The interaction of CO2 lasers with concrete and cement materials.
This thesis investigates the use of CO2 laser radiation to treat concrete surfaces. Specimens were
treated with varying laser parameters, and the resultant surfaces were analysed mechanically
and chemically. A glass was formed by laser interaction, with underlying decomposition of both
the cement paste and aggregate. The application of a cement-based coating prior to processing
protects the concrete from excessive temperature rises during treatment. Processing of the
coated material resulted in a glazed surface with no decomposition of the concrete substrate.
With low energy density, OPC concrete exhibits only surface dehydration. However, when the
energy density is increased, a glassy layer, with surrounding and underlying dehydration, is
formed. Increasing the spot size results in a change in behaviour when the material is laser
treated: several mm of concrete are removed, leaving either rough, bare concrete or a glazed
trench. The resulting surface condition is dependent on the laser power.
Thermal analysis techniques were used to identify the degradation reactions and the
temperatures at which they occur during laser treatment. These are dehydration of the ettringite
and ferrite phases at 1149C, dehydration of Ca(OH)2 at 462C, decarbonation of CaCO3 and
ejection of material from 8129C onwards and the formation of a fiised glass layer at 1283 `C.
The strength of attachment of the glass to the concrete decreases with increasing power or
decreasing traverse speed due to the dehydration of the underlying material. The strength also
decreases with time after treatment, due to rehydration of CaO. Mechanical failure occurs
several mm below the glassy area into the dehydrated substrate, where dehydration of Ca(OH)2
has caused disruption to the structure of the material.
The temperature rise in the material was monitored using embedded thermocouples at various
depths. A one dimensional theoretical model agrees well with the experimental results over only
a limited range of depth and time. A three dimensional finite difference model shows close
agreement with experimental results over a range of operating parameters equivalent to those
determined experimentally. Operating maps were generated which predict the depths to which
the identified reactions occur.
A combination of pozzolanic Portland cement, chamotte, sand and waterglass can be successfully
applied to the concrete surface. It acts both as a thermal insulator and provides vitrifiable
material for laser treatment. Low power levels drive water out of the coating resulting in
dehydration and colour changes, whilst higher power levels result in the formation of a glass on
the coating surface. The attachment of the glass shows an area of maximum strength when
power levels are below 150 Watts and traverse speeds below 2mm/s. Beyond these parameters
the attachment becomes progressively weaker.
Thermal analysis of the coating material shows no evidence of Ca(OH)2 dehydration and no
decarbonation, resulting in no ejection of material. The underlying concrete is unheated, and
therefore undergoes no decomposition reactions. Mechanical failure occurs at the limit of the
glassy region rather than several mm below it as with bare concrete. Thus, the weakest point is
the interface of the glazed-unglazed regions now that no significant Ca(OH)2 dehydration occurs.