LAB-3b: Concrete Strength Specimen Preparation

Objective

To prepare specimens for un-reinforced, hardened concrete tests.

To determine the effect of water-to-cementitious materials ratio (w/cm), aggregate content, and aggregate type on the workability of concrete.

Definitions

Concrete: Particle reinforced composite.

Slump: A measure of workability, resistance to flow, of concrete.

Workability: Ease of placement.

Apparatus

Figure 3B-1 Cylinder molds: Diameter 6" and height 12", Beam molds: 6"x6"x20"

Procedure

Four factors are considered in this experiment:

  1. Water-to–cementitious materials ratio (w/cm)
  2. Amount of coarse aggregate used (CAF – coarse aggregate factor: loose [DRUW] volume of coarse aggregate per volume of concrete)
  3. Amount of cement in mix (CF – cement factor: number of sacks per cubic yard)
  4. Type of coarse aggregate used in the analysis of the test results obtained during this lab exercise

Table 3B-1 shows the factors and coarse aggregate types used, and the mix corresponding to each group and day. Note that they must be the same as in Lab 3a, so that workability, unit weight and air content can be correlated to strength.. For your group, record the quantities of each material per batch in Table 3B-3.

Table 3B-1. Workability Factors and Mixture Design Combinations
Factor Level Definition
Levels / w/cm / CAF / CF / Coarse Agg. Type
Low / 0.42 / 0.60 / 4 / River Gravel (RG)
High / 0.48 / 0.70 / 6 / Limestone (LS)
Other Cementitious Materials, or Mineral and Liquid Admixtures
Fly Ash / 25% by wt. of Portland Cement (substitution)
Air Entrainer / 0.5 mL / lb of cementitious materials
Factor Combination Plan
Day / Group / Mix # / w/cm / CAF / CF / Fly Ash / Air Ent. / Coarse Agg. Type
Monday / 1 / 1 / Low / Low / High / Yes / Yes / River Gravel (RG)
2 / 2 / High / Low / High / Yes / Yes / River Gravel (RG)
3 / 3 / Low / High / High / Yes / Yes / River Gravel (RG)
4 / 4 / High / High / High / Yes / Yes / River Gravel (RG)
Tuesday / 1 / 5 / Low / Low / High / Yes / Yes / Limestone (LS)
2 / 6 / High / Low / High / Yes / Yes / Limestone (LS)
3 / 7 / Low / High / High / Yes / Yes / Limestone (LS)
4 / 8 / High / High / High / Yes / Yes / Limestone (LS)
Wednesday / 1 / 1 / Low / Low / High / Yes / Yes / River Gravel (RG)
2 / 2 / High / Low / High / Yes / Yes / River Gravel (RG)
3 / 3 / Low / High / High / Yes / Yes / River Gravel (RG)
4 / 4 / High / High / High / Yes / Yes / River Gravel (RG)
Thursday / 1 / 5 / Low / Low / High / Yes / Yes / Limestone (LS)
2 / 6 / High / Low / High / Yes / Yes / Limestone (LS)
3 / 7 / Low / High / High / Yes / Yes / Limestone (LS)
4 / 8 / High / High / High / Yes / Yes / Limestone (LS)
Friday / 1 / 1 / Low / Low / High / Yes / Yes / River Gravel (RG)
2 / 2 / High / Low / High / Yes / Yes / River Gravel (RG)
3 / 3 / Low / High / High / Yes / Yes / River Gravel (RG)
4 / 4 / High / High / High / Yes / Yes / River Gravel (RG)

Each group is assigned to prepare 1.10ft3 of concretefor every mix number assignedenough forthe specimens shown in Table 3B-2:

  1. 3 cylindrical specimens (6 in diameter  12 in height)
  2. 1 flexural beam specimen (6620 in)
  3. Quality Control Tests (these tests were performed in Lab 3a; the instructor will indicate whether they need to be repeated to confirm results):
  4. 1 slump test
  5. 1 unit weight test
  6. 1 air content test

