Concrete Tests

Q: What is Concrete Testing?
A: A concrete test is performed to learn more about the properties of a specific sample of concrete. A number of different tests can be performed on concrete both, on a job site and in a laboratory. Because concrete is an important structural element, testing is mandatory in many regions of the world, and construction companies must provide documentation of their testing and results when asked to do so by government representatives. Test results are also kept on file in case a problem develops in the future. There are a dozen different test methods for freshly mixed concrete and at least another dozen tests for hardened concrete, not including test methods unique to organisations like the Army Corps of Engineers, the Federal Highway Administration and state departments of transportation. 

 

Q: What are the different types of Concrete Testing?
A: One of the most classic concrete tests is a slump test. In a slump test, concrete is tamped into a cone, which is then removed to see how far the concrete sinks or slumps without the support of the cone. Changes in slump height between batches of concrete can indicate a consistency problem. There are also slump requirements for different types of construction; low slump height concrete, for example, cannot be used to build roadways. Thus, the slump concrete test is both a quality control measure and a consistency measure.
Other types of concrete test can include laboratory analysis of the components of concrete, to confirm that concrete contains what the manufacturer claims it contains and to learn more about properties, such as durability and permeability. Concrete must meet specific standards, especially for high stress projects like bridges and high-rise buildings, which makes lab testing important to confirm that concrete is safe to use and that it will withstand years of stress.
Material testing, in general, is designed to make sure that materials meet quality control standards, comply with any laws and contain the components people claim they contain. If concrete fails a concrete test, it may mean that part of a construction project needs to be redone to remove the faulty concrete or that a batch of concrete needs to be discarded.
People have been testing concrete for strength, durability, and workability since Roman times. Curiously, reports about falsification of concrete test results pop up in the news on a regular basis, with construction companies being accused of cutting corners to avoid having to replace work performed with faulty concrete. Reputable companies use third parties to test their concrete so that the results are neutral, ensuring that there will be no doubts about the safety of the concrete in the future.

 

Q: What are the most common tests for fresh concrete?
A: Slump, air content, unit weight and compressive strength tests are the most common tests for fresh concrete.
Slump is a measure of consistency, or relative ability of the concrete to flow. If the concrete can’t flow because the consistency or slump is too low, there are potential problems with proper consolidation. If the concrete won’t stop flowing because the slump is too high, there are potential problems with mortar loss through the formwork, excessive formwork pressures, finishing delays and segregation.
Air content measures the total air content in a sample of fresh concrete, but does not indicate what the final in-place air content will be, because a certain amount of air is lost in transportation, consolidating, placement and finishing. Three field tests are widely specified: the pressure meter and volumetric method are ASTM standards and the Chace Indicator is an AASHTO procedure.
Unit weight measures the weight of a known volume of fresh concrete.
Pouring cylinders of fresh concrete and measuring the force needed to break the concrete cylinders at proscribed intervals as they harden, test compressive strength. According to Building Code Requirements for Reinforced Concrete (ACI 318), as long as no single test is more than 500psi below the design strength and the averages of three consecutive tests equals or exceeds the design strength, then the concrete is acceptable. If the strength tests don’t meet these criteria, steps must be taken to raise the average.

 

Q: How is Self-Consolidating Concrete (SCC) tested?
A: Test Methods
SCC is characterised by its unique fresh concrete properties. Therefore, new test methods are being developed to characterise SCC properties. See table for test methods approved or under consideration at ASTM:


Test Method

Description

Slump Flow

ASTM C1611
Test is performed similar to the conventional slump test (ASTM C143) using the Abrams cone (use of inverted cone possible). However, instead of measuring the slumping distance vertically, the mean spread of the resulting concrete patty is measured horizontally. This number is recorded as the slump flow.

