Aug 02

How to Minimize Dust Exposure in Shotcrete Placement

Dust generation may seem to be part and parcel of any construction job and nothing to worry about but long-time exposure and inhalation of dust can lead to damaging health effects especially when working in tunnels and mining jobs.

Let’s first look into factors that cause excess dust and how we can avoid or reduce them. Take for instance, the process of building tunnels. It requires going through a series of steps that generate huge amounts of dust: drilling, namely: blasting, crushing, extracting and shotcrete. While the first four processes have to control the dust generated and remove it from the atmosphere through special ventilation, water sprays and dust collectors, shotcrete placement doesn’t.  However, hiring a shotcrete crew that is experienced in working underground will minimize dust generation because they will know how to control it at the source.


Where is silica found:
Silica is found in asphalt, brick, cement, concrete, drywall, grout, mortar, stone, sand, and tile. OSHA regulation 29 CFR 1926.55(a) requires that exposures must be below a Permissible Exposure Level (PEL) of 0.1 mg/m3. The National Institute for Occupational Safety and Health has a lower Recommended Exposure Level of 0.05 mg/m3.

Health risks from exposure to Silica:
Repeated exposure to cement dust will lead to irritation to eye, nose, throat and upper respiratory system. When skin is directly exposed to cement, further irritation can occur and skin cracking can result from chemical burns. Rinse eyes or skin with water and soap if it comes into contact with cement dust and if the burning continues consult a doctor.

Another serious risk for workers is lung injuries from Silica exposure. This can lead to chronic obstructive pulmonary disease (COPD), lung cancer, tuberculosis and Silicosis. There are about 3,600 to 7,300 new cases of silicosis every year. Note: This is a progressive illness that will continue to worsen even after exposure to silica has stopped. While it may take years for these diseases to show up, workers should be on the lookout for certain symptoms like a chronic dry cough and shortness of breath. These disabling, irreversible and at times fatal diseases occur when silica particles are inhaled by workers. As a result, nodules start growing and can become so large that they impede normal breathing.

concrete safety

Dust Control Checklist

How to protect your crew:
1. Prevent dust from being released in the air by using water or vacuums at the source.
2. When water and vacuums are not available use a respirator.
3. Replace sand with aluminum oxide for blasting.
4. Use a wet abrasive blaster that mixes water with media before leaving the nozzle.
5. Avoid eating, drinking and smoking in dusty area to reduce, wash face and hands first.
6. Use disposable clothing or wash work clothes at the work site.
7. Shower and change into clean clothes before leaving the job site to avoid contamination.
8. Do a periodic lung screening.

With over 40 years of experience in the industry, we know the importance of safety at work. We also know that using the right pump for the job means even greater safety, better results and more savings in the long run. So if you are looking for a great deal on concrete or plaster pumps, give us a call at (503)283-2105  or visit our website.

Additional reading:
Concrete Safety procedures
Maintenance Tips for gunite
Work Safely with Silica 

Jun 30


In our previous article, we investigated slump testing as one of the most commonly used test method by site engineers to ensure that fresh poured concrete has the right consistency and workability. We also learned that this method is not optimal for all forms of concrete. There are 2 other methods for testing consistency, they are Flow test (using vibration) and Ball penetration test.

Fresh Concrete Testing with a Flow Table

Flow test:
Also known as Flow Table Test is often performed when fresh, unhardened self-consolidating concrete arrives on site and the operator needs to monitor its consistency before pouring can start. It is simple, inexpensive and can be easily reproduced and conducted on any site.

How does it works:  Viscosity is what determines the rate of spread.
1. Make sure the flow table is perfectly flat on the ground.
2. Clean the surface of the flow table.
3. Place the cone in the center of the flow table in between your feet and proceed as you would for a standard slump test (fill it with fresh concrete applying 2 equal layers.that each have been tamped down a dozen times). Level the top of the mold removing any excess. Clean up the table of any extra concrete.
4.Wait 30 seconds, then lift the cone.
5. Raise up the tabletop 40 mm and drop it 15 times forcing the concrete to flow.
6. You can now measure the diameter with a rule in both directions to the nearest 10 mm. Watch a video of a flow table test being conducted.

Tools for performing a flow test

Best applications:
This testing methods works best for high concrete workability using coarse aggregates no larger than 25 mm (1 in.) and with a slump of more than 175 mm.The reason why is that anything larger will not produce flowable and nonsegregating self-consolidating concrete.

