BEFORE THE ARCHITECT – CUSTOM HOME DESIGNING BACKGROUND – HOUSE DESIGN ARTICLES

ALL ABOUT HOUSE FOUNDATION PROBLEMS and YOUR HOME DESIGN FOUNDATION CONTROL JOINTS, OR CONTRACTION JOINTS, AND ISOLATION JOINTS OR EXPANSION JOINTS

By Before The Architect  Copyright 2009

YOU MAY FREELY QUOTE THE AG WITH PROPER ATTRIBUTION

Introduction

                  Masonry wall control joints shall be sealed with an elastomeric conforming to not less than most recent ASTM C920 (a/k/a American Society for Testing and Materials, "Standard Specification for Elastomeric Joint Sealants") in a manner conforming to not less than most recent ASTM C7793 (a/k/a American Society for Testing and Materials, "Standard Guide for Use of Joint Sealants") and joint sealant shall be tooled to secure complete seal of joint. 

Masonry wall control joints shall be located at not less than changes in wall height or thickness, chases, recesses, abutment with columns or pilasters, intersection of partitions and load-bearing walls at openings (except where reinforcing is provided above and below) at not greater than 20 linear feet in a wall at expansion, or isolation, joints in clay brick veneer. 

Control Joints or Contraction Joints

              Control, or contraction joints shall involve linear places in concrete placements where cracking of slabs-on-grade are desired, or controlled, as opposed to places, usually nonlinear, where cracking and separating of slabs-on-grade are neither controlled nor desired and which, to a greater extent (albeit short of perfection), can be limited with careful design and construction shall include in surface area not greater than 275 square feet. 

Comment:  This home designer has seen this number as high as 300 square feet. 

Control joints shall be in width of slab-on-grade section not greater than 1.5-times length. 

Comment:  This width-to-length rule combined with the total area rule immediately preceding effectively limits a given slab-on-grade face to not greater than about 13 linear feet-6 linear inches x 20 linear feet-6 linear inches. 

Control joints shall not be greater than 15 linear feet on it shorter side. 

Comment:  Tricky.  You’d think that this wasn’t helpful given the first two rules, but think again.  This tells us that a short side can be as long as 15 linear feet which leaves us about 18 linear feet-2 linear inches on the longer side, still staying within the length-to-width rule. 

Control joints shall be formed as nearly square as possible (though not at all necessarily same-sized or symmetrical in layout) at 90 degrees intersection ideally but not less than 60 degrees if at all possible to intersect at floor drains and other penetrations of a slab-on-grade. 

Comment:  This square rule can be toughest of all, if you don’t give up symmetry.  The lesson: thou shalt not offset slab-on-grade wall lines with abandon. 

Control joints shall be in a perpendicular, horizontal dimension of linear feet not greater than 3-times slab-on-grade depth in linear inches. 

Comment:  The inch-to-feet rule restated practically is:  a 4 linear inches slab-on-grade shall have a side dimension not greater than 12 linear feet, and a 6 linear inches slab, 18 linear feet. 

Control joints shall be in configuration straight and true and continuous. 

Comment:  When you combine this rectilinear rule with the others, you get the sense that simple slab-on-grade layouts are downright virtuous. 

Control joints shall be in depth of joint not less than 1/3 of the slab-on-grade’s depth, but in any case not less than 1 linear inch depth, shall be in width not greater than 1 linear inch.  If applied by insertion of hardboard or plastic strips, then the control joints shall be done before finishing.  If hand-tooled, then the control joints shall be cut while the slab-on-grade is still plastic (which, in some applications, can mean within an hour or so after bull-floating and not greater than 4-6 hours after final finishing) – done twice  - once during finishing and once shortly after finishing.  If saw-cut (which is preferable), control joints shall be cut 4-6 hours after finishing while the placement is still plastic, paying careful attention to faster curing in hotter climes or hotter placements.  When control joints bridge reinforced rebar, the rebar may be only severed at every other joint-crossing point or sleeved or smooth.  In any case every effort shall be made to cross reinforced rebar only perpendicular to the bridged line. 