Additional information about the mixture can be obtained by measuring the time it takes for the patty to reach 500mm (20in). This is called the T50 value and is a measure of viscosity.
A final visual observation of the patty can be compared to an index called the Visual Stability Index, which rates the degree of segregation during the test.
Measured characteristic: Filling ability (deformability), viscosity & stability

J-Ring

ASTM C1621
The J-Ring consists of a ring of reinforcing bars such that it will fit around the base of a standard slump cone. The slump flow with and without J-Ring is measured and the difference calculated.
An index, called the Visual Blocking Index, is available to rate the segregation of the mixture during the test; however, this is not part of the ASTM standard.
Measured characteristic: Passing ability

Column Segregation

ASTM C1610
This test evaluates the static stability of a concrete mixture by quantifying aggregate segregation. A column is filled with concrete and allowed to sit for a while after placement. The column is then separated into three or four pieces. Each section is removed individually and the concrete from that section is washed over a No. 4 sieve and the retained aggregate weighed. A non-segregating mix will have a consistent aggregate mass distribution in each section. A segregating mix will have higher concentrations of aggregate in the lower sections.
Measured characteristic: Stability

There are many other accepted tests for SCC that have not been standardised within ASTM. The following table summarises some of the more recent or commonly used methods:


Test Method

Description

L-Box

This test evaluates the passing ability of the mixture. Concrete is placed inside the vertical portion of the testing apparatus. A grill placed at the entrance of the horizontal portion simulates
reinforcement. Once the concrete has flowed to the resting position, the heights of concrete in the vertical portion (H1) and the end of the horizontal portion (H2) of the apparatus are measured. The ratio H2/H1 is used as a measurement of passing ability. Values of 0.75 and higher are considered acceptable.
Measured characteristic: Passing ability
(photo courtesy of BASF Construction Chemicals)

V-Funnel

The V-Funnel consists of a V-shaped apparatus with an opening at the bottom. The time taken to empty the funnel is regarded as a measure of the viscosity of the mixture.
Measured characteristic: Viscosity
(photo courtesy of BASF Construction Chemicals)

Segregation Probe

The segregation probe test is a quick and easy test that measures the thickness of the mortar layer on the surface. A standard 150mm x 300mm (6in x 12in) cylinder is filled in one lift and allowed to sit for two minutes. A wire ring with a graduated stem is then placed on the top surface of a cylinder and allowed to settle for one minute before the depth of penetration is recorded. Penetration values range from less than 4mm (1/6in) for very stable mixtures to more than 25mm (1in) for highly unstable mixtures.
Measured characteristic: Stability

 

Q: What items should one look for in a fresh concrete test set? 
A: The ASTM 138 deals with the Test Methods for Density, Yield and Air Content of Concrete
The equipment required is 
1. Balance
2. Tamping Rod
3. Internal Vibrator
4. Measure - A cylindrical container made of steel or other suitable material
5. Strike off Plate
 6. Mallet

 

Q: Why is it important to test concrete?
A: Therehas always been, and will always be, a need to test products and materials to prevent disasters. Critical physical parameters must be measured to quantify performance and strength, ensuring our safety and the safety of our environment.

 

Q: Why is the concrete cube tested for weight before compression test?
A: The main reason for calculating the weight of concrete block before testing in the compression machine is to check whether the weight is approximately equal to the value, which is calculated by the multiplication of density and volume.  This is a way to see approximately whether the concrete being used is correct or not. Now the exact results can be calculated by the concrete compression-testing machine, which will give the strength the block can hold.
Mass = density x volume
The concrete block should weigh about 8 ~ 8.5kg for a 150*/150*150mm3. If the value does not come as the given value then the concrete mixture is not correct and the compressive testing is of no use, as the concrete being taken cannot be used in building structures.

 

Q: What is the volume of mould in VICAT's Apparatus for cement test as per Indian Standards?
A: This test is conducted to determine the percentage of water required for preparing cement test.

  • The volume of mould required is 300gm of cement and 30 by weight or 90gm of water is added to it. These ingredients are mixed thoroughly.
  • Thus the required mould is prepared n then filled to the vivant apparatus.
  • Time interval should be 3 ¾ to 4 ¼ minutes.

It also consists of a plunger and the settlements are noted. If the penetration is between 5mm and 7mm from the bottom of mould, the water added is correct.

 

Q: Can rebound hammers reliably measure compressive strength? 
A: Rebound hammers test the surface hardness of concrete, which cannot be converted directly to compressive strength. ASTM C 805-97, Standard Test Method for Rebound Number of Hardened Concrete, states, “because of the inherent uncertainty of estimating strength with a rebound number, the test is not intended as the basis for acceptance or rejection of concrete.”
There are many factors other than concrete strength that influence rebound hammer test results, including surface smoothness, finish, moisture content, coarse aggregate type and the presence of carbonation. Although rebound hammers can be used to estimate concrete strength, the rebound numbers must be correlated with the compressive strength of moulded specimens or cores taken from the structure. The procedure used to develop this relationship is described in ACI 228.1R-03, In-Place Methods to Estimate Concrete Strength, American Concrete Institute.

