Determining the roughness of precast concrete slabs correctly: first real-life tests analyzed

To date, the sand-area method has been applied most frequently for determining the mean roughness of the rough-grained bond surface of precast wall panels and floor slabs designed as waterproof concrete structures. This conventional approach is increasingly being complemented by laser profile measurements.

The German Guideline on Waterproof Concrete Structures provides the option of also using precast wall panels or floor slabs as waterproof structures. For this purpose, a cavity-free bond must be created between the precast slabs and the cast-in-place concrete infill. To achieve this goal, the guideline specifies that the insides of precast slabs be designed as having a rough-grained surface and a mean surface roughness of at least 1.5 mm. To date, the sand-area method has been applied most frequently for verifying this value. However, this conventional approach is increasingly being complemented by laser profile measurements because laser-based methods bring about major benefits compared to the sand-area method.

Building with precast floor slabs and wall panels has been a tried-and-tested method for a long time. Precast floor slabs comprise a precast concrete slab and the lattice girder reinforcement required for their transport and installation. On the construction site, a cast-in-place concrete infill is applied to the precast slab so as to ensure the structural performance of the entire component after hardening of the cast-in-place portion. In its finished state, the precast slab’s structural capacity will not differ from that of a monolithic floor slab. The same applies to precast wall panels that were originally derived from precast floor slabs. Such walls consist of two precast concrete slabs between which a connecting lattice girder structure is inserted to ensure an appropriate spacing. The free space between these two concrete shells is filled with in-situ concrete after installation on the construction site. Once this concrete has hardened, the precast wall unit’s structural performance will be equivalent to that of a reinforced concrete wall installed using the cast-in-place method. Precast floor slabs and walls have been used in a wide range of buildings and structures since the mid-1990s, and their share is still on the increase because this method is associated with significant economic benefits. Both clients and specifiers are particularly convinced of the shorter time to completion and high standard of workmanship that this method ensures based on the prefabrication at the precast plant. Among other fields, precast walls are also used for constructing waterproof concrete structures.

 

Building with waterproof concrete structures

In 2003, the German Committee for Structural Concrete published the first edition of the Guideline on Waterproof Concrete Structures. Since its adoption, precast wall panels have consistently been included as an option in this generally accepted specification because they have stood the test of time for over 20 years in terms of constructing safe waterproof concrete structures. However, using waterproof concrete alone will not suffice for ensuring the water tightness of the structure.

The Guideline on Waterproof Concrete Structures specifies additional requirements to be met for ensuring proper functioning of the structural precast walls even in the presence of pressing water. For instance, the core mix must be impervious to water, and the insides of the precast wall panels making contact with the cast-in-place infill concrete must have a consistently rough-grained surface in order to ensure a perfect bond between the core mix and the precast slabs. This bond is designed to prevent water from penetrating into the joint between the cast-in-place infill and the precast elements. In addition, all butt joints and openings included in the system must be sealed by appropriate means.

 

Focusing on the joint between the precast shell and the cast-in-place infill

The water impermeability of precast walls essentially depends on creating an appropriate bond and cavity-free connection between the precast shell and the subsequently poured cast-in-place infill. Besides ensuring the optimal interaction of physical phenomena such as adhesion and friction, the steel proportion in the lattice girders is another influential factor. The key criterion, however, is the roughness of the surfaces. A distinction is made between four different roughness categories, namely very smooth, smooth, rough, and interlocked. The Guideline on Waterproof Concrete Structures requires precast wall panels to have “rough-grained” surfaces, which can be created either by designing a custom concrete mix with a specific consistency or by roughening the precast panel’s fresh concrete surface using a steel rake prior to compaction. However, rakes are difficult to use in the area of the lattice girders, thus creating the risk of remaining smooth surface portions.

The Guideline focuses on the surface roughness of precast elements with cast-in-place infills. Its revised edition published in late 2017 specifies more demanding requirements for the roughness of surfaces because the mean surface roughness Rt must now be at least 1.5 mm, as opposed to the previous 0.9 mm. Appropriate test methods must always be applied in order to prove compliance with the specified surface roughness of the precast element, including consideration of the lattice girder areas. To date, the sand-area method has been applied most frequently, but it is unsuitable for the lattice girder zone.

Sand-area method according to Kaufmann

The sand-area method also referred to as sand patch test or Kaufmann method (according to N. Kaufmann, who introduced this method in Germany modeled on a UK test standard) is applied for determining the roughness or texture of a surface volumetrically. This approach is thus suitable for capturing the condition of surfaces in road construction or the roughness of the base surface prior to coating for the purpose of concrete repair. The sand-area method determines the mean roughness of the tested surface. For this purpose, the person carrying out the test pours a known volume of sand onto the clean and dry surface and spreads it using a wooden or metal disc so that it forms a circular patch until all sand has settled in the surface cavities. In the next step, the diameter of the circle is measured, and the mean surface roughness Rt is calculated by dividing the volume of the spread sand by the sand patch diameter.

For several decades, the sand-area method has been the most widespread approach to assessing the roughness of a base surface prior to coating because it is easy to apply and requires a relatively small amount of material. However, though this test method is easy to use, it is also associated with some major weaknesses. For instance, the conventional sand-area method is suitable only for horizontal or slightly inclined surfaces. Only by deriving the gel-sand method from the conventional sand patch test did it become possible to also test vertical or overhead surfaces. None of these methods, however, is suitable for testing at greater depths inside precast wall panels without destroying one of the shells of the double wall. Yet the test method is associated with additional issues because its outcome will also strongly depend on the person performing the test. As part of a research, scientists requested different test engineers to perform the sand patch test on one and the same test slab. In some cases, the values determined for the mean surface roughness differed from each other by over 25%, for a wide range of reasons. One of the variables, for example, was the pressure applied when spreading the sand used in the test over the concrete slab. In other cases, the sand patch strongly deviated from the circular shape and its edges were so irregular that no accurate diameter measurement was possible. In addition, the test result may also be influenced by non-compliance with the specified sand particle size ranges.

The Guideline on Waterproof Concrete Structures also provides the option of performing laser-based measurements for the purpose of determining the mean surface roughness. Laser-based tests provide a useful alternative to the sand-area method with its strongly subjective influences. Besides overcoming these shortcomings, state-of-the-art laser profile measuring instruments can also deliver additional information on the characteristics of the concrete surface.

Laser profile measurement

DIN EN ISO 13473-1 (“Characterization of pavement texture by use of surface profiles – Part 1: Determination of mean profile depth“) includes the option of applying contactless electro-optical measurement methods, primarily laser profile measurements, to replace volumetric methods. Laser-based methods provide a number of advantages compared to the sand patch test. They are not only accurate and easily reproducible, but they also deliver detailed results describing the texture of the surface. Moreover, the mobile, handheld measuring instruments currently available on the market can also be used on vertical or overhead surfaces. Last but not least, laser profile measurements function almost independently of the person performing the test, which is why they are hardly affected by subjective influences.

Yet laser-based methods are currently still associated with a few shortcomings, including the relatively high equipment cost and the risk of potential measurement errors on very steep profiles and edges that may occur due to shadow effects, although such inaccuracies can be rectified by using a suitable software.

Such adverse effects can be mitigated by positioning the laser perpendicular to the direction of travel. Other distorting factors include strong external light effects and strongly reflecting (e.g. mica) or translucent particles present on the surface. As far as possible, measurements should be carried out on dry surfaces because moisture can also have an impact on the result.

The Rp values measured using the laser-based method must be converted to the Rt values of the volumetric method because the mean surface roughness specifications continue to refer to the sand-area method. Comparative analyses demonstrated that the surface roughness values Rt determined in the sand patch test were about 10% higher than the values measured applying the laser-based method. In addition, it is important for the laser profile measurement method to become widely accepted that it should ensure comparability of results by standardizing the analysis and interpretation of measured values because although the Guideline on Waterproof Concrete Structures mentions the option of applying laser-based methods, it fails to provide any guidance on how to perform such measurements.

Development of a new laser profile measuring system

Laser-based measuring instruments have been continuously improved and optimized on the basis of comprehensive studies of the laser profile measurement method. The laser profile measuring device recently presented by Syspro Group in collaboration with Prof. Dr.-Ing. Rolf-Rainer Schulz, of Frankfurt University of Applied Sciences, allows for roughness measurements even on the insides of precast wall panels and in the lattice girder area. In contrast, previously available devices were unsuitable for performing such measurements on the insides of precast walls because of their size or owing to the required measuring distance, for instance when using line lasers. This is why Syspro Group and Prof. Dr.-Ing. Schulz jointly decided to design an innovative, compact and slender instrument that can also be used on-site in confined spaces.

This endeavor gave rise to the SL Laser Profilometer. The linear drive of the laser is located in the front section of a 2 m long aluminum profile, enabling measurements within cavities down to depths of several meters. This arrangement requires a minimum clearance of 10 cm between the precast concrete shells and a width of at least 15 cm between the rows of lattice girders. The integrated microcomputer makes it possible to analyze the measured values in accordance with DIN EN ISO 13473-1 on-site either after each measurement or after a series of measurements, including statistical analyses. This measurement method determines not only the surface roughness but also other key parameters, thus improving the reliability of the results and options for interpretation. DIN EN ISO 13473-1 prescribes that at least ten individual measurements at a length of at least 100 mm per test section are necessary in any given test area, which is equivalent to at least five individual measurements to be performed at a given measuring length of 200 mm. However, it appears to be useful to double the number of measurements particularly on heterogeneous surfaces.

As a matter of course, the specification of a consistently rough-grained surface in the Guideline on Waterproof Concrete Structures also applies to the area underneath the lattice girders in order to prevent remaining untreated, smooth surface portions. However, the concrete surfaces underneath lattice girders are difficult to reach for the purpose of manual roughening, which is why implementing this requirement has previously been a time-consuming, labor-intensive process. Testing these difficult-to-access areas has been equally difficult, which is why the efforts at precast plants and on construction sites were usually limited to visual inspections and comparisons with reference surfaces. In contrast, the new SL Laser Profilometer also enables scanning of the surface profile underneath lattice girders with the aid of an oblique laser beam routed near the bars of the lattice girders.

 

Successful real-life tests

Syspro Group is currently testing several new laser-based measuring instruments under real-life conditions, with the following preliminary conclusions: The profile measurement method using the SL Laser Profilometer meets all requirements for practical use. Not only can it completely replace the sand-area method – it also allows for measurements in locations that were previously inaccessible for inspection. These include, for example, deeper zones of precast wall panels as well as the areas underneath and in-between lattice girders. In addition, this new system makes it possible to carry out measurements on installed walls prior to pouring the cast-in-place infill.

Moreover, additional roughness parameters can be determined to permit a more reliable and plausible assessment of the surface texture. Measuring speeds are basically equal to those reached by the sand-area method, and the software that comes with the instrument is easy to use.

CONTACT

Syspro-Gruppe Betonbauteile e. V.

Postfach 90 11 53

63420 Hanau/Germany

info@syspro.org:info@syspro.org%3C/a%3E">

www.syspro.de

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