Flow test results can be difficult to interpret, so they are used primarily as a qualitative index of workability. The good news is that there are several other tests relying on vibration such as compaction test (Waltz test), Vebe consistometer, Thaulow tester etc. that can also measure the rheological properties of cement mixtures.

Understanding the results:
If the concrete is too pasty, it which will cause cavities and corrosion of the rebar in the medium term and weaken the concrete’s ability to resist stress.
A mix that tends to segregate will produce a non-circular pool of concrete.
If a ring of clear water appears after a few minutes, the mix indicates a bleeding problem.
NOTE: concrete slump and final mortar spread correlate linearly when the concrete slump is greater than sever inches.

Diameter of flow (cm)-25
———————————— X 100 = FLOW %

% of Flow












Dick Hibbard

Combining a slump test with a flow table test will ensure that your mix has the right consistency and workability on the job. So don’t cut corners and do the right thing, it will save you money and time. And if you are looking for a great deal on a concrete pump, give us a call at: (503)283-2105. We would love to hear from you!
ASTM Standards
Sampling & testing fresh concrete (UK)
Flow Test Evaluation (Georgia Department of Transportation)

Jun 20


Fresh Concrete Testing

Considering the many factors that can affect the quality of fresh concrete, it is understandable that dozens of tests methods are available and being used today. However, we will focus today on one of the most prevalent test method called Slump test.

What are the main properties of fresh concrete that should be measured?
Primarily its strength and durability. Both factors are highly dependable on the degree of compaction and affected by transportation, placement and curing. So creating and maintaining the consistency of the mix are essential to a successful job and regular testing will help achieve the desired results.

There are 4 main areas that need to be tested:
1. Consistency
2. Workability (compacting factor test, WeBe Time test)
3. Segregation
4. Bleeding water test

Consistency or fluidity of concrete:
Think of it as the degree of wetness but beware, the wetter the mix doesn’t equate to greater workability.  Too much water can lead to segregation, too much bleeding, sand streaking on the surface. Too dry of a mix and holes and cracks (another form of segregation) will form as a sign of low plasticity.

What kinds of Test should be done
The most commonly used method is called Slump test. It can easily be performed and doesn’t require complicated equipment. It measures the behavior of an inverted cone of concrete (focusing on workability, consistency & wetness).

What does the process entail: Use a lightly moisten slump cone (metallic mould open at both ends with a handle). Fill it in three successive layers that have been each temped +20 times. It is recommended to hold the mold with feet to make sure that it doesn’t move during the pouring of the concrete. Quickly lift the mold right after it has been filled to the top, place the mold next to it. You can now measure the decrease in height (usually in increments of ¼ in).

Main Slump test results

Types of Slump
Collapse Slump
: it indicates a mix that is too wet or that the high workability mix you need shouldn’t be tested with this method.

Shear Slump: either the top or half of the cone tapers off. You should repeat the test. If the same result keeps happening, your mix needs more cohesion.

True Slump: most of the original shape remains, indicating a dry mix with stiff consistency.
Note: Slump test is not reliable for lean mixes (lower cement content to liquid ratio used for base layers).

Action to take:
Any variation in slump results is a signal to the mixer operator that he needs to make a change. If you experience an increase in slump, your moisture level could have suddenly increased or you may not be adding enough sand to your mix without you realizing it.


SLUMP (mm)



Very low



Super dry mixes for making road with power operated vibrated machines




For foundations with reinforcement and road with hand operated machines




Manually compacted flat slabs and reinforced concrete




great for void filling, underwater applications, pumping over long distances, large flat areas, not appropriate for vibration

When to perform a Slump Test?
On site on a day-to-day or even hour-to-hour basis when material is being brought into the mixer.

Is there an easier way to conduct the test?
You can use a K-Slump tester (best for in-situ measurement and in form testing). It looks like a syringe.

There are digital Slump Meters that use sensors and controls. They will save operators time to clean, pour, temper and measure the slump throughout the job.

There are other more sophisticated systems that combine devices and software to monitor the consistent quality of concrete from the plant and during transportation in the ready mix truck (concrete process control)

Are there any limitations to the slump test?
This test is best for testing medium to high workability (between 5-260 mm). It should only be performed on concrete using 1.5 inch aggregates or less and not be used for stiff mixes with zero slump or for wet mixes which resulted with a collapsed slump.

