THE RUDOLF REPORT

GERMAR RUDOLF · THE RUDOLF REPORTrior surfaces of approximately 200 m 2 per gram). 397Fresh concrete and cement mortars—which are identical from achemical point of view—are relatively strongly alkaline (pH approximately12.5). It later falls, however, due to the binding of carbon dioxidefrom the air. Depending on the special chemistry of the cementmortar, this process proceeds very slowly in the depth of the material.According to the composition of the cement mortar, this may last froma few months to many decades, until the pH value of such a mortar orconcrete becomes neutral, even in the deepest layers. 396-398 This chemicalbehavior explains the entire secret of the stability of reinforcedconcrete, which prevents the embedded steel from rusting further in theenvironment within the concrete, which remains alkaline for lengthyperiods of time. 399The water content of concrete and cement mortars depends on thetemperature and relative humidity of the air and fluctuates between 1%and less at 20°C and 60% relative humidity up to 10% in air saturatedwith humidity. 395 In case of permanently high humidity, penetratingwetness from outside, a huge part of the pore system can be filled withwater. 400Poorly insulated rooms built underground always have cool andhumid walls due to their great exchange surface area with the ground:partly because of their absorption of humidity from the ground, andpartly because of the condensation of humidity in the air on the coolwalls when the temperature falls below the dew point. The water contentof these walls therefore lies around 10%, i.e., a factor of approximately10 or more above that of dry walls of heated rooms built above ground.6.7.2.2. ExcursusThe chemical composition of Portland cement, the most frequentlyused cement for concrete and water mortar, can be seen in Table 6.397 W. Czernin, Zementchemie für Bauingenieure, Bauverlag, Wiesbaden 1977, pp. 49f.398 N.V. Waubke, Transportphänomene in Betonporen, Dissertation, Braunschweig 1966.399 In the strongly alkaline environment, iron is passivated by a passive layer of Fe(OH) 3 . ‘Botchwork’ on building sites, i.e., rusting reinforcement rods and cracking concrete after only a fewyears or decades, due to overly low pH value in the vicinity of the embedded reinforcementrods, is caused by a) an incorrect composition of the concrete (too little cement—it’s cheaperthis way—and/or too much or too little water—incompetence), or b) by installing the reinforcementrods too close to the surface of the concrete, where the pH value falls strongly aftera few years or decades; see notes 396f.400 K. Wesche, Baustoffe für tragende Bauteile, volume 2, Bauverlag, Wiesbaden 1981, pp. 51f.182