Germann Instruments A/S. GI. Emdrupvej 102, DK-2400 Copenhagen, Denmark Phone: +45 39677117 Fax: +45 39673167 E-mail: [email protected] Web: www.germann.org. Early age strength assessment with LOK-TEST and COMA-Meter. For timing of safe and early loading operations
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Lok-Test for compressive strength timed by COMA-Metermaturity readings
A steel disc cast into the concrete is pulled against a counterpressure on the surface with a calibrated, hydraulic pullmachine.
A sample pullout LOK-TEST cone, removed for inspection is illustrated in figure (f).
The concrete between the disc and the counterpressure is being compressed (d), hence the pullforce is a direct measure of the compressive strength.
Pulling is done only to failure of the concrete or to a required strength. The disc is usuallyleft in the structure (e) leaving the surface almost undamaged.
From left to right, inserts for pullforce of 0-50 kN:
L-40 Control insert for nailing to wooden formwork
L-42 Early form stripping insert with L-44 steel plate
L-45 Insert, disc and stem, thread locked and coated
L-49 Floating insert
High strength (0-100 kN) versions of the above, using the L-46 disc and stem, are available, but require high strength pullbolt and accessories.
L-40 Control insert is an L-45 with a Masonite plate screwed to the stem.
Four nails are used to attach the plate to the wooden formwork.
When the formwork is removed, the plate breaks, leaving the L-45 insert in the concrete.
The screw and stem are removed for LOK-TEST testing.
L-42 Early form stripping insert is an L-45 with a metal washer, nut and screw.
The L-44 steel plate and insert are bolted to a plug fitted to a hole cut into the formwork.
When the proper maturity time is reached, the plug and hardware are removed for access to the insert for LOK-TEST testing.
L-49 Floating insert is an L-45 with a plastic cup, metal plate, washer and screw attached for testing on the surface.
The insert is tilted at an angle between 15-25 degrees.
Care must be taken to properly place this insert so that the aggregates that flow into the pullout failure zone represent the actual distribution of the aggregates in the mix.
Pullout testing is the only reliable and repeatable method for on-site determination of the compressive strength of concrete. Unlike the poor correlations derived from methods likeultrasound or the rebound hammer, the LOK-TEST readings are converted directly into compressive strength.
Numerous major correlations taken over the past 20 years have consistently shown that the stability of the LOK-Test correlation is unaffected by variations in types of cement, w/c-ratio, age, curing conditions, air entrainment, admixtures, flyash, and shape/type and size of aggregates up to maximum of 40 mm. Only the use of lightweight aggregates has produced a significantly different correlation.
L-10 Bourdon Gauge Pullmachine in use. illustrated in figure (f).
L-11 Electronic Pullmachine in use.
Timing of early loading operations and early stripping are strong points of the pullout system. Increased safety, cost savings, and fast track construction are the main benefits when using the LOK-TEST for new structures.
LOK-TEST is minimally invasive. Coring creates holes in a structure that weakens the structure and requires patching.
Sixteen correlations between pullout force and standard cylinder compressive strength
One of LOK-TEST’s strongest advantages is the stability of the correlation between the LOK-force in kN and the compressive strength, cylinders, or cubes,in MPa.
Especially at early ages, the sensitivity of the LOK-Test reacts to even small differences in concrete quality
Recommended correlation between pullout force and standard cylinder compressive strength.
The 95% confidence levels between the LOK-TEST and cylinder strength, for aggregates sizes 16 mm and 32 mm.
As shown, changes in the LOK-force in kN’s, causes proportional changes in compressive strength in MPa.
Other methods, such as ultrasound and rebound hammers, have a large change in compressive strength for a modest increase inthe velocity or rebound number.
L-10 LOK-TEST kit
L-11 LOK-TEST kit
The COMA-Meter (Concrete Maturity-Meter) measures, in the most simple manner, the maturity of in-place concrete.
A capillary tube contains a liquid that evaporates according to the Arrhenius equation for maturity. The tube is placed on a scale in a container. Following casting of the concrete the capillary tube is broken at the zero mark, threaded into the container and the container is pressed into the fresh concrete.
The temperature in the container will quickly stabilize with the temperature of the concrete. Evaporation of the liquid in the container will take place at a rate determined by the temperature. The level of the liquid - readable on the scale - indicates the maturity, the number of days (M20), the concrete is old at 20ºC (68ºF).
Germann Instruments’ newly formed cylinder compressive strength.NDT-Titans group offer the following services on a worldwide basis.
1) General NDT Seminars 2) Training Courses 3) Implementation Support 4) Testing On-site
Group members are all highly trained and experienced professionals (an average of over 20 years) who are consultants and system developers. Years of practical NDT field work have honed their skills to the highest level.
Training on the systems, along with NDT seminars and support, before, during, and after projects are available to our clients on the following web site: