Concrete pumping has emerged during the last decade as a major means of transporting concrete on the jobsite. Although pumping of grout and mortar has been with us for many years, the use of pumps to transport large sized aggregate concrete is comparatively new. During a recent concrete pumping job, an unusual situation occurred. A state highway department had approved the use of pumped concrete for one section of a bridge deck. Pumped concrete being new to the area, a number of technical men from the department were on hand to observe the performance of the equipment and the concrete. The pump was a modern high-quality machine. It transported the mix through 300 feet of 4 inch diameter aluminum pipe. The mix design employed coarse aggregate of crushed granite, and a moderate slump and air content. The first indication of a problem with the pumped concrete came when the men making the strength cylinders noted that this concrete "grew" after the cylinder had been struck off. The contractor's finishers soon discovered they were having difficulty in maintaining true grade on the section with pumped concrete. Later, when 28 day compressive strength tests were made, the pumped concrete demonstrated lower strength than the conventionally placed concrete in every case. What was the matter? The experts at the jobsite and those in charge of the concrete pumping demonstration ventured to guess that aluminum particles had been abraded from the interior surface of the pipe. During the test, however, it was apparent that a chemical reaction was occurring within the concrete emerging from the aluminum pipe. When the aluminum pipe was allowed to rest within the mound of concrete, a noticeable amount of bubbling and gurgling was seen and heard. The bubbles appearing on the surface of the concrete exploded when a match was held near them. Apparently the following guidelines should be heeded: the coarse aggregate should be abrasive; the mix should be low slump; a low-sand aggregate ratio should be employed; and no air should be entrained in the concrete.