Properties Of Concrete Structures Research Articles

Drying shrinkage of mortars with limestone filler and blast-furnace slag

During the 1990's the use of cements made with port land clinker and two mineral admixtures, called ternary or blended cements, has grown considerably. Nowadays, cements containing several combinations of fly ash and silica fume, blast-furnace slag and silica fume or blast-furnace slag and limestone filler are commonly used. There are numerous works on the influence of blended cements on the fresh state and mechanical properties of mortar and concrete, but the their deformations due to drying shrinkage are not so well described. Analysis of drying shrinkage is relevant because this property influences the possibility of cracking occurrence and, hence, the deterioration of mechanical and durable properties of concrete structures. This paper evaluates the influence on the drying shrinkage of mortars of variable contents of limestone filler and/or blast-furnace slag in Portland cement. Additionally, flexion strength and non evaporable water content were evaluated. Test results show that the inclusion of these mineral admixtures, Joint or separately, increments drying shrinkage of mortars at early ages. Despite this fact, mortars made with limestone filler cement are less susceptible to cracking than mortars made with cements incorporating blast-furnace slag or both admixtures.

Radar systems for examining concrete structures by transmitting power control

Two radar systems, with respective centre frequencies of 800 MHz and 1.1 GHz, are developed to examine concrete structures. The former radar is for the non-destructive inspection of relatively thick concrete structures such as tunnel walls. The latter is to detect rebars and/or probe behind the rebars for reinforced concrete structures. To improve the inspection ability, transmitting pulse amplitude was controlled by changing the feed signal to an impulse generator for higher power at greater depths. As reflected data are acquired by sequentially synchronising to the transmitting pulses, the radar could probe a surface region using a lower transmitting amplitude, and it could probe the deep region at a higher amplitude while reducing attenuation of the radio wave propagation. Antenna sensitivity was improved by using FD-TD (Finite Difference-Time Domain) analysis to match antenna impedance to electric properties of concrete Structures. The systems were successfully applied to probing artificial defects and metal wires or pipes. The 800 MHz radar demonstrated high sensitivity which was more than 10 times that of earlier equipment. The 1.1 GHz radar could locate the position of rebars and detect metal wires and pipes behind rebars as three-dimensional colour images.

Concrete at temperatures above 1000°C

Concrete is the major building material for many structures. It is of primary importance that fire fighters or fire investigators have a full understanding of the properties of concrete so that better control of the fire scene is achieved. This, in turn, not only helps to ensure a safer fire-fighting job but also a more successful fire investigation. Very little attention has been paid in the literature to the effects of temperatures above 1000°C on the structural properties of concrete. In this study, ordinary concrete samples made from the most common Type I portland cement as well as some uncommon glossy-looking concrete lumps collected from a real fire scene were examined by using thermogravimetry (TG), differential thermal analysis (DTA), X-ray diffraction and scanning electron microscope/energy dispersive X-ray spectrometry (SEM/EDX) with the aid of an unsealed furnace. The changes of crystallography, colour, appearance, shape, density, and elemental composition of the concrete with respect to temperature were carefully observed. The study yields a rationalization of the interesting lump-formation phenomenon.

Thermal Considerations in the Design of Concrete Shields

The behavior and properties of concrete structures for shielding atomic power plants are discussed briefly. Methods are presented for estimating thermal effects associated with linear and non-linear distributions of temperature, nuclear heating, and shield cooling. These data and procedures are subsequently used for analyzing several design problems and thermal criteria.