Determination of Slump of Hydraulic-Cement Concrete

 Experiment No: 12

Determination of Slump of Hydraulic-Cement Concrete

 

Introduction

 

The slump of fresh concrete is a quantitative measure of the ease with which the concrete mixture flows during placement. Since the test for slump is straightforward and trustworthy, most mix-design procedures use slump as a basic indicator of workability.

AS the slump test is an indicator of workability when evaluating similar mixtures this test consists of filling a truncated cone with concrete, removing the cone, then measuring the distance the concrete slumps (ASTM C143). The slump is increased by adding water, air entrainer, water reducer, superplasticizer, or by using round aggregates. Table 1 provides recommendations for the slump of concrete used in different types of projects. For batch adjustments, slump increases about 25 mm (1 in.) for each 6 kg of water added per m³ (10 lb per cubic yard) of concrete.

Table 1 Recommended Slumps for Various Types of Construction (ACI)

Concrete construction	Slump, mm (in)
	Maximum**	Minimum
Reinforced foundation walls and footings	75 (3)	25 (1)
Plain footings, caissons, and substructure walls	75 (3)	25 (1)
Beams and reinforced walls	100 (4)	25 (1)
Building columns	100 (4)	25 (1)
Pavements and slabs	75 (3)	25 (1)
Mass concrete	75 (3)	25 (1)

 

** May be increased 25 mm (1 in.) for consolidation by hand methods such as rodding and spading. Plasticizers can safely provide higher slumps.

The equipment for the slump test is indeed very simple. It consists of a tamping rod and a truncated cone, 300 mm height and 100 mm diameter at the top, and 200 mm diameter at the bottom. The cone is filled with concrete and then slowly lifted. The unsupported concrete cone slumps down by its own weight; the decrease in the height of the slumped cone is called the slump of concrete.

In the slump test, there are four main types of slumps that can occur, each indicating different characteristics of the concrete’s workability and cohesion. Here's a discussion of the types of slumps, along with descriptions of how each look:

 

True Slump:

·       Description:

This is the type of slump, where the concrete subsides evenly and maintains its overall shape. The concrete mass retains uniformity, indicating that the mix has a good balance between water content and aggregate proportions.

·        Indication:

True slump indicates that the concrete has good workability and cohesion without segregation or excessive bleeding.

Zero Slump

·        Description:

Occurs when the concrete mix retains its shape completely after the slump cone is lifted, showing no measurable subsidence. This type of slump is typical in very stiff or dry concrete mixes where the water content is extremely low, and the mixture is very dense.

·        Indication:

A zero slump indicates that the concrete has very low workability and is often used in applications where concrete needs to be placed with minimal movement, such as in pavements or where mechanical compaction methods like roller compaction are used.

Collapse Slump:

·        Description:

 In a collapse slump, the concrete completely collapses after the cone is lifted, losing all shape. This occurs when the concrete is too wet and lacks cohesion, which can result in segregation of the aggregates and water bleeding out of the mix.

·        Indication:

A collapse slump indicates that the mix has too much water, making it unsuitable for most structural applications. Excessive water reduces the strength and durability of the hardened concrete.

Shear Slump:

·        Description:

In a shear slump, the concrete shears off and slides down one side of the cone instead of subsiding evenly. This type of slump suggests that the concrete has some degree of cohesiveness but may also indicate insufficient water content.

·        Indication:

Shear slump indicates that the mix is too stiff or dry, and it may result in difficulties during placement and compaction.

 

Figure: Types of Slumps

Scope

This test method covers determination of slump of hydraulic-cement concrete, both in the laboratory and in the field.

 

Purpose

 

To determine the slump of freshly mixed portland cement concrete, both in the laboratory and in the field. The purpose of this test is to evaluate the consistency of freshly mixed concrete to ensure that it has the right workability for the specific construction application. The slump test helps detect variations in water content or aggregate proportion that may affect the strength and durability of the concrete. It also serves as an on-site check to adjust the mix during construction as needed.

 

 

ASTM Designation

 

ASTM C143—Slump of Portland Cement Concrete.

 

 

Terminology

 

hydraulic cement —

a cement that sets and hardens by chemical reaction with water and is capable of doing so under water.

