The new Guideline on Waterproof Concrete Structures (December 2017 edition, ) has been available since February 2018. DBV Heft [Volume] 43  contains a summary of the revised specifications included in the new version of the guideline. This article elaborates on the more sophisticated design and structural requirements for waterproof precast wall units contained in  ]. In addition, it will report on initial experience gained in applying these new specifications.
the minimum thickness of precast waterproof wall units produced according to the new guideline  can exceed the minimum thickness of cast-in-place walls.
Requirement of a “rough grain texture of the complete bond surface“ and increase of the so-called mean surface roughness to 1.5 mm
 requires the surface to be subjected to testing to be clean and dry. The diameter d of the sand area resulting from the circular distribution of a given sand volume V must be measured at three points equally distributed across the sand area, and the mean value of d must be calculated with millimeter precision. Equation (1) must then be applied to calculate the surface roughness expressed in millimeters.
Surface roughness = 4 × V/(π × d²) =
V [ml]/d [mm]² × 1272 (1)
With regard to Equation (1), it should be noted that the concepts of “mean surface roughness” and “surface roughness” are generally taken to be synonyms. DIN EN 1766:2000-03 , for instance, only uses “roughness”, unlike the Guideline on Waterproof Concrete Structures. This interpretation also becomes apparent when looking more closely at the wording of the DAfStb Repair Guideline  and the ZTV-ING .
The DAfStb Repair Guideline, Section 3.2.5, says:
“The mean surface roughness Rt is defined as the height of the notional cylinder with the diameter d and the sand volume V. […] Applying the sand volume V (cm³) and the diameter d (cm) of the approximately circular sand area, we obtain the mean surface roughness Rt (mm) as Rt = 40 × V / (π × d²).“
Compare this to the wording of ZTV-ING, Part 1-3, Section 4:
Thus, the specified increased (mean) roughness of the bond surface automatically requires a more stringent quality control system at the precast plant, primarily with regard to testing as part of in-process quality control and its documentation.
The above requirements result in the following process sequence for construction sites:
Order precast wall units explicitly in accordance with the new Guideline on Waterproof Concrete Structures, December 2017 edition ,
Recommendation: As part of the order, request documentation of the visual inspection performed at the precast plant
Random visual inspections on the construction site make it possible to ascertain if the concrete exhibits a relatively even surface texture. Based on some degree of prior experience, precast waterproof wall units with suspected non-conformance with the specified (mean) surface roughness can be identified. In such a case, surface roughness measurements must be performed on-site using the sand-area method or, if and when required, laser-based methods.
Requirements for clearance dimensions bw,iin waterproof precast wall units
Section 7, Table 1 of the new Guideline on Waterproof Concrete Structures  contains recommended minimum thicknesses for precast waterproof concrete elements depending on their exposure class, type of component, and design (see Table 1).
The recommended minimum total thicknesses of precast waterproof wall units and cast-in-place walls are identical for exposure class 1 (pressing water) (Table 1, Line 1). In exposure class 2 (moist earth), permissible thicknesses of cast-in-place walls are smaller than those of precast waterproof wall units (Table 1, Line 2).
, Section 7.2 (3) also includes requirements for the clearance dimensions bw,i between the inside surfaces of precast walls for ensuring effortless concrete pouring and appropriate installation of inside waterstops. These dimensions must generally be adhered to for exposure class 1 (pressing water). In the following paragraphs, the requirements for the clearance dimensions bw,i between the inside surfaces of precast walls are referred to as “standard requirements”.
In precast wall units, the actual wall reinforcement is usually installed in the precast panel. Within the meaning of the Guideline on Waterproof Concrete Structures, the concept of “supplementary reinforcement” includes all reinforcement layers located in the cast-in-place portion of the element (core layer), i.e. a vertical connecting reinforcement and, in some cases, an additionally inserted horizontal reinforcement for load distribution purposes (cf. Fig. 3). If and when required, stirrups or reinforcing mesh with a horizontal component will also be installed in the butt joint area of the wall panels. Such reinforcement also falls under the “supplementary reinforcement” category in accordance with the Guideline on Waterproof Concrete Structures.
, Section 7.2 (3) specifies the values for the clearance dimensions bw,i depending on the maximum aggregate size (Dmax), as follows:
Dmax = 8 mm: bw,i ≥ 120 mm,
Dmax = 16 mm: bw,i ≥ 140 mm,
Dmax = 32 mm: bw,i ≥ 180 mm.
Specified bw,i values (special requirements) apply only to exposure class 1 (pressing water) in accordance with , Section 7.2 (3).
The special requirements for the clearance dimensions (bw,i between the reinforcement layers) should ensure appropriate concrete pouring in the core cross-section, i.e. effortless insertion of the concrete pumping hose, for instance to a maximum height of 1 m, and of the vibrator down to the base area. Supplementary reinforcement installed in the cast-in-place portion makes insertion of the hose and vibrator more difficult. To ensure either cavity-free embedment of inside waterstops (e.g. metal strips) or sufficient compaction of cast-in-place concrete in the base area, the requirements for the clearance dimensions bw,i generally distinguish between
inside waterstops and
Irrespective of the location of waterstops, care must be taken to ensure that the waterproof cast-in-place concrete (core mix) is always sufficiently compacted and protected against water ingress across the entire height of the wall exposed to pressing water.
Read Part 2 of this article in the August 2019 issue of BFT International.