Markku Leivo

Experimental study on the basic phenomena of shrinkage and cracking of fresh mortar

Abstract Evaporation, horizontal shrinkage, settlement, setting and capillary pressure of mortar mixes were measured during first hours with samples which were exposured to wind (velocity 4 m/s, T = 20 °C and RH 40%). The effects of different admixtures (super plasticizer, accelerating, retarding, air-entraining agent and one type of fibre) on the shrinking behaviour of the mortars were studied accordingly. The well-known fact that proper long-term wet curing is vital for the crack-free surfaces of concrete or mortar was confirmed experimentally. The beginning of setting can be regarded as a critical moment. After that the mortar has capacity to resist the capillary forces and thus shrinkage. The mix modified by super plasticizer behaved unexpectedly, and the surface of the sample cracked. This cracking was indicated by zig-zag behaviour in the measured horizontal shrinkage and capillary pressure. Air- entraining agent reduced horizontal shrinkage considerably. Use of fibre reduced shrinkage about 30% when compared to mix without fibre. Based on the results some conclusions are drawn concerning properly limed trowelling on horizontal shrinkage.

Radio wave heater for concrete

Abstract In dielectric heating, the specimen is placed in an electromagnetic field that alternates at high frequency. Electrically charged molecules and ions start to vibrate and the temperature of the material rises. The domestic microwave oven is an example of this phenomenon. Microwave radiation penetrated fresh concrete to a depth of about 2 cm only. Penetration with radio frequency was far better. A radio frequency heater was found to be an effective heater for concrete. The following conclusions can be drawn from the results: A whole mass of concrete can be heated simultaneously. With this heater it is possible to raise the concrete temperature to 50–60°C in just a few minutes (3–4 minutes).