Prelab

• Bring your datasheet from lab #1 where you recorded the details of your cement mix.

Problem

A.P. Struct-gineering  is sourcing a new supplier for concrete used in concrete pours as the flooring in houses built on rocky areas of N.S. 

The National Building code of Canada (accessible here: NBC Canada 2020 ) defines the minimum requirements for concrete used in building.

From Section 9.3.1.6. found on Page 818. 

Determine which concrete samples would be appropriate for Walls and Floors, which are only appropriate for Walls, and which samples Fail to meet the standard. 

Present an organized chart with the sample numbers in one column, ranked by maximum stress. 

Next to that column indicate "Floors & Walls", "Walls", or "Fail"

From Section 9.3.1.7 

From the maximum water allowances for concrete blends. Determine if your sample meets the standard.

• Be sure to compare you water mass to cementing materials mass only (this excludes sands, aggregates, and other components).

Hydration ratio is given by | mass of water : mass of dry mix

e.g: 770g sample mass - 700g dry mix = 70g.    Then 70g water : 700g dry mix or 10% hydration.

Comment on the 3 weakest samples (ultimate stress) and 3 strongest in terms of hydration ratio.

See image below for Concrete Blend Specifics.

Procedure

1. Use pliers to peel away the can from your concrete sample.

2. File your sample to create a neat flat surface on both the top and bottom.

3. Measure the dimensions and mass of your sample and record them on the data sheet.

4. Set the Instron moving downward, watch the compression and load as the sample is compressed.

5. Record the peak extension and load. Save your data to the computer.

Results

Assume that your sample dimensions are true for all runs. This is an approximation but works fine in general.

All mixes have 700g of Dry mix, with the final mass provided in excel data sheet labels.

• Calculate: Volume and then Density of concrete test sample.

• Create a Stress-Strain Diagram from the provided data for all runs.

  • Plot all runs on a single chart.

• Calculate or find for each sample:

  • Ultimate Stress and its corresponding Strain (σu and εu).

    • The original sample height is the gauge length.

• • Young's Modulus of Elasticity using (CSA A23.3) (Canadian Standards Association) methodology as shown below.

    • Use the equation for high strength concrete (Eq. 6).

      • Ec = Youngs’s Modulus of Elasticity for concrete (MPa)

      • Fc’ = Ultimate Stress of Concrete (MPa) -> Found via Load / CrossSectional Area 

      • Wc = Density of Concrete (kg/m3)

      • Note: ignore unit cancellation in the calculation, but be sure to enter them numerically in listed units above

• What was the average  σu  considering all of the samples? 

• If the manufacturer of this blend of cement and aggregate claims 20MPa after 28 days cure, does it's advertising stand up to our testing?

  • If not, suggest a reason why it may not. Here you should comment on the differences in professional concrete production versus our. What are the largest differences in your estimation?

• Determine graphically (via rise/run in linear region) the typical Modulus of Elasticity of the specific concrete blend tested.

  • State your methodology and thinking clearly. Which runs did you consider, ignore, weight more heavily etc...

  • Is there a pattern or general behavior that gives you confidence in this assumption?

  • Compare your result to the typical performance of High strength concrete from the textbook or online sources and comment on any reasons you feel may lead to the discrepancy. Comment specifically on:

    • Young's modulus (Manufacturer Source Data Indicates ~20GPa)

    • Ultimate stress (Manufacturer Source Data Indicates ~30MPa)

• Be sure to answer the overall problem statement based on the findings above.

Questions

1. For samples with similar hydration and significantly different strength what might be at fault?

2. Explain the yielding behaviour shown in your plots despite brittle materials like concrete have little to no malleability or flexibility.

3. Some common failure modes are shown below, which looks most like your sample?

   • • • (a) All cracks and fissures propagate from the upper or lower surface

       • (b) Top and Bottom surfaces intact with median structure broken away from core.

       • (c) Some cracks appear at locations away from upper and lower loading surfaces.