Air-void parameters measurement of fresh concrete and hardened concrete

Air content, spacing factor and specific surface of fresh concrete and hardened concrete with different air contents, slumps and mineral admixtures (fly ash, slag, fly ash + slag, fly ash + slag + silica fume composite) were studied by the air-void analyzer (AVA) method and the microscopical method. The correlations between the test results obtained from different methods were analyzed. The results show that, there is a close correlation of air content and spacing factor between the fresh concrete and the hardened concrete, but the specific surface correlation is weak. The air content of concrete measured by the AVA method is smaller than that of the pressure method and the microscopical method, because AVA device captures only the air voids with the size smaller than 3 mm. Spacing factor of the fresh concrete measured by the AVA method is greater than that of the hardened concrete measured by the microscopical method, while the specific surface is smaller. When the criterion of 4%-7% air content measured by the pressure method and microscopical method is acceptable for concrete freezing-thawing (F-T) durability in cold weather, the air content measured by the AVA method should be 2.4%-4.6%. For the concrete F-T durability, when the criterion of the spacing factor measured by the microscopical method is 300 μm, the spacing factor measured by the AVA method should be 360 μm.

Production and Properties of Superplasticized Concrete

The objective of this research is to study the effect of grinding powdered superplasticizer, Portland cement, sand, and silica fume on the properties of fresh and hardened concrete. Lose Angeles Machine was used to grind these constituents. The program was arranged to determine the effect of cycles＇ number, superplasticizer type and dosage, silica fume dosage and condition, and gravel to sand ratio on properties of concrete. Naphthalene sulphonated formaldehyde （NSF） superplasticizers in the forms of liquid and powdered were used. Silica fume may be grinded with the other constituents （grinded）, or added to concrete mixer （normal）. The water/cement （w/c） ratio varied from 0.35 to 0.55 to achieve a constant slump （50-90 mm）. Slumps, bulk density and mechanical properties of concrete were measured. Scanning electron microscope （SEM） was also used to show the differences between traditional and superplasticized concrete. The results showed that grinding the mixture enhances fresh and hardened concrete properties. It is also observed that grinding the mixture for 500 cycles is more effective than other numbers of grinding. In addition, superplasticized concrete exhibits compressive strength higher than traditional one at varied ages. Moreover, using powdered superplasticizer has a remarkable effect on enhancing concrete properties rather than using it in a liquid form. A dosage of 1% by weight of cement gave the highest results of compressive strength. Silica fume has an essential role in improving concrete strength and durability since it acts as very efficient void filler and as a super pozzolana. SEM observations illustrate that grinding the mixture enhances transition zone （TZ） properties and makes it denser. On the other hand, grinded mixture can be packaged in bags and transported for use in crowded cities, and so, enhances quality control, since the only requirement to obtain superplasticized concrete is to add water and gravel. This technique has many benefits such as; saving cement, labor and noise, high quality control, and enhancing concrete permeability and durability. There are many fields of application of superplasticized concrete such as; in locations which are not easily accessible by ordinary concreting techniques, in repairing and strengthen, thin coating, and for small projects when ready mix supply is not feasible.

Testing the Setting and Hardening of Concrete Using the Prism Technique

In non-destructive testing of concrete, the ultrasonic pulse-echo technique has proven to be a reliable method for locating cracks and other internal defects. An electro-mechanical transducer is used to generate a short pulse of ultrasonic stress waves that propagate into the object being inspected. Reflection of the stress pulse occurs at boundaries separating materials with different densities and elastic properties. The reflected pulse travels back to the transducer that also acts as a receiver. The received signal can give many insights to the properties of materials under test. The question arises how the setting and hardening of cement paste, mortar, and concrete can be measured continuously. Continuous measurement asks for non-destructive methods. The current testing method such as the vicat needle for cement paste and penetration resistance test for concrete methods measure at intervals. These methods can be applied before the end of setting. All these methods do not allow continuous measurements and axe partly destructive In this paper a special pulse-echo technique called ＂prism technique＂ is used to evaluate the evolution of proprieties of concrete over time. For that, an automatic system has been developed with LabVIEW program in order to monitor the time of flight of the reflected p-wave. Two mixtures of mortar are made using different proportions, of water, cement, and sand. The mortar is poured into a mould that has a form of a prism. Measurements are taken every 10 minutes over a period of 24 hours. This enabled us to plot the evolution of p-wave velocities over time for the samples and compare the results.