Concrete expansion joints: cutting vs wood

[James & Shalise Cash]

Can the requirement for expansion joints in a concrete slab and flatwork be
satisfied by cutting the concrete surface? I am building a house and a
50'X60' outbuilding slab, wondering if this method is worth the expense. I
have seen cuts made about 1/4" wide and deep.

[Shannon Pate]

I seldom use expansion joints anymore, whether they be wood or tar based
material. We regularly use cuts. The concrete is going to move and
therefore crack. What you are doing is controlling where the concrete will
crack. However, I will warn you that no matter how hard you try, concrete
usually finds a way to sneak a crack in around your control joints.

Shannon Pate
ASP Homebuilding, Inc.
Birmingham, AL

James & Shalise Cash wrote in message ...

[Nnails456]

You are talking about two different types of joints, expansion and control.

A control joint is used to "control" the slabs craking. We know concrete will
crack and we would prefer if it was not cracked into a spider web
configuration, so we saw control joints into it. This encourages the concrete
to crack at the joint versus wherever it chooses. The spacing of control
joints varies depending on several factors such as concrete thickness,
reinforcing, etc. A sidewalk will generally have control joints every 4 feet
or so, a driveway slab may be sawed into 9 foot squares, a industrial slab
maybe as large as 20 foot squares.

Expansion joints are used to relieve the pressures caused by expansion and
contraction and shoul be of fiber expansion joint material. Expansion joints
should go against where ever a concrete slab abuts another surface (foundation
wall for instance) or when slabs reach a certain size. Pay attention as you
walk down a sidewalk and you will see both types of joints. A control joint
sawed in every 4 feet or so, and expansion joints against curbs, driveways, and
maybe in the center of long stretches.

[Bob Morrison]

In article <s50gu7...@corp.supernews.com>, James & Shalise Cash
says...

> Can the requirement for expansion joints in a concrete slab and flatwork be
> satisfied by cutting the concrete surface? I am building a house and a
> 50'X60' outbuilding slab, wondering if this method is worth the expense. I
> have seen cuts made about 1/4" wide and deep.

Cutting is actually the preferred method if you want the joint to be
straight. The depth should be 1/4 the the slab thickness (1" deep for 4"
slab). Cutting should be done as soon as the surface is hard enough to
walk on without marring the finish and in any case within 24 hours of the
time the slab is poured.
--
Bob Morrison
R L Morrison Engineering Co
Structural and Civil Engineering
Poulsbo WA

[VINCENT CAMPANELLA]

James & Shalise Cash wrote in message ...

>Can the requirement for expansion joints in a concrete slab and flatwork be
>satisfied by cutting the concrete surface? I am building a house and a
>50'X60' outbuilding slab, wondering if this method is worth the expense. I
>have seen cuts made about 1/4" wide and deep.
>

>No, the expansion joint requirement allows for movement of the slab
sections during expansion, where as the 1/4 deep cuts are for control joints
which allow for controlled cracking of the slab. I would check with your
architect to see if expansion joints are needed other than at the perimeter
of the slab.

[Jim Nelson]

The term "expansion" joint is a misnomer. The concrete never has a volume larger
than the day it is placed. The correct terminology is "control" joint. The idea
for the joints is that the concrete will crack. The joints are cut deep enough
(normally 25% of the slab thickness) so the slab will crack at the joint. You
then get a nice, straight, crack rather than a random, crooked, crack. If the
joints are only cut 1/4" deep the slab will still crack at random. The slab can
be divided into sections with wood strips, instead of cut joints, but I wouldn't
recommend it. The wood is softer than the concrete and will wear faster, leaving
dirt-catching voids in the slab. Better to cut the joints after the slab is
done, or tool them (better yet) at the time the slab is being finished.
It is also a good idea to put pre-formed joint material against any existing
items the concrete is to be placed around or against, such as a foundation or
existing concrete flatwork, or, say, a steel post or something the concrete will
surround. The idea is to isolate the new concrete from the existing, so there is
no bond between them, and the new concrete can have some lateral movement.

[Scott Tafoya]

I would have to disagree with the statement that concrete never has a volume larger
than the day it is placed. I distinctly remember doing the calculations in my
college structures classes to determine how many inches a concrete bridge will
expand in the summer and contract in the winter. It wasn't my strongest class, so I
can't quote the formula, but if your really interested, I could try to look it up.
Here in Nebraska, where it can be 110degrees in summer and -40 in winter, it is
pretty easy to see. The expansion joints on bridges can be a good 6-9 inches wide
in winter, and completely closed in the heat of the summer.

Scott Tafoya

[Jim Nelson]

Scott, you're not reading my post clearly. I said it doesn't exceed the volume it is on
the day it is placed. I didn'y say it doesn't expand and contract. After it is placed
and cured, the concrete dries and shrinks. The expansion from heating never equals the
shrinkage from drying. The concrete definitely expands as it heats and shrinks as it
cools. There are special cements that, when used, produce a concrete which may exceed
it's original volume, but they are not normally used in residential construction and are
beyond the original scope of this thread.
Jim Nelson

[Jim Nelson]

I hope you are suggesting 1/4" WIDE cuts, not 1/4" deep. The cuts should be a
minmum of 25% of the slab thickness, in depth.

RCKLEWIS wrote:

> The general rule of thumb for control joints in slabs on grade per the American
> Concrete Insitute is to saw cut the concrete three times the thickeness of the
> concrete slab in feet. I.E. for a 5" concrete slab the joints should be 15" on
> center.
> 1/4" saw cut joints will be fine and should minimize cracking for your new
> concrete floor.

[RCKLEWIS]

[Matthew Whiting]

RCKLEWIS wrote:
>
> The general rule of thumb for control joints in slabs on grade per the American
> Concrete Insitute is to saw cut the concrete three times the thickeness of the
> concrete slab in feet. I.E. for a 5" concrete slab the joints should be 15" on
> center.

Do you mean 15'? I've never heard of having control joints anywhere
near this close together. Typically, I hear 10'-20'.

> 1/4" saw cut joints will be fine and should minimize cracking for your new
> concrete floor.

I believe a depth of about 1/4 the slab thickness is more typically
recommended.

Matt

[Joe McIntyre]

Jim Nelson wrote:
>
> I hope you are suggesting 1/4" WIDE cuts, not 1/4" deep. The cuts should be a
> minmum of 25% of the slab thickness, in depth.
>

Actually the ACI has now recognized that in the attempt to control
shrinkage cracking the length of time between pouring and saw cutting is
as important if not more important than the depth of cut. For early cut
saws (within 8 hours of the final finishing pass) the specified 1/4d has
been waived. Tests have proven that a 1-1/4" deep cut made by an early
cut saw is sufficient for slab thickness of up to 10" and even 12".

Joe McIntyre
Ivan Rice & Sons Inc.
Concrete Flooring Contractors

[Jim Nelson]

Thanks, Joe, I wasn't aware of that. I think you would agree, however, that no
matter the method, on a typical residential 4" concrete slab, you would not want
less than a 1" deep cut.

[Joe McIntyre]

Absolutely, also we do not recommend joints at a larger spacing than 15'
x 15' even in a 6" SOG w/wwf or sfr. in a 4" SOG our recommendation is
no more than 12' x 12'.

Joe McIntyre

[Scott Tafoya]

Jim-
Thanks for setting me straight. I stand corrected.