Portal: General

Where a section of shotcrete is left incom-plete at the end of a shooting shift, some provision must be made to ensure that the joint will not develop a plane of weakness at this point. According to the American Concrete Institute™s œGuide to Shotcrete1 and the œUnified Facilities Guide Specifications,2 construction joints should be tapered to a shallow edge form, about 25 mm thick. (1 in.) Therefore, in North America, contract specifications will typically require that shotcrete construction joints be tapered to a 45-degree edge and be thoroughly cleaned and wetted prior to the application of additional shotcrete. When welded wire mesh re-inforcement is used, some contractors will also overlap two layers of mesh at the expected location of construction joints in the antici-pation that the shotcrete at the joint may be weaker than the unjointed portions of the shotcrete.

The use of calcium aluminate cement as the binder in refractory systems in general, and refractory shotcretes in particular, is widely approach to high-performance refractory shotcrete involves the use of calcium aluminate aggregates along with the calcium aluminate cement. The addition of this unique aggregate imparts enhanced properties to shotcretes, enabling them to endure the toughest of industrial environments.

After a brief history of different installation modifications, this paper introduces a novel installation technique, which enables form-free placement of low cement and fully dispersed castable compositions. The method does not require the typical wet pumping equipment, as is needed for shotcreting, but delivers similar lining properties.

Whenever possible, try to pre-site all construc-tion projects before scheduling personnel and equipment. A little extra planning ahead of the actual work can save a lot of headaches later on.

I only recently joined the American Shotcrete Association. But, in reading through the issues of Shotcrete and ASA™s excellent brochure, I was struck by how little seems to be known about Carl E. Akeley, the man to whom we all owe our livelihoods. What™s more, I™m told by the litera-ture that he invented the cement gun œto apply mortar over skeletal matrices to form the shapes of prehistoric animals (not true); that he was a Doctor (his schooling was limited to three or four years of grade school); and that the œdevelopment of the original cement gun started in 1895 (it actually started twelve years later).

About fifteen years ago, we had a project that involved doing some extensive shotcrete (dry process) window infills. The job required shooting from the inside of the building onto plywood forms and involved about 5 stories of window.

The proper encasement of reinforcement in shotcrete is a criti-cal issue with respect to the quality and the durability of a shotcrete application. One simply has to refer to the Shotcrete Nozzleman Training Course offered by the ASA (see Shotcrete Magazine, Vol. 1, No. 4) or the recently available Nozzleman Certification Program of ACI to confirm this statement. Although many concerns have been raised, mostly by owners, regarding the presence of voids around reinforcement and their potential effects on structural per-formance, there is, unfortunately, little technical data available to back up the shotcrete industry on this problem.
Several years of experience with a certification program for shotcrete nozzlemen, as well as early research (Studebaker, 1939) suggest that the best approach is to apply dry-mix shotcrete1 at its wettest stable consistency, which is defined as œthe consistency at which the moisture content is the maximum, the maximum being determined by the stability of the fresh gunite (shotcrete). How-ever, observations on many job sites and training of nozzlemen show that many apply shotcrete with a relatively dry (stiff) shooting con-sistency, which may adversely affect rebar encapsulation as well as increase rebound.
The Industrial Chair on Shotcrete and Concrete Repairs of Laval University (City of Quebec, Canada) has initiated a thorough inves-tigation into the Evaluation of reinforcement encasement quality and its effect on shotcrete quality. This research program is financed by the partners of the Industrial Chair, the Concrete Research Council of ACI, as well as by the American Shotcrete Association (ASA). It has several objectives, including: