Nano indentation test is ...still.. running

With a bit of smallish problems last Tuesday we started our first batch of Nano Indentation tests.

First batch covers samples from Nautesund Bridge Original mix 1 (Prisms 1-2-3) and Nautesund aggregate Standard AAR-4 testing (Prism 5). All samples were subjected to 60°C and %100 humidity for 140 days. A total of 850 indent points selected for 4 samples (roughly 15mm X 15 mm).

On the 6th day of experiments, Indentation still continues... Needle is on the final indent group though.

Hopefully, by the end of today, we will get our samples our for further investigation

Nano indentation

Recently I have been working on ways to identify material properties of phases in my concrete samples. As of yesterday we started with our first Nano indentation test. The main aim of this study will be to identify dissolved properties of aggregate and paste in the matrix with known exposure to Accelerated test conditions. Here is a short photo series of how we prepared samples for nano-indenter.

We cut sections from expansion prism (roughly 1.5 cm thick)

Sections are organized named and further selections are marked

Smaller Specimens are prepared to fit the sample holder for Nano Indentation

Timing of Damage - Cutoff level in simulation results

Lattice simulations are capable of simulating crack pattern due to various loading modes. Either Aggregate, ITZ or paste (open cracks). As simulations run till the system failure (or predeifned failure threshold strain) there is a challenge on deciding on the cut-off level of damage progress in 2D system. Thanks to help from my colleague Branko Savija, now we introduced a itterative way to calculate total crack opening on every step. This approach allows us to compare damaged images (with known duration, environmental conditions (accelerated test) mix properties and aggregate geometry distribution) with threshold cracks (based on pixel counts) with relative damage in the system of lattice beams. Total crack width calculations shall be smooted for outliers but comparison graphs look like tese:

Sample 1: Core from Nautesund Bridge S31-3
Simulations are based on loading modes; AGG, ITZ, AGG+ITZ, Paste (20% randomly distributed). Based on image analyses cut-off line is 6%

Sample 2: Cube from accelerated tests (60°C) duration 140 days - original mix design from Nautesund Bridge (w/c=0.5) - No Alkali boosting (Expansion value: 0.24%)
Simulations are based on loading modes; AGG, ITZ, AGG+ITZ, Paste (20% randomly distributed). Based on image analyses cut-off line is 6%

Sample 3: Cube from standard accelerated tests (60°C) duration 140 days - original aggregates from Nautesund Bridge (w/c=0.5) - Based on standard grading curve - No alkali boosting. (Expansion value: 0.14%)
Simulations are based on loading modes; AGG, ITZ, AGG+ITZ, Paste (20% randomly distributed). Based on image analyses cut-off line is 10%

 This approach allows us indetify which loading modes progress faster. But there is still room for progress but initial results seems promising. Next step will be intentifying crack width growth by phase types and recalibration of material properties with Nanoindentation techniques.

SIMULATION: Crack formation under Aggregate Expansion

We have been trying to run simulations on various loading conditions and material properties to investigate crack formation in 2D sections. While we are busy with sample analyses and running simulations, here is a taster (if you may call it a result) video of how cracks grow under expanding aggregates. This simulation is based on a real test specimen (Upside down though).

Simulations continue till the specimen fails (or reaches a predefined stain level). You may realise some simplification on the section identification. Here are how the original sections look like. These specimens have been kept in 60°C reactor (RH 100%) for 20 weeks before sample preparation process.

 1- Normal Light Image

2- UV Light Image

Specimens are based on original mix design of Nautesund Bridge. you can find more information about this testing campaign here.

I have also done damage rating index analyses on this (and various other) specimens based on method defined by Grattan-bellew (1992). Some results regarding These analysis are as follows:

 Damage rating index - calculated based on Grattan Bellew (1992)

Distribution of Crack types observed on our samples

DRI analyses and simulations are continuing in full speed. Soon I will provide more data (and definitely more results) on our blog.

Busy months ahead..