Physical properties of concrete at early ages

The introduction of the motor vehicle and the subsequent rapid expansion of the highway systems of the United States have brought with them many problems. The Civil Engineer, in his attempts to provide adequate safe avenues of travel for the populace, has been faced with many problems. Handicapped by the need for speed in design and lack of information of the trends in motor vehicle design, the engineer has been forced at times to utilize design methods inconsistent with good engineering practices. Of the many types of pavement sections which have been designed in an effort to provide structurally and economically suitable transportation avenues, the rigid type portland cement section represents one of the best known and most durable in use today. Rigid and durable as they may be, these pavements are still cracking and breaking up under the action of traffic. Engineers in an effort to adapt this design to increasingly heavy axle loadings and higher traffic volumes and eliminate cracking and subsequent shortening of pavement life, have followed two separate design trends. One of these has been the use of short, un-reinforced slabs, whose action under load would be as a complete unit in the transference of stresses to the subgrade. Also, attempt has been made to thoroughly and completely compact the sub-grade so as to obviate any necessity for so-called bridging” and subsequent possibility of cracking. Still other designs have thought to handle heavier loadings by increasing the depth of paving section, thus lowering the sections flexibility, however, this procedure has increased the cost prohibitively. These attempts have failed to eliminate completely cracking, infiltration of moisture to the sub-grade, and the relatively short pavement life. Investigations have been, and are being conducted by various organizations throughout the United States in an effort to analyze pavement action under load, the effect of temperature change, and the possibility of developing new criteria for design. Effort is also being made to investigate the properties of the various materials used in highway construction so as to utilize them to better advantage. The last two mentioned items have led to this investigation. Development of pre-stressing procedures and technique and better understanding of this useful construction method has suggested its application to pavement design. If prestressing our highway slabs is to be practical, we must immediately consider the properties of the concrete to be used. Modern highway construction methods require continuity of action, no lost motion, no duplication of effort, and if we are to maintain the time sequence which has become standard we must understand the early age properties of the material we use. It is hoped that the result of this project and others of similar nature may be utilized for the development of better, safer, more durable highways for the use of future generations --Introduction, pages 1-2