If the on-site slump test fails, should engineers allow the contractor continue the concreting work?
In some cases, they can order to stop the job. Depending on which school of thought they follow, some engineers may trust compression tests more than slump tests. If this is the case, they will let the contractor continue their work but demand that the finished work complies with the agreed upon compression strength. However, if the compression test fail, the work will have to be entirely demolished and redone. This approach is both costly in terms of time, resources and will break the budget. So why risk it when you can do it right from the beginning?

Using the right concrete pump for the job is also essential. With over 40 years in the concrete pumping industry, United Equipment Sales stand by its products, can help you solve challenges on the job and give you great deals on your next machine. Call Dick at (503)283-2105

Suggested Further Reading:
Fresh Concrete Properties and Its Standard Tests
The Importance of Testing Concrete
Slump Test  


Mar 31

Maintenance Tips for Gunite/Dry-mix Equipment

A little planning and precaution always goes a long way in the success of a project and the life of your equipment.

For any given job, a gunite contractor and its crew will have, as a very minimum, to operate and maintain a gunite machine, a material nozzle and hose, water hose, air compressor and an air hose.

Lubrication is an essential part of maintenance

A good part of maintenance on new and used machines will involve regularly oiling various part of your equipment and replacing certain parts over time. That’s just the reality of operating heavy equipment! Another good thing to remember is that every part of the machine that touches material flowing through it will wear out the fastest. So let’s see what some of these are.

If you have a bowl-type gunite machine, you will eventually have to replace the wear plate, the wear pad and material outlet. But before you get to that point, you will need to oil the pockets of the bowl after having cleaned it first after each job so that you don’t get material accumulation. If you have a steel bowl, you will need to use a hammer and something like a chisel to dislodge dried leftover material. If you have a steel/poly bowl, flip it on its belly and pound it  all over using a rubber mallet then chip any small pieces left, after every job.

Over time, the wear plate will also start to degrade. How can you tell?  Grooves will be showing up on the plate and/or in the pocket surfaces which can affect the rotation of the plate. When this happens, you should have a company resurface the plate.

Another part that will also show sign of deterioration is the wear pad due to its contact with the material. Don’t wait to replace it because it will cause faster wearing out of the wear plate if left damaged. It will also affect the quality of the air flow into the system.

DON’T: Tighten the hold clamp to try to extend the life of the wear!
DO:  Replace the wear pad as soon as it shows signs of slight deterioration.

There is also a piece of felt that protects the wear plate from the bottom of the hopper that should be lubricated until it becomes flexible and seal. No need to take anything apart to reach the felt, just use the holes in the hopper to do the lubricating. Having well oiled seals will prevent dust from escaping and create a safer work environment for your crew.

DON’T: Ignore an accumulation of dust next to your gunite machine. This is a sign that something is not working properly and needs to be checked.
DO: Replace the felt piece with a new one.

What about the air motor? The air lines comes with a filter and lubricator. Without the filter, the air that goes into the motor wouldn’t be clean and the lubricator puts a small amount of oil into the air. Both are required for the motor to function properly. As with all filters, you will need to check it to see whether or not they need to be replaced. Make sure to regularly inspect and adjust the oil level of the air lubricator.

Most gunite machines are connected to a gear box which will also need to be oiled. Some models come with an exhaust chamber that needs to be emptied out on a daily basis. The idea is to remove anything that could end up in the hopper base and could prevent rotation in the feed bowl.

DON’T: ever reach inside the hopper with your hands or arms while the air line is connected because the agitator could be moving.

DO: Connect an extra exhaust hose to the machine to bring down even more the level of dust on the job site.


What about hose accessories and nozzle maintenance?
The one more important piece to take care of in a nozzle is the water ring, usually made of brass or aluminum. You must inspect the tiny holes in the ring before starting any job and make sure there is nothing clogging them from a previous job. When cleaning the holes is no longer possible it is time to replace the ring.

DO: coat the water ring with some oil for extra protection
DON’T: ever drill holes in the water ring if the original holes are no longer working.

To eliminate any potential defects which cannot be seen by visual examination or in order to determine certain characteristics of the hose while it is under internal pressure, we suggest you conduct non-Destructive Hose Pressure Tests

Airplaco PG25 Gunite Machine

If you are looking for a  great gunite machine for your next job, give us a call at 503-281-2105 and we can help you find a perfect fit.