“Cracks Happen” Concrete's gonna crack. Since concrete is weak in ...

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29 Νοε 2013 (πριν από 3 χρόνια και 6 μήνες)

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“Cracks Happen”
Concrete's gonna
crack
.
Since concrete is weak in tension ... even a little bit of ...
shrinkage (contraction) ...
flexure ...
settlement ...
... will cause it (concrete) to crack.
So ...
1.
We can
lessen
shrinkage by demanding low
water
content (low slump values) ...
... low water/cement ratio ...
... low overall cement (less `paste' means less shrinkage)
... larger aggregate size (less relative surface area for paste to cover)
But less cement also means less strength.
2.
We can
allow
for shrinkage (and contraction) with
contraction
joints.
Concrete is
gonna wanna
shrink after placement (due to moisture loss). Once

cured, it is also gonna wanna expand and contract (due to temperature changes).
So, let's let it.
We will let walls move, to some extent, ... and slabs float.
For example, a large slab ... around the boundaries we may want to detail an

`expansion/contraction' joint. (See sketch.) Such a joint will allow the slab to

shrink or contract (or expand) without getting `hung up' on the wall.
Often such joints are formed using a `filler' that expands and contracts with the

joint (so there isn't just a `space').
Note: some foundation details show slabs poured monolithic with foundation

walls (e.g. Frost Protected Shallow Foundations). Scary. Expect a crack. Expect

a
big
crack. If the slab wants to contract, it will crack. If there is some

differential settlement, it will crack.
Sometimes I will detail a slab tied to a foundation (wall or footing) ... but only

with a
lot

of

reinforcement
.
So, some details show reinforcement tying the slab to the foundation. Fine.

Expect cracks
somewhere else
.
Here is a number ...
Overall shrinkage can be as much as 600 x 10
-6
in./in. in structural concrete.

(And even more in junk-quality.)
What does this number mean?
It means that for every linear inch of concrete – it will overall shrink 600

millionths of an inch.
So? That's `nothin'.
Exactly.
But let's look at a 50 ft long slab, or wall, or sidewalk. It will shrink 600

millionths of an inch x 600 inches ... equals ... 0.36 in.
(That's 3/8ths of an inch.)
That

is
significant.
So, that's why, every 50 ft or so, ... in a sidewalk, slab, or wall, ... put in ½

in. contraction joints.
(Whether or not there's reinforcement.)
The one exception to this is a basement wall.
3.
We can lessen the
size
of cracks with
reinforcement
.
It takes huge amounts of reinforcement to keep concrete from cracking
at all
. So,

what we will do – is put in enough reinforcement so that the cracks are ...
many,
distributed, and
small.
So, if I am going to tie a slab to a wall, I will use reinforcement for the tie – and I

will specify at least temperature-shrinkage (t/s) amounts of reinforcement

throughout
the slab. Because I know that the slab is gonna crack – especially if I

have constrained it (at the perimeters).
Other note: if I am using the slab to restrain the bottom of the wall – for example

– a basement retaining wall (discussed earlier in the course) – I will spec the slab

cast against the wall – and I will NOT allow contraction joints throughout the slab

... otherwise the wall
will push the slab
and close the gaps. And we don't want

walls to move.
So my concrete is gonna crack – but with reinforcement – the cracks are small

(hopefully `hairline'), dispersed, hardly noticeable.
Some designers and contractors like to use `fibers' for reinforcement.
Assignment
(not graded): do a search on fiber reinforcement for concrete slabs

and put a summary or summary article in your notebooks.
4.
Concrete is gonna crack ... so we will control
where
it cracks with
control
joints.
A control joint is something we put in our concrete to tell it
where
to crack.
For example – in slabs – as the Contractor is `finishing' – we have him/her/it

`knife' a grove into slab. (Or cut (saw) it in later.)
This groove should be at least
one-third
the depth to be effective. (See sketch.)
What it does is it forms a
local weakness
in the slab – and so theoretically it (the

slab) will crack there first.
If the control joint is not effective – it will crack somewhere else.
Here is the thing – if our control joint is straight – the crack will be straight (and

hidden in the `groove').
If we let the slab crack on it's own – you guessed it – it/they (the cracks) will

likely be ragged, probably ugly looking.
For things like walls, where we don't have direct access to the surface we want to

control (due to the forms), we will cast features into the concrete that will leave

planes of weakness ... (see sketch).
Here is a cool rule of thumb ...
5.
Concrete cracks
IN SQUARES
.
So, if you have a 5 foot wide
sidewalk
... expect cracks every five feet. If you let

the sidewalk crack on it's own – then expect ragged, ugly cracks. If you tool

control joints into the sidewalk – it will crack in the joints (and look beautiful).
So, if you have a 24 x 36 garage
slab
... specify control joints every 12 ft ...

(allowing the slab to crack into (six) perfect squares).
(I generally specify control joints every 12 – 15 ft.)
So, if you have an 8 ft tall
wall
... specify control joints every ... you guessed it ...

8 ft along the wall. (Though some spec them farther apart than this ...
at their

peril in my opinion.
)
6.
Deal with potential cracking at
discontinuities
(`sharp corners') in your concrete ...

such as ...
steps in walls
openings in walls
(inside) corners
Some designers detail 45 deg pieces of rebar placed across the corner at the

bottoms of wall steps. (See sketch.)
Code calls for (2) # 5 rebar at concrete wall openings (windows and doors)

extended 24 in. each way past the corner of the opening. (See sketch.)
Corners are also good places for control or contraction joints.
7.
So ... in summary,
... when I detail concrete ... I know it's gonna crack ... I
let
it crack with

contraction joints ... say every 50 ft, or more. I
control
where it cracks

with control joints ... say every 12 – 15 ft, ... in `squares'. I deal with

potential cracking at
corners
. And I use lots of
reinforcement
.
Additional notes ...
Sometimes I will let the Owner and Contractor decide about reinforcement in

slabs, if the slabs are not `structural' to other parts of the building.
Sometimes I will let the Contractor decide the specifics about crack control,

control and contraction joints – but make it sure it's clear that the end product and

it's `look', durability, and performance is the Contractor's responsibility.
Drying (shrinkage) occurs much slower than curing (hydration). At 28 days the

almost all of the curing has taken place ... but we may only be 1/3
rd
of the way

through the shrinking process. Overall shrinking takes
years
.
Construction issue ...
* Don't let the concrete dry out too fast, as
differential
drying plus the concrete

not curing will cause what I call `drying' cracking.*