Five On-site tests you should know as a Civil Engineer

As a Civil Engineer, we often need to perform various tests on site. These tests help us to know about quality of materials and works. Every test has their own purposes to serve. Today we are going to discuss Five on-site test procedure that you should know as a civil engineer.

Slump Test:

The Concrete Slump Test is one of the most used tests at the site to measure the consistency of the fresh concrete before it sets.

We perform this test to check the workability and flow of freshly made concrete. It helps us to know about the improperly mixed batch of concrete. This test needs a very simple apparatus to perform with a very simple procedure. We perform the slump test to ensure the uniformity of concrete for different loads under field conditions. We use a metal mold in the shape of a conical frustum which we call the slump cone. The cone is open at both ends with an internal diameter of 100 mm on the top and 200 mm at the bottom with a height of 305 mm.

To perform the test, we should place the cone on a hard non-absorbent surface.

Then we should fill the cone with fresh concrete in three stages. Each time, we should tamp the concrete layer 25 times with a 600mm long bullet-nosed metal rod with 16mm dia. We should strike off the concrete flush with the top of the mold. Then we should carefully lift the mold vertically upwards. The concrete then slumps. Then we place the mold just beside the concrete and measure the distance between the top of the slumped concrete to the level of the top of the slump cone. This value helps us determine the concrete consistency, we can decide if the concrete is good to use for structures or not. 

The slump test has some its limitations. This test is suitable only for the slumps of medium to low workability which is in between the range of 5 to 260 mm. The test fails to determine the difference in workability in stiff mixes which have zero slumps or for wet mixes that have a collapse slump.  

Soil Field Density Test:  

Soil Field Density Test or FDT is another common test procedure in construction. Where there is earthwork, there is FDT. At the construction site, we need to excavate soil to continue the works. Then we again backfill the excavated area. This backfilling procedure is done layer by layer with proper compaction of each layer by proper compacting equipment. The layer thickness may vary under on-site conditions and equipment’s compaction energy. So, how can we be sure about the proper compaction of each layer? Here comes the FDT. This test measures the relative density of compacted soil comparing the maximum dry density obtained from the field test with the maximum dry density of the laboratory test as a standard reference.   

 The most common procedure to perform FDT on the construction site is the sand cone procedure. In this procedure of FDT, at first, we measure the weight of the empty cone. Then we fill the cone with dry sand and take the weight again.  

 Then, we make a hole where we want to perform the test. We should measure the excavated soil (from the hole) weight and keep a record of these weight measurements. After that, we should collect specimens from it to measure the water content in it. After that, we should place the base plate over the test hole and place the sand cone on it. Then we should open the control valve to allow the sand to fill the test hole. When the sand stops running, we should close the control valve and weigh the cone again with the remaining sand.   

 Then using the equation, we can easily measure the dry density of the compacted soil.  

 Dry Density = Wet Density x 100%/ Moisture Content of soil + 100;  

 where,  

 The Volume of the test hole = Mass of sand in the test hole/ Density of used sand  

 Wet Density = Mass of excavated soil/ volume of the test hole  

 To determine the degree of compaction, we can use the below equation:  

 Degree of compaction = Dry Density x 100% / Maximum Dry Density  

 The procedure of sand cone test method is a standard procedure approved by various International Standardization organizations, such as ASTM refers to this method in ASTM D1556/D1556M  

Pile Integrity Test:  

Pile is a slender cement cast deep foundation technique that helps a structure to transmit its load to the underground. It is mostly invisible as we construct or drive it direct underground. So how can we be so sure about its integrity? As site experience shows us that all kinds of flaws may occur during pile construction which can be a great problem for the structure.

The main purpose of the Pile Integrity Test is to find out those flaws and rectify those before it becomes bigger problems for structures. This test is very cost-effective but not much time-consuming, so construction engineers perform this test worldwide.

We normally perform Pile Integrity Test by using a Motion Sensor and a special hammer that weighs less than 1 kg with a special impact tip made of plastic. At first, we need to make the surface of the pile head very clean by removing all loose concrete and any kind of contamination. The surface should be dry to perform this test. Then we should set the motion sensor on the top of the clean surface of the pile head. After that, we strike on top of the pile head with a special hammer to generate a compressive stress wave that can travel through the pile.

The pile toe or any other anomalies within the pile should reflect the stress wave on its way back to the pile head. The travel of this compressive stress wave throughout the pile creates a graph on the screen and shows us if there is any impurity or abnormality within the pile. We should repeat this procedure at least three times in three different positions for a single pile to be sure about its quality and integrity.   

 ASTM refers to this test in ASTM D5882.  

High Strain Dynamic Load Test for Pile:  

High strain Dynamic Load test for piles helps us to obtain information on piles about capacity, integrity, and installation. It provides a fast, reliable, and cost-effective method for the evaluation of cast or driven pile capacity.

To perform this test, we need to attach a minimum of two pairs of strain transducers and accelerometers on opposite sides diagonally of the pile at a minimum distance of 1.5 times the pile diameters below the pile top. We use an impacting hammer weigh typically 1% to 4% of the static test load. We drop this hammer on top of the pile head from a pre-determined height to measure the strain induce under the impact of heavy falling with the help of strain transducers attached to the pile, whereas accelerometers record the accelerations generated in the pile.

The Pile Driving Analyzer then converts strain to force and acceleration records to velocities. The pile develops a resistance which is then a function of force and velocity. We should input a few assumed factors based on the soil type. We can find the maximum pile top compression by integrating the pile top velocity. We continue this testing procedure at the site by increasing the dropping height in stages until we achieve the test load or the pile shows us a permanent set of at least 4 mm per blow for several blows. After we complete the field testing, we can achieve a more accurate value of these parameters from CAPWAP analysis using the field data. We can find this test procedure on ASTM D4945 – 2016.  

Dye Penetration Test:  

We often perform Dye Penetration tests on-site to check the surface quality. Mostly we perform this test to check the welding quality as it helps us find the narrow openings on the surface which may not be visible to the eyes.

So, if the welding work does have an opening, this test helps us to find that out. This test is also known as liquid penetrant inspection or liquid penetrant testing.

We can use this test to inspect almost all non-porous materials. This test is based on the principles of Capillary Action. As we know, liquid tends to flow or seep into any narrow opening even against external forces which occur due to molecular attraction.

The test procedure for Dye Penetrant Test can vary depending upon the factors such as the used penetrant system and the testing environment and conditions. We must clean the testing surface properly by removing dirt, oil, paint, grease, water, or any other contaminants. We can use dry cloths, solvents, cleaner, et. Depending on the condition of the testing surface. After a thorough cleaning, we can apply the penetrant which is a red-colored liquid by spraying or brushing. We should maintain the surface temperature between 5 degree C to 52 degree C.

We should maintain the ambient temperature, otherwise, elevated temperature can evaporate the penetrant .

Then we should leave the penetrant on the surface for a minimum period which may vary from 5 min to 60 min, depending on the surface material and its condition. The manufacturer of the penetrant also mentions this period, which we can call as Dwell Period. In this period, the penetrant seeps into the flaws due to Capillary Action. After the dwell time, we should clean the excess penetrant by using penetrant removal. We should maintain unidirectional cleaning throughout the surface.

Once the surface is cleaned properly, we can apply the developer by spraying. We should maintain a distance of 10-12 inches from the surface while spraying the developer. Then w should wait at least 10-60 minutes, which we call the development time. In this period, the developer sucks the trapped penetrant on the surface from the flaws (if there are any). That is the penetrant bleeds out on the surface and we can identify it by its sharp red color as the developer is a white color liquid.