Question:
When using match curing to determine the release strength of prestressed concrete beams, where should the temperature sensor in the beam be placed?
Answer:
Match curing is a procedure for curing concrete cylinders at the same temperature as that monitored at a specific location in a concrete member. Consequently, the compressive strength of the cylinder more accurately represents the in-place concrete strength of the member. The method is particularly useful for determining the concrete compressive strength at early ages in the fabrication of prestressed concrete products and is permitted by some state departments of transportation.
The temperature sensor in the beam needs to be placed at the most critical location for strength development. This generally means the location with the slowest and least concrete temperature rise. This location depends on the beam cross section and method of curing. For most I-beams and bulb-tee beams that are cured without the use of external heat, the coolest temperature is likely to be in the top flange since it has the highest ratio of surface area to volume compared to the web or bottom flange and, therefore, cools more rapidly. It is also cooler on the surface than on the inside. For beams where the predominant amount of heat is supplied externally to the beam, the least temperature rise in most I-beams and bulb-tee beams is likely to be in the middle of the bottom flange since this is furthest from the heat source. However, if there is also heat of hydration present, the center may not be the coolest location. Consequently, it is recommended that the producer should determine the critical location within the cross section based on the plant procedures. To avoid subsequent disputes, the location should be approved by the Engineer.
The critical location along the length of the beam should also be defined. Generally, the last concrete placed in the forms has the coolest temperature until peak temperatures are reached. It has also been observed that the ends of beams closest to the ends of the precasting line may not achieve as high a temperature as at other locations along the line. The prestressing strands projecting from the ends of the beam act as conductors removing heat from the beam. Therefore, a location near the end of a beam closest to the end of the precasting bed where the last concrete was placed may be more critical than other locations along the bed. Irrespective of the selected location, it is important to measure the concrete temperature and not the temperature of the surrounding air, steam, or formwork.