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November 30, 2016, 02:56 PM

Test wall used half lime mortar half OPC from 2,352 stone types

By Thornton Kay

The NBS bookmatched natural ashlar stone test wall [photo NIST

 
Bristol marble in OPC mortar [photo NIST
 
Bristol marble in OPC mortar [photo NIST
 
Bristol marble in lime mortar [photo NIST
   

Maryland, USA - A test stone sample wall constructed in 1948 by the National Bureau of Standards in Washington DC was subsequently relocated (possibly in two sections) in 1977 to the National Institute of Standards and Technology in Gaithersburg MD.

The south-facing 12m long by 4m high wall was built using 2,032 varieties of US domestic stone from 47 states, and 320 stone samples from abroad, including Cotham marble from England and a range of Scottish sandstones. Each half of the wall is symmetrically 'bookmatched' with a granite base, limestone coping, marble quoins on the West end, and sandstone quoins on the East end.

A lime mortar was used on the east (right) half and a Portland cement mortar on the west (left) half.

Two types of stone-setting mortar were used in the front of the wall. All stones above the base in the east half were set in a 1:3 lime mortar, using a high calcium hydrate. The other parts, including the stonework on the back of the wall, were set in a 1:0.4:3 portland cement, whiting, and sand mortar. All stones set in lime mortar were wedged at front and back to prevent settling and the wedges were left in place for about a week before removal. The consistencies of the mortars were judged by the mason and both mortars had excellent workability. The vertical joints were poured full, and for these a creamy consistency was used. All joints in the south face of the wall were raked out to a depth of 1 inch before the mortar had set. After all stones in this part of the wall had been set, the joints were pointed with mortar of the same mixtures as those used in the setting, that is, lime mortar for the right half and cement mortar for the rest. A pointing tool of rounded contour was used to obtain slightly concave joints.

Virginia slate and copper DPCs were installed.

The wall was built by the Louis Perna Co. of Washington, D.C., with one mason, Vincent Di Benedeto, setting all the stonework. He was an exceptionally careful workman and took a special interest in doing a thorough job. William Amoroso, who had charge of the work, devised several experiments of his own on the back of the wall. The contractor supplied most of the stone for this face.

There seems to be no report on the performance of the stone, nor differing weathering of the lime mortar compared to the OPC (ordinary Portland cement) mortar. From photographs of the individual pieces it can be seen that the Bristol marble of stromatolite fossil algae, aka Cotham marble, forest or field or even landscape marble, not the most likely candidate for walling stone and more normally seen as a polishable limestone in decorative items such as fireplaces. However it seems to have performed fairly well in both types of mortar.

NIST: THE NIST STONE TEST WALL
Masonry mag: Stutzman and Raz Building Stones of America

Story Type:  Feature

ID: 97931

        
 
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