Avoidance of Blockages in Concrete Pumping Process

This paper describes a joint research project about concrete pumping. After a careful literature review was carried out, an experimental 148 m-closed-loop pumping circuit was built near Roissy, France. Over 68 concrete loads were pumped-a number of which were subject to blockage. These experiments led to significant advances in the understanding of the blockage process. Four types of blockages were pointed out. The most frequent happens at priming. It is due to coarse aggregate particles, which tend to leave the concrete front and to flow through the grout section, eventually forming a dense plug ahead of the flow. The tendency to segregate is correlated with bleeding, a phenomenon that is easier to reproduce in a small-scale test. Such a bleeding test was developed by modifying a conventional air meter, providing a simple and cheap site test. The initial bleeding rate was correlated with the likelihood of blockage in the pumping circuit. Finally, a broad analysis of the blockage formation could be proposed, giving clues to practitioners for a better control of the concrete pumping process.

Keywords: bleeding; blockage; process; pumped concrete; test.

(ProQuest Information and Learning: ... denotes formulae omitted.)

INTRODUCTION

Pumping is a technique used to transport fresh concrete and allows that fresh concrete to be placed in the formwork without using a concrete bucket or a conveyor belt. The technique of concrete pumping has existed for almost 70 years. Concrete pumps in which concrete moves through metal pipes have been used in the U.S. since 1933. Pumping technology has undergone considerable development and many improvements have been made, particularly to the pumps-for example, the introduction of long boom pumps, high pressure pumps, etc. The concrete pump is used a great deal nowadays in commercial construction; it has many advantages over other techniques, such as:

* Rapidity of placement (a pump can convey between 10 and 150 m^sup 3^/h of concrete depending on its size, power plant, and type of concrete);

* Concreting can be performed where access is difficult (for example, in tunnels); and

* The rate of placement is constant so concrete crews can be used more efficiently.

The technique of pumping involves pushing concrete through pipes by means of a pump. The two main types of pumps are piston pumps and rotary or squeeze type pumps.1 In the first type, a piston lowers the pressure in one of the two cylinders. As a result, concrete is drawn in. In modern pumps, an agitating blade is fitted to the feed hopper. Its role is to maintain good intake. Its shape and mixing speed are designed to ensure maximum filling. Despite this, the cylinder is never full at the end of intake. The filling coefficient is the ratio between the volume of concrete drawn up and the total volume of the piston. The nearer this ratio is to unity, the greater the efficiency of the pump. Obviously, for a given pump, the filling coefficient depends on the concrete. The second phase consists of pushing the concrete into the pipe. The pump configuration allows alternation between the two pistons to occur; when one piston pushes concrete into the circuit, the other draws it in from the hopper. In the case of rotary pumps, delivery is based on a different principle: two or three rollers squeeze a nonrigid delivery tube in the stator, the latter being evacuated.

Pumpability can be defined as the aptitude of concrete to be placed using a pump. The concept of pumpability therefore relates to the formation of blockages and does not refer to such pumping parameters as flow and pressure.

To be pumpable, concrete must satisfy a number of criteria. It must remain homogenous throughout pumping and it must be deformable so it can negotiate elbows and tapers. The pumpability of concrete is quite difficult to forecast. Generally, pumpable concrete will be successfully manufactured if empirical mixture design rules are followed.2,3 If the concrete has been designed using other methods, it is difficult to predict whether it will be pumpable or not. Pumping conditions are not easy to reproduce in the laboratory and a full-scale test is relatively expensive. For this reason a number of researchers have tried to develop a simple test that makes it possible to model the behavior of concrete during pumping. But none of these tests are currently applied by practitioners. Concrete pumping remains to date an empirical, trial-and-error process, involving frequent troubleshooting on construction sites.

RESEARCH SIGNIFICANCE

The aim of the research presented in this paper, which was carried out in the framework of the French national project CALIBE, is to make a careful literature review of concrete pumpability, to understand the process of plug formation based upon a series of full-scale pumping tests, to develop a simple test usable on site for predicting the risk of blockage, and to provide recommendations for avoiding blockage at the various stages of the concrete pumping process.