 

Q. Why do we need concrete testing?
A. Testing of concrete is required to know whether the concrete made is conformable with the required specifications or not. Testing is a party in Quality control system.

 

Q. What is Inspection?
A.  Inspection is simply a subsystem of the quality plan. It may be employed by the owner to evaluate future acceptance of the work or by contractors and material producers for quality control purposes. It includes visual observations, measurements and field and laboratory testing of activities, components and materials to specified requirements along with the recording and evaluation of much data.

 

Q. What is the significance of inspection and testing in construction?
A. Inspection and testing requirements vary based on the specific space and needs of construction and should be tailored to each project. The content of an inspection plan is dependent on the type and complexity of the project, special features informed, quality level desired, building code requirements and the responsibilities of the inspection organisation performing the work.
Inspection activities generally fall into on of the categories – owners’ inspection, Quality control inspection, contractor and compliance inspection by third party.

 

Q. What are the field tests for concrete in fort state?
A. The most common test is ‘Slump contest’ meant for workability of concrete. There are several other tests that include – test for cement content, w/c ratio, temperature of concrete, density, yield, air content, setting time, compatibility test etc. All tests need not be carried out for every batch of lament. The number of tests will be specified depending upon the intensity.

 

Q. What is the usefulness of Slump test of concrete?
A. Slump test of fresh concrete is an indication of the following:
If grading of the aggregates is uniform, it will indicate variations of water context pr water/ cement ratio.
If the amount of water added to the mix is accurate and surface mixture of fine aggregates is constant, then it will indicate variations in grading or variations in batching of element or aggregates.
If the cohesiveness of the mix for a particular workability is most adequate, due to inadequate concrete mix design and batching, the concrete core will collapse or shear off. In general, slump test provides an idea of consistency of concrete batch to batch of concrete. It should be carried out as soon as possible, at about the same time after mixing let us 5/10min. The method of slump test depends upon slump of concrete. For very law slumps – compacting factor method, foe low medium slumps- slump core and for very high slumps veer-bee flow method. All these methods are applicable to concrete without admixtures (in general)

 

Q. What are three types of slumps? What does it signify?
A. Please include 8.0 content with figures from slump test for quality control Ambuja Tech. Literature (p23) small book. Slump test to be carried out as per IS: 1199–1999

 

Q. What are the tolerance limits of work ability for acceptance?
A. As per is 4926, the tolerance limits of workability-
-           Slumps:  + - 25mm or + - 1/3 of the specified value whichever is less 
-           Compacting factor: + - 0.03 for specified value >- 0.9
+ - 0.04 for specified value <- 0.9 >- 0.8
+ - 0.05 for specified value < - 0.8
Flow test: Acceptance criteria to be established between supplier (in case of RMC) and purchaser.

 

Q. How do you estimate ‘Cement Content’ in fresh concrete in forth concrete?
A. National council for concrete and building materials (NCB) has developed a simple field test estimate current content in berth concrete. The apparatus contains a wooden box with a 2l plastic beaker in the centre, surrounded by non-conductive materials. It is provided with a lid and provisions for inserting thermometers and stirrers. The test procedure is as follows: Take 1kg of far concrete and pan it into the 2l plastic container. Add 800ml of water and allow to equilibrate to room temperature and initial temperature is recorded (+1) after stirring the concrete well. Then add 500g/435ml of HCL acid quickly and stir well with a wooden stick to ensure complete dissolution of cement. Due to the exothermic reaction, the temperature of concrete rises to its maximum extent and tends to fall. The beak temperature to the recedes (+2), (T2 – T1) gives rise in temp. The cement content in 1kg of fresh concrete may be calculated by using the formula.
C = 3 (T1 – T 2) + 12.28g/1kg of concrete.
Cement content in grams
Personal error in this method may be minimised to a greater extent by carrying out reneval mixer with known quantities of cement and known grade of courte. Results +- 10%.
This method is applicable for OPC based concrete.

 

Q. What is yield of concrete? Why do we need it?
Q. How do you determine ‘ setting time of concrete’? How does it different from setting time of lement?
Q. How do you carry out sampling of beth concrete? What is the minimum frequency to be adopted?
Q. What do you understand by Acceptance criteria for concrete?

Q. What are compressive strength compliance requirement for different grades of concrete?