Expansive fracture agent behaviour for concrete cracking

Abstract

Increasing concerns regarding litigation and terrorism provide a strong dual motivation to decrease high explosives usage in the construction industry. This paper provides parameter considerations and initial guidelines for the application of expansive fracture agents, typically used for concrete and soft rock removal. This approach may be especially appropriate near environmentally and historically sensitive sites. Thirty-three unreinforced blocks (approximately a cubic metre each) of varying strengths, composed of sand, cement, and fly ash, were tested under various temperature environments, with differing expansive agents, confinement levels and post-cracking treatments. Cracking characteristics such as crack initiation and crack expansion were analysed. Although the performance of expansive cement was dependent on a highly complex set of variable interactions, higher ambient temperatures, higher agent mixture temperatures and chemical configuration designed for colder temperatures decreased the time to first crack and hastened the extent of cracking. Conversely, higher strength material required more time to first crack, as well as an extended time to achieve a 25 . 4 mm wide crack. Manual interference with the normal material volume expansion slowed the cracking process but did not truncate it, while the manufacturer's recommendation to introduce water post-cracking actually reduced and slowed the extent of cracking.

An Alternative to Conventional Rock Fragmentation Methods Using SCDA: A Review

“…SCDA also known as soundless cracking agents, expansive demolition agents, expansive concrete, non-explosive demolition agents [34][35][36][37][38][39][40], are such alternative rock fragmentation agents. The material used in the soundless cracking is a cementitious powdery substance and when it is hydrated at an appropriate temperature its quantity expands through the formation of ettringite (3CaO·Al 2 O 3 ·3CaSO 4 ·32H 2 O) and calcium oxide (CaO) hydration.…”

“…This allows the material to volumetrically expand over a period of several hours. Under the existing confinement in a borehole, this volumetric expansion generates an expansive pressure and when it exceeds the tensile strength of the rock cracks form [39]. Thus, SCDAs can eliminate almost all the drawbacks associated with explosive demolition.…”

“…Technical data relevant to the performance of SCDA is limited to the manufacturer specifications, those may highly deviate from the actual conditions. According to the conducted Energies 2016, 9, 958 4 of 31 experiments on concrete fracturing, Laefer et al [39], the time required for the first crack varied from 8 to 21 h at 38 and 24 • C, temperatures respectively, whereas the product specification indicates cracking occurs after 10~12 h of injection (Bristar, Table 1). This implies that the product specifications which are given on borehole diameter show extreme variation with experimental data.…”

“…Boreholes with diameter 6.35 mm were successfully used by Gambatese [36] to generate controlled cracks in concrete, despite that requirement of minimal 32 mm diameter given in the product specification (Betonamit, Table 1). Though there are many variations in the composition of SCDA reported in the literature [37,[39][40][41][42]51], a more common chemical composition (as shown in Table 2) can be found in any type of SCDA with the major proportion of CaO. A low content of MgO is also used to facilitate the formation of clinker in the production process of the material [52] and a low Sulphuric anhydrite (SO 3 ) content is used to regulate the quick and irregular setting of the slurry [53].…”

“…In Addition to the borehole spacing, the bore hole pattern also significantly affects the fragmentation characteristics using SCDA and therefore Laefer [39] using 38 mm diameter boreholes in large 32 concrete blocks with varying compressive strengths studied the crack propagation in concrete. The observed crack shape variations are shown in Figure 16.…”