International Concrete Abstracts Portal

In connection with extensive laboratory investigations of auto-claved, foamed cellular concretes data have been secured on the effects of variations in the curing on the compressive strength and elasticity. The concretes were made of portland cement. Type I or III, with one of several Kansas pozzolanic materials, or with a fly ash used for comparison. In each of 53 laboratory batches, one curing factor was varied between the 2 or 3 sets of cylinders. This inves tigation covered the effects of each of the four curing variables (in order of extent of coverage): maximum autoclaving pressure, moist storage prior to autoclaving, duration of the maximum pressure, and rate of heating and cooling. The effects of each variable are found to be dependent on the properties of the pozzolan, especially fineness. For the proportions of volcanic ash used and the curing conditions studied, Type III cement was more effective than Type I, but for one fly ash the reverse was true. The analyses are discussed and tentative conclusions are summarized.

The effect of temperature on the physical and chemical properties of hydrating cement is considered from the effect temperature has on both rate of hydration and on the composition of the hydration product. The increased rate of hydration produced by elevated temperature is discussed in terms of its apparent effect on the uniformity of distribution of the hydration product within the paste matrix and the significance of degree of uniformity of distribution on strength. The general chemical and physical nature of the hydration products formed below 212 F (1OOC) is essentially unaffected by curing temperature. In high pressure steam curing above 212 F the properties of the product can be considerably altered depening upon the temperature and the composition of the cement. The various phases produced for different temperatures and compositions are described in terms of the inherent nature of the basic calcium silicates. The strength of high pressure steam cured paste is shown to depend upon the phase composition of the product primarily through the influence fo the phase composition on its relative ability to reduce the capillary porosity of the paste.

Automation in the block industry is at a primitive stage compared to some other industries. Within the block manufacturing process, the greatest advancement of automation has developed in proportioning and mixing. Those plants constructed in recent years that have been the most automated have all utilized single-stage autoclaving in some form. The efficiency of autoclave production depends on using a maximum portion of each 24 hr for curing, and a minimum I for loading and unloading the autoclave. Thus, efficient and b rapid movement of block from one end of the flow diagram to the other is important. It may be accomplished either by using transfer cars in a shuttle system or by using trains that operate over curved tracks and switches. Disadvantages of these systems suggest the desirability of a completely circular system if technical difficulties could be surmounted.

The disruptive expansion of concrete which is experienced with either early or rapid application of heat in the autoclave is caused almost entirely by the expansion of the air and water vapor enclosed within the concrete. Deformations in concretre speciments caused by this gaseous expansion during autoclaivng have ben measured. It has been found that if external steamair environment presure is applied properly, these deformations can be prevented. Te method used is to seal the autoclave immediately after introduction specimens, and not vent the air during steaming. The result is that the pressure buildup inside the specimen is actually offset by the couter-pressure of the environment. Higher strenghs were obtained on 10-cm cubes cured in this manner thatn those cured in the usal way. An additional benefit of the method is aht mositure is "locked into" the specimen. Apparently the lack of air migration toward the exteriro helps retain moisture, as indicated by the lower moisture loss meaused when curing with steam-and-using a pressure of 1 atmosphere of air show that pressure reduces moisture loss and increases strength.

The report data are from three investigations on autoclave curing of block-type concrete mixes. Variables were presteaming time, heating time, heating rate, and time at maximum temperature and pressure of laboratory and plant-manufactured specimens of three aggregate types. The principal finding was the high strength and low drying shrinkage generally obtained with two hours of curing at 365 F. A one-hour cure at 400 F produced optimum strength in the test series investigating higher temperatures. With the industry trend to faster rail charging systems, data indi-cate the feasibility of four curing cycles per autoclave per day. However, author recommends that time at maximum temperature should not be shorter than five hours unless long-term individual plant tests have consistently demonstrated good results.