Portland cement—

a hydraulic cement produced by pulverizing clinker, consisting essentially of crystalline hydraulic calcium silicates, and usually containing one or more of the following: water, calcium sulfate, up to 5 % limestone, and processing additions.

Workability –

Workability of concrete is defined in ASTM C-125 as the property determining the effort required to manipulate a freshly mixed quantity of concrete with minimum loss of homogeneity. The term manipulate includes the early-age operations of placing, compacting, and finishing. The effort required to place a concrete mixture is determined largely by the overall work needed to initiate and maintain flow, which depends on the rheological property of the lubricant (the cement paste) and the internal friction between the aggregate particles on the one hand, and the external friction between the concrete and the surface of the formwork on the other.

water-cement ratio —

the ratio of the mass of water, exclusive only of that absorbed by the aggregates, to the mass of portland cement in concrete, mortar, or grout, stated as a decimal.

Concrete —

a composite material that consists essentially of a binding medium within which are embedded particles or fragments of aggregate; in hydraulic-cement concrete, the binder is formed from a mixture of hydraulic cement and water.

concrete, fresh —

concrete that possesses enough of its original workability so that it can be placed and consolidated by the intended methods.

concrete, hardened —

concrete that has developed sufficient strength to serve some defined purpose or resist a stipulated loading without failure.

Consistency (fresh cementitious mixture)—

the relative mobility or ability to flow.

Curing —

action taken to maintain moisture and temperature conditions in a freshly-placed cementitious mixture to allow hydraulic cement hydration and (if applicable) pozzolanic reactions to occur so that the potential properties of the mixture may develop.

cementitious material (hydraulic) —

an inorganic material or a mixture of inorganic materials that sets and develops strength by chemical reaction with water by formation of hydrates and is capable of doing so under water.

 

Significance and Use

This method measures the slump of freshly mixed Portland concrete cement (PCC). To some extent, this test indicates how easily concrete can be placed and compacted, or the workability of concrete. The slump test is significant in evaluating the concrete's behavior under field conditions. It helps ensure that the concrete mix meets the design requirements and can be easily placed and compacted. The test is crucial for maintaining quality control during construction and for adjusting mixes as needed on-site to avoid problems such as segregation or excessive bleeding. Consistency in concrete mix also affects the strength, durability, and finish of the final structure.

 Apparatus

Mold:

The test specimen must be formed in a mold made of metal or plastic that is resistant to cement paste and non-absorbent. The mold should have an average thickness of 0.060 in. (1.5 mm) and no individual thickness measurement less than 0.045 in. (1.15 mm). Plastic molds should be ABS plastic [Acrylonitrile Butadiene Styrene: properties: Tensile modulus of elasticity, at 23 °C is 320 000 psi, Tensile strength is 5670 psi, Percent Elongation at Break at 23 °C is 40%] or equivalent with a minimum average wall thickness of 0.125 in. (2.5 mm). The manufacturer or supplier must certify the materials used in mold construction. The mold should be in the form of the lateral surface of a cone with a base of 8 in. (200 mm) in diameter, a top of 4 in. (100 mm) in diameter, and a height of 12 in. (300 mm). The base and the top shall be open and parallel to each other and at right angles to the axis of the cone.

Tamping Rod:

A round, smooth steel rod with a diameter of 5/8 in.  ± 1/16 in. is required for tamping. The rod should be at least 4 in. longer than the mold depth but not exceed 24 in. in overall length. The tamping ends should be rounded to a hemispherical tip of the same diameter.

Measuring Device:

A measuring device is a rigid or semi-rigid length instrument marked in increments of 1⁄4 in. or smaller, with a minimum length of 12 in. [300 mm].

Scoop:

A scoop is a large enough size to represent each amount of concrete obtained from the sampling receptacle and small enough to prevent spillage during mold placement.

Sample

·        The elapsed time shall not exceed 15 min. between obtaining the first and final portions of the composite sample.

·        Calculate concrete materials for making freshly mix concrete using different W/C ratio.

·        Prepare a fresh batch of hydraulic-cement concrete in accordance with the project specifications.

·        Ensure the concrete is well-mixed, homogeneous, and free from any visible segregation or bleeding.

·        Mix concrete either manually or with a mechanical mixer. If a large quantity of mixed concrete exits, obtain a representative sample.

