Civil Engineering Reference
In-Depth Information
showing better than 8% with an average of better than 2% over a wide
range of bar sizes, lapped bars, etc. A survey of a car park carried out by the
author, where cover to reinforcement was in contention, showed results on
average within ±1% with a worst error of 3 mm, over a range of bar sizes,
including lapped bars (calibrated against numerous breakouts). However,
the cheapest covermeters are unlikely to achieve such an accuracy.
Figure
1.20
illustrates one of the more expensive and competent covermeters.
The standard also lists a number of extraneous factors which are
potential sources of error. Those concerned with magnetic effects from
the aggregates or the concrete matrix, and those due to variations in cross-
sectional shape of the bars should not affect the modern covermeter, but
care must always be taken when dealing with multiple bars and the effects
of adjacent steel such as window frames, crash barriers or scaffolding as
mentioned earlier.
The Hilti Ferroscan Covermeter now provides an image of the
reinforcement under the concrete surface (
Figure 1.21)
and a simulated 2D
diagram of the reinforcement can also be obtained using the Profometer 5+
with a scanlog accessory, at significantly lower cost, but with less information.
1.4.3 Ultrasonic pulse velocity measurement (Pundit)
Introduction
The velocity of ultrasonic pulses travelling in a solid material depends on
the density and elastic properties of that material. In concrete, the pulse
velocity is related to its elastic stiffness so that measurement of ultrasonic
pulse velocity can often be used to indicate concrete quality as well as to
determine its elastic properties. Pulse-echo type surveys have only recently
become available as a technique, which is known as ultrasonic tomography.
Such surveys provide data not unlike that of radar and can be visualised
by computer interpretation to give a 2D or 3D plot of features within the
concrete (Lorenzi et al., 2010, Pinto et al., 2010).
Figure 1.22
illustrates the
PUNDIT 7 UPV equipment, available from Proceq.
UPV can be used to detect areas of concrete with defects (honeycombing,
voidage, or simply high w/c ratio) as these will show a lower pulse velocity
than sound areas. By appropriate calibration (and testing cube or cylinder
samples ultrasonically prior to compression testing is a good way to achieve
this) concrete can easily be tested to show whether it is of consistent quality
throughout a member, or from member to member, provided these are of
similar age. Considerable use of this technique was made in the evaluation
of structures made with high-alumina cement, for example, to follow any
loss in strength with time. Similarly, there is no reason why such a tool
cannot be used for routine quality assessment on new structures to ensure
consistent quality between pours and between different parts of a structure.
The technique is detailed in British Standard BS EN 12504-4:(BSI, 2004).
