thawing period when the chamber temperature was around +5 °C. Then the maximum tension occurred in the middle of the concrete cylinder in the axial direction of the cylinder. This maximum tensional stress was over 2.5 MPa in the air-entrained concrete cured in the relative humidity of 96%. The thermodynamic pumping effect at the end of the thawing phase in every cycle can increase the pore water amount remarkably if free water or moisture is available on the surface of the structure or in the environment vapor. The thermodynamic pumping effect seems to be remarkably greater and more dangerous in air-entrained concretes.

Effects of high temperature on the pore structure and strength of plain and polypropylene fiber reinforced cement pastes

The effect of high temperature on the residual properties of plain and polypropylene fiber reinforced Portland cement paste was investigated. Plain Portland cement paste having water/cement ratio of 0.32 was exposed to the temperatures of 20, 50, 75, 100, 120, 150, 200, 300, 400, 440, 520, 600, 700, 800, and 1000°C. Paste with polypropylene fibers was exposed to the temperature of 20, 120, 150, 200, 300, 440, 520, and 700°C. Residual compressive and flexural strengths were measured and pore structure of the pastes was determined by mercury porosimetry. The total porosity of the pastes more than doubled when exposure temperature was increased from 20°C to 1000°C. The gradual heating coarsened the pore structure. The most notable coarsening of pore structure—together with strength loss—took place at exposure temperatures exceeding 600°C. At 600°C, the residual compressive capacity (fc600°C/fc20°C) was still over 50% of the original. Strength loss due to the increase of temperature was not linear. Polypropylene fibers produced a finer residual capillary pore structure, decreased compressive strengths, and improved residual flexural strengths at low temperatures. According to the tests, it seems that exposure temperatures from 50°C to 120°C can be as dangerous as exposure temperatures 400–500°C to the residual strength of cement paste produced by a low water cement ratio.

Synthesis of carbon nanotubes and nanofibers on silica and cement matrix materials

In order to create strong composite materials, a good dispersion of carbon nanotubes (CNTs) and nanofibers (CNFs) in a matrix material must be obtained. We proposed a simple method of growing the desirable carbon nanomaterial directly on the surface of matrix particles. CNTs and CNFs were synthesised on the surface of model object, silica fume particles impregnated by iron salt, and directly on pristine cement particles, naturally containing iron oxide. Acetylene was successfully utilised as a carbon source in the temperature range from 550 to 750°C. 5-10 walled CNTs with diameters of 10-15nm at 600°C and 12-20nm at 750°C were synthesised on silica particles. In case of cement particles, mainly CNFs with a diameter of around 30nm were grown. It was shown that high temperatures caused chemical and physical transformation of cement particles.

Direct Synthesis of Carbon Nanofibers on Cement Particles

Carbon nanotubes (CNTs) and nanofibers (CNFs) are promising candidates for the next generation of high-performance structural and multifunctional composite materials. One of the largest obstacles to creating strong, electrically or thermally conductive CNT–CNF composites is the difficulty of getting a good dispersion of the carbon nanomaterials in a matrix. Typically, time-consuming steps are required in purifying and functionalizing the carbon nanomaterial. A new approach under which CNTs–CNFs are grown directly on the surface of matrix and matrix precursor particles is proposed. Cement was selected as the precursor matrix, since it is the most important construction material. A novel cement hybrid material (CHM) was synthesized in which CNTs and CNFs are attached to the cement particles by two different methods: screw feeder and fluidized bed reactors. CHM has been proved to increase the compressive strength by two times and the electrical conductivity of the hardened paste by 40 times.

Drying of lightweight concrete produced from crushed expanded clay aggregates

When crushed lightweight aggregates are used in lightweight aggregate concretes very small water amounts evaporate from the concrete even in a dry environment. It is shown that a large portion of the free batch water is absorbed into the pore structure of the crushed aggregates. The relative humidity in concrete pore structure diminishes faster than in normal weight aggregate comparison concretes. Lightweight aggregate concrete is drying internally and therefore the structure thickness has a minor effect on concrete drying times. The exceptional drying properties of lightweight concretes produced from crushed aggregates enables a fast execution of the floor covering works without time consuming drying operations.

CHH Cement Composite

The compressive strength and electrical resistivity for hardened pastes produced from nanomodified Portland SR cement (CHH- Carbon Hedge Hog cement) were studied. The nanomodification included growing of carbon nanotubes (CNTs) and carbon nanofibers (CNFs) on the cement particles. Pastes having water to binder ratio of 0.5 were produced. The obtained hardened material was characterized by increased compressive strength in comparison with the reference specimens made from pristine SR cement, which was attributed to reinforcing action of the CNTs and CNFs. The electrical resistivity of CHH composite was lower by one order of magnitude in comparison with reference Portland cement paste.

Study of heat and moisture transport for concrete sandwich panel wall construction

This paper presents a mathematical model for simulation of transient heat and moisture transfer in concrete sandwich panel wall constructions with variable outdoor temperature and humidity as boundary conditions. The objective is to evaluate the possibility of diminishing moisture content in constructions in order to reduce deterioration risks. Some control strategies involving installation of heating cables in the air gap are proposed and investigated. Simulations demonstrate that the moisture condition of the construction can be improved by using the control regulation developed in this paper. This study improves the understanding of the heat and moisture transport properties of concrete sandwich panels and provides useful information on the decision making in solving the constructions moisture problems.