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Tests on Fresh Concrete

Overview

Source: Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA

Concrete is one of the most common construction materials and consists of two phases: the mortar phase, comprised of concrete, water and air, and the aggregate phase, comprised of coarse and fine aggregates. There are two key considerations when designing a concrete mix. First, the concrete must be workable and easy to cast in the forms in its fresh condition, even when the forms are packed with steel reinforcement. In this condition, it is the rheology of concrete that is important. Second, the mix must produce a hardened concrete of specified strength at 28 days (or similar specified time) that is durable and provides good serviceability.

In this laboratory exercise, a method of concrete mixture proportioning, named the trial batch method, will be explored. The concrete produced will be used in conducting typical tests to determine the principal characteristics of fresh concrete, including slump, flowability, air content, and density. The trial batch method is a simple, empirical approach to mixture design.

The objectives of this experiment are fourfold: (1) to use the trial batch mix method to determine optimum proportions of aggregates, cement, and water for concrete to meet specified slump requirements, (2) to learn concrete mixing practice in a laboratory environment, (3) to observe the characteristic properties of fresh concrete, and (4) to prepare 4"x8" concrete cylinders for subsequent evaluation.

Procedure

The procedure below first describes the mixing process and then the typical tests (slump, density, and air content) used in the field to determine workability, consistency and quality. The procedure described here has been found to work well with a small concrete mixer.

1. Mixing Concrete by the Trial Method

  1. Weigh quantities of coarse aggregate and fine aggregate and store them in separate containers. Record the exact weights on the data sheet.
  2. Weigh the quantity of cemen

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Results

In general, mixes such as the one described above will have slumps of 3 to 4 inches. Such values are common for small jobs with little steel congestion in the forms. In modern construction, the widespread use of superplasticizers has meant that it is economical to get much higher slumps (6 to 10 inches, i.e., self-leveling concrete). Non-air-entrained mixes will show air contents below 2%, while air-entrained mixes, depending on admixture dosage, will show 5% to 8% air content. The uni

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Application and Summary

Slump cone and flow table tests are in-situ test results used to determine if the concrete being delivered to the site has the specified workability. These tests are meant to ensure an adequate rheology for the mix, i.e., a good initial "viscosity" that lasts long enough for the concrete to get from the batching plant to its final position in the forms without leaving large voids or similar defects around the reinforcement. Additionally, the air-content test is key to ensuring long-term durability in areas where

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Tags
ConcreteConstruction MaterialConcrete MixtureRheologyTrial Batch MethodFresh ConcreteViscosityBatching PlantConstruction SiteIn Situ TestLaboratory TestingCementWaterCoarse AggregatesFine AggregatesAir ContentWater to cement RatioStrength Of Concrete28 Days After Casting

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0:08

Overview

1:17

Principles of the Trial Batch Method

4:24

Mixing Concrete by the Trial Batch Method

7:10

Preparing Concrete Test Cylinders

8:05

Results

9:29

Applications

10:58

Summary

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