Comment:  Perpendicularity herewith is crucial to focus (but not always available), because elsewhere herein there is a prescript for rebar reinforcement of open, interior corners to slabs-on-grade, which prescript has been amended by placing 2-#5 Grade 50 rebar not less than 6 linear feet long rebar in-line with and perpendicular to the control joint and not at a 45° angle to the joint, the latter angularity being at odds with clean, evenly tensioned contraction. 

Welded wire mesh shall be severed not less than every other wire line. 

Comment: Dimension contraction joints from stem wall side of face, not footing side of face.  By the time it’s ok to form the joints, the footings will be below the slab-on-grade. 

Isolation Joints or Expansion Joints

                  Isolation joints X "isolation joints" , or expansion, joints shall involve separating concrete elements to permit independent movement over time on the horizontal or the vertical, notably, but not exclusively, applied at the joining of exterior slabs-on-grade to perimeter foundations.  These joints shall rim a slab-on-grade at its perimeter joints to walls and at its perimeter and interior joints to piers and other protrusions and obstructions, as columns, for example, and shall run continuous and without interruption from slab-on-grade top of face to slab-on-grade bottom of face at slab-on-grade sides of face.  Isolation joints or expansion joints shall not be bridged by rebar, welded wire mesh or any other form of reinforcement without severing every other one or, for rebar alone also either sleeving or applying smooth bar thereat. 

Isolation, or expansion, joints shall be sealed  with backer rod or equivalent of appropriate size below, to ½ linear inches depth then filled in on top smooth and to full depth with an elastomeric joint sealant, i.e., silicones, urethanes, and polysulfides – each in conformance with not less than ASTM C-920 (a/k/a American Society for Testing & Materials, “Standard Specification for Elastomeric Joint Sealants”) of lesser likelihood, a bituminous-based, hot-poured sealant. 

Comment: Sealants for consideration may include Sonneborn NP1 for large gap filler and Sonneborn SL1 (a self-leveler) for above-rod application. 

Control Joints or Contraction Joints and Isolation Joints or Expansion Joints

Control joints or contraction joints and isolation joints or expansion joints at perimeter abuts overall to structure shall be cleaned (before and after backer rod insertion for isolation joints or expansion joints), primed well and shall be not greater than 1 linear inch in width.  

Control joints or contraction joints and isolation joints or expansion joints subject to smaller movement, e.g., masonry abuts to door and window casings and frames shall be sealed with a solvent-based sealant, e.g., an acrylic or butyl caulk oil-based sealant shall be a prohibited material in masonry applications. 

Comment: So far, the most comprehensive detail on concrete joints this home designer’s seen is from England by way of http://www.pavingexpert.com/concjnt1.htm.   

Comment:  This home designer respectfully notes that there are, in fact, three types of concrete joints: Control, or contraction, joints; Isolation, or expansion, joints; and Construction joints. 

While the literature provides direction on construction joints in slab-on-grade applications, this custom home designer considers such construction joints generally beyond common building practice in home foundation building, except in slab-on-grade, wall, and foundation extensions from a cold joint.  In such instance, all effort shall be taken to extend with materials and methods consistent with existing, including but not limited to substrates, barriers, reinforcements, and mix.  The joints themselves shall be mended with not less than doweling in accord with Simpson Strong-Tie materials and methods or similar and, as needed, engineering latitude.  Additionally, slab-on-grade extensions shall be undermined at existing not less than 2x existing slab-on-grade depth for subsequent, reinforced placement, again, with methods and materials consistent with existing structure. 

The form of construction joint in home building work that is quite common arises in non-monolithic placements, as, for example, in a key joint or keyed joint wherewith a reinforced concrete footing is placed and initially cured before a continuous concrete stem wall is placed atop it.  This joint – cold placements with essentially horizontal joints – should involve a non-wicking bond break.  This bond break could be a continuous sheet along the key’s entirety of HDPE (a/k/a high-density polyethylene sheeting qualifying as Class A in ASTM E-1745 (a/k/a American Society for Testing and Materials, “Standard Specification for Plastic Retarders Used in Contact with Soil or Granular Fill Under Concrete Slabs” Slabs”) in continuous sheets at cold joints pre-placement of the wall element.