Table 3B-2. Volume and Types of Tests per mix number
Yield (ft3) / Comp. Strength (fc’) / Split-Tensile Strength (ft) / Elastic Modulus (Ec) / Modulus of Rupture (MOR)
1.10 / / / /
Table 3B-3. Mix Quantities per Batch (Yield = 1.10 ft3 per mix#)
Day / Group / Mix # / Water (lb) / Cement (lb) / Fly Ash (lb) / Coarse Agg. (lb) / Sand (lb) / Air Entrainer (mL)
1 / 1
2 / 2
3 / 3
4 / 4
1 / 5
2 / 6
3 / 7
4 / 8

Batch Preparation (ASTM C192)

  1. Mix-designs are to be prepared by each team and include fly ash and air entrainer, or as indicated by instructor. No intermediate aggregates will be used.Unless otherwise indicated ahead of time, the TA is responsible for preparing the proportions for all groups before each lab session, except for the High-Strength Competition. Groups are required to become familiar withthe class website’s worksheet to calculate the proportions during the first week of concrete labs.
  2. Weigh out selected materials according to Table 3A-2.
  3. Obtain concrete mixer.
  4. Moisten sides with water, a color change is sufficient. No standing water should be present.
  5. Pour the aggregates into the mixer (coarse aggregate and sand). Turn on the mixer and mix thoroughly.
  6. Stop the mixer. Add the cement and cementitious materials after the aggregates have blended. Turn on the mixer and mix thoroughly.
  7. Mix liquid admixtures with the water in a separate bucket.
  8. With the mixer on, add water with the admixtures by thirds. Do not let the paste or the mortar stick to the walls of the mixer. Failure to take care of this will result in a bad mix.
  9. Mixing is complete when the aggregates are fully covered by paste. The fresh concrete shall not remain in the mixer for more than 2 minutes after it is ready.
  10. Transport concrete by wheelbarrow to perform tests on staging yard.
  11. IMPORTANT: Do not flush concrete down any drain. Use trash bins as indicated by instructor. Failure to follow properly dispose of excess concrete will result in severe penalty to the lab grade.

Slump Test (ASTM C143)

  1. Wash the mold and place it on the board provided. Wash the board as well, but make sure there is no stagnant water.
  2. Hold the mold firmly in place by stepping on the foot pieces. Place the funnel on top of the mold.
  3. Fill the mold in 3 equal layers and rod it 25 times for each layer.
  4. Remove the funnel, strike off the surface and level the mix with a straight edge or trowel.
  5. Hold the mold firmly with your hands, step off the flaps, and carefully remove the mold vertically and place it upside down next to the sample.
  6. Measure the vertical difference between the top of the mold and the top of the specimen. Record this difference as the slump.
  7. Return concrete material to the wheelbarrow (to be reused in other tests).

Unit Weight(ASTM C138)

  1. Weigh the empty unit weight bucket and record.
  2. Fill the bucket in three layers; rod each layer 25 times.
  3. After filling in 3 layers up to the top, level the surface with the tamping rod or a trowel.
  4. Wipe-off clean the outside of the bucket. Weigh the bucket with the concrete mix and record it.
  5. Find the volume of the cylinder.
  6. Calculate the unit weight of the mix.
  7. Return concrete material to the wheelbarrow (to be reused in other tests).

Air Content Test using a Roll-A-Meter (ASTM C172)

  1. Obtain enough concrete for the bucket provided. Retain an extra portion of the same volume available in another container in case this test fails and has to be repeated.
  2. Fill the bucket in three layers; rod each layer 25 times.
    After filling in 3 layers up to the top, level the surface with the tamping rod.
  3. Strike off the top of the concrete with a straight edge. Wipe the top rims perfectly clean and clamp the top on.
  4. Slowly fill with water using the funnel with a baffle bottom so the sample will not be disturbed. Make sure no leaking occurs. If it does, discard sample, clean the equipment, and repeat steps 1 through 3.
  5. When water is up to the glass on meter neck, remove funnel and use the squirt bottle to add water up to the zero mark. Put top cap on.
  6. Invert the meter and shake until concrete settles free from base.
  7. While holding the neck, roll the meter for 5 minutes.
  8. Set the meter upright until air has fully risen in the neck.
  9. Read percent of air, at the liquid level, through the glass. If a thick layer of foam is present (>0.5 in), use 23 mL of alcohol to dilute it. Repeat if foam is very thick. For every 23 mL of alcohol present, add 0.5% air to the reading.
  10. Record results and discard sample. Do not reuse in other tests.

