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Selecting appropriate repair materials for concrete
MC-CMC
DEC
-21993
Fall
1993
A
publication of NRC's
Institute for Research
in
Construction
Issue
No.
10MATERIALS
N
E
W
S
Evaluating and selecting repair materials is
complex. An IRC study reveals how
manufacturers and product users can simplify the process and ensure that repairs are
effective.
Selecting
Appropriate
Repair Materials
for
Concrete
Arecent IRC study will help engineers and building owners select appropriate materials for building repair. No single repair method or material was superior in the study. Instead, early repair using appropriate materials and methods was identified
as the most effective action in building maintenance. Researchers gathered important data on the long -term
behaviour of new and conventional materials
-
mainlycementitious and polymer -based building products
-
and critically assessed current test procedures. They also evaluated commonly used repair materials inrelation to site conditions and structure. The materials underwent wear, abrasion and mechanical action involving load, chemical attack, temperature and
moisture changes, shrinkage and weathering.
The study revealed that the proper application of the many products available is not always clearly defined. Products designed for specific uses are sold as general purpose materials. and vice versa. Performance criteria, product certification and regulation of
specialized services are lacking. Manufacturers fail to
explain how surfaces should be prepared for the installation of their products, and product data sheets are confusing, quoting a mixture of standards, modified standard or ad hoc tests. Moreover, current standard tests do not evaluate the in- service performance capability, and the pressure to produce design information quickly leaves no time for product appraisals.
Researchers also found that early repair is important. The most common repair method, patch and
waterproof, is viable only if workers apply the product
carefully to ensure that it bonds well with the existing concrete. Unfortunately, product limitations are often not identified before selection, and materials are mismatched. Similarly, waterproofing membrane
systems protect against the ingress of moisture and chloride ions, but inadequate wearing courses were found to be a major weakness.
The study proposed several recommendations:
interdisciplinary research to identify the causes of
material deterioration and to understand material compatibility
developing tests to simulate in- service conditions and ascertain the suitability of products for specific environments
encouraging those commissioning repairs to
understand how materials are evaluated and then to
properly assess products themselves
ensuring that designers and engineers base material selection and specification on structural use and
maintenance
using a checklist (see chart page 2) to investigate the structural compatibility between concrete and the patch material selected
regular maintenance programs, including periodic inspections, surface washing and solving existing problems, to ensure longevity of the repair using products that emphasize environmental concerns and safety
using repair materials with improved fire safety characteristics, specifically related to smoke and flame hazards.
Information: M. Lacasse
Researchers applied various physico- chemical techniques to
understand how alkalis affect the properties of cement.
M
A
T
E
R
I
A
L
SN
E
W
SSelecting Appropriate Repair Materials... continued from page 1
Degradation Factors
Materials
Grouts Patching Materials Overlays/ toppings
Surface Hardeners
Bonding Agents
Cementitious Polymeric Fast setting Polymeric Cementitious
Temperature Cycling thermal shock freeze /thaw
x x x
x
UV /solar radiation /natural x x
Chemical attack x x x
Carbonation /acid gases x x
Abrasion x Impact Biological x x x x Incompatibility chemical physical Sustained load point Moisture
Adequate repair demands compatibility between concrete and the
patch materials(s). The factors listed can deteriorate repair materials in
their operational environments.
r
Role
of Alkalis
in
Cement Investigated
he role of alkalis in cement is attracting increasing attention. Alkalis in cement exist as
soluble sulphates and in major clinker phases. Excessive amounts of alkalis cause alkali- aggregate expansive reactions and also affect other properties in
concrete, such as bleeding, setting, shrinkage, and strength development. Changes in the clinker processing technology, demands for energy conservation, limited accessibility to low- alkali raw
materials. and increased environmental concerns are shifting attention to alkalis in cement.
To understand how alkalis affect the properties of
cement paste, IRC researchers recently investigated the effect of alkalis on individual cement minerals. Mineral tricalcium silicate, the predominant component
of cement, largely determines many of the properties of
concrete. Researchers systematically applied various physico -chemical techniques to study how 3% sodium hydroxide (NaOH) affects the properties of tricalcium silicate paste.
Adding small amounts of NaOH accelerates the hydration of tricalcium silicate by reducing the induction period. What's more, NaOH maintains the higher rate of hydration for up to 28 days.
Symbol Meaning
x
Relevant
Not relevant
The strength of the test and reference pastes was simi-
lar for up to three days after addition of Na0H. After six days, the strength of the reference paste was consis- tently higher than that of the paste containing NaOH. The total porosity of the paste, with or without NaOH, was similar at any particular period of hydration. However, the paste with NaOH had a larger number of
finer pores. Nevertheless, at any particular porosity. after seven days of hydration the reference paste was stronger than the paste containing NaOH.
Microstructural examination revealed that the reference paste comprised mainly fibres, whereas the paste formed with NaOH consisted mainly of plates. The low
strength of the paste containing NaOH may be due to
the poor bond strength between the plates, which have
low surface areas. In the reference paste, higher strength is developed by the fibrous matrix. There was evidence for the incorporation of sodium ions in the C -S -H phase in the presence of NaOH. The accelerating effect of NaOH on hydration may involve the formation of metastable C -S -H (containing NaOH)
on the hydrating surface. This complex may be more permeable and have less adhesive strength than the C- S-H normally formed without additives present. Information: V.S. Ramachandran
