Identifying and Evaluating Concrete Defects
Concrete structures are regularly constructed without complications.
However, defects can occur that can be traced to problems related
to environmental conditions during construction or with the concreting
procedures used. In order to determine a repair method, it is necessary
to identify what caused the defect. Evaluation of deficiencies helps
ensure that repairs will be effective and the defect will not extend
into the surrounding concrete.
Identifying Concrete Defects
Many concrete defects are immediately recognized and others are
not. Being able to properly identify the defect will aid in choosing
the best evaluation methods and subsequently the best repair methods.
Concrete defects can be broken down into four broad groups based
on visual observation: deformation of the surface, cracking of the
surface, disintegration of the surface, and other defects.
Deformation of the Surface
Three defects cause deformation of the concrete surface, but may
not exhibit any other symptoms. The first is curling or warping.
This is the deformation of the edges and corners of a slab-on-ground
in the absence of any loads. When caused by moisture, this deformation
is called warping; when caused by temperature, it is called curling.
The second defect is the delamination of surface mortar from underlying
concrete. It is difficult to visually observe a delamination before
it becomes dislodged from the surface. However, in small discrete
locations, the surface may exhibit convex rising called blistering.
Blisters are generally isolated, but may be closely spaced and can
combine to form a large blister or delamination. A third defect
occurs when isolated low spots on the surface collect water and
have no means of drainage. These surface water pools are known as
Cracking of the Surface
Cracks appear in concrete for many reasons. Some cracks can appear
as secondary symptoms of other defects, such as a long rounded crack
following the structural failure of a warped slab. Discussed here
are cracks that are primary symptoms of distress, caused by volume
changes and structural failure.
cracks have many different looks and can be difficult to distinguish
from cracks caused by other mechanisms. Discreet, parallel cracks
that look like tearing of the surface are caused by shrinkage while
the concrete is still fresh, called plastic shrinkage. Fine random
cracks or fissures that may only be seen when the concrete is drying
after being moistened are called crazing. This defect may also
evident when a translucent coating is applied to the concrete surface.
Cracking that occurs in a three-point pattern is generally caused
by drying shrinkage. Large pattern cracking, called map-cracking,
can be caused by alkali-silica reaction within the concrete. Structural
failure cracking may look like many other types of cracking; however,
in slabs they are often associated with subsequent elevation changes,
where one side of the crack is be lower than the other.
of the Surface
Disintegration of the surface is generally caused by three types
of distress. When laitance forms on the surface, it is called dusting.
This can be caused by a number of reasons, which include carbonation
of the surface by unventilated heaters or by applying water during
finishing. Raveling or spalling at joints occurs when aggregates
or pieces of concrete from the joint edges are dislodged. The last
form of disintegration is the breaking of pieces from the surface
of the concrete generally caused by delaminations and blistering.
Popouts are conical fragments that come off the surface, typically
leaving a broken aggregate at the bottom of the hole. Popoffs, or
mortar flaking, is similar to popouts, except that the aggregate
is not broken and the broken piece is generally smaller. Flaking
of the concrete surface over a widespread area is called scaling.
Other defects include discoloration of the concrete, which is covered
in depth by PCA’s Removing
Stains and Cleaning Concrete Surfaces, IS214; bugholes,
which are small voids in the surface of vertical concrete placements;
and honeycombing, which is the presence of large voids in concrete
caused by inadequate consolidation.
Evaluating Concrete Defects
Visual examination typically does not provide enough information
to determine the cause or causes of a defect. In some cases, it
may not provide evidence of a defect at all. In order to narrow
the scope of an investigation to probable causes and suitable repair
methods, the appropriate information factors and the proper evaluation
methods need to be identified.
Evaluation Information Factors
Information about the structural capacity and loading of a concrete
placement may provide important details as to the cause of defect
or failure. The thickness and the loading of the slab will allow
assessment of the inplace strength versus actual loads in use. Crack
properties, such as its width and location of reinforcement, will
provide information that can be used to determine activity and origin.
Joint characteristics, such as spall dimensions and activity, will
provide information on the effectiveness of the joint. Many other
factors that improve the quality of an investigation are listed
in PCA’s Concrete
Floors on Ground, EB075.
visual examination is typically the first method of evaluation used
as it identifies that a problem exists and that further evaluation
is needed. Surface elevation profile measurements taken according
to ASTM E1155, Standard Test Method for Determining FF
Floor Flatness and FL Floor Levelness Numbers, can
be used to quantify deformations of the surface. Sounding according
to ASTM D4580, Standard Practice for Measuring Delaminations
in Concrete Bridge Decks by Sounding, can locate delaminations.
Coring can be used for a variety of reasons including strength testing
according to ASTM C42, Standard Test Method for Obtaining and
Testing Drilled Cores and Sawed Beams of Concrete, or petrographic
examination according to ASTM C856, Standard Practice for Petrographic
Examination of Hardened Concrete.
There are many different evaluation methods and each provides specific
information that can be used to identify or help identify the cause
of a specific defect; thereby allowing for proper design of a repair
method to ensure a quality repair and mitigation of further damage.
Using visual examination as the initial evaluation method allows
for identification of the information factors that are needed to
complete the evaluation and repair.
More information on concrete defect causes, prevention and repair,
see PCA’s Concrete
Floors on Ground, EB075, Concrete
Finisher’s Guide, EB122, and Concrete
Slab Surface Defects: Causes, Prevention, Repair, IS177.