·        The sample should be large enough to fill the slump cone and should be representative of the entire batch.

Procedure

 

 

[1] To fill a mold, dampen it and place it on a level, level, moist, nonabsorbent surface.

 

[2] Hold it firmly during filling and perimeter cleaning by the operator standing on the two-foot pieces or by a clamping arrangement to a base plate.

 

[3] Immediately fill the mold in three layers, each approximately one-third of the volume of the mold. [One third of the volume of the slump mold fills it to a depth of 25⁄8 in. (70 mm); two thirds of the volume fill it to a depth of 61⁄8 in. (160 mm)]

 

[4] Use a scoop to Place the concrete in the mold and evenly distribute the concrete.

 

[5] Rod each layer 25 times equally over the cross section using the rounded end of the rod. For the bottom layer, this will involve inclining the rod slightly and making approximately half of the strokes around the periphery, and then progressing with vertical strokes spirally into the center.

 

[6] Rod the bottom layer across its depth. For each topmost layer, allow the rod to penetrate through the layer being rodded and into the layer below about 1 in.

 

[7] To fill and rodd the top layer of a mold, heap concrete above it before starting the rodding process. If the rodding results in subsidence, add more to maintain an excess above the top.

 

[8] After the top layer is rodded, use a screeding and rolling motion of the tamping rod to strike off the surface.

 

[9] Hold the mold down firmly and remove concrete from the surrounding area to prevent interference with slumping concrete movement.

 

[10]           Remove the mold immediately from the concrete by raising it carefully in a vertical direction. Raise the mold a distance of 12 in.  in 5 ± 2 s by a steady upward lift with no lateral or torsional motion.

 

[11]           Complete the entire test within an elapsed time of 2.5 min, from the start of the filling through removal of the mold without interruption.

 

Calculation

·        Immediately measure the slump by determining the vertical difference between the top of the mold and the displaced original center of the top surface of the specimen.

·        If two consecutive tests on a sample of concrete show a falling away or a shearing off of a portion of concrete from the mass of the specimen, the concrete probably lacks the necessary plasticity and cohesiveness for the slump test to be applicable and the test results will not be valid and make a new test on another portion of the sample.

Report

Report the slump value to the nearest 1/4 in. (5 mm).

 

LAB Assignment Questions:

Basic Understanding

1.   What is the slump test, and why is it performed on hydraulic-cement concrete?

2.   What does the slump value represent in concrete?

3.   Why is the workability of concrete important?

Procedure and Equipment

4.   Can you describe the procedure for conducting the slump test on fresh concrete?

5.   What is the purpose of the slump cone, and what are its dimensions?

6.   Why is the concrete placed in three layers in the slump cone during the test?

7.   What is the significance of rodding the concrete after each layer is placed in the slump cone?

8.   How do you measure the slump after removing the cone?

9.   What precautions should be taken while performing the slump test to ensure accurate results?

Types of Slump and Interpretation

10.  What are the different types of slumps (true slump, shear slump, collapse slump), and what do they indicate about the concrete mix?

11.  What would you infer if the concrete exhibits a shear or collapse slump?

12.  What does a zero slump indicate about the concrete mix?

13.  What factors can lead to a higher or lower slump value in concrete?

14.  What are the ideal slump values for different types of concrete applications (e.g., pavements, foundations, columns)?

Standards and Specifications

15.  Which standards (e.g., ASTM, IS) govern the determination of slump in hydraulic-cement concrete?

16.  What are the standard limits for slump in various construction practices?

17.  What is the relationship between the water-cement ratio and the slump of concrete?

Application and Practical Considerations

18.  How does the slump of concrete affect its placement, compaction, and durability?

19.  How can the workability of concrete be adjusted if the slump is too high or too low?

20.  What are the possible reasons for an unexpected slump result in a given concrete mix?

21.  How does the addition of admixtures affect the slump of concrete?

22.  In what situations would a higher slump be preferred, and when would a lower slump be more suitable?

23.  What role does the aggregate size and shape play in determining the slump of concrete?

24.  What precautions should be taken to maintain consistency in slump test results on a construction site?

 

References

ASTM C143: Standard Test Method for Slump of Hydraulic-Cement Concrete.