Molding of Cylinders and Beams for Hardened Concrete Tests(ASTM C39)

  1. Clean the molds, making sure they are free of dust or old concrete.
  2. Properly tighten all assembling parts of each mold.
  3. With a small amount of lubricating oil, brush the insides of the molds. Do not let oil collect on the bottom of the molds. With a towel, wipe off the excess oil from the mold.
  4. Place the prepared molds as near as practicable to the place where the fresh specimens are to be stored during the first 24 hours (this is done outside the lab).
  5. Pour the fresh concrete in the molds using a scoop, a blunted trowel or shovel, making sure that you achieve symmetrical distribution of concrete and to minimize segregation of coarse aggregate inside the mold.
  6. Fill the mold according to the following table. Use a 5/8" rod.

Table 3B-4. Specimen Preparation
Specimen Type / No. Of Layers / No. Of Roddings per Layer / No. of Mallet hits on sides per Layer
Cylinder / 3 equal / 25 / 15
Beam / 2 equal / 60 / 15
  1. Ensure that the rod penetrates the bottom layer throughout its depth. Distribute the strokes uniformly over the cross section of the mold and for each upper layer allow the rod to penetrate at least 1/2 in into the underlying layer.
  2. After each layer is rodded, tap the outside of the molds lightly 15 times with the mallet to close any holes left by rodding and to release any large air bubbles that may have been trapped. After tapping the last (overflowing) layer, spade and level the top with a trowel.
  3. Insert an identification tag in the specimen(not on the mold) to identify your specimen later. Include the following information:

Group Number

Section Number

Mix Number

Specimen/Test type

Specimen number or letter (for the same test type)

Date Mixed

  1. Cure all specimens by covering them with polyethylene sheeting for the first 24 hours.
  2. IMPORTANT:Do NOT discard any excess concrete unless all specimens have been made and fresh concrete tests performed.
  3. IMPORTANT: After 24 hours of curing, specimens must be stripped from molds, and samples moved to the 100% humidity room. Label all your specimens with a permanent marker with the same information as the label tag. Failure to do this will result in severe penaltyto the lab grade.
  4. IMPORTANT: wash all equipment and return it to original location. Wipe clean and reassemble molds after demolding. Failure to do this will result in severe penaltyto the lab grade.
  5. Cure each specimen for 7 days, or as indicated by instructor.

Data Collection

Table 3B-5. Test Results*
Day / Group / Mix # / Slump (in) / Air Content (%) / Unit Weight (lb/ft3)
1 / 1
2 / 2
3 / 3
4 / 4
1 / 5
2 / 6
3 / 7
4 / 8

*These tests were performed in Lab 3a; the instructor will indicate which or whether they need to be repeated to confirm results)

Minimum Required Data from Lab 3b to be Presented in Lab Report§†

§In general, a statistical analysis (averages, standard deviations, and coefficients of variation) is required for every test performed more than once, especially when data from other sections is needed to supplement your section’s data. Your instructor will explain the details of this requirement.

†Your instructor will indicate the required format and guidelines for all lab reports.

  1. Effect of w/cm, CF, CAF, and Aggregate type on slump, unit weight, and air content (only if these tests were repeated from Lab 3a or to confirm results).

Lab-3b Engineering Report Individual Discussion Questions

  1. Why is concrete tamped or vibrated when it is poured into the structure or control samples?
  2. Why should concrete be properly cured? List 3 methods of curing used in practice today?

1