Civil engineering and maintenance of infrastructures
Effect of non-linear differential shrinkage distribution on restrained stresses in bonded concrete overlays
Concrete overlays constitute a versatile method to restore and strengthen concrete pavements. Meticulous operations with removal of deteriorated concrete, surface preparation, overlay casting, compaction and curing are of outermost importance for a good and sustainable behaviour but it is also important to understand the phenomena that may cause high stresses and eventual cracking. Differential shrinkage, i.e., the shrinkage difference between the new-cast concrete overlay and the mature base layer concrete, causes restrained stresses in the bonded overlay, tensile stresses that may lead to vertical cracking. They may also lead to local shear stresses at boundaries, stresses that may cause debonding. Differential shrinkage has been studied during more than 60 years, but still there is no final solution that is accepted by the entire research society. The condition that the overlay shrinks more at the top than at the bottom is discussed. This non-homogeneous shrinkage causes stresses and stress distributions that differ from the calculated stresses determined for the simple case where the shrinkage is assumed to be constant across the entire overlay thickness or height. Calculations show that the maximum tensile stress in the overlay increases with increased degree of non-linearity of the differential shrinkage. Simultaneously, the tensile stresses are more and more local. The local nature of the tensile stress and the beneficial effect of overlay concrete creep are shown to be the probable reason for the absence of cracking in laboratory tests on composite concrete beams as well as in real cases.
Contact: Johan Silfwerbrand (profile pages)
Bulk hydrophobic civil engineering concrete for Nordic conditions: Freeze thaw action
Concrete is a composite building material which long term function can be modified for example by changing the water to cement ratio (w/c) or by adding in other chemical admixtures to change the fresh and hardened properties of the concrete. The overall goal of this study is to reduce the water absorption capacity of the cement paste/microstructure by at least 85 %. This is achieved by using bulk hydrophobic agents in the mixing phase rather than post hardened surface application. Numerous commercial agents and vegetable oils were tested and showed promising results at a dosage equal to 3% of cement weight. This though affected compressive strengths negatively. As these concretes will be exposed to Nordic winter conditions, the concrete should perform well under repeated salt water freezing and thawing. This continued study will show how a selection of these bulk hydrophobic concretes performed during this part of the study. The concrete has a w/c = 0.4 with a cement content (CEM I) of 430 kg/m3.
Continuous preventive bridge maintenance in Sweden – Field experiment on the effect of washing on concrete bridges
Bridges are an important part of the infrastructure. For the bridges to have the longest possible service life with minimum repairs, the maintenance is of great importance. One type of bridge maintenance that is rarely researched is the continuous preventive maintenance. The continuous preventive maintenance consists of removal of vegetation, cleaning of bridge joints and drainage systems as well as high-pressure washing of the structure. The effects of washing is heavily discussed but not properly researched. A study on the effectiveness of washing concrete is therefore being conducted. A field experiment has been initiated where concrete specimens are installed on an edge beam of a road bridge. The specimens are of two recipes where one represents an old bridge with rather high water-cement ratio and the other one represents a new bridge with a low water-cement ratio. 50% of the specimens are washed annually, while the others are not. Each year samples are collected and tested for a chloride profile. The results for the first year of exposure have been determined. They are promising but are still only very preliminary. The effect of washing, if any, will be visible after a longer exposure.
Contact: Louise Andersson , Johan Silfwerbrand (profile pages)