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In investigation into the durability and performance effects of CFRP strand in permanent ground anchors
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posted on 16.01.2017by Sentry, Matthew Robert
Ground anchors are a form of ground improvement technique that strengthens and stabilises structures to the surrounding ground, whether it be in soil or rock. Steel strand ground anchors have many advantages when transferring applied loads into the surrounding sub-base; however the environments in which they can be constructed in are aggressive and can cause corrosion and premature failure of the anchor should they not be adequately protected. Current ground anchor best practices require a complex double protection boundary layer system to encase the bond length, free length and the anchor head and may not guarantee complete protection against corrosion.
Fibre Reinforced Polymer (FRP) materials in the construction and infrastructure industry has seen significant interest and development in recent years. Unfortunately these developments have not extended to the ground anchor industry. As such, little is known about material performance under adverse ground anchoring conditions, where exposure to extreme alkaline, acidic or briny environments is not uncommon. This thesis investigated two different types of Carbon Fibre CFRP strands to assess if they are suitable for ground anchor applications and what environments they a can perform safely and successfully unprotected.
Absorption, Scanning Electron Microscopy (SEM), spectroscopy and tensile performance tests were conducted in aggressive alkaline and acidic groundwater environments and neutral solution for a period up to six months at elevated temperatures of 30 degrees Celcius and 60 degrees Celcius. These tests were conducted unstressed and stressed. Results showed that although absorption of aggressive environments were observed and localised penetration of the aggressive solution had occurred (without damage or deterioration of the composite structure) the tensile performance of the CFCC product was unaffected by the aggressive ground environments in both a stress-cured or unstressed-cured state. Absorption levels and penetration of aggressive solutions were higher for the HS25x2 material. This penetration showed signs of composite and fibre breakdown over the six months curing. Tensile performance was still efficient, with most results performing better than control results; however, inconsistencies in the tensile results were reported as a result of gripping issues developed during material curing.
Ground anchor bond assessment was conducted under confined conditions on a single CFRP strand over various bond lengths, bonded using a standard ground anchor cementitious based grout mixture with no additives. Pull-out tests were conducted on both CFCC and HS25x2 materials. Results identified the minimum bond length required to induce strand rupture for the CFCC specimens was calculated to be 775mm. No minimum bond length was established for the HS25x2 due to a large scatter and variance in results over the numerous bond lengths tested. Finite Element Analysis (FEA) was conducted on the CFCC strand to develop greater understanding of the progressive debonding during axial loading and bond stress distribution along the bonded length of strand. FEA model was able to predict ultimate loads capacity and bond slip prior to failure accurately. FEA model was compared to full scale test results and strong linearity between bond stresses and progressive debonding distribution were identified. No FEA modelling was developed for the HS25x2 product as a result of the inconsistent bond pull-out results.
CFRP anchors are restricted with the methods available to lock the anchor off and the head. Bond type anchor heads are good reliable method for anchorage, but minimum bond length requirements are long and restrictive for ground anchor applications. This project identified the issues with current bond type anchor heads and developed a reliable and compact “modified bond type” anchor head system suitable to CFRP material similar to the profile of CFCC strand. The anchor head was successfully tested on CFCC strand where a four and 10 strand anchor head assessment was conducted. Results from the test identified specimen failure occurred as a result of strand rupture in the bond length and no movement or cracking at the distal end of the anchor head. The success of this anchor head enabled the development of a full scale gun barrel test to be conducted.
Using the anchor head technology developed a four strand CFCC gun barrel test was conducted with a 3.0 meter bond length and 2.0m free length. Testing was conducted as per an acceptance test program for steel strand ground anchors with cyclic loading over 10 equal load increments up to proof load of 800kN. Results showed a linear elastic extension over each load increment. Creep was observed at each cycles peak load (held for 50 minutes). The net anchor extension was a direct result of the creep. There were no signs of strand deformation from the data collected. No strand movement was observed at the anchors distal end, nor any movement, cracking or pull-out of the wires within the anchor head system. Strain gauges mounted along the strands free and bond lengths plotted load and bond stress distribution along the strand throughout the loading process. Bond stress distribution results replicated FEA model results obtained from the single strand bond model. An apparent free length of 3.5 meters was calculated. Progressive debonding of each strand throughout the test was established. Strain results verified the calculated apparent free length of 3.5 meters.
The research conducted of CFRP strand identified that there are limitations for exposure to aggressive ground environments and bond strengths for selected FRP material. CFRP strand which can be securely anchored (using bond or other methods) and resist aggressive ground environments, such as CFCC, could perform as a permanent ground anchor in an unprotected state for the duration of the anchors design life. Guidelines and standards needs to be urgently developed to assist in designers utilising the knowledge developed in research the superior performance of CFRP strand in aggressive environment compared to steel strand ground anchors.