Page 32 - Masonry and Concrete

Understanding Concrete
31
UNDERSTANDING CONCRETE

Slab Slump 3–5 inches Slump Less
Thickness, in. Maximum size Maximum size Than 4 in.
3
aggregate less aggregate /4 in.
3
than /4 in. and larger
4 8 10 12
5 10 13 15
6 12 15 18

FIGURE 2-15
Recommended concrete control joint spacing, ft.

2.5.2 Construction Joints

Construction joints are installed wherever a concrete pour is inter-
rupted for more than half an hour or stopped at the end of the day.
Construction joints are usually coated with oil to prevent bond with
the next pour, and located so that they can also act as control joints.
For slabs that are only 4-in. thick, a straight-edged butt joint is ade-
quate, but for thicker slabs, a tongue-and-groove joint is required
(Figure 2-16). The tongue-and-groove joint transfers loads in such a
way that the adjoining panels remain level with one another but can
still expand and contract independently. A tongue-and-groove joint is
shaped by attaching a beveled wood, metal, or molded plastic form to
a temporary wooden bulkhead. Construction joints should be square
or rounded at the surface to match saw-cut or tooled control joints,
respectively.
2.5.3 Isolation Joints

Isolation joints are used to separate new concrete from existing or adja-
cent construction, which might expand and contract differently or
experience different soil settlement or other movement. If the fresh
concrete were not separated from these elements by an isolation joint,
1
a crack could form where the two meet. Isolation joints should be /4
1
in. to /2 in. wide, and filled with a molded fiber, cork, or rubber strip
1
that is set /4 in. below the surface (Figure 2-17). Do not use caulking
or materials that might be squeezed out of the joint when it contracts,
as this could cause someone to trip and fall. Figure 2-18 shows an
example of control joint and isolation joint locations.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.