TEST RESULTS 



The first experimental panels were 2 by 4 feet and were constructed 

 out-of-doors (Figure 2). They were cast into simulated footings to 

 enable them to stand upright without bracing. In Figure 2 the final 

 layer of mortar had been applied to the panel on the left, while only 

 the first lift of mortar had been applied to the one on the right. 

 These panels were cured under wet burlap for 2 days and were then allowed 

 to dry under ambient climatic conditions. 



At first mechanical strain gage data seemed to be of reasonable 

 magnitude, but after a few weeks very peculiar readings began to appear; 

 in addition, some of the reference points loosened. The earlier readings 

 had shown rather high shrinkage strains immediately following the curing 

 period; at first, they seemed to be reasonable considering the rather 

 thin concrete sections involved (approximately 1 inch). It is highly 

 probable that screws used as the reference points were not securely 

 bonded to the concrete (probably because they had to be pushed into the 

 wet concrete after it had been brought to the proper level). The screws 

 could not be pre-attached without tying them to the mesh, which would 

 have defeated the purpose. It was eventually concluded that the mechanical 

 strain gage data on these two panels were incorrect, and the information 

 was discarded. Strains in the steel mesh, measured on reference points 

 soldered to the steel, were also erratic. 



Immediately following construction of the 2 by 4-foot panels, a 

 series of 1 by 2-foot panels were constructed and instrumented for 

 measurement with a mechanical strain gage. One of these panels can be 

 seen in Figure 3. These panels were cured in fog for either 9 or 16 

 days prior to being placed in controlled temperature and humidity rooms 

 (50% or 75% RH) for shrinkage determination. As with the larger panels, 

 mechanical strain gage data seemed to be realistic for a while and then 

 began to vary beyond reasonable limits. 



By the time 20 panels had been constructed and it had been concluded 

 that mechanical strain gage measurements were unreliable, financial 

 support for the work was severely reduced. For FY-TQ, FY-77, and FY-78 

 measurements were continued on existing panels, but no additional exper- 

 imental panels were constructed except for one additional 1 by 2-foot 

 panel instrumented with embeddable resistance strain gages. The panel 

 was cured for 16 days in fog and then placed in 50% RH. The embedded 

 strain gages performed very well. Figure 4 shows shrinkage strains in 

 50% RH for this panel as well as average shrinkage strains for several 

 of the panels in which the strain was measured with a mechanical strain 

 gage. In terms of desired results (i.e., adequate expansion during the 

 curing period to overcome subsequent shrinkage) , the upper curve is much 

 better than the lower curve. When the shrinkage curve drops below the 

 horizontal zero strain line, it is in the tension or potential cracking 

 zone. 



Neither of the curves in Figure 4 shows sufficient expansion. In 

 the case of the upper curve (measured with embedded resistance gages), 

 the expansion should be about 200 microstrain (29%) higher to overcome 

 the drying shrinkage expected in a dry climate (20% RH) . 



