Omer Arioz

Physical, Mechanical and Micro-Structural Properties of F Type Fly-Ash Based Geopolymeric Bricks Produced by Pressure Forming Process

Geopolymer is a new class of three-dimensionally networked amorphous to semi-crystalline alumino-silicate materials, and first developed by Professor Joseph Davidovits in 1978. Geopolymers can be synthesized by mixing alumino–silicate reactive materials such as kaolin, metakaolin or pozzolans in strong alkaline solutions such as NaOH and KOH and then cured at room temperature. Heat treatment applied at higher temperatures may give better results. Depending on the mixture, the optimum temperature and duration vary 40-100 °C and 2-72 hours, respectively. The properties of geopolymeric paste depend on type of source material (fly ash, metakaolin, kaolin), type of activator (sodium silicate-sodium hydroxide, sodium silicate-potassium hydroxide), amount of activator, heat treatment temperature, and heat treatment duration. In this experimental investigation, geopolymeric bricks were produced by using F-type fly ash, sodium silicate, and sodium hydroxide solution. The bricks were treated at various temperatures for different hours. The compressive strength and density of F-type fly ash based geopolymeric bricks were determined at the ages of 7, 28 and 90 days. Test results have revealed that the compressive strength values of F-type fly ash based geobricks ranged between 5 and 60 MPa. It has been found that the effect of heat treatment temperature and heat treatment duration on the density of F-type fly ash based geobricks was not significant. It should be noted that the spherical particle size increased as the heat treatment temperature increased in the microstructure of F-type fly ash based geobricks treated in oven at the temperature of 60 °C for 24 hours.

Some Factors Influencing Effect of Core Diameter on Measured Concrete Compressive Strength

Various concrete mixtures were produced using crushed limestone and river gravels with four different maximum sizes of 10, 15, 22, and 30 mm (0.39, 0.59, 0.87, and 1.18 in.). The 28-day cube strength of concrete mixtures ranged between 28 and 43 MPa (4061 and 6236.6 psi). Beam specimens were cast by these concrete mixtures and cores were drilled from the beams. Compressive strength tests were performed at the ages of 7, 28, and 90 days on a total of 2268 core and cube specimens and the effect of core diameter on concrete core strength was examined. The strength correction factors were determined to convert the strength of a core with a diameter of 144, 69, or 46 mm (5.67, 2.72, or 1.81 in.) to that of a core having a diameter of 94 mm (3.70 in.). As the maximum aggregate size increased, the strength of the core decreased and, consequently, the correction factors increased. The effect was more pronounced for smaller diameter cores. The correction factors were somewhat higher for cores drilled from river gravel concrete. The age of the concrete was found to be an important factor in the strength correction of different diameter cores, that is, the older the concrete, the lower the correction factor. It should be noted, however, that it is very difficult to use age-versus-correction factor relation in practical terms. Additionally, test results showed that the length-to-diameter ratio of the specimen is more significant for small-diameter cores.

Effect of Length-to-Diameter Ratio of Core Sample on Concrete Core Strength—Another Look

Cores with diameters of 144, 94, 69, and 46 mm and length-to-diameter (l/d) ratios of 0.75, 1, 1.25, 1.5, 1.75, and 2, were removed from beam specimens produced from eight different concrete mixtures. Compressive strength tests were conducted on a total of 1876 core specimens. Strength correction factors were determined for converting the strength of the cores with l/d ratios ranging from 1.75 to 0.75 to the strength of an equivalent standard specimen with a l/d ratio of 2. The effects of type and maximum size of aggregate and core diameter on these correction factors were examined. As it was expected, the correction factors gradually decreased with the decrease in l/d. The effect was found to be more pronounced for cores with a smaller diameter. The results also revealed that the effects of type and maximum size of the aggregate on correction factors were not significant. The proposed strength correction factors differ only slightly from those currently recommended in ASTM C 42/C 42M-04 [1] and they are very close to those proposed by Bartlett and MacGregor cited by Arioglu [2–4].

An Experimental Study on the Evaluation of Concrete Test Results

The compressive strength test applied to standard samples is one of the most important tests indicating the quality of concrete in structures. The results of the standard tests are compared with the values used in design calculations to check for specification compliance and quality assurance. Although the standard tests are well accepted by the construction industry, they may not represent the in-situ strength of concrete due to the differences between the degree of compaction and curing conditions of concrete and those of standard samples. The quality of concrete can be assessed by means of minor-destructive methods. Pull-out test is an example of minor-destructive tests. The test causes only minimal destruction to the structure and the hole is repaired easily after testing. In the present study, the results from pull-out tests, and maturity method were extensively analysed for the assessment of concrete strength.