Effects of Specimen Size on Efficiency of Shape Memory Polymers in Self-healing Concrete

This research investigates the effects of change in the size of concrete specimens on the efficiency of shape memory polymers in self-healing concrete. It aims to explore concrete crack healing using polyethylene terephthalate shape memory polymers and understand the causes of changes in efficiency with different specimen sizes.

Effects of Specimen Size on Efficiency of Shape
Memory Polymers in Self-healing Concrete
 Background:
In construction, concrete is the most preferred material because of its high compressive
strength, ease of molding and low cost. However, concrete is susceptible to cracking due to its
low tensile strength. Reinforcement is usually embedded in it to upturn the tensile strength of
the concrete. Cracking affects the durability of the structure and leads to degradation because
of corrosion of the steel reinforcements. Nearly half of the construction budget in the Europe is
spent on repair and maintenance of concrete structures (Cailleux and Pollet 2009). Therefore,
there is a need to minimize the cracks developing in concrete to avoid frequent repairs on the
structure. Cracking negatively affects concrete durability by creating passageways for materials
that cause steel reinforcement corrosion. Under certain conditions, concrete is capable of
autogenous or natural healing. When steel bars exposed and corrode due liquids and gasses the
structure will collapse. Therefore, the maintains and crack repairing is indispensable. Therefore,
Concrete Self-Healing is one of the suggestions solutions for this problem. Self-healing means
that concrete heals and close the cracks by its self. It can be classified into three groups:
vascular self-healing, intrinsic self-healing and capsule-based self-healing(Van Tittelboom and
De Belie 2013). Nevertheless, there are wide numbers of research which concern on a different
type of concrete self-healing and their abilities to be used in the large scale of the structure.
The concept of use of shape memory polymers to generate compressive stresses at the face of
tensile cracks in cement-based materials have been proved by various researchers. This
research seeks to look into the effects of change in the size of concrete specimens on the
effectiveness of shape memory polymers in healing induced concrete cracks.
 The Topic:
As a partial fulfilment for the requirement for the award of a degree, this research seeks to
answer the question; does change in the size of concrete specimen influence the efficiency of
shape memory polymers in self-healing concrete?
 Aims and objectives:

 To investigate concrete crack healing by use of polyethylene terephthalate shape
memory polymers.
 To investigate the causes of changes in efficiency of the shape memory polymers
autogenous healing technique with change in specimen sizes.
 Methodology:
 This study will adopt experimental research system to gather evidence to achieve the
set research aim. Experimental tests will be carried out within a Cardiff university
laboratory. Materials required in this research will be acquired locally and any relevant
tests will be carried out on materials to determine their fitness for this research.
 Tasks/resources:
Materials:
Cement
Ordinary Portland cement acquired locally.
Fine Aggregates
River sand or quarry dust with less than 30% silt content.
Coarse Aggregates
Crushed stone coarse aggregates free from dirt and silt with a maximum aggregate size of 20
mm.
Water
Tap water safe for human consumption.
Polyethylene Terephthalate (PET) Tendons
To be acquired from local industries.
Experimental Method
Polyethylene Terephthalate (PET) Tendons
PET tendons of lengths 400 and 900 mm for 500x100x100 and 1000x150x100 and each tendon
containing 200 PET strands. PET strands are to be manufactured using commercial grade PET.
The filaments will be manufactured and die-drawn as described in (Teall et al. 2018). with
strand diameters of 0.9 mm and draw ratio of 4.0.

Figure 1 PET tendon schematic diagram (Teall et al. 2018)
Restrained Shrinkage Test
a) Raise the input voltage until the internal thermocouple records 90°C.
b) Regulate the temperature by switching electric input on and off ensuring it remains with
± 5°C of 90°C for 15 minutes. A higher fluctuation is expected in external thermocouple
due to higher heat rate loss on the external face of the tendon.
c) Turn off electric input and allow the tendon to cool at room temperature.
d) Measure temperature and load at a 2Hz rate using thermocouple and load cell
respectively.
e) Do trio tests in this case.
Figure 2 SMP Testing rig arrangement (Teall et al. 2018)
Concrete Preparation
a) Mix concrete using proportions provided in table 3-1 below in a concrete mixture until
you attain a uniform mix.
b) Place concrete in the specimen moulds in three layers while compacting using any
appropriate method.

Table 3-1 Concrete C30 Mix Design
Cement (Kg/m3) w/c ratio Coarse aggregates (Kg/m3) Fine aggregates (Kg/m3) water (Kg/m3)
350 0.5 1050 700 175
Beam Specimens Fabrication
a) Cast all beams and leave to cure in air.
b) After 24 hours remove beams from moulds and submerse in water at 20° C.
c) After 6 days from casting, remove the beam specimens from water, give 5 mm notch
and knife edges in preparation for testing.
Table 3-2 Beam specimens’ description
Sample Number Description
500x100x100 3 Reinforced concrete beam
containing 1 PET tendon.
1000x150x100 3 Reinforced concrete beam
containing 1 PET tendon.
Concrete Crack Closure Tests
a) After 7 days subject beams to three point bending to form cracks. Load beams at a rate
of 0.001 mm/s to 0.5 mm CMOD. Hold the CMOD constant at 0.5 mm and take
microscope images on both sides of the beam at location of crack. Unload the beam and
take additional microscope images.
Figure 3 Figure5 500x100x100 Beam specimen three-point bending representation (Teall et al. 2018)
b) Immediately after three-point bending, activate the tendons by passing current through
the nichrome wire. Increase the input voltage until a temperature of 90°C is recorded by
the internal thermocouple or wire temperature reaches 150°C. Keep the voltage at a
constant rate for 1 hour after which turn off the current and allow the tendon to cool at
room temperature.

c) Subject the beams to three-point bending. Reload the beams at a rate of 0.001 mm/s to
1 mm CMOD.
Crack Width and Strain Measurements
a) Take microscope images of the crack along with a graduated scale on both sides at peak
load and after unloading on both the initial three-point bending and the reload three-
point bending tests.
b) Take additional images after tendons activation. Measure crack width perpendicular to
the crack face in each image at three points 0.75 mm apart using ImageJ software. Use
average crack width from both sides of the beam specimen for further comparison.
c) Use DIC system with two LA Vision Imager X-lite 8M CCD cameras and Davis analysis
software to determine displacement and strain. take images every at intervals of 30s
during loading , activation and reloading using the DIC system. Process these images in
DaVis software to track changes over producing 2D strain images to visualise closure of
crack.
• Expected Results:
The first research objective is to investigate concrete crack healing by use of polyethylene
terephthalate shape memory polymers. When heated PET will shrinks creating forces which will
be applied on the concrete crack formed through three-point bending. The specimens are
expected to regain their flexural strength and probably exceed their original strength flexural
strength. The second objective in this research is to investigate the causes of changes in the
efficiency of the shape memory polymers autogenous healing technique with the change in
specimen sizes. The efficiency of this method is expected to decrease with increase in specimen
size. As the specimen size increases a larger shape memory polymer is required which leads to
reduction if cross sectional area of concrete resisting flexural forces, thus shape memory
polymers generating high shrinkage stresses for small replacement of concrete are necessary in
large concrete specimens (Teall et al. 2018).

 Gantt Chart:
Assignments 24-Jun 1-Jul 8-Jul 15-Jul 22-Jul 29-Jul 5-Aug 12-Aug 19-Aug 26-Aug 2-Sep 9-Sep
1 conceptualize idea.
2 Defining specifications.
3 Lay down procedures and methodologies.
4 Assemble materials and equipment.
5 Preparation and casting.
6 Curing of cubes.
7 Application of three-point bending.
8 Investigating cracks and Recording of results.
9 Report writing.

References:
Cailleux, E. and Pollet, V. 2009. Investigations on the development of self-healing properties in
protective coatings for concrete and repair mortars. 2nd International Conference on Self
Healing Materials 32(0), pp. 1–4. Available at: http://www.j-act.org/headers/171.pdf
%5Cnhttp://www.cstc.be/homepage/download.cfm?
dtype=research&doc=INVESTIGATIONS_ON_THE_DEVELOPMENT_OF_SELF_HEALING_PROPERT
IES.PDF&lang=en.
Teall, O. et al. 2018. A shape memory polymer concrete crack closure system activated by
electrical current. Smart Materials and Structures 27(7), p. 75016. Available at:
http://dx.doi.org/10.1088/1361-665X/aac28a.
Van Tittelboom, K. and De Belie, N. 2013. Self-healing in cementitious materials-a review. doi:
10.3390/ma6062182.