Types of concrete

Summary

Concrete is produced in a variety of compositions, finishes and performance characteristics to meet a wide range of needs. Mix design Modern concrete mix designs can be complex. The choice of a concrete mix depends on the need of the project both in terms of strength and appearance and in relation to local legislation and building codes. The design begins by determining the requirements of the concrete. These requirements take into consideration the weather conditions that the concrete will be exposed to in service, and the required design strength. The compressive strength of a concrete is determined by taking standard molded, standard-cured cylinder samples. Many factors need to be taken into account, from the cost of the various additives and aggregates, to the trade offs between the "slump" for easy mixing and placement and ultimate performance. A mix is then designed using cement (Portland or other cementitious material), coarse and fine aggregates, water and che

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Quantification of effects of fly ash type on concrete early-age cracking

The mechanisms that contribute to early-age cracking are complex. Determining the relative importance of each mechanism as well as the combined cracking potential for a given concrete material is essential for the concrete industry to construct structures with a long service life. A method for quantifying the cracking risk of a concrete mixture is presented. The method involves testing for the concrete heat of hydration, setting time free thermal and autogenous movement, restrained stress, and mechanical property development. The concrete uniaxial stress under restrained conditions is measured using a rigid cracking frame. This test setup was used to quantify the effects of using fly ash on the concrete cracking risk using four different fly ashes with varying calcium oxide contents. All fly ashes reduced the cracking risk because of the decrease in the heat of hydration of the cementitious materials and, to a lesser extent, the increased early-age creep.

American Concrete Institute2008

The production of Portland cement accounts for 5 to 8% of the global anthropogenic CO2 emissions. In the past years, the cement industry has attempted to reduce this contribution by the use of mineral additions on the cement or in the concrete mix. The typical additions in cement are materials with pozzolanic properties such as fly ash, silica fume, etc. Other attempts have been done using materials obtained as byproducts from other industrial processes. This is the case of the chemical gypsums i.e. calcium sulfates. Calcium sulfate can be obtained in the production of several acids such as boric (borogypsum), hydrofluoric (fluorgypsum or fluoroanhydrite), phosphoric (phosphogypsum), etc. or in desulfurization process in coal burning power plants. In this work, the reaction mechanisms of a binder made of 60% fluoroanhydrite and 40% Portland cement clinker have been studied. This binder is currently being produced in Mexico in the preparation of aerated concrete mixes intended to be used in low-cost housing. This work aims to gain insight on the mechanisms of reaction of the binder made of fluoroanhydrite- Portland cement clinker on the one hand and the dimensional stability aspects related with this binder and its application in the construction industry on the other. Model mixes prepared with different proportions of materials were also studied to understand the effect of the fluoroanhydrite on the properties of the binder. The hydration kinetics of the binder was studied by means of isothermal calorimetry and chemical shrinkage. The microstructural development was studied by a combination of techniques such as SEM, XRD, mercury intrusion and others. The dimensional stability of the binder was approached mainly by length change tests carried under different conditions. As this product is a calcium- sulfate based binder the results under high relative humidity conditions were highlighted. The reaction kinetics of the binder were found to be affected by the impurities present in the fluoroanhydrite; hydrofluoric acid traces particularly retard the hydration of both fluoroanhydrite and the clinker. Other aspects such as aging and the ethanolamine-based grinding agent used on the production of the binder were found responsible of the delayed reaction of the binder compared to Portland cement clinker. The hydration of the binder in water immersion is not very different from that observed under medium relative humidity; however, the dimensional stability of the binder is importantly affected by this parameter. The composition of the binder was found to play an important role.

EPFL2010

A simplified method of measuring concrete resistivity, as an index of permeability, has been developed that is similar to ASTM C1202 or the rapid chloride permeability test (RCPT). It is significantly faster and easier to perform, however In this test, cylinders 100 x 200 mm (4 x 8 in.) were cured in 100% relative humidity and tested using the same solutions, test cells, and rubber gaskets as specified in ASTM C1202. To eliminate the problem of the temperature rise of the sample during the test, only one current reading was taken (after 5 minutes) that could be used to calculate the concrete resistivity. Testing was conducted on various different concrete mixtures after 91 days of moist curing using both the new quicker method and the standard ASTM 0202 method. An empirical correlation between the new method and the standard method demonstrates the validity and promise of the new method.

2